Astronomija

Kako dolgo so planeti TRAPPIST-1 v bivalnem območju?

Kako dolgo so planeti TRAPPIST-1 v bivalnem območju?


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Planeti, ki krožijo okoli TRAPPIST-1, so v orbiti okoli svoje zvezde veliko bližje kot Zemlja okoli Sola. Ker pa je TRAPPIST-1 hladen rjavi pritlikavec, so nekateri planeti v "bivalnem območju", kjer temperatura ne bi bila preveč ekstremna za življenje, kot ga poznamo.

Sprašujem se, kako dolgo so ti planeti v bivalnem območju? Na strani wikipedije piše, da je TRAPPIST-1 starejši od 1 Gyr. Toda, koliko časa je bil to hladen rjavi pritlikavec?

Z drugimi besedami, ker sta življenjska abiogeneza in razvoj živih organizmov odvisni od časa, koliko časa se je življenje lahko razvijalo v bivalnem območju zvezde?

Pri tem vprašanju me ne skrbi kemija ali druge značilnosti samih planetov.


Ocenjeno je, da je svetilnost Trappist-1 5,25 USD krat 10 ^ {- 4} L _ { odot} $, vendar ni bila vedno takšna.

Svetilnost rjavega pritlikavca se s časom zmanjšuje in ta izmerjena svetilnost (skupaj s spektralnim tipom) omogoča oceno mase in a spodnja meja do starosti z zvezdnimi evolucijskimi modeli.

Če pogledam Baraffe in sod. (2015) evolucijski modeli z majhno maso in si oglejte mesto svetilnosti v primerjavi s časom za 0,08 $ M _ { odot} $ zvezdo, kot je Trappist-1, lahko vidite, da trenutna svetilnost pomeni starost $ sim 500 milijonov USD letih. Če pa se vrnete v preteklost, je bila zvezda bolj svetleča in iz tega razloga planeti, ki so trenutno v bivalnem območju (planeti e, f, g) v preteklosti niso bili takšni.

Podrobnosti o izračunu bivalnega območja (HZ) so lahko zapletene, vendar je v bistvu polmer bivalnega pasu omejen kot kvadratni koren svetilnosti. Če sta planeta d in h ne trenutno v HZ, potem jih lahko uporabimo kot konzervativno definicijo meje HZ.

Iz tega (in z uporabo objavljenih orbitalnih polmerov planetov) vidim, da če se svetilnost poveča za faktor 9, potem nobenega planetov b-g v HZ, je večja od vseh njihovih orbit. Trappist-1 je imel svetilnost, ki je bila devetkrat večja, ko je bila mlajša od 27 milijonov let. Po drugi strani pa, če želim premakniti HZ tik izven orbite planeta e (in hkrati vključiti planet h znotraj HZ), bi se to zgodilo, ko bi bil Trappist-1 v starosti 206 milijonov let. Kot zadnja misel lahko iz tega posebnega modela vidite, da lahko Trappist-1, ko se stara, zbledi še za dvakrat. To zmanjšuje polmer HZ za faktor 1,41 in bi pomenil, da bi g (in morda f) padel zunaj HZ, medtem ko bi bil d (in morda c) vnesen v HZ.

Vendar je treba opozoriti, da: različni modeli dajejo nekoliko različne rezultate, ti lokusi so odvisni od mase in masa ni znana, iz istih modelov se sklepa z uporabo ocene temperature (ki je prav tako negotova). Torej, medtem ko so moji kvalitativni zaključki o preteklosti lokacija HZ verjetno pravilna (čeprav so podrobne starostne številke odvisne od modela), je prihodnje vedenje HZ bolj negotovo, ker je Trappist-1 morda nekoliko masivnejši od predvidenega in že dosegla najnižjo svetilnost.

Razvoj svetilnosti Trappist-1, ob predpostavki mase 0,08 milijona dolarjev _ { odot} $ in modelov Baraffe in sod. (2015). Vodoravna črtkana črta označuje najboljšo oceno njene trenutne svetilnosti, za katero naj bi bili planeti e-g v HZ. Če se vrnemo v preteklost, skrajna desna navpična črtkana črta označuje starost, pod katero se svetilnost poveča do te mere, da e postane prevroče, da bi bilo vseljivo. Nato naprej levo navpična črtkana črta označuje točko, kjer vsi trenutno znani planeti (b-h) postanejo neprimerni za bivanje.

Odgovor na vaše vprašanje je torej precej negotov in je kritično odvisen od starosti Trappist-1 zdaj in seveda ali so bili planeti vedno v polmeru orbite, kot so zdaj$ ^ {*} $. Kot lahko vidite iz zgornje ploskve (upoštevajte logaritemsko lestvico na osi x), zgoraj omenjeni razvoj svetilnosti poteka zgodaj. Če bi bil Trappist-1 star samo 500 milijonov let, bi bilo življenje na planetu e mogoče le 300 milijonov let. Če pa je zvezda nekoliko bolj masivna in je stara 10 milijard let, je življenje moralo začeti 9,8 milijarde let.

Če govorite o planetu f, je imel znotraj HZ nekoliko daljši čas ($ sim + 100 milijonov dolarjev let) in planet g spet nekoliko daljši ($ sim + 70 milijonov dolarjev let). Planet h bo v HZ preživel razmeroma malo časa (v preteklosti), manj kot nekaj sto milijonov let.

$ ^ {*} $ Povzetek dokumenta o odkritju Gillon et al. (2017) na kratko razpravlja o možnosti selitve planetov navznoter po nastanku s postopkom "diskovne migracije". Če je tako, potem bo to ne spremeni zgornjo razpravo. Disk okoli zvezd z zelo majhno maso je lahko daljši od tistih okoli zvezd z višjo maso, vendar se je v bistvu razpršil po 10 20 milijonih dolarjih let (Kennedy & Kenyon 2009; Dawson et al. 2013; Binks & Jeffries 2017) , in planetarno konfiguracijo bi bilo treba do trenutka, ko disk izgine, urediti tam, kjer je zdaj.


TRAPPIST-1 & # 8217s & # 8216habitable & # 8217 planeti so morda zanič vse življenje

Ta članek lahko delite z licenco Attribution 4.0 International.

Dve novi študiji bi lahko astronomi na novo opredelili bivalno območje za TRAPPIST-1, sistem, kjer sedem zemeljskih skalnatih planetov kroži okoli hladne zvezde.

Od odkritja leta 2016 so planetarni znanstveniki navdušeni nad TRAPPIST-1. Trije planeti so v bivalnem območju, območju vesolja, kjer lahko tekoča voda teče na planetih in # 8217 površinah.

Relativna velikost planetov TRAPPIST-1 in njihovih orbit. Celoten sistem TRAPPIST-1 bi se lahko prilegal orbiti Merkurja, z veliko prostora. Rdeči pas označuje orbite, kjer je prostor pretopel, da bi se tekoča voda lahko združila, modri pas označuje, kje je prostor prehladen, da bi bila voda tekoča, in zeleni pas označuje bivalno območje. (Zasluge: NASA / JPL-Caltech)

Trije planeti v bivalnem območju se verjetno soočajo z močnim nasprotnikom življenja, ugotavljajo nove raziskave: visokoenergijski delci, ki jih bruha zvezda. Raziskovalci so prvič izračunali, kako močno ti delci prizadenejo planete.

Medtem pa druga študija ugotavlja, da gravitacijsko vlečenje vrvi, s katero se igrajo planeti TRAPPIST-1, dviguje plimovanje na njihovih površinah, morda poganja vulkanske aktivnosti ali ogreva z ledom izolirane oceane na planetih, ki so sicer premrzli, da bi podprli življenje .

Tako članek o visokoenergijskih delcih kot prispevek o plimovanju se pojavita v Astrofizični časopis.

Visokoenergijski protonski dež

Zvezda & # 8217s zvezda, TRAPPIST-1A, je manjša, manj masivna in hladnejša od 6000 stopinj Celzija od našega 10.000-stopinjskega sonca. Je tudi izjemno aktiven, kar pomeni, da oddaja ogromno količino visokoenergijskih protonov - enakih delcev, ki povzročajo polarne svetlobe na Zemlji.

Federico Fraschetti, predavatelj teoretične fizike astro delcev na oddelkih za planetarne vede in fiziko na Univerzi v Arizoni, je s svojo ekipo simuliral potovanje teh visokoenergijskih delcev skozi magnetno polje zvezde. Ugotovili so, da četrti planet - najbolj notranji svet znotraj bivalnega območja TRAPPIST-1 - morda doživlja močno bombardiranje protonov.

& # 8220Tok teh delcev v sistemu TRAPPIST-1 je lahko do 1 milijon krat večji od pretoka delcev na Zemlji, & # 8221 pravi Fraschetti.

