Astronomija

Ali je zemeljska gravitacija trdne površine 1 g nenavadno velika za eksoplanete?

Ali je zemeljska gravitacija trdne površine 1 g nenavadno velika za eksoplanete?


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Površinske gravitacije Sončevih planetov so blizu 1g, 0,38-2,53 (okoli faktorja e po naključju). Tukaj gravitacija plinskih velikanov v oblaku ni preveč zanimiva, vendar mislim, da gravitacija na njihovih trdnih površinah (če sploh) ni večja. Zdi se, da tako imenovani superzemeljski eksoplaneti na splošno kopičijo atmosfere lahkih plinov, kar zmanjša njihovo pričakovano gravitacijo trdne površine glede na njihovo velikost, izmerjeno med na primer tranzitnimi poti (vseeno mislim, da tako).

Ali obstajajo tehtni razlogi za pričakovanje, da ima naša Zemlja v živalskem vrtu planetov v vesolju visoko lestvico površinske gravitacije? Da je nenavadno, če stojimo na trdnih nepremičninah, ki imajo bistveno (kot v nekaj desetink) večjo težo, kot jo imamo tukaj?

Površinske gravitacije iz Wikipedije:

  • Živo srebro 0,38
  • Venera 0,90
  • Zemlja 1.00
  • Mars 0,38
  • Jupiter 2,53
  • Saturn 1.07
  • Uran 0,89
  • Neptun 1.14

Površinska gravitacija je odvisna od dveh stvari:

  1. Maša
  2. Polmer

Masa je sorazmerna s kocko polmera, pomnoženo s srednjo gostoto. Gravitacija je sorazmerna s kvadratom polmera. Če imate dva planeta z enako gostoto, bo večji planet imel večjo težo.

Toda povprečna gostota je lahko velik dejavnik.

Zemlja:

  • Povprečna gostota 5,515 g / cm3
  • Povprečni polmer 6371,0 km

Venera:

  • Povprečna gostota 5,243 g / cm3
  • Povprečni polmer 6051,8 km

Jupiter:

  • Povprečna gostota 1,326 g / cm3
  • Povprečni polmer 69911 ± 6 km

(vir: strani Wikipedije za Zemljo, Venero in Jupiter)

Venera je torej nekoliko manjša in nekoliko manj gosta od Zemlje, kar ji daje 90% gravitacije Zemlje. Jupiter je veliko večji (več kot 10-krat), vendar veliko manj gost (približno ena četrtina), kar mu daje 10/4 ali približno 250% zemeljske gravitacije.

Razumem, da je Zemlja dokaj majhna, a nenavadno gosta. Od planetov, ki jih lahko zaznamo s trenutno tehnologijo, bi pričakoval, da bi jih bilo več večjih in / ali težjih, saj bi jih zaradi teh lastnosti lažje zaznali.

Pričakujem, da bomo od vseh planetov v vesolju našli porazdelitev, ki ni preveč drugačna od našega Osončja in da bo Zemlja padla ravno na visoko stran povprečja.


Po tem prispevku je sl. 4, trdni planeti poljubne mase (do 3000-krat večje od Zemljine mase) ne zrastejo približno približno tri- ali štirikrat več od Zemlje v premeru. To je zato, ker notranji deli planeta zaradi visokega pritiska stisnejo. Planeti, ki so težji od 3000-krat mase Zemlje, so v prehodnem območju do zvezd. V primeru kamnitih planetov bi postali podzvezdni predmeti, podobni po svojih fizikalnih lastnostih (elektronska degeneracija v jedru) belim ali črnim palčkom, ki zaradi pomanjkanja lahkih elementov v jedru ne morejo začeti jedrske fuzije.

Površinska gravitacija je sorazmerna s srednjo gostoto $ rho $ in polmerom $ r $ planeta: $ g = frac {4 pi} {3} G rho r $, pri čemer je $ G $ gravitacijska konstanta .

Planet z približno sestavo Zemlje in 3000-kratno maso Zemlje bi imel približno 3-krat večji zemeljski polmer, torej 27-kratni volumen in 3000/27 = 111-kratno gostoto. Njegova površinska gravitacija bi torej znašala 333 g. To je blizu zgornje meje površinske gravitacije tistega, kar bi imenovali planet. S čisto železnim planetom bi lahko šli nekoliko dlje. Vsaka površinska gravitacija pod to zgornjo mejo je za trdni planet možna vsaj v teoriji.

Glede na stran 1284 prispevka:

Masivni trdni eksoplaneti s stotinami do tisočimi masami Zemlje bodo morda lahko nastali okoli masivnih zvezd (zvezde B in O; 5-120 sončnih mas), kjer bi protoplanetarni disk vseboval dovolj težkih elementov.

Zvezde tipa B in O so redke (0,13% zvezd glavnega zaporedja) in kratkotrajne (manj kot 100 milijonov let; natančneje manj kot 10 USD {10} cdot 18 ^ {- 2,5} USD let, za maso> 18 sončne mase). Zato bodo tudi veliki trdni planeti opisanega tipa redki, čeprav vnaprej nemogoče.

Planet z doslej najvišjo ocenjeno površinsko težo (8. marec 2014) je CoRoT-Exo-3b (v tabelo eksoplanetov dodajte stolpec "površinska gravitacija" in razvrstite po tem stolpcu):

CoRoT-Exo-3b ima polmer 1,01 ± 0,07 R_Jup in prehaja okoli svojega primarnega tipa F3 vsakih 4,26 dni v sinhroni orbiti. Njegova masa 21,66 ± 1,0 M_Jup, gostota 26,4 ± 5,6 g cm-3 in površinska gravitacija logg = 4,72 ga jasno ločujejo od običajne populacije blizu planeta, zaradi česar je do zdaj najzanimivejši prehodni podzvezdni objekt.

Površinska gravitacija logg = 4,72 pomeni, da je površinska gravitacija enaka $ 10 ^ {4,72} mbox {cm} / mbox {s} ^ 2 = 52 480 mbox {cm} / mbox {s} ^ 2 = 53,5 ~ g. $ Ta raziskava je uporabila bazo podatkov Exoplanet Orbit in Exoplanet Data Explorer na exoplanets.org.

CoRoT-3b na Wikipediji.


Končno o eksplanetah ne vemo dovolj, da bi bili prepričani; za zdaj so vsi naši podatki nagnjeni k masivnejšim planetom, ki jih je lažje zaznati z Dopplerjevimi nihanji, ali planetom z velikim premerom (skoraj zagotovo plinskim velikanom), ki jih je enostavno zaznati z zatemnitvijo gostiteljske zvezde, ko jo zasenčijo v primerjavi z nami. Vsak dan prihaja več podatkov in tako fantastičen, kot je bil Keplar, mislim, da moramo vsaj zdržati, da bo James-Webb na spletu, preden bomo naredili kakšne res težke zaključke iz podatkov.