Vtis umetnika o večnem sončnem vzhodu, ki bi lahko pozdravil obiskovalce na površju planeta TRAPPIST-1f. Če je planet plimno zaklenjen, je lahko & # 8220terminatorska regija & # 8221, ki deli nočno in dnevno planoto, kraj, kjer se lahko življenje zaživi, ​​tudi če dnevno stran zasipajo energični protoni. Na tej sliki je TRAPPIST-1e viden kot polmesec v zgornjem levem kotu slike, d je srednji polmesec in c je svetla pika ob zvezdi. (Zasluge: NASA / JPL-Caltech)

To je presenetilo znanstvenike, čeprav so planeti veliko bližje svoji zvezdi kot Zemlja soncu. Magnetna polja prenašajo visokoenergijske delce skozi vesolje, magnetno polje TRAPPIST-1A & # 8217s pa se tesno vije okoli zvezde.

& # 8220 Pričakujete, da bi se delci ujeli v te tesno ovite črte magnetnega polja, če pa vnesete turbulenco, lahko uidejo in se premikajo pravokotno na povprečno zvezdno polje, & # 8221 pravi Fraschetti.

Plameni na površini zvezde povzročajo turbulenco v magnetnem polju, kar omogoča protonom, da odplujejo stran od zvezde. Kam gredo delci, je odvisno od tega, kako je magnetno polje zvezde nagnjeno stran od osi vrtenja.

V sistemu TRAPPIST-1 bo najverjetneje poravnava tega polja energetske protone pripeljala neposredno na obraz četrtega planeta, kjer bi lahko razločili kompleksne molekule, ki so potrebne za izgradnjo življenja - ali pa bi lahko služili kot katalizatorji za ustvarjanje teh molekul.

Medtem ko magnetno polje Zemlje ščiti večino planeta pred energičnimi protoni, ki jih oddaja naše sonce, bi moralo biti polje, ki je dovolj močno, da odbije protone TRAPPIST-1 & # 8217s, neverjetno močno - stotine krat močnejše od Zemlje. Toda to ne pomeni nujno smrti v sistemu TRAPPIST-1.

Planeti TRAPPIST-1 so najverjetneje plimsko zaklenjeni, kar pomeni, da je ista polobla vsakega planeta vedno obrnjena proti zvezdi, medtem ko večna noč zavije drugega.

& # 8220 Mogoče je nočna stran še vedno dovolj topla za življenje in je ne bombardira sevanje, & # 8221 pravi Benjamin Rackham, raziskovalni sodelavec z astronomskega oddelka, ki ni sodeloval v nobeni študiji.

Oceani bi se lahko tudi zaščitili pred uničujočimi visokoenergijskimi protoni, saj lahko globoka voda absorbira delce, preden raztrgajo gradnike življenja. Plimovanje v teh oceanih in celo v skalah planetov ima lahko druge zanimive posledice za življenje.

Plimno vlečenje vrvi

Na Zemlji Luna ne dviguje samo plime in oseke v oceanih - plimovalne sile deformirajo tudi sferično obliko plašča in skorje Zemlje. V sistemu TRAPPIST-1 so planeti dovolj blizu, da so znanstveniki domnevali, da svetovi morda dvigujejo plimo in oseko, tako kot Luna na Zemljo.

& # 8220 Ko se planet ali luna deformira od plime in oseke, bo trenje v njej ustvarilo ogrevanje, & # 8221 pravi Hamish Hay, podiplomski študent v Lunarnem in planetarnem laboratoriju in vodilni avtor druge študije.

Z izračunom, kako bi se gravitacija planetov TRAPPIST-1 & # 8217s vlekla in deformirala, je Hay raziskoval, koliko plimovanja v sistem prinese plimovanje.

TRAPPIST-1 je edini znani sistem, pri katerem lahko planeti dvignejo pomembne plime in oseke, ker so svetovi tako tesno nabiti okoli svoje zvezde.

& # 8220To je tako edinstven postopek, o katerem še nihče ni podrobno razmišljal, in neverjetno je, da se dejansko zgodi, & # 8221 pravi Hay. V preteklosti so znanstveniki razmišljali le o plimovanju, ki ga je vzgojila zvezda.

Hay je ugotovil, da se notranja planeta sistema dovolj približata, da drug drugega dvigujeta močne plime in oseke. Možno je, da je poznejše plimovanje dovolj močno, da spodbudi vulkanske aktivnosti, ki lahko nato vzdržujejo ozračje. Čeprav so najgloblji planeti TRAPPIST-1 na svoji dnevni plati verjetno preveč vroči, da bi ohranili življenje, bi lahko vzdušje, ki ga poganja vulkan, premaknilo nekaj toplote na svojo sicer prehladno nočno stran, ki bi jo dovolj ogrelo, da živa bitja ne bi zmrznila.

Šesti planet v sistemu, imenovan TRAPPIST-1g, doživlja plimovanje tako od zvezde kot od drugih planetov. To je edini planet v sistemu, kjer je ogrevanje plimovanja zaradi drugih planetov tako močno, kot to povzroča osrednja zvezda. Če je TRAPPIST-1g ocean oceanov, kot sta Evropa ali Enceladus v našem lastnem sončnem sistemu, lahko ogrevanje plimovanja ohrani tople vode.

M-pritlikavi zvezdni sistemi, kot je TRAPPIST-1, astronomom ponujajo najboljšo priložnost za iskanje življenja zunaj sončnega sistema, študije Fraschetti in Hay & # 8217s pa lahko znanstvenikom pomagajo pri izbiri načina raziskovanja sistema v prihodnosti.

& # 8220 Moramo resnično razumeti primernost teh sistemov za življenje, energijski pretoki delcev in plimovanje pa so pomembni dejavniki, ki omejujejo našo sposobnost za to, & # 8221 pravi Rackham.


Ti potencialno vseljivi eksoplaneti lahko vidijo Zemljo, ko se razvija

Ko je človeška civilizacija začela cveteti na Zemlji pred približno 5000 leti, bi lahko v tem času naš planet videlo 1.715 zvezdnih sistemov v 326 svetlobnih letih od Zemlje. V naslednjih 5000 letih bo Zemljo lahko videlo še 319 zvezdnih sistemov.

Če obstajajo eksoplaneti, ki krožijo okoli teh bližnjih zvezd, bi lahko bili priča našemu planetu, ko se križa pred soncem. Opazovanje prehoda planeta pred gostiteljsko zvezdo se imenuje tranzit in je ena glavnih metod, ki jo astronomi uporabljajo za odkrivanje eksplanetov s pomočjo zemeljskih in vesoljskih teleskopov. In to je, kako bi lahko druga življenja na drugih planetih, če obstajajo, opazovala Zemljo.

Nekateri planeti, ki krožijo okoli teh zvezd, bi lahko bili vseljivi. Ko planeti krožijo na določeni razdalji od svojih zvezd, lahko ti planeti podpirajo tekočo vodo na svojih površinah. Astronomi to razdaljo imenujejo bivalno območje.

V zadnjih sto letih je Zemlja poslala tudi sporočila, ki razkrivajo inteligentno življenje, ki ga podpira, v obliki radijskih valov, ki jih je ustvaril človek & # 8212 in 75 zvezd je v 100 svetlobnih letih, kar pomeni, da so dovolj blizu, da lahko ti valovi dosežejo njim.

Lisa Kaltenegger, izredna profesorica astronomije in direktorica Inštituta Carl Sagan na univerzi Cornell, raziskuje idejo o tem, kako lahko Zemlja vidi druge planete.

V svoji prejšnji raziskavi, objavljeni oktobra, & # 8220 je raziskovala, kdo nas lahko trenutno vidi kot prehodni planet & # 8212 na enak način, kot iščemo druge svetove, & # 8221 je dejala.

V svoji zadnji raziskavi, objavljeni v sredo v reviji Nature, sta Kaltenegger in astrofizičarka Jackie Faherty, višja znanstvenica iz Ameriškega muzeja naravne zgodovine, želela raziskati idejo o spreminjanju razglednih točk Zemlje skozi čas z vidika bližnjih zvezdnih sistemov .

Zvezde niso v mirovanju. Premikajo se, tako kot se naše sonce vrti okoli središča naše galaksije, pri čemer se Kaltenegger sprašuje, kako to & # 8220 vpliva, kdo bi nas lahko opazil kot & # 8216zemljane. '& # 8221

& # 8220Vselo je dinamično in vse se premika, & # 8221 Kaltenegger je zapisal po e-pošti. & # 8220Tako je ta vesoljski sprednji sedež, da bi videl Zemljo kot prehodni planet, ki preprečuje sončno svetlobo, minljiv. Lahko se pridobi in izgubi. Želeli smo vedeti, kako dolgo zvezde hranijo to izhodišče in tudi, katere zvezde so jo imele in katere zvezde jo bodo dobile & # 8212 v 5000-letnem časovnem okviru. 5000 let v preteklosti, ko so civilizacije začele cveteti, in v naslednjih 5000 letih, saj sem optimističen, da bomo še vedno ugotovili, kako preživeti. & # 8221

Raziskovalci so uporabili podatke iz baze podatkov Evropske vesoljske agencije & # 8217s Gaia, kataloga bližnjih astronomskih objektov, ki se nahajajo v približno 300 svetlobnih letih našega sonca. Ekipa je želela vedeti, kdaj zvezde vstopijo v tako imenovano tranzitno območje Zemlje & # 8212, kjer je Zemljo mogoče zaznati s tranzitom & # 8212, kako dolgo ostanejo v tem območju in kdaj izstopijo.

& # 8220Gaia nam je priskrbela natančen zemljevid galaksije Rimske ceste, ki nam omogoča, da gledamo nazaj in naprej v času ter vidimo, kje so bile zvezde in kam gredo, & # 8221 je dejal Faherty v izjavi.

Znanstveniki so ugotovili, da je skozi to območje v 10.000 letih šlo 2.034 zvezdnih sistemov.

& # 8220Naša sončna soseska je dinamično mesto, kjer zvezde vstopajo in izstopajo s tega popolnega razglednega mesta, da vidijo Zemljo, kako hitro prehaja Sonce, & # 8221 je dejal Faherty.

Znotraj zvezdnih sistemov na pravi razdalji in izhodiščni točki za opazovanje Zemlje & # 8212 v preteklosti, sedanjosti in prihodnosti & # 8212 znanstveniki poznajo sedem, ki gosti eksoplanete. Če na teh eksoplanetih živi življenje in lahko odkrijejo druge planete tako, kot to počnejo astronomi na Zemlji, bi lahko ugotovili, da lahko Zemlja gosti življenje.

Ko se planet prečka pred svojo zvezdo, je ozračje planeta v bistvu osvetljeno z ozadjem. Zemeljsko ozračje bi razkrilo, da ima kemijske podpise življenja.

Raziskovalci so ugotovili, da bi lahko sistem Ross 128, drugi najbližji sistem, ki vključuje zemeljski eksoplanet, ki kroži okoli rdeče pritlikave zvezde, oddaljene 11 svetlobnih let, Zemljo morda videl v tranzitu skozi sonce že 2158 let. Okno njihove perspektive se je odprlo pred približno 3.057 leti in zaprlo pred 900 leti.

V prihodnosti bo sistem Trappist-1, ki gosti sedem planetov velikosti Zemlje, vključno s štirimi potencialno vseljivimi, čez 1642 let v območju tranzita Zemlje. Ti planeti bodo lahko Zemljo videli 2.371 let.

Pred približno sto leti so ljudje začeli ustvarjati radijske valove.

Tako kot vozila tudi svetloba potrebuje čas za potovanje. Torej so radijski valovi, ki so jih ljudje ustvarili že zgodaj, od Zemlje potovali le približno 100 svetlobnih let, je dejal Kaltenegger.

& # 8220 Na našem seznamu zvezd je 75 zvezd, ki bi lahko videle ali zdaj vidijo Zemljo, ki prehaja Sonce, ki bi hkrati tudi prejemalo radijske valove od nas. In poznali bi naš glasbeni okus, v dobrem in v slabem, & # 8221 je dejala.

Glede tega, zakaj nismo prejeli povratnega signala, lahko obstaja veliko razlogov, je dejal Kaltenegger. Sto let je na astronomski časovni lestvici kratek čas in z razvojem drugih tehnologij se vse manj zanašamo na radio.

& # 8220In vsa sporočila & # 8212, če so poslana & # 8212, so poslana s tisto prihodnjo tehnologijo, ki je še ne moremo najti, & # 8221 je dejala.

Ko se bo NASA & # 8217s vesoljski teleskop James Webb lansiral oktobra, bo uporabljen za ogled v ozračje eksoplanetov, astronomi pa lahko s temi podatki olajšajo ozračje eksoplanetov.

Dolgotrajno iskanje eksoplanetov redko raziskuje ekliptično ravnino ali ravnino Zemlje, ki kroži okoli sonca, galaksije Rimske ceste, ker je tako natrpana z zvezdami, zaradi česar je planete težje opazovati, je dejal Kaltenegger.

& # 8220Ampak s tem novim seznamom in dodano motivacijo, da so to zvezdni sistemi, ki bi nas že lahko opazili, ki se spreminja, & # 8221 je dejala. & # 8220Naša raziskava ponuja najboljši ciljni seznam za iskanje nezemeljske inteligence. & # 8221

Kaltenegger je član znanstvene ekipe za misijo NASA & # 8217s za lov na planete TESS, ki se je začela leta 2018 in išče eksoplanete, ki krožijo v bližini zvezd. Transiting Exoplanet Survey Satellite je aprila začel iskati skalnate eksoplanete v tej regiji in upa, da bo zdaj, ko imajo ta seznam zvezd, našel še veliko bolj zanimivih planetov.

& # 8220Ko ko misija NASA TESS in opazovalci s tal najdejo nove planete okoli teh zvezd, jih želim modelirati, da ugotovijo, ali bi lahko bili podobni Zemlji, in nato prosim za čas (vesoljski teleskop James Webb), da jih poiščejo če so v njihovem ozračju znaki življenja. To bi bilo fascinantno, znaki življenja na planetu, ki bi že lahko opazil tudi nas, & # 8221 je dejala.


V coni

Vseh sedem planetov je po velikosti podobno zemlji. Najmanjši h je 75,5% premera zemlje, največji g pa 112,7% premera zemlje. Zemeljska velikost je pomembna pri iskanju planetov, kjer bi lahko obstajalo življenje. Če je planet premajhen (kot je Merkur), mu primanjkuje zadostne gravitacije, da bi se zadržal v ozračju. Po drugi strani pa, če je planet prevelik (kot je Jupiter), bo njegova gravitacija premočna in bo zadrževal napačne pline. Zato se vseh sedem teh planetov zdi obetavno. Vendar ustrezna velikost ne zagotavlja življenjskega vzdušja.

Poleg tega pravilna velikost sama po sebi ne zadostuje. Če bi bila na primer zemlja veliko bližje soncu, bi bila zemlja preveč vroča, da bi obstajala tekoča voda. Če pa bi bila zemlja veliko bolj oddaljena od sonca, bi bilo prehladno, da bi obstajala tekoča voda. Znanstveniki lahko izračunajo območje okoli zvezde (bivalnega območja), kjer bi lahko tekoča voda obstajala na površini planeta. Vendar ta izračun temelji na predpostavki ozračja, kakršno je zemeljsko. Če se atmosfera planeta bistveno razlikuje, bi bilo dejansko bivalno območje drugačno in bivalnega območja morda sploh ne bi bilo. Kakšno je ozračje na teh sedmih planetih? Ne vemo, lahko pa naredimo nekaj ocen.

Te ocene so odvisne od sestave planetov, vendar jih tudi ne poznamo. Če pa poznamo maso in velikost planeta, lahko določimo njegovo gostoto, iz katere lahko sklepamo na njegovo sestavo. Tranzitna opazovanja običajno ne razkrivajo mase zunajsolarnega planeta, vendar v tem primeru obstaja subtilen učinek, ki ga lahko uporabimo za ugotavljanje mase. Šest najglobljih planetov ima orbitalne resonance. To pomeni, da so razmerja med njihovimi orbitalnimi obdobji razmerja med celo število. Na primer, vsakič, ko planet b kroži osemkrat, planet c kroži petkrat. Vsakič, ko planet c kroži petkrat, planet d kroži trikrat. Resonance so verjetno posledica gravitacijskih interakcij.


So tuji svetovi v TRAPPIST-1 bolj vseljivi kot mislili?

Eden najbolj očarljivih zvezdnih sistemov onkraj sončnega sistema je na našem galaktičnem dvorišču. Morda je običajna rdeča pritlikava zvezda, toda eksplaneti, ki jih ima, so vse prej kot običajni. Dejansko so nova opažanja pokazala, da so tuji svetovi TRAPPIST-1 morda res zelo posebni.

TRAPPIST-1, ki se nahaja 40 svetlobnih let od Zemlje, je mini različica našega sončnega sistema. Ob ultrahladnem rdečem palčku kroži vsaj sedem znanih majhnih eksoplanetov zemeljskih dimenzij, od katerih tri krožijo okoli svoje zvezde v bivalnem območju. To je območje okoli zvezde, na katerem ni niti vroče niti prehladno, da bi na površini eksoplaneta obstajala tekoča voda.

Odkritje katerega koli bivalnega območja eksoplaneta, zlasti majhnega in skalnatega, vpliva na možnost iskanja nezemeljskega življenja. Toda preprosto iskanje svetov, ki krožijo znotraj bivalnega območja majhne zvezde, še ne pomeni, da so ti eksoplaneti resnično bivalni. Območje ponuja le vodnik, kje iskati. Kemijo življenja v teh eksplanetarnih ozračjih (če imajo celo ozračja) je treba preučiti, preden lahko eksoplanet resnično šteje za življenjske lastnosti.

Zdaj so astronomi začeli postopek za svetove, ki krožijo okoli TRAPPIST-1, in ugotovili, da lahko vsebujejo vodo. Veliko in veliko in veliko od vode.

Svetovi pare, tekočine in ledu?