Brez podatkov se lahko zanesemo le na naše teorije o nastanku planetov, v katerih smo dokaj dobri.
Zemlja je verjetno bolj gosta od povprečnega planeta svoje velikosti, zaradi česar je že v začetku svojega razvoja trčila v približno mars velik objekt (vzdevek Theia). Jedro Theia bi bilo absorbirano v zemeljsko jedro, vendar sta bila oba zunanja sloja odstranjena, kar je ustvarilo obroč, ki bi se združil v našo luno. Tako bi Zemlja dobila jedro z večjo maso, kot bi jo imel planet, ki se oblikuje na njegovi razdalji.

To lahko vidimo v gostotah zemeljskih planetov;

- Objekt ------- Gostota (g cm − 3) ----- Pol glavna os (AU) -

------------- Srednje ---- Nestisnjeno -----------------------

-Živo srebro ----- 5,4 --------- 5,3 ------------- 0,39 ------------

-Venera ------- 5.2 --------- 4.4 ------------- 0.72 ------------

-Zemlja ------- 5,5 ---------- 4,4 ---------------- 1,0 -------------

-Mars -------- 3.9 ---------- 3.8 -------------- 1.5 -------------

Zasluge, Wikipedia

Planeti, ki so bližje svoji zvezdi, bodo naravno imeli večje gostote kot posledica diferenciacije mase; gostejši material, ki se usede v jedro planeta ali središče sončnega akrecijskega diska.

Če pogledamo z vidika bivalnosti,
Vemo, da je gostota pozitivno povezana s površinsko gravitacijo, zato lahko pričakujemo, da bi imela zemlja nekoliko višjo od povprečne površinske teže za planet v bivalnem območju okoli zvezde v kategoriji ena sončna masa.

Kot rečeno, večina zvezd nima ene sončne mase, večina zvezd v vesolju so rdeči palčki, ki so veliko bolj zatemnjeni in lažji od našega sonca in bi imeli bližje, ožje bivalno območje. Bivalni planet okoli rdečega pritlikavca bi bil verjetno manjši in lažji, vendar gostejši zaradi manjšega množičnega oblaka in bližine zvezde.

Mislim, da bi lahko pričakovali, da bo večina eksoplanetov planetov, podobnih živemu srebru, ki kroži okoli rdečih palčkov.
Če je temu tako, lahko pričakujemo, da bi imela zemlja visoko površinsko težnost glede na zemeljske planete (čeprav obstajajo daljši masivnejši zemeljski planeti s podobnim premerom) in približno povprečno težo, če upoštevamo vse planete.


Masa, polmer in temperatura Uredi

K2-229b je razmeroma velik zemeljski planet, prvič identificiran s tranzitno metodo, kjer planet prehaja pred gostiteljsko zvezdo in blokira majhen delček njene svetlobe. Ko je bil planet prvič odkrit, je bil znan le njegov polmer. Ugotovljeno je bilo, da znaša 1,165 R ali približno 16,5% večja od Zemlje. [1] Planet te velikosti je najverjetneje skalnat s trdno površino, kot je Zemlja sama. Meritve radialne hitrosti s spektrografom HARPS pa so pokazale, da je bil K2-229b veliko gostejši in bolj masiven, kot je bilo sprva pričakovano. Masa planeta je 2,59 M in izjemno visoko gostoto približno 8,9 g / cm 3, [1] kar ji daje približno 91% večjo površinsko težo kot Zemlja. Nenavadno visoka masa in gostota K2-229b kažeta na živosrebrno sestavo, v kateri prevladuje železo, ki zavzema približno 70% mase planeta. [4] | masa = 0,837 + 0,019
−0.025 [1]

Zaradi izredno tesne orbite je K2-229b eden najbolj vročih planetov, ki so jih še našli. Ima ravnotežno temperaturo 1.960 K (1.690 ° C 3.070 ° F), [1] dovolj vročo, da lahko stopi železo. Dnevna stran ima še višjo temperaturo nad 2.330 K (2.060 ° C 3.730 ° F). [4]

Uredi orbito in vrtenje

K2-229b ima eno najkrajših znanih orbitalnih obdobij, pri čemer je ena polna orbita trajala le 0,584 dni (14 ur). Planet kroži okoli svoje gostiteljske zvezde na razdalji 0,012888 AU, skoraj 100-krat bližje od Zemlje. [1] Za primerjavo, najbolj notranji planet našega Osončja, Merkur, potrebuje 88 dni, da kroži z 0,39 AU. K2-229b ima orbitalno ekscentričnost 0 in je najverjetneje zaklenjen s svojo gostiteljsko zvezdo.

Gostiteljska zvezda Edit

K2-229b kroži okoli oranžne pritlikave zvezde K2-229, kar je približno 79% polmera in 84% mase Sonca, s temperaturo 5185 K in starostjo približno 5,4 milijarde let. [1] Za primerjavo, Sonce ima temperaturo 5778 K in je staro 4,5 milijarde let. K2-229 ima vizualno magnitudo 10.985, ki je preslaba, da bi jo lahko videli brez teleskopa. Znano je, da je izredno aktiven. [4]

Odkritje velike mase in gostote K2-229b je bilo nepričakovano. "Ko smo videli ta planet, ki je velik kot Zemlja, smo mislili, da bo imel zemeljsko sestavo. Izkazalo pa se je, da je bolj kot Merkur", astrofizičarka Jessie Christiansen, ki ni bila del ekipe, ki je odkrila K2-229b, povedal Newsweek. [5] Nenavadna sestava, podobna Merkurju, K2-229b naj bi ponujala vpogled v to, kako so lahko nastali ta in drugi planeti, podobni Merkurju z visoko gostoto.

Obstaja več hipotez o tem, kako je K2-229b postal tako gost, pri čemer ena navaja, da je velik del ozračja planeta razjedlo zvezdno sevanje bližnje aktivne zvezde. Druga hipoteza kaže, da je K2-229b nastal, ko sta trčila dva planeta v sistemu, podobno kot teorija o tem, kako je Luna nastala zaradi trka med Zemljo in drugim planetom. [4] Od marca 2018 so vse te teorije še vedno v igri in ni dovolj dokazov, da bi jih dokazali ali ovrgli. [5]

Raziskovalci so opazili tudi položaj K2-229b v njegovem planetarnem sistemu. "Zanimivo je, da je K2-229b tudi najbolj notranji planet v sistemu vsaj treh planetov, čeprav vsi trije krožijo veliko bližje svoji zvezdi kot Merkur. Več takšnih odkritij nam bo pomagalo osvetliti nastanek teh nenavadnih planetov. kot sam Merkur ", je komentiral dr. David Armstrong, eden od članov skupine iz skupine za astronomijo in astrofiziko Univerze v Warwicku, ki je odkrila planet. [4] [6]

K2-229b je znan po tem, da je precej podoben Merkurju, z približno enakim masnim deležem jedra 68 +17
−25 %. Vendar je prvi precej bližje svoji gostiteljski zvezdi in je bolj dovzeten za izhlapevanje plašča kot drugi. Pri dnevni temperaturi nad 2.330 K (ob predpostavki, da je plimovanje zaprto s svojo gostiteljsko zvezdo) naj bi K2-229 imel vsaj tanko atmosfero silikatne pare, ki je nastala zaradi visokih temperatur na strani, obrnjeni proti zvezdi. . Kljub visoki aktivnosti K2-229 naj ne bi planet popolnoma izgubil tega ozračja. Če ima K2-229b magnetno polje, se lahko upre atmosferski eroziji, kar pomeni, da je zelo malo verjetno, da bi izhlapevanje plašča povzročilo sestavo, bogato z železom. To in K2-229b naj bi izgubil le 1,3 × 10 −5 M na leto premalo, da bi v življenju odstranili več kot nekaj odstotkov celotne mase. [2]


Zelo nenavaden "Hot Neptune" Exoplanet 260 svetlobnih let stran, ki "ne bi smel obstajati"

Ta vtis umetnika prikazuje LTT 9779b v bližini zvezde, ki jo kroži, in osvetljuje izredno vročo (2000 Kelvinov) dnevno planoto in njeno nočno stran (okoli 1000 K). Zasluge: Ethen Schmidt

Skupina, ki jo vodi astronom z univerze v Kansasu, je drobila podatke NASA-jevih vesoljskih teleskopov TESS in Spitzer, da bi prvič prikazala ozračje zelo nenavadne eksoplanete, imenovane "vroč Neptun".

Ugotovitve glede nedavno najdenega planeta LTT 9779b so bile danes objavljene v časopisu Astrophysical Journal Letters. Prispevek podrobno opisuje prvo spektralno atmosfersko karakterizacijo katerega koli planeta, ki ga je odkril TESS, prvi globalni temperaturni zemljevid katerega koli planeta TESS z ozračjem in vročim Neptunom, katerega emisijski spekter se bistveno razlikuje od številnih večjih "vročih Jupitrov", ki smo jih prej preučevali.

"Prvič smo izmerili svetlobo s tega planeta, ki ne bi smela obstajati," je povedal Ian Crossfield, docent za fiziko in astronomijo na KU in vodilni avtor prispevka. »Ta planet je tako močno obsevan s svojo zvezdo, da ima njegova temperatura več kot 3000 stopinj Celzija in bi lahko ozračje popolnoma izhlapelo. Kljub temu pa nam Spitzerjeva opazovanja kažejo njegovo ozračje prek infrardeče svetlobe, ki jo oddaja planet. "

LTT 9779b je sicer izjemen, nekaj pa je gotovo: ljudem tam verjetno ne bi bilo všeč.


& # 8220 Prvič smo izmerili svetlobo, ki prihaja s tega planeta, ki ne bi smel obstajati, & # 8221 je povedal Ian Crossfield, docent za fiziko in astronomijo na KU in vodilni avtor prispevka. & # 8220Ta planet je tako močno obsevan s svojo zvezdo, da ima njegova temperatura več kot 3000 stopinj Celzija in bi lahko ozračje popolnoma izhlapelo. Kljub temu pa nam Spitzerjeva opazovanja pokažejo njegovo ozračje prek infrardeče svetlobe, ki jo oddaja planet. & # 8221 Zasluge: Ethen Schmidt, Univerza v Kansasu

"Ta planet nima trdne površine in je v našem sončnem sistemu celo bolj vroč kot Merkur - ne samo, da bi vodil talino v ozračju tega planeta, temveč tudi platino, krom in nerjaveče jeklo," je dejal Crossfield. »Leto na tem planetu je manj kot 24 ur - to pomeni, kako hitro se šiba okoli svoje zvezde. To je precej ekstremen sistem. "

Vroči Neptun LTT 9779b je bil odkrit šele lani in je postal eden prvih planetov velikosti Neptuna, ki jih je odkrila NASA-ina misija TESS za lov na planete TESS. Crossfield in njegovi soavtorji so za analizo atmosferske sestave eksoplaneta uporabili tehniko, imenovano "fazna krivulja".

Ta vtis umetnika prikazuje sistem LTT 9779 približno v merilu, z vročim Neptunovim planetom na levi in ​​svetlo, bližnjo zvezdo na desni. Sled materiala, ki odteka s planeta, je hipotetična, a verjetno temelji na intenzivnem obsevanju tega planeta. Zasluge: Ethen Schmidt

"Izmerimo, koliko infrardeče svetlobe je oddajal planet, ko se vrti za 360 stopinj na svoji osi," je dejal. »Infrardeča svetloba vam pove temperaturo nečesa in kje so vroči in hladnejši deli tega planeta - na Zemlji ni najbolj vroče opoldne, nekaj ur popoldne je najbolj vroče. Toda na tem planetu je pravzaprav najbolj vroče ravno opoldne. Večino infrardeče svetlobe vidimo iz dela planeta, ko je njegova zvezda naravnost nad glavo, veliko manj pa iz drugih delov planeta. "

Odčitki temperature planeta & # 8217s se obravnavajo kot način za karakterizacijo njegove atmosfere.

"Planet je veliko hladnejši, kot smo pričakovali, kar kaže na to, da odseva velik del vpadne zvezdne svetlobe, ki ga zadene, verjetno zaradi oblakov ob dnevu," je povedal soavtor Nicolas Cowan z Inštituta za raziskave eksoplanetov (iREx) in Univerza McGill v Montrealu, ki je pomagal pri analizi in interpretaciji meritev krivulje termične faze. "Planet tudi ne prenaša veliko toplote na svojo nočno stran, vendar mislimo, da razumemo naslednje: Zvezdina svetloba, ki se absorbira, se verjetno absorbira visoko v ozračju, od koder se energija hitro seva nazaj v vesolje."

Po Crossfieldu so rezultati le prvi korak v novo fazo eksoplanetarnega raziskovanja, saj se preučevanje zunanjih planetov vztrajno premika proti manjšim in manjšim planetom.

Ta prikaz umetnika prikazuje LTT 9779b, ki prehaja zvezdo, ki jo obkroži. Ta tranzit na kratko blokira zajeten del svetlobe zvezde & # 8217s in tako je planet prvič odkrila misija NASA & # 8217s TESS. Zasluge: Ethen Schmidt

"Ne bi rekel, da zdaj razumemo vse o tem planetu, vendar smo izmerili dovolj, da bomo vedeli, da bo to res ploden predmet za prihodnje študije," je dejal. »Že zdaj je namenjen opazovanjem z vesoljskim teleskopom James Webb, ki je Nasin naslednji velik večmilijardski vodilni vesoljski teleskop, ki se dviguje čez nekaj let. Dosedanje meritve nam kažejo tisto, kar imenujemo spektralne absorpcijske lastnosti - in njegov spekter označuje ogljikov monoksid ali ogljikov dioksid v ozračju. Začenjamo razumevati, katere molekule tvorijo njegovo atmosfero. Ker to vidimo in zaradi tega, kako je videti ta globalni temperaturni zemljevid, nam nekaj pove tudi o tem, kako vetrovi krožijo energijo in material po atmosferi tega mini plinskega planeta. "

Crossfield je razložil izjemno redkost svetov, podobnih Neptunu, ki jih najdemo blizu svojih gostiteljskih zvezd, regiji, ki je običajno tako brez planetov, ki jo astronomi imenujejo "vroča puščava Neptuna".