Prve eksoplanete TRAPPIST-1 so odkrili v opazovanjih teleskop TRAPPIST-South na lokaciji La Silla pri Evropskem južnem observatoriju v Čilu leta 2016. Zelo velik teleskop ESO (tudi v Čilu) in NASA-in vesoljski teleskop Spitzer so nato postavili in določili da v sistemu ni bilo manj kot sedem majhnih eksoplanetov. Svetovi so dobili ime TRAPPIST-1b, c, d, e, f, g in h, v vedno večji oddaljenosti od osrednje zvezde. Pet eksoplanetov (TRAPPIST-1b, c, e, f in g) je približno Zemljine velikosti, dva (TRAPPIST-1d in h) pa sta manjša. V območju bivanja zvezde so TRAPPIST-1e, f in g. Odkritje zvezdnega sistema s sedmimi eksoplaneti je brez primere in bivalni potencial sistema je očaral svet.

V študiji TRAPPIST-1, ki bo objavljena v reviji Astronomy & amp Astrophysics, so znanstveniki povzeli vse informacije, ki jih poznamo o fascinantnem sistemu, jih prenesli skozi računalniške modele in z veliko natančnostjo določili gostoto eksoplanetov. To pomeni, da lahko planetarni znanstveniki zabodejo svoje sestave in zato ugibajo o tem, katere kemikalije so prisotne. Lahko celo ugotovijo, kaj so poglej všeč.

& quot; Planeti TRAPPIST-1 so tako blizu, da se med seboj gravitacijsko motijo, zato se časi, ko preidejo pred zvezdo, nekoliko premaknejo, & quot; Simon Grimm, je dejal v izjavi ESO. Grimm dela na univerzi v Bernu v Švici in je vodil študijo. & quot; Ti premiki so odvisni od mas planetov, njihovih razdalj in drugih orbitalnih parametrov. Z računalniškim modelom simuliramo orbite planetov, dokler se izračunani tranziti ne ujemajo z opaženimi vrednostmi in tako izpeljemo planetarne mase, "je nadaljeval.

Ko je Grimmova ekipa skrbno sestavila vse to skupaj, je ugotovila, da gostota eksoplanetov razkriva, da niso neplodni, kamniti svetovi pa so prisotni v velikih količinah hlapnega materiala. Hlapne kemikalije vključujejo vodo, ogljikov dioksid, metan in druge, vendar so astronomi iz predhodnega opazovanja protoplanetarnih diskov okoli mladih zvezd vedeli, da prevladuje spektroskopski podpis vode. Zato raziskovalci ugotavljajo, da bodo hlapne snovi v svetovih TRAPPIST-1 v glavnem sestavljene iz vode, in sicer veliko. V nekaterih primerih študija ocenjuje, da do 5 odstotkov eksoplanetarne mase sestavlja voda - to je 250-krat več vode kot vsi oceani na Zemlji!

& quotDensities, čeprav so pomembni namigi o sestavi planetov, ne govorijo nič o bivalnosti. Vendar pa je naša študija pomemben korak naprej, saj še naprej raziskujemo, ali bi ti planeti lahko podpirali življenje, "je v isti izjavi ESO dodal soavtor Brice-Olivier Demory, prav tako z Univerze v Bernu.

Ker ti sveti Trappist-1 krožijo okoli zvezde na različnih razdaljah, bo voda v različnih fazah, odvisno od sveta. Zdi se, da so najbolj notranji (in zato najtoplejši) eksplaneti kamniti in imajo verjetno zelo gosto in soparno ozračje, medtem ko bodo najbolj oddaljeni sveti zamrznjeni in prekriti s plastjo ledu. TRAPPIST-1e velja za najbolj "zemeljski" svet v sistemu, verjetno ima gosto železno jedro, skalnato notranjost in po možnosti tanko atmosfero.

Exoplanetary Presenečenja

Vsi ti izračuni gostote so dali nekaj presenetljivih vpogledov. Na primer, najgostejši planeti v sistemu Trappist-1 niso tisti, ki so najbližji zvezdi. Poleg tega se zdi, da hladnejši planeti ne morejo imeti goste atmosfere.

Obe opazovanji sta prišli od soavtorice Caroline Dorn, ki dela na univerzi v Zürichu v Švici.

Čeprav so te ugotovitve prepričljive, druga opazovanja s pomočjo vesoljskega teleskopa Hubble niso uspela zaznati prisotnosti vodika v eksplanetarnih ozračjih TRAPPIST-1. Odkrivanje vodika bi dodalo dokaze o prisotnosti vode. Zdi se, da bomo morali počakati na naslednjo generacijo opazovalnic, kot je NASA-in vesoljski teleskop James Webb, ki bo dovolj zmogljiva za odkrivanje opozorilnih znakov vode.

Tudi če ima TRAPPIST-1 vse sestavine za resnične bivalne eksoplanete, se narava sistemov rdečih pritlikavih zvezd zelo razlikuje od našega sončnega sistema. Ker so bivalna območja rdečih pritlikavk bližje svojim zvezdam, bodo vsi eksoplaneti v bivalnih območjih izpostavljeni povečani ravni sevanja. Če ti svetovi nimajo močnih magnetnih polj in debelih atmosfer, da bi odvrnili in absorbirali vesoljske vremenske navale, se lahko življenje, kakršno poznamo, izziva razvijati. Ker so ti sistemi tako kompaktni, bo še eno vprašanje zaklepanje ob plimovanju. Tam se bo ena polobla katerega koli krožečega eksplaneta nenehno soočala z zvezdo. Težko si je predstavljati bivalni svet, ko je ena stran zamrznjena v večni noči.

Toda v TRAPPIST-1 je verjetno voda, zato, če življenje najde pot drugje v naši galaksiji, bomo težko našli primernejši kraj, kjer bi se lahko tujerodna biologija uveljavila.

Če bi eksplanete TRAPPIST-1 krožile okoli sonca, bi vsi zasedli območje znotraj Merkurjeve orbite, najglobljega planeta sončnega sistema.


Sonde Hubble Atmosfere planetov TRAPPIST-1 s bivalnimi območji

Astronomi z uporabo vesoljskega teleskopa Hubble NASA / ESA so izvedli prvo spektroskopsko raziskavo eksoplanetov znotraj bivalnega območja okoli TRAPPIST-1. Objavljeno v reviji Astronomija narave, rezultati nadalje podpirajo zemeljsko in potencialno bivalno naravo treh od preučevanih planetov.

Vtis tega umetnika prikazuje več planetov TRAPPIST-1. Zasluga za podobo: M. Kornmesser / ESO.

Ultrahladna pritlikava zvezda TRAPPIST-1 gosti skupno sedem znanih planetov velikosti Zemlje.

Trije planeti & # 8212 TRAPPIST-1e, f in g & # 8212 krožijo znotraj bivalnega območja sistema, območja na oddaljenosti od zvezde, kjer bi lahko tekoča voda obstajala na površini planeta. Četrti planet & # 8212 TRAPPIST-1d & # 8212 kroži v mejni regiji na notranjem robu bivalnega pasu.

Novi podatki o Hubblu izključujejo atmosfero, bogato z vodikom, bogato z oblaki za tri planete & # 8212, vendar za TRAPPIST-1g take atmosfere ni mogoče izključiti.

"Prisotnost zabuhlih atmosfer, v katerih prevladuje vodik, bi lahko pokazala, da so ti planeti bolj verjetno plinasti svetovi, kot je Neptun," je povedal astronom z MIT-a dr.

"Pomanjkanje vodika v njihovem ozračju nadalje podpira teorije o planetih, ki so zemeljske narave."

"To odkritje je pomemben korak k ugotavljanju, ali lahko planeti na svojih površinah zadržujejo tekočo vodo, ki bi jim lahko omogočila podporo živim organizmom."

Opazovanja so bila opravljena, ko so bili planeti v tranzitu pred TRAPPIST-1. V tej konfiguraciji majhen del zvezdine svetlobe prehaja skozi ozračje planeta in komunicira z atomi in molekulami v njem. Tako v spektru zvezde ostane šibek prstni odtis ozračja.

Medtem ko rezultati izključujejo eno vrsto ozračja, so številni alternativni atmosferski scenariji še vedno skladni s podatki, ki so jih zbrali avtorji.

Ti spektri prikazujejo kemično sestavo atmosfere štirih planetov velikosti Zemlje, ki krožijo znotraj bivalnega območja zvezde TRAPPIST-1 ali v njegovi bližini. Za pridobitev spektrov so astronomi uporabili Hubble za zbiranje svetlobe iz TRAPPIST-1, ki je šla skozi atmosfero eksoplanetov, ko so eksoplaneti prečkali pred zvezdo. Vijolične krivulje kažejo predvidene podpise plinov, kot sta voda in metan, ki absorbirajo določene valovne dolžine svetlobe. Te pline najdemo v zabuhli atmosferi, v kateri prevladuje vodik, podobni atmosferi plinastih planetov, kot je Neptun. Rezultati Hubbla, označeni z zelenimi križi, ne kažejo nobenega dokaza o razširjeni atmosferi na treh eksoplanetih (TRAPPIST-1d, f in e). Additional observations are needed to rule out a hydrogen-dominated atmosphere for the fourth planet (TRAPPIST-1g). The evidence indicates that the atmospheres are more compact than could be measured by the Hubble observations. Image credit: NASA / ESA / Z. Levy, STScI.