& # 8220Mislimo, da je to zato, ker vroči Neptuni niso dovolj masivni, da bi se izognili znatnemu izhlapevanju zraka in izgubi mase, «je dejal. "Torej, najbolj vroči eksoplaneti so bodisi masivni vroči Jupitri bodisi skalnati planeti, ki so že zdavnaj izgubili večino svojega ozračja."

Spremni članek k tej raziskavi, ki jo je vodila Diana Dragomir, docentka za fiziko in astronomijo Univerze v Novi Mehiki, raziskuje atmosfersko sestavo eksoplaneta in # 8217s s pomočjo sekundarnih opazovanj mrkov z infrardečo kamero Spitzer (IRAC) vročega Neptuna.

Čeprav LTT 9779b ni primeren za kolonizacijo s strani človeka ali katere koli druge znane oblike življenja, je Crossfield dejal, da bi ocenjevanje njegovega ozračja izpopolnilo tehnike, ki bi jih nekoč lahko uporabili za iskanje bolj prijetnih planetov za življenje.

"Če bo kdo verjel temu, kar astronomi pravijo o iskanju znakov življenja ali kisika na drugih svetovih, bomo morali pokazati, da lahko to dejansko storimo najprej na enostavnih stvareh," je dejal. "V tem smislu ti večji, bolj vroči planeti, kot je LTT 9779b, delujejo kot vadbena kolesa in dokazujejo, da dejansko vemo, kaj počnemo, in lahko vse uredimo."

Crossfield je dejal, da je bil njegov pogled v ozračje tako čudnega in oddaljenega planeta dragocen že sam po sebi.

"Kot nekdo, ki to preučuje, lahko naredimo veliko zanimivih planetarnih znanosti pri merjenju lastnosti teh planetov - tako kot ljudje preučujejo atmosfero Jupitra, Saturna in Venere - čeprav ne mislimo, da bodo te gostile življenje ," rekel je. "Še vedno so zanimivi in ​​lahko spoznamo, kako so nastali ti planeti, in širši kontekst planetarnih sistemov."

Crossfield je dejal, da je še veliko dela, da bi bolje razumeli LTT 9779b in podobne vroče Neptune, ki še niso odkriti. (Hkrati objavljamo spremljevalni članek o sestavi atmosfere LTT 9779b z analizo njegovega "spektra" sekundarnega mrka, ki ga je Crossfield napisal v soavtorstvu.)

"Še naprej ga želimo opazovati z drugimi teleskopi, da bomo lahko odgovorili na več vprašanj," je dejal. »Kako lahko ta planet obdrži svoje ozračje? Kako je sploh nastala? Je bil sprva večji, a je izgubil del prvotnega vzdušja? Če je odgovor pritrdilen, zakaj potem njegovo ozračje ni le pomanjšana različica ozračja izjemno vročih večjih eksoplanetov? In kaj se še lahko skriva v njenem ozračju? "

Nekateri soavtorji raziskovalca KU v prispevku načrtujejo tudi nadaljevanje preučevanja neverjetnega eksoplaneta.

"V njeni atmosferi smo zaznali ogljikov monoksid in da je stalna dnevna stran zelo vroča, medtem ko se na nočno stran prenaša zelo malo toplote," je dejal Björn Benneke iz iREx in Université de Montréal. “Zaradi obeh ugotovitev LTT 9779b trdi, da je treba opaziti zelo močan signal, zaradi česar je planet zelo zanimiv cilj za nadaljnjo podrobno karakterizacijo z JWST. Zdaj načrtujemo tudi veliko podrobnejša opazovanja fazne krivulje z NIRISS-om na JWST. "

Referenca: & # 8220 Phase Curves of Hot Neptune LTT 9779b Suggest a High-metallicity Atmosphere & # 8221 Ian JM Crossfield, Diana Dragomir, Nicolas B. Cowan, Tansu Daylan, Ian Wong, Tiffany Kataria, Drake Deming, Laura Kreidberg, Thomas Mikal- Evans, Varoujan Gorjian, James S. Jenkins, Björn Benneke, Karen A. Collins, Christopher J. Burke, Christopher E. Henze, Scott McDermott, Ismael Mireles, David Watanabe, Bill Wohler, George Ricker, Roland Vanderspek, Sara Seager in Jon M. Jenkins, 26. oktober 2020, Astrophysical Journal Letters.
DOI: 10.3847 / 2041-8213 / abbc71


Nova tehnika za & # 8220Gledanje & # 8221 površin Exoplanet na podlagi vsebine njihovih atmosfer

Novembra 2021 je Vesoljski teleskop James Webb (JWST) bo svojo težko pričakovano pot v vesolje. Ta observatorij naslednje generacije bo vesolje opazoval s svojo napredno infrardečo zbirko in razkril številne še nikoli videne stvari. Do leta 2024 se bo pridružila Rimski vesoljski teleskop Nancy Grace (RST), naslednik Hubblove misije, ki bo imel 100-krat vidno polje Hubble & # 8217s in hitrejši čas opazovanja.

Ti instrumenti bodo močno prispevali k številnim področjem raziskav, nenazadnje pa je tudi odkrivanje in karakterizacija zunaj sončnih planetov. Toda tudi z napredno optiko in zmogljivostmi te misije ne bodo mogle natančno pregledati površin eksoplanetov. Vendar pa je skupina UC Santa Cruz (UCSC) in Space Science Institute (SSI) razvila naslednjo najboljšo stvar: orodje za zaznavanje površine eksoplaneta, ne da bi jo neposredno videli.

Članek, ki opisuje njihove raziskave, z naslovom & # 8220Kako prepoznati površine eksplanetov z uporabo vrst atmosferskih sledi v atmosferah, v katerih prevladujejo vodik, & # 8221 se je nedavno pojavil v Astrofizični časopis. Kot so navedli, je ekipa poskušala razviti načine za preučevanje površin eksoplanetov glede na njihovo atmosfersko sestavo. To je potrebno, ker noben od prihajajočih vesoljskih teleskopov nima možnosti posredno preučevati površinskih značilnosti eksoplaneta.