“Although Hubble did not find evidence of hydrogen, we suspect the planetary atmospheres could have contained this lightweight gaseous element when they first formed,” they said.

“The planets may have formed farther away from their parent star in a colder region of the gaseous protostellar disk that once encircled the infant star.”

“The system is dynamically stable now, but the planets could not have formed in this tight pack,” said co-author Dr. Nikole Lewis, from the Space Telescope Science Institute.

“They’re too close together now, so they must have migrated to where we see them. Their primordial atmospheres, largely composed of hydrogen, could have boiled away as they got closer to the star, and then the planets formed secondary atmospheres.”

In contrast, Solar System’s rocky planets likely formed in the hotter, dryer region closer to the Sun.

“There are no analogs in our Solar System for these planets,” said co-author Dr. Hannah Wakeford, also from the Space Telescope Science Institute.

“One of the things researchers are finding is that many of the more common exoplanets don’t have analogs in our Solar System. So the Hubble observations are a unique opportunity to probe an unusual system.”

By ruling out the presence of a large abundance of hydrogen in the planets’ atmospheres, Hubble is helping to pave the way for the NASA/ESA/CSA James Webb Space Telescope.

“Spectroscopic observations of the TRAPPIST-1 planets with the next generation of telescopes will allow us to probe deeper into their atmospheres,” said co-author Dr. Michael Gillon, from the University of Líege.

“This will allow us to search for heavier gases such as carbon, methane, water, and oxygen, which could offer biosignatures for life.”

Julien de Wit et al. Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1. Astronomija narave, published online February 5, 2018 doi: 10.1038/s41550-017-0374-z


Atmospheres of exoplanets in TRAPPIST-1 habitable zone probed

Astronomers using NASA's Hubble Space Telescope have conducted the first spectroscopic survey of the Earth-sized planets (d, e, f, and g) within the habitable zone around the nearby star TRAPPIST-1. This study is a follow-up to Hubble observations made in May 2016 of the atmospheres of the inner TRAPPIST-1 planets b and c.

Hubble reveals that at least three of the exoplanets (d, e, and f) do not seem to contain puffy, hydrogen-rich atmospheres similar to gaseous planets such as Neptune.

Additional observations are needed to determine the hydrogen content of the fourth planet's (g) atmosphere. Hydrogen is a greenhouse gas, which smothers a planet orbiting close to its star, making it hot and inhospitable to life. The results, instead, favor more compact atmospheres like those of Earth, Venus, and Mars.

By not detecting the presence of a large abundance of hydrogen in the planets' atmospheres, Hubble is helping to pave the way for NASA's James Webb Space Telescope, scheduled to launch in 2019. Webb will probe deeper into the planetary atmospheres, searching for heavier gases such as carbon dioxide, methane, water, and oxygen. The presence of such elements could offer hints of whether life could be present, or if the planet were habitable.

"Hubble is doing the preliminary reconnaissance work so that astronomers using Webb know where to start," said Nikole Lewis of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, co-leader of the Hubble study. "Eliminating one possible scenario for the makeup of these atmospheres allows the Webb telescope astronomers to plan their observation programs to look for other possible scenarios for the composition of these atmospheres."

The planets orbit a red dwarf star that is much smaller and cooler than our Sun. The four alien worlds are members of a seven-planet system around TRAPPIST-1. All seven of the planetary orbits are closer to their host star than Mercury is to our Sun. Despite the planets' close proximity to TRAPPIST-1, the star is so much cooler than our Sun that liquid water could exist on the planets' surfaces.

Two of the planets were discovered in 2016 by TRAPPIST (the Transiting Planets and Planetesimals Small Telescope) in Chile. NASA's Spitzer Space Telescope and several ground-based telescopes uncovered five additional ones, increasing the total number to seven. The TRAPPIST-1 system is located about 40 light-years from Earth.

"No one ever would have expected to find a system like this," said team member Hannah Wakeford of STScI. "They've all experienced the same stellar history because they orbit the same star. It's a goldmine for the characterization of Earth-sized worlds."

The Hubble observations took advantage of the fact that the planets cross in front of their star every few days. Using the Wide Field Camera 3, astronomers made spectroscopic observations in infrared light, looking for the signature of hydrogen that would filter through a puffy, extended atmosphere, if it were present. "The planets are close enough to their host star, and they have very short orbital periods, which means there are lots of opportunities to make observations," Lewis said.

Although Hubble did not find evidence of hydrogen, the researchers suspect the planetary atmospheres could have contained this lightweight gaseous element when they first formed. The planets may have formed farther away from their parent star in a colder region of the gaseous protostellar disk that once encircled the infant star.

"The system is dynamically stable now, but the planets could not have formed in this tight pack," Lewis said. "They're too close together now, so they must have migrated to where we see them. Their primordial atmospheres, largely composed of hydrogen, could have boiled away as they got closer to the star, and then the planets formed secondary atmospheres."

In contrast, the rocky planets in our solar system likely formed in the hotter, dryer region closer to the Sun. "There are no analogs in our solar system for these planets," Wakeford said. "One of the things researchers are finding is that many of the more common exoplanets don't have analogs in our solar system. So the Hubble observations are a unique opportunity to probe an unusual system."

The Hubble team plans to conduct follow-up observations in ultraviolet light to search for trace hydrogen escaping the planets' atmospheres, produced from processes involving water or methane lower in their atmospheres.

Astronomers will then use the Webb telescope to help them better characterize those planetary atmospheres. The exoplanets may possess a range of atmospheres, just like the terrestrial planets in our solar system.

"One of these four could be a water world," Wakeford said. "One could be an exo-Venus, and another could be an exo-Mars. It's interesting because we have four planets that are at different distances from the star. So we can learn a little bit more about our own diverse solar system, because we're learning about how the TRAPPIST star has impacted its array of planets."


Using NASA’s Hubble Space Telescope, astronomers conducted the first spectroscopic survey of the Earth-sized planets (d, e, f, and g) around TRAPPIST-1, including three that are in the habitable zone (e, f, and g). Hubble reveals that at least three of the exoplanets (d, e, and f) do not seem to contain puffy, hydrogen-rich atmospheres similar to gaseous planets such as Neptune.

Additional observations are needed to determine the hydrogen content of the fourth planet’s (TRAPPIST-1g) atmosphere. Hydrogen is a greenhouse gas, which smothers a planet orbiting close to its star, making it hot and inhospitable to life. The results, instead, favor more compact atmospheres like those of Earth, Venus, and Mars.

As a planet in the TRAPPIST-1 system passes between us and the star, it blocks out a tiny portion of the star’s light. Precise telescopes like Hubble can look at changes in specific wavelengths of light, which provide clues to the composition and size of the planet’s atmosphere. Hubble observations in May 2016 of TRAPPIST-1 b and c showed that these planets do not seem to have thick, puffy hydrogen-rich atmospheres. This indicates a higher chance that they are rocky, terrestrial planets rather than mini gas-giants.

These two planets are considered not in the habitable zone, though. Hubble then observed planets d, e, f, and g in December 2016 and January 2017 in near-infrared wavelengths, and the results were similar: Hubble found no sign of thick, puffy hydrogen-rich atmospheres for any of the four planets. The data suggest that there isn’t this gas-giant like atmosphere for planets d, e, and f. The data from this round of observations was not as strong for planet g, which is also in the habitable zone like e and f, so while there’s no evidence for a thick, hydrogen-rich atmosphere on TRAPPIST-1g, the researchers are not yet ruling it out.

Planets e, f, and g orbit at distances where temperatures would allow for liquid water, while d is likely a little too hot because it is so close to the star. Hubble has yet to take observations of planet h, which is outside the system’s habitable zone (too far from the star). But it’s worth noting though, that even the planets outside of the habitable zone might be able to have liquid water somewhere on its surface in certain conditions.

It’s also worth noting that if any of these planets have high-altitude clouds and hazes, that would block Hubble’s ability to detect a tick, hydrogen-rich atmosphere, but such an atmosphere does not likely exist on these exoplanets. Many possibilities remain for what types of atmospheres these planets have, or whether they even have atmospheres. The TRAPPIST-1 planets could have compact atmospheres similar to Mars, Venus, and Earth, or something entirely different.

TRAPPIST-1 system compared to Earth (February 2018)

The combination of atmospheric gases is important. Find oxygen and the methane in the same atmosphere, and you’ve got something special. There are ways to build up oxygen or methane in a planetary atmosphere, but the only way you get them both in the same atmosphere at the same time is if you produce them both super rapidly. And the only way we know how to do that is through life.

Researchers hope to use Hubble’s ultraviolet capabilities to look for evidence of water vapor or methane, and NASA’s upcoming James Webb Space Telescope will look in the far-infrared to further characterize these atmospheres. Future telescopes also hope to look for hints of whether the planets are habitable and if life could be present. The TRAPPIST-1 system provides the best opportunity we currently have to study Earth-size exoplanets. Over the next few years, Hubble and other telescopes will work together, each contributing important observations. For the first time ever, we’ll have an in-depth understanding of a set of terrestrial planets outside our solar system.