Vendar pa bodo ti isti teleskopi izvrstno orodje za določanje sestave atmosfer eksoplanetov. Onkraj James Webb in Rimski vesoljski teleskopi, bodo v prihodnjih letih začele delovati številne zemeljske opazovalnice naslednje generacije, ki bodo imele podobne zmogljivosti. Sem spadajo izredno velik teleskop (ELT), velikanski Magellanov teleskop (GMT) in trideset metrski teleskop (TMT).

S svojo kombinacijo visoko občutljivih, koronografov in prilagodljive optike bodo ti observatoriji lahko izvajali neposredne slikovne študije eksoplanetov, kjer bodo za določanje atmosferske sestave preučevali svetlobo, ki se odbija neposredno iz atmosfere eksoplaneta. To bo astronomom in astrobiologom pomagalo postaviti strožje omejitve, kateri eksoplaneti so & # 8220potencialno bivalni & # 8221 in kateri ne.

Pogoji, za katere menimo, da so predpogoji za življenje, vključujejo tudi geološke procese, kot so vulkanska dejavnost in tektonika plošč, ki jih je mogoče razbrati po njihovih značilnostih površja. Čeprav jih v bližnji prihodnosti ne bomo mogli zaznati, so Xinting Yu (podoktorska sodelavka za Zemljo in planetarne vede na UCSC) in njeni kolegi predlagali nov način za določanje površinskih značilnosti na podlagi številčnosti atmosferskih plinov.

Kot je dr. Yu pojasnil za Universe Today po e-pošti, sta navdih za to metodo dobila dva telesa našega sončnega sistema & # 8211 Jupiter in Titan (največja luna Saturna in # 8217s). Obe telesi imata gosto plinasto ozračje z dvema kemijskima vrstama & # 8211 amoniakom (NH 3) in metanom (CH 4) & # 8211, ki igrata glavno vlogo v atmosferskih procesih. Rekel je Yu:

& # 8220Titan ima hladno in plitvo površino z skoraj nič (ali naj ne bi bilo) amoniaka in metana, medtem ko ima Jupitrovo ozračje veliko amoniaka in metana. Zakaj se to dogaja? V zgornji atmosferi Jupitra in Titana amoniak in metan nenehno uničujejo UV-fotoni, pri čemer nastajajo dušik (za amoniak) in bolj zapleteni ogljikovodiki (za metan). Na Titanu se fotokemijski dušik in kompleksni ogljikovodiki nenehno tvorijo in kopičijo. & # 8221

Cassinijeva slika Saturna & # 8217s največje lune Titan. Zasluge: NASA / JPL-Caltech / Space Science Institute

Skratka, metan in amoniak se v atmosferi Titanov uničita in nato porabita za tvorbo dušika in ogljikovodikov. Prav zaradi tega je dušik postal prevladujoči plin v atmosferi Titan-a (98 vol.%) In veliko odlaganje ogljikovodikov na njegovi površini, kar je privedlo do nastanka okolja, bogatega z organskimi snovmi. Zaradi izjemnega mraza površine Titan & # 8217s je ta postopek pretvorbe nepovraten.

Po drugi strani pa ima Jupiter v gosti atmosferi tudi amoniak in metan, vendar nima površine, o kateri bi lahko govoril. Kot je pojasnila Yu, ima to za posledico precej drugačen postopek, pri katerem sodelujejo kemične vrste:

& # 8220 Ker na Jupitru ni površine, se ozračje razširi vse do tisoč zemeljskih površinskih pritiskov in tisoče kelvinov. Fotokemijski dušik in kompleksni ogljikovodiki v zgornjih slojih atmosfere se lahko prenašajo v ta globok, vroč del ozračja. Tam bi lahko kombinirali vodik za predelavo metana in amoniaka. Reformirani metan in amoniak se nato "reciklirata" nazaj v zgornje ozračje. Ta cikel še naprej dopolnjuje uničeni metan in amoniak. & # 8221

Druga ključna točka, ki sta jo Yu in njena ekipa obravnavali, je povezana s trenutnim popisom eksoplanetov. Do danes je bila večina odkritih eksplanetov mini Neptun & # 8211, torej planetov, ki so manj masivni od Neptuna, vendar imajo gosto atmosfero, v kateri prevladujeta vodik in helij. Dejansko je bilo od 4.401 potrjenih eksoplanetov do danes 1488 identificiranih kot & # 8220Neptune-like & # 8221 z masami, ki se gibljejo od 9-krat večje od Zemljine do nekoliko manjše od Jupitra.

Jupitrovo ozračje, kot ga je posnela misija Juno (obarval Kevin M. Gill). Zasluge: NASA / JPL-Caltech / SwRI / MSSS / Kevin M. Gill

Zaradi njihovih plinastih ovojev in vključenih razdalj je nemogoče ugotoviti, ali so se ti planeti pojavili in kje se nahajajo. Zaradi svoje statistične pomembnosti sta se Yu in njena ekipa odločili, da bosta še posebej uporabila enega za preizkus njihovega novega pristopa. To je bil K2-18b, mini Neptun z približno 8-kratno maso Zemlje, ki kroži znotraj bivalnega območja (HZ) rdeče pritlikave zvezde (K2-18), ki se nahaja 124 svetlobnih let od Zemlje.

K2-18b, ki ga je vesoljski teleskop Kepler prvotno odkril leta 2015, je prvi eksplanet HZ, za katerega je bilo ugotovljeno, da ima v svoji atmosferi znatne količine vodne pare. Z uporabo fotokemičnega modela sta Yu in njena ekipa simulirala, kako bi prisotnost površine na tem eksoplanetu vplivala na atmosferski razvoj K2-18b. Upoštevali so tudi različne atmosferske tlake in temperature, dejavnike, ki so povezani z višino površine.

& # 8220 Sprašujemo se, ali lahko z obilico vrst, kot sta amoniak in metan, ugotovimo, ali ima eksoplanet površino ali ne, & # 8221 je dejal Yu. & # 8220 Hladna in plitva površina bi zmanjšala vse reakcije "recikliranja", ki zahtevajo visoke temperature in tlake v globokih planetarnih atmosferah za preoblikovanje metana in amoniaka. Tako pričakujemo, da bomo na eksoplanetu s hladno in plitvo površino videli malo metana in amoniaka, na eksoplanetu brez površine ali globoke in vroče površine pa veliko metana in amoniaka. & # 8221

Ugotovili so, da sta amonijak in metan, kot je bilo napovedano, občutljiva tako na prisotnost kot na višino površine. To je v skladu s tistim, kar smo opazili pri eksoplanetih z mrzlo in plitvo površino, kjer se kemične vrste, kot so voda, vodikov cianid in težji ogljikovodiki, razgradijo zaradi izpostavljenosti UV-žarkom. Medtem se ohranijo vrste, kot sta ogljikov monoksid in ogljikov dioksid (ki sta manj nagnjeni k UV-uničenju).