Since Hubble’s observations in December 2016 and January 2017, NASA’s Kepler Space Telescope has also observed the TRAPPIST-1 system, and Spitzer Space Telescope began a program of 500 additional hours of TRAPPIST-1 observations, which will conclude in March. This new body of data helped study authors paint a clearer picture of the system than ever before.


TRAPPIST-1 Exoplanets Reveal Clues About Habitable Worlds

Artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018. Credit: NASA/JPL- Caltech

TRAPPIST-1 is an ultra-cool red dwarf star that is slightly larger, but much more massive, than the planet Jupiter, located about 40 light-years from the sun in the constellation Aquarius.

Among planetary systems, TRAPPIST-1 is of particular interest because seven planets have been detected orbiting this star, a larger number of planets than have been than detected in any other exoplanetary system. In addition, all of the TRAPPIST-1 planets are Earth-sized and terrestrial, making them an ideal focus of study for planet formation and potential habitability.

ASU scientists Cayman Unterborn, Steven Desch and Alejandro Lorenzo of the School of Earth and Space Exploration, with Natalie Hinkel of Vanderbilt University, have been studying these planets for habitability, specifically related to water composition. Their findings have been recently published in Nature Astronomy.

The calculations equal water

The TRAPPIST-1 planets are curiously light. From their measured mass and volume, all of this system’s planets are less dense than rock. On many other, similarly low-density worlds, it is thought that this less-dense component consists of atmospheric gasses.

“But the TRAPPIST-1 planets are too small in mass to hold onto enough gas to make up the density deficit,” geoscientist Unterborn explained. “Even if they were able to hold onto the gas, the amount needed to make up the density deficit would make the planet much puffier than we see.”

So scientists studying this planetary system have determined that the low-density component must be something else that is abundant: water. This has been predicted before, and possibly even seen on larger planets like GJ1214b, so the interdisciplinary ASU-Vanderbilt team, comprised of geoscientists and astrophysicists, set out to determine just how much water could be present on these Earth-sized planets and how and where the planets may have formed.

All seven planets discovered in orbit around the red dwarf star TRAPPIST-1 could easily fit inside the orbit of Mercury, the innermost planet of our solar system. Credit NASA/JPL- Caltech

But how much is there?

To determine the composition of the TRAPPIST-1 planets, the team used a unique software package, developed by Unterborn and Lorenzo, that uses state-of-the-art mineral physics calculators. The software, called ExoPlex, allowed the team to combine all of the available information about the TRAPPIST-1 system, including the chemical makeup of the star, rather than being limited to just the mass and radius of individual planets.

Much of the data used by the team to determine composition was collected from a dataset called the Hypatia Catalog, developed by contributing author Hinkel. This catalog merges data on the stellar abundances of stars near to our sun, from over 150 literature sources, into a massive repository.

What they found through their analyses was that the relatively “dry” inner planets (“b” and “c”) were consistent with having less than 15 percent water by mass (for comparison, Earth is 0.02 percent water by mass). The outer planets (“f” and “g”) were consistent with having more than 50 percent water by mass. This equates to the water of hundreds of Earth-oceans. The masses of the TRAPPIST-1 planets continue to be refined, so these proportions must be considered estimates for now, but the general trends seem clear.

“What we are seeing for the first time are Earth-sized planets that have a lot of water or ice on them,” said Steven Desch, ASU astrophysicist and contributing author.

A slice through a model composition of TRAPPIST-1 “f” which contains over 50 percent water by mass. The pressure of the water alone is enough to cause it to become high-pressure ice. The pressure at the water-mantle boundary is so great that no upper mantle is present at all instead the shallowest rocks would be more like those seen in Earth’s lower mantle. Credit: ASU

But the researchers also found that the ice-rich TRAPPIST-1 planets are much closer to their host star than the ice line. The “ice line” in any solar system, including TRAPPIST-1’s, is the distance from the star beyond which water exists as ice and can be accreted into a planet inside the ice line water exists as vapor and will not be accreted. Through their analyses, the team determined that the TRAPPIST-1 planets must have formed much farther from their star, beyond the ice line, and migrated in to their current orbits close to the host star.

There are many clues that planets in this system and others have undergone substantial inward migration, but this study is the first to use composition to bolster the case for migration. What’s more, knowing which planets formed inside and outside of the ice line allowed the team to quantify for the first time how much migration took place.

Because stars like TRAPPIST-1 are brightest right after they form and gradually dim thereafter, the ice line tends to move in over time, like the boundary between dry ground and snow-covered ground around a dying campfire on a snowy night. The exact distances the planets migrated inward depends on when they formed.

“The earlier the planets formed,” Desch said, “the farther away from the star they needed to have formed to have so much ice.” But for reasonable assumptions about how long planets take to form, the TRAPPIST-1 planets must have migrated inward from at least twice as far away as they are now.

This graph shows the minimum starting distances of the ice-rich TRAPPIST-1 planets (especially f and g) from their star (horizontal axis) as a function of how quickly they formed after their host star was born (vertical axis). The blue line represents a model where water condenses to ice at 170 K, as in our solar system’s planet-forming disk. The red line applies to water condensing to ice at 212 K, appropriate to the TRAPPIST-1 disk. If planets formed quickly, they must have formed farther away (and migrated in a greater distance) to contain significant ice. Because TRAPPIST-1 dims over time, if the planets formed later, they could have formed closer to the host star and still be ice-rich.

Too much of a good thing

Interestingly, while we think of water as vital for life, the TRAPPIST-1 planets may have too much water to support life.

“We typically think having liquid water on a planet as a way to start life, since life, as we know it on Earth, is composed mostly of water and requires it to live,” Hinkel explained. “However, a planet that is a water world, or one that doesn’t have any surface above the water, does not have the important geochemical or elemental cycles that are absolutely necessary for life.”

Ultimately, this means that while M-dwarf stars, like TRAPPIST-1, are the most common stars in the universe (and while it’s likely that there are planets orbiting these stars), the huge amount of water they are likely to have makes them unfavorable for life to exist, especially enough life to create a detectable signal in the atmosphere that can be observed. “It’s a classic scenario of ‘too much of a good thing,’” said Hinkel.

So, while we’re unlikely to find evidence of life on the TRAPPIST-1 planets, through this research we may gain a better understanding of how icy planets form and what kinds of stars and planets we should be looking for in our continued search for life.

Publication: Cayman T. Unterborn, et al., “Inward migration of the TRAPPIST-1 planets as inferred from their water-rich compositions,” Nature Astronomy (2018) doi:10.1038/s41550-018-0411-6


Vsebina

The star at the center of the system was discovered in 1999 during the Two Micron All-Sky Survey (2MASS). [28] [29] It was entered in the subsequent catalog with the designation "2MASS J23062928-0502285". The numbers refer to the right ascension and declination of the star's position in the sky and the "J" refers to the Julian Epoch.

The system was later studied by a team at the University of Liège, who made their initial observations using the TRAPPIST–South telescope from September to December 2015 and published their findings in the May 2016 issue of the journal Narava. [20] [12] The backronym pays homage to the Catholic Christian religious order of Trappists and to the Trappist beer it produces (primarily in Belgium), which the astronomers used to toast their discovery. [30] [31] Since the star hosted the first exoplanets discovered by this telescope, the discoverers accordingly designated it as "TRAPPIST-1".

The planets are designated in the order of their discovery, beginning with b for the first planet discovered, c for the second and so on. [32] Three planets around TRAPPIST-1 were first discovered and designated b, c in d in order of increasing orbital periods, [12] and the second batch of discoveries was similarly designated e do h.

TRAPPIST-1 is an ultra-cool dwarf star of spectral class M8.0 ± 0.5 that is approximately 8% the mass of and 11% the radius of the Sun. Although it is slightly larger than Jupiter, it is about 84 times more massive. [33] [12] High-resolution optical spectroscopy failed to reveal the presence of lithium, [34] suggesting it is a very low-mass main-sequence star, which is fusing hydrogen and has depleted its lithium, i.e., a red dwarf rather than a very young brown dwarf. [12] It has a temperature of 2,511 K (2,238 °C 4,060 °F), [7] and its age has been estimated to be approximately 7.6 ± 2.2 Gyr . [10] In comparison, the Sun has a temperature of 5,778 K (5,505 °C 9,941 °F) [35] and an age of about 4.6 Gyr. [36] Observations with the Kepler K2 extension for a total of 79 days revealed starspots and infrequent weak optical flares at a rate of 0.38 per day (30-fold less frequent than for active M6–M9 dwarfs) a single strong flare appeared near the end of the observation period. The observed flaring activity possibly changes the atmospheres of the orbiting planets on a regular basis, making them less suitable for life. [8] The star has a rotational period of 3.3 days. [8] [37]

High-resolution speckle images of TRAPPIST-1 were obtained and revealed that the M8 star has no companions with a luminosity equal to or brighter than a brown dwarf. [38] This determination that the host star is single confirms that the measured transit depths for the orbiting planets provide a true value for their radii, thus proving that the planets are indeed Earth-sized.