Umetnikov vtis o planetu Super-Zemlja, ki kroži okoli Soncu podobne zvezde. Zasluge: ESO / M. Kornmesser

Nepričakovano pa je bilo, kako so različne kemikalije na različne načine občutljive na različne stopnje višine. Po mnenju Yu je to posledica dejstva, da imajo vrste ogljika in dušika & # 8220sladko mesto & # 8221, kjer jih je mogoče v celoti reciklirati. Medtem ko so amoniak in vodikov cianid (HCN) občutljivi na ozračja z gostoto 100 barov na površini (100-krat več kot Zemljina, podobno kot Venera), so metan, ogljikov monoksid in ogljikov dioksid občutljivi na tlake pod 10 barov na površini. (desetkrat več od Zemlje).

Te ugotovitve imajo več posledic za preučevanje eksoplanetov, med katerimi je predvsem dejstvo, da so planetarne površine pomembne. Rekel je Yu:

& # 8220Prehodno so znanstveniki z uporabo termokemičnih ravnotežnih modelov napovedovali atmosfersko sestavo eksoplanetov. Atmosferske sestave določajo izključno tlak in temperatura ozračja. Toda naša študija kaže, tudi če sta tlak in temperatura enaka, lahko dodajanje površine drastično spremeni atmosfersko sestavo eksoplaneta! & # 8221

Druga implikacija te študije je, da lahko astronomi na podlagi njihove atmosferske sestave spoznajo površine eksoplanetov. & # 8220 Na primer, ko opazovalci vidijo izpraznjene količine amoniaka in HCN, lahko ugotovimo, da ima ta eksoplanet površino manj kot 100 barov, & # 8221 je dodal Yu. & # 8220Potem, če vidimo tudi osiromašene količine metana, ogljikovodikov in povečano količino ogljikovega monoksida, to pomeni površino, manjšo od 10 barov. To je precej obetavno za prepoznavanje bivalnih eksoplanetov! & # 8221

Poleg karakterizacije mini-Neptuna ima ta raziskava posledice tudi za vse druge vrste eksplanetov & # 8211, vključno s kamnitimi in & # 8220zemeljskimi & # 8221. V bistvu, dokler ima zadevni planet ozračje in je v zgornjem ozračju izpostavljen ultravijoličnemu sevanju, velikost eksoplaneta ni pomembna. V vseh primerih bodo astronomi opazili enake razlike v kemijski številčnosti, odvisno od tega, ali obstaja površina ali ne.

Umetnikov vtis o zemeljskem skalnatem eksoplanetu GJ 1132 b, ki se nahaja 41 svetlobnih let stran okoli rdeče pritlikave zvezde. Zasluge: NASA, ESA in R. Hurt (IPAC / Caltech)

According to Yu, it is the smaller colder exoplanets that are more promising testing targets for this method since they are more likely to have shallow and cold surfaces. However, smaller planets are also more likely to have interior or surface processes that will affect the abundance of certain chemicals in their atmospheres – such as volcanic activity and plate tectonics. The smaller they are, the more significant these processes could be.

These and other concerns are things that Yu and her team look forward to studying in greater detail in the future to determine the robustness of their results and how it might be affected by different perturbations from the surface/interior of the exoplanets. Their efforts, and those of astrobiologists in general, will benefit greatly from the launch of the JWST, which is currently scheduled to take place sometime in November of 2021. Said Yu:

“Our study points out an exciting science angle for JWST. It is fine to have solely atmospheric characterization data. Without direct surface observations, we can still tell if an exoplanet has a surface, and even roughly where the surface is located. Knowing whether an exoplanet has a surface is also undoubtedly important for astrobiology. A liquid or a solid surface is likely necessary for sustaining complex lifeforms. Thus, the existence of a surface would be an essential thing to look for when assessing an exoplanet’s habitability.”

The ability to study exoplanets directly, combined with the ability to constrain their surface conditions, will advance the study of astrobiology considerably. The field will also benefit from innovative methods that could allow scientists to search for life (aka. biosignatures) based on different levels of entropy in an environment or different levels of complexity with organic particles. Little by little, we are narrowing the focus and tightening the constraints!


Was 'Star Wars' Right? Can We Walk On Most Exoplanets?

When you saw "Star Wars: The Force Awakens" for the fourth time in theaters, did you see any niggling issues with gravity? Or the fact that alien worlds had breathable atmospheres? The characters were conveniently zipping from planet to planet and walking on each surface with no apparent problem.

Is this actually true for most exoplanets we know of? After all, several of the planets in our solar system aren't that friendly for a stroll. Jupiter's pressure would crush you long before you reached the surface &mdash if there je a "surface" buried beneath the gas layers. Venus-landing spacecraft had to be reinforced against the immense surface pressure from its thick clouds. The moon and Mars &mdash past and future destinations for astronauts &mdash are both possible for astronauts to walk on, however, though those gravitational fields would take some getting used to.

A new paper in the journal Astrobiology (also available in preprint version on Arxiv) says the "Star Wars" problem is understandable because "filming is done on Earth, and an accurate representation of other gravity fields would be technically difficult and expensive." But is it representative of exoplanets generally? Led by the University of Valencia&rsquos Fernando Ballesteros, the authors say reality may actually be not too far off.

The authors say more than 2,000 extrasolar worlds (both confirmed and plausible) have been found, at a rate of about three per week since 2011. That's mostly thanks to NASA's prolific Kepler space telescope. While Kepler finds planets by looking at the dip in light they produce when passing across a star, other telescopes measure the gravitational wobble these planets create in the star instead. That wobble gives an estimate of how massive the planet is.

The authors classify found exoplanets into three categories: 1) masses below Earth (like Mars), 2) a transition zone with super-Earths, Neptunes and some solar system planets, 3) gas giants with masses hundreds of times that of Earth. Surprisingly, that "transition zone" has several planet analogs in our own solar system with surface gravities similar to Earth: Venus, Uranus, Neptune, and Saturn. (Note again that the gravity of Venus is similar to Earth, but its atmosphere can quickly crush unprotected spacecraft.)

But it's still unclear what the precise relationship is between planetary masses and their diameters. "For a given mass one could expect a diversity of sizes depending on the planetary composition and atmospheric size," the authors write, "and we do not even know whether all that we call super-Earths have a solid surface."

The authors add that in theory, you could have a huge rocky planet with no natural atmosphere, but that is challenged by current planetary formation models. Generally it is believed that planets assembled with bits of rock and gas attracting each other over time.

"One could in principle propose a rocky planet as big and massive as one would wish, with no atmosphere at all, but no natural process produces it. The accretion process and the competition for materials during planetary formation impose severe constraints on feasible planets," the authors wrote.

But the authors note that several super-Earths that are both rocky, and that have surface gravities similar to our own planet, have already been spotted by telescopes. So perhaps the "Star Wars" strolls are not too far-fetched, they said.

"If while viewing 'The Force Awakens' the reader sees Harrison Ford walking on Takodana as if he were strolling down Hollywood Boulevard, do not be too critical," they said at the end of the paper. "After all, this may not be so wrong."


Is Earth's 1g solid surface gravity unusually high for exoplanets? - astronomija

Can life exist on any of the recently discovered planets that orbit other stars?

We sure hope so! The search for life is an exciting motivator that gets us to find new planets, build new telescopes and study their atmospheres. We have only one example of life on a planet in the Universe. One way to search for life on other planets is to look for their effect on the atmosphere - does it produce oxygen and methane the way life on the Earth does? If it has a solid surface, liquid water and atmosphere like the Earth, it could be a sign of Earth-like life. There is a big focus on planets that are in the so-called habitable zone - the location where an Earth-like planet is not too hot and not too cold so it can have liquid water, a requirement for all life we know of.

There is a growing list of planets that are in the Habitable Zone, one catalog is here:

Of course we don't know yet if any of these planets host life, nor even if they have a solid surface or an ocean. Sometime in the future, we may be able to measure these properties and also to look for bio-signatures (such as methane and oxygen) in there atmosphere. This list should just keep growing and including more and more Earth-like planets.

UPDATED by Everett Schlawin, July 18, 2015

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.


Experimental Determination of Mantle Solidi and Melt Compositions for Two Likely Rocky Exoplanet Compositions

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1029/2020JE006731

Povzetek

For rocky exoplanets, knowledge of their geologic characteristics such as composition and mineralogy, surface recycling mechanisms, and volcanic behavior are key to determining their suitability to host life. Thus, determining exoplanet habitability requires an understanding of surface chemistry, and understanding the composition of exoplanet surfaces necessitates applying methods from the field of igneous petrology.

Piston-cylinder partial melting experiments were conducted on two hypothetical rocky exoplanet bulk silicate compositions. HEX1, a composition with molar Mg/Si = 1.42 (higher than bulk silicate Earth’s Mg/Si = 1.23) yields a solidus similar to that of Earth’s undepleted mantle. However, HEX2, a composition with molar Ca/Al = 1.07 (higher than Earth Ca/Al = 0.72) has a solidus with a slope of ∼10°C/kbar (versus ∼15°C/kbar for Earth) and as result, has much lower melting temperatures than Earth. The majority of predicted adiabats point toward the likely formation of a silicate magma ocean for exoplanets with a mantle composition similar to HEX2. For adiabats that do intersect HEX2’s solidus, decompression melting initiates at pressures more than 4x greater than in the modern Earth’s undepleted mantle. The experimental partial melt compositions for these exoplanet mantle analogs are broadly similar to primitive terrestrial magmas but with higher CaO, and for the HEX2 composition, higher SiO2 for a given degree of melting.

This first of its kind exoplanetary experimental data can be used to calibrate future exoplanet petrologic models and predict volatile solubilities, volcanic degassing, and crust compositions for exoplanets with bulk compositions and ƒO2 similar to those explored herein.

Plain Language Summary

The composition of rocky exoplanets can be approximated, to first order, from the composition of the stars they orbit. In this work, we conducted experiments to determine the types of minerals found in the interior of two hypothetical exoplanets that orbit stars with compositions different from our star, and the magma compositions produced from melting them. Our results suggest that in many ways exoplanets with these potential compositions have melting behavior similar to that of the Earth’s interior. A key difference is that one composition produced melt at much lower temperatures than Earth, such that the exoplanet may form a magma ocean early in its history or begin melting at depths greater than are common in Earth today. These results provide constraints for modeling geologic processes on exoplanets and their ability to support life this study underlines the need for further experimentation of this kind. The geochemical models that can be derived from this and similar studies will allow us to better interpret the data returned by satellites that will observe exoplanets in the near future.

This article is protected by copyright. Vse pravice pridržane.

Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Accepted, unedited articles published online and citable. The final edited and typeset version of record will appear in the future.


The Pressure and Temperature Limits of Likely Rocky Exoplanets

The interior composition of exoplanets is not observable, limiting our direct knowledge of their structure, composition, and dynamics. Recently described observational trends suggest that rocky exoplanets, that is, planets without significant volatile envelopes, are likely limited to <1.5 Earth radii. We show that given this likely upper limit in the radii of purely rocky super-Earth exoplanets, the maximum expected core-mantle boundary pressure and adiabatic temperature are relatively moderate, 630 GPa and 5000 K, while the maximum central core pressure varies between 1.5 and 2.5 TPa. We further find that for planets with radii less than 1.5 Earth radii, core-mantle boundary pressure and adiabatic temperature are mostly a function of planet radius and insensitive to planet structure. The pressures and temperatures of rocky exoplanet interiors, then, are less than those explored in recent shock-compression experiments, ab initio calculations, and planetary dynamical studies. We further show that the extrapolation of relevant equations of state does not introduce significant uncertainties in the structural models of these planets. Mass-radius models are more sensitive to bulk composition than any uncertainty in the equation of state, even when extrapolated to terapascal pressures.


6 Future Directions

Approximately 15 years since their first discovery, the nature and origins of sub-Neptune size exoplanets have started to come into focus. The global picture that has recently emerged from population level studies is that most close-in, sub-Neptune size planets are actually large terrestrial bodies, with the absence (“true super-Earths”) or presence (“gas-rich super-Earths”) of hydrogen-dominated atmospheres separating them into two classes. These objects are likely poor in volatiles (≲10% by mass), and their final assembly occurred close to their host stars in the presence of a gas-rich disk. It has been tempting to compare these objects to the solar system ice giants Uranus and Neptune because they have hydrogen-dominated atmospheres as a common factor (e.g., Atreya et al., 2020 Wakeford & Dalba, 2020 ). However, the more we learn about these objects the less appropriate this comparison is. Uranus and Neptune have roughly 10 times more hydrogen and helium by mass than the typical gas-rich super-Earth, their bulk and envelopes are likely rich in volatiles, and their formation histories must be quite different to have arrived at very different orbital distances.

The distinct internal structures of gas-rich super-Earths, that is, rock overlaid by thick, hydrogen-dominated atmospheres, leads us to propose that these objects are the first fundamentally new type of planetary object identified from the study of exoplanets. There are a number of observations that can be done to test this hypothesis. One ongoing area of work is the precise measurement of masses and radii for sub-Neptune size planets orbiting stars with a range of masses and ages, and with a wide range of orbital separations. These observations should seek to determine how the planet radius gap varies with stellar mass (Cloutier & Menou, 2020 Hardegree-Ullman et al., 2020 ) and age (Berger et al., 2020 ), and to ultimately reveal the statistical distribution of planet densities in the multi-dimensional parameter space. Early results on this topic from further analysis of Kepler/K2 data have yielded tentative evidence that super-Earths form in gas poor disks around low-mass stars (Cloutier & Menou, 2020 ), and that the mass-loss timescale for these planets around stars with masses ≳1 M is approximately a Gyr, which is a potential signpost to the core-powered mechanism (Berger et al., 2020 ). Continuing work on this topic is currently enabled through the detection of transiting planets around bright stars by NASA's TESS mission (launched 2018 Ricker et al., 2015 ) and ESA's CHEOPS mission (launched 2019 Benz et al., 2020 ), and will be furthered by ESA's PLATO mission (scheduled for launch in 2026 Rauer et al., 2014 ).

Another key observation that can be done for sub-Neptune size planets is precise spectroscopy to reveal their atmospheric compositions. While such observations have been mostly stymied so far, the increased sensitivity and spectral range of the Vesoljski teleskop James Webb (Beichman et al., 2014 Greene et al., 2016 ) and the next generation of ground-based Extremely Large Telescopes (Gandhi et al., 2020 Hood et al., 2020 ) are expected deliver breakthroughs on this topic. Spectroscopy of gas-rich super-Earths should seek to determine if the metallicities of their atmospheres follow the trend of increasing metallicity with lower planet mass that is expected from extrapolating from giant planet formation (Fortney et al., 2013 ). These observations may also reveal atmospheric carbon-to-oxygen abundance ratios, which are a tracer of formation location and migration (Madhusudhan et al., 2014 Öberg et al., 2011 ). Gas-rich super-Earths are expected to have deep magma oceans in contact with their atmospheres, thus yielding unique chemistry in atmospheric gases that could be detectable (Kite et al., 2020 ), as well as potentially sculpting the populations statistics at large sizes (Kite et al., 2019 ).

On the theoretical side, work combining models of photoevaporation and core-powered mass loss into a unified picture of hydrodynamic escape is necessary. This modeling should help identify the regions of parameter space that each mass-loss mechanism dominates. Further, observations of atmospheric escape for the emerging class of very young planets that are the likely antecedents of mature sub-Neptune size planets (David, Cody, et al., 2019 David, Petigura, et al., 2019 David et al., 2016 Newton et al., 2019 Plavchan et al., 2020 Rizzuto et al., 2020 ) offer the hope of distinguishing between the photoevaporative and core-powered atmospheric loss mechanisms. Ultimately, our quantitative insights into how these planets formed, such as the core-mass function and how much H/He these planets accreted, depend strongly on the assumed mass-loss model.

The main uncertainty in our understanding of the formation of sub-Neptune systems is where large cores (planetary embryos) form. Do they originate past the snow line and undergo large-scale migration, or very close to their stars and only migrate to a limited extent? Future advances will likely be aided by a better understanding of the bulk compositions of close-in planets, in particular their volatile contents (e.g., Gupta & Schlichting, 2019 J. G. Rogers & Owen, 2020 ). Improved observations and models of the structure and evolution of planet-forming disks will also play a role, as the disk determines how fast pebbles drift, where and when they accumulate to form planetesimals (Dra̧żkowska & Alibert, 2017 ), and how fast and in what direction growing planets migrate (Bitsch et al., 2019 ).

Putting our Solar System—and its lack of close-in super-Earths or sub-Neptunes—in the context of extrasolar planets is a challenge (for a discussion, see Raymond, Boulet, et al., 2018 ). Jupiter is the only Solar System planet that would be detectable if the Sun were observed with present-day technology. Understanding where our system fits within the bigger picture may thus rest on demographic studies that correlate the nature of inner and outer parts of planetary systems, including super-Earths and sub-Neptunes, Jupiter-like gas giants, ice giant analogs, and even debris disks (Barbato et al., 2018 Bryan et al., 2019 Clanton & Gaudi, 2016 Moro-Martín et al., 2015 Raymond et al., 2011 Suzuki et al., 2016 Zhu & Wu, 2018 ). Fortunately, we are in a golden era of extrasolar planetary astronomy where the observational tools needed for these studies are rapidly advancing. The next 15 years are sure to bring dramatic surprises and insights to match those of the first 15 years of sub-Neptune planet discovery and characterization.


Vsebina

K2-18b was identified as part of the Kepler space telescope program, one of over 1,200 exoplanets discovered during the "Second Light" K2 mission. [9] The discovery of K2-18b was made in 2015, orbiting a red dwarf star (now known as K2-18) with a stellar spectral type of M2.8 about 124 light-years (38 pc) from Earth. The planet was detected through variations in the star's light curve caused by the transit of the planet in front of the star as seen from Earth. [1] [10] The planet was designated "K2-18b" as it was the eighteenth planet discovered during the K2 mission. The predicted relatively low contrast between the planet and its host star would make it easier to observe K2-18b's atmosphere in the future. [1]

In 2017, data from the Spitzer Space Telescope confirmed that K2-18b orbits in the habitable zone around K2-18 with a 33-day period, short enough to allow for observations of multiple K2-18b orbital cycles and improving the statistical significance of the signal. This led to widespread interest in continued observations of K2-18b. [11]

Later studies on K2-18b using the High Accuracy Radial Velocity Planet Searcher (HARPS) and the Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrographs (CARMENES) instruments also identified a likely second exoplanet, K2-18c, with an estimated mass of 5.62 ± 0.84 M in a tighter, 9-day orbit, [4] but this additional planet has not yet been confirmed, and may instead be due to stellar activity. [2]

K2-18 is in the constellation of Leo, but outside its lion asterism. [12] When first discovered, K2-18's distance from Earth was estimated to be 110 light-years (34 pc). [1] However, more precise data from the Gaia star mapping project has shown K2-18 to be at a distance of 124.02 ± 0.26 light-years (38.025 ± 0.079 pc). This improved distance measurement helped to refine the properties of the exoplanetary system. [4]

K2-18b orbits K2-18 at about 0.1429 au (21.38 million km), which lies within the calculated habitable zone for the red dwarf, 0.12–0.25 au (18–37 million km). [8] The exoplanet has an orbital period of about 33 days, [11] which suggests it is tidally locked, with the same face to its host star. [13] The planet's equilibrium temperature is estimated to be around 265 ± 5 K (−8 ± 5 °C 17 ± 9 °F), [4] due to its stellar irradiance of approximately 94% of Earth's. [11] K2-18b is estimated to have a radius of 2.279 ± 0.025 R and a mass of 8.63 ± 1.35 M , based on analysis using HARPS and CARMENES instruments as well as followup observations from Spitzer. [4] [11] It was initially considered a mini-Neptune on its 2015 discovery, [1] but improved data on K2-18b has classified it as a super-Earth. [11] A later study from 2019 classified the planet as a sub-Neptune. [14] [3]

Artist's impression of the K2-18 star system

Diagram of the K2-18 planetary system, showing the orbits of K2-18b and the unconfirmed candidate K2-18c, and the star's habitable zone

A comparison of K2-18b's size, orbit, and other features to other detected exoplanets suggests that the planet could support an atmosphere that contains additional gases besides hydrogen and helium. [15]

External video
Hubblecast Light on the discovery of water vapor on K2-18b
(video/1:19 11 September 2019)
On the discovery of water vapor on K2-18