Owing to its low luminosity, the star has the ability to live for up to 12 trillion years. [39] It is metal-rich, with a metallicity ([Fe/H]) of 0.04, [7] or 109% the solar amount. Its luminosity is 0.05% of that of the Sun ( L ), most of which is emitted in the infrared spectrum, and with an apparent magnitude of 18.80 it is not visible with basic amateur telescopes from the Earth.

The TRAPPIST-1 planetary system [5] [40] [7]
Družabnik
(po zvezdici)
Maša Polmajorna os
(AU)
Orbitalno obdobje
(dnevi)
Eccentricity [40] Inclination [7] [41] Polmer
b 1.374 ± 0.069 M 0.01154 ± 0.0001 1.51088432 ± 0.00000015 0.006 22 ± 0.003 04 89.56 ± 0.23 ° 1.116 +0.014
−0.012 R
c 1.308 ± 0.056 M 0.01580 ± 0.00013 2.42179346 ± 0.00000023 0.006 54 ± 0.001 88 89.70 ± 0.18 ° 1.097 +0.014
−0.012 R
d 0.388 ± 0.012 M 0.02227 ± 0.00019 4.04978035 ± 0.00000256 0.008 37 ± 0.000 93 89.89 +0.08
−0.15 °
0.778 +0.011
−0.010 R
e 0.692 ± 0.022 M 0.02925 ± 0.00025 6.09956479 ± 0.00000178 0.005 10 ± 0.000 58 89.736 +0.053
−0.066 °
0.920 +0.013
−0.012 R
f 1.039 ± 0.031 M 0.03849 ± 0.00033 9.20659399 ± 0.00000212 0.010 07 ± 0.000 68 89.719 +0.026
−0.039 °
1.045 +0.013
−0.012 R
g 1.321 ± 0.038 M 0.04683 ± 0.0004 12.3535557 ± 0.00000341 0.002 08 ± 0.000 58 89.721 +0.019
−0.026 °
1.129 +0.015
−0.013 R
h 0.326 ± 0.020 M 0.06189 ± 0.00053 18.7672745 ± 0.00001876 0.005 67 ± 0.001 21 89.796 ± 0.023 ° 0.775 ± 0.014 R

On 22 February 2017, astronomers announced that the planetary system of this star is composed of seven temperate terrestrial planets, of which five (b, c, e, f in g) are similar in size to Earth, and two (d in h) are intermediate in size between Mars and Earth. [41] At least three of the planets (e, f in g) orbit within the habitable zone. [41] [42] [23] [43]

The orbits of the TRAPPIST-1 planetary system are very flat and compact. All seven of TRAPPIST-1's planets orbit much closer than Mercury orbits the Sun. Razen b, they orbit farther than the Galilean satellites do around Jupiter, [44] but closer than most of the other moons of Jupiter. The distance between the orbits of b in c is only 1.6 times the distance between the Earth and the Moon. The planets should appear prominently in each other's skies, in some cases appearing several times larger than the Moon appears from Earth. [43] A year on the closest planet passes in only 1.5 Earth days, while the seventh planet's year passes in only 18.8 days. [41] [37]

The planets pass so close to one another that gravitational interactions are significant, and their orbital periods are nearly resonant. In the time the innermost planet completes eight orbits, the second, third, and fourth planets complete five, three, and two. [45] The gravitational tugging also results in transit-timing variations (TTVs), ranging from under a minute to over 30 minutes, which allowed the investigators to calculate the masses of all but the outermost planet. The total mass of the six inner planets is approximately 0.02% the mass of TRAPPIST-1, a fraction similar to that for the Galilean satellites to Jupiter, and an observation suggestive of a similar formation history. The densities of the planets range from

1.17 times that of Earth (ρ, 5.51 g/cm 3 ), indicating predominantly rocky compositions. The uncertainties are too large to indicate whether a substantial component of volatiles is also included, except in the case of f, where the value ( 0.60 ± 0.17 ρ ) "favors" the presence of a layer of ice and/or an extended atmosphere. [41] Speckle imaging excludes all possible stellar and brown dwarf companions. [46]

On 31 August 2017, astronomers using the Hubble Space Telescope reported the first evidence of possible water content on the TRAPPIST-1 exoplanets. [47] [48]

Between 18 February and 27 March 2017, a team of astronomers used the Spitzer Space Telescope to observe TRAPPIST-1 to refine the orbital and physical parameters of the seven planets using updated parameters for the star. Their results were published on 9 January 2018. Although no new mass estimates were given, the team managed to refine the orbital parameters and radii of the planets within a very small error margin. In addition to updated planetary parameters, the team also found evidence for a large, hot atmosphere around the innermost planet. [7]

On 5 February 2018, a collaborative study by an international group of scientists using the Hubble Space Telescope, the Kepler Space Telescope, the Spitzer Space Telescope, and the ESO's SPECULOOS telescope released the most accurate parameters for the TRAPPIST-1 system yet. [49] They were able to refine the masses of the seven planets to a very small error margin, allowing the density, surface gravity, and composition of the planets to be accurately determined. The planets range in mass from about 0.3 M to 1.16 M , with densities from 0.62 ρ (3.4 g/cm 3 ) to 1.02 ρ (5.6 g/cm 3 ). Planeti c in e are almost entirely rocky, while b, d, f, g, in h have a layer of volatiles in the form of either a water shell, an ice shell, or a thick atmosphere. Planeti c, d, e, in f lack hydrogen-helium atmospheres. Planet g was also observed, but there was not enough data to firmly rule out a hydrogen atmosphere. Planet d might have a liquid water ocean comprising about 5% of its mass—for comparison, Earth's water content is < 0.1% —while if f in g have water layers, they are likely frozen. Planet e has a slightly higher density than Earth, indicating a terrestrial rock and iron composition. Atmospheric modeling suggests the atmosphere of b is likely to be over the runaway greenhouse limit with an estimated 10 1 to 10 4 bar of water vapor. [40] [50]

Stellar spectrum study, performed in early 2020, has revealed the TRAPPIST-1 star rotational axis is well aligned with the plane of planetary orbits. The stellar obliquity was found to be 19 +13
−15 degrees. [51]

Data chart Edit

Other characteristics
Družabnik
(po zvezdici)
Stellar flux [5]
(⊕)
Temperature [3]
(equilibrium, assumes null Bond albedo)
Surface gravity [5]
(⊕)
Približno
orbitalni
resonanca
razmerje
(wrt planet b)
Približno
orbitalni
resonanca
razmerje
(wrt next planet inwards)
b 4.153 ± 0.16 397.6 ± 3.8 K (124.45 ± 3.80 °C 256.01 ± 6.84 °F)
≥1,400 K (1,130 °C 2,060 °F) (atmosphere)
750–1,500 K (477–1,227 °C 890–2,240 °F) (surface) [40]
1.102 ± 0.052 1:1 1:1
c 2.214 ± 0.085 339.7 ± 3.3 K (66.55 ± 3.30 °C 151.79 ± 5.94 °F) 1.086 ± 0.043 5:8 5:8
d 1.115 ± 0.043 286.2 ± 2.8 K (13.05 ± 2.80 °C 55.49 ± 5.04 °F) 0.624 ± 0.019 3:8 3:5
e 0.646 ± 0.025 249.7 ± 2.4 K (−23.45 ± 2.40 °C −10.21 ± 4.32 °F) 0.817 ± 0.024 1:4 2:3
f 0.373 ± 0.014 217.7 ± 2.1 K (−55.45 ± 2.10 °C −67.81 ± 3.78 °F) 0.851 ± 0.024 1:6 2:3
g 0.252 ± 0.0097 197.3 ± 1.9 K (−75.85 ± 1.90 °C −104.53 ± 3.42 °F) 1.035 ± 0.026 1:8 3:4
h 0.144 ± 0.0055 171.7 ± 1.7 K (−101.45 ± 1.70 °C −150.61 ± 3.06 °F) 0.570 ± 0.038 1:12 2:3

Orbital near-resonance Edit

The orbital motions of the TRAPPIST-1 planets form a complex chain with three-body Laplace-type resonances linking every member. The relative orbital periods (proceeding outward) approximate whole integer ratios of 24/24, 24/15, 24/9, 24/6, 24/4, 24/3, and 24/2, respectively, or nearest-neighbor period ratios of about 8/5, 5/3, 3/2, 3/2, 4/3, and 3/2 (1.603, 1.672, 1.506, 1.509, 1.342, and 1.519). This represents the longest known chain of near-resonant exoplanets, and is thought to have resulted from interactions between the planets as they migrated inward within the residual protoplanetary disk after forming at greater initial distances. [41] [37]

Most sets of orbits similar to the set found at TRAPPIST-1 are unstable, causing one planet to come within the Hill sphere of another or to be thrown out. But it has been found that there is a way for a system to migrate into a fairly stable state through damping interactions with, for example, a protoplanetary disk. After this, tidal forces can give the system a long-term stability. [18]

The tight correspondence between whole number ratios in orbital resonances and in music theory has made it possible to convert the system's motion into music. [19] [52]

Formation of the planetary system Edit

According to Ormel et al. previous models of planetary formation do not explain the formation of the highly compact TRAPPIST-1 system. Formation in place would require an unusually dense disk and would not readily account for the orbital resonances. Formation outside the frost line does not explain the planets' terrestrial nature or Earth-like masses. The authors proposed a new scenario in which planet formation starts at the frost line where pebble-size particles trigger streaming instabilities, then protoplanets quickly mature by pebble accretion. When the planets reach Earth mass they create perturbations in the gas disk that halt the inward drift of pebbles causing their growth to stall. The planets are transported by Type I migration to the inner disk, where they stall at the magnetospheric cavity and end up in mean motion resonances. [53] This scenario predicts the planets formed with significant fractions of water, around 10%, with the largest initial fractions of water on the innermost and outermost planets. [54]

Tidal locking Edit

It is suggested that all seven planets are likely to be tidally locked into a so-called synchronous spin state (one side of each planet permanently facing the star), [41] making the development of life there much more challenging. [16] A less likely possibility is that some may be trapped in a higher-order spin-orbit resonance. [41] Tidally locked planets would typically have very large temperature differences between their permanently lit day sides and their permanently dark night sides, which could produce very strong winds circling the planets. The best places for life may be close to the mild twilight regions between the two sides, called the terminator line. Another possibility is that the planets may be pushed into effectively non-synchronous spin states due to strong mutual interactions among the seven planets, resulting in more complete stellar coverage over the surface of the planets. [55]

Tidal heating Edit

Tidal heating is predicted to be significant: all planets except f in h are expected to have a tidal heat flux greater than Earth's total heat flux. [37] With the exception of planet c, all of the planets have densities low enough to indicate the presence of significant H
2 O in some form. Planeti b in c experience enough heating from planetary tides to maintain magma oceans in their rock mantles planet c may have eruptions of silicate magma on its surface. Tidal heat fluxes on planets d, e, in f are lower, but are still twenty times higher than Earth's mean heat flow. Planeti d in e are the most likely to be habitable. Planet d avoids the runaway greenhouse state if its albedo is ≳ 0.3 . [56]

Possible effects of strong X-ray and extreme UV irradiation of the system Edit

Bolmont et al. modelled the effects of predicted far ultraviolet (FUV) and extreme ultraviolet (EUV/XUV) irradiation of planets b in c by TRAPPIST-1. Their results suggest that the two planets may have lost as much as 15 Earth oceans of water (although the actual loss would probably be lower), depending on their initial water contents. Nonetheless, they may have retained enough water to remain habitable, and a planet orbiting further out was predicted to lose much less water. [26]

However, a subsequent XMM-Newton X-ray study by Wheatley et al. found that the star emits X-rays at a level comparable to our own much larger Sun, and extreme ultraviolet radiation at a level 50-fold stronger than assumed by Bolmont et al. The authors predicted this would significantly alter the primary and perhaps secondary atmospheres of close-in, Earth-sized planets spanning the habitable zone of the star. The publication noted that these levels "neglected the radiation physics and hydrodynamics of the planetary atmosphere" and could be a significant overestimate. Indeed, the XUV stripping of a very thick hydrogen and helium primary atmosphere might actually be required for habitability. The high levels of XUV would also be expected to make water retention on planet d less likely than predicted by Bolmont et al., though even on highly irradiated planets it might remain in cold traps at the poles or on the night sides of tidally locked planets. [57]

If a dense atmosphere like Earth's, with a protective ozone layer, exists on planets in the habitable zone of TRAPPIST-1, UV surface environments would be similar to present-day Earth. However, an anoxic atmosphere would allow more UV to reach the surface, making surface environments hostile to even highly UV-tolerant terrestrial extremophiles. If future observations detect ozone on one of the TRAPPIST-1 planets, it would be a prime candidate to search for surface life. [58]

Spectroscopy of planetary atmospheres Edit

Because of the system's relative proximity, the small size of the primary and the orbital alignments that produce daily transits, [60] the atmospheres of TRAPPIST-1's planets are favorable targets for transmission spectroscopy investigation. [61]

The combined transmission spectrum of planets b in c, obtained by the Hubble Space Telescope, rules out a cloud-free hydrogen-dominated atmosphere for each planet, so they are unlikely to harbor an extended gas envelope, unless it is cloudy out to high altitudes. Other atmospheric structures, from a cloud-free water-vapor atmosphere to a Venus-like atmosphere, remain consistent with the featureless spectrum. [62]

Another study hinted at the presence of hydrogen exospheres around the two inner planets with exospheric discs extending up to seven times the planets' radii. [63]

In a paper by an international collaboration using data from space and ground-based telescopes, it was found that planets c in e likely have largely rocky interiors, and that b is the only planet above the runaway green-house limit, with pressures of water vapour of the order of 10 1 to 10 4 bar. [40]

Observations by future telescopes, such as the Vesoljski teleskop James Webb or European Extremely Large Telescope, will be able to assess the greenhouse gas content of the atmospheres, allowing better estimation of surface conditions. They may also be able to detect biosignatures like ozone or methane in the atmospheres of these planets, if life is present there. [14] [64] [65] [66] As of 2020, the TRAPPIST-1 is considered a most promising target for transmission spectroscopy using the Vesoljski teleskop James Webb. [67]

Impact of stellar activity on habitability Edit

The K2 observations of Kepler revealed several flares on the host star. The energy of the strongest event was comparable to the Carrington event, one of the strongest flares seen on the Sun. As the planets in TRAPPIST-1 system orbit much closer to their host star than Earth, such eruptions could cause 10–10000 times stronger magnetic storms than the most powerful geomagnetic storms on Earth. Beside the direct harm caused by the radiation associated with the eruptions, they can also pose further threats: the chemical composition of the planetary atmospheres is probably altered by the eruptions on a regular basis, and the atmospheres can be also eroded in the long term. A sufficiently strong magnetic field of the exoplanets could protect their atmosphere from the harmful effects of such eruptions, but an Earth-like exoplanet would need a magnetic field in the order of 10–1000 Gauss to be shielded from such flares (as a comparison, the Earth's magnetic field is ≈0.5 Gauss). [8] The study in 2020 have found super-flare (defined as flare releasing at least 10 26 J - twice the Carrington event) rate of TRAPPIST-1 is 4.2 +1.9
−0.2 year −1 , insufficient to permanently deplete ozone in the atmosphere of habitable-zone planets. Also, flare UV emission of TRAPPIST-1 is grossly insufficient to compensate the lack of quiescent UV emission and to power prebiotic chemistry. [68]

Probability of interplanetary panspermia Edit

Hypothetically, if the conditions of the TRAPPIST-1 planetary system were to be able to support life, any possible life that had developed through abiogenesis on one of the planets would likely be spread to other planets in the TRAPPIST-1 system via panspermia, the transfer of life from one planet to another. [69] Due to the close proximity of the planets in the habitable zone with a separation of at least

0.01 AU from each other, the probability of life being transferred from one planet to another is greatly enhanced. [70] Compared to the likelihood of panspermia from Earth to Mars, the likelihood of interplanetary panspermia in the TRAPPIST-1 system is thought to be about 10,000 times higher. [69]

Radio signal searches Edit

In February 2017, Seth Shostak, senior astronomer for the SETI Institute, noted, "[T]he SETI Institute used its Allen Telescope Array [in 2016] to observe the environs of TRAPPIST-1, scanning through 10 billion radio channels in search of signals. No transmissions were detected." [22] Additional observations with the more sensitive Green Bank Telescope did not show evidence of transmissions. [71]

Existence of undiscovered planets Edit

One study using the CAPSCam astrometric camera concluded that the TRAPPIST-1 system has no planets with a mass at least 4.6 M J with year-long orbits and no planets with a mass at least 1.6 M J with five-year orbits. The authors of the study noted, however, that their findings left areas of the TRAPPIST-1 system, most notably the zone in which planets would have intermediate-period orbits, unanalyzed. [72]

Possibility of moons Edit

Stephen R. Kane, writing in Časopis Astrofizični časopis, notes that TRAPPIST-1 planets are unlikely to have large moons. [73] [74] The Earth's Moon has a radius 27% that of Earth, so its area (and its transit depth) is 7.4% that of Earth, which would likely have been noted in the transit study if present. Smaller moons of 200–300 km (120–190 mi) radius would likely not have been detected.

At a theoretical level, Kane found that moons around the inner TRAPPIST-1 planets would need to be extraordinarily dense to be even theoretically possible. This is based on a comparison of the Hill sphere, which marks the outer limit of a moon's possible orbit by defining the region of space in which a planet's gravity is stronger than the tidal force of its star, and the Roche limit, which represents the smallest distance at which a moon can orbit before the planet's tides exceed its own gravity and pull it apart. These constraints do not rule out the presence of ring systems (where particles are held together by chemical rather than gravitational forces). The mathematical derivation is as follows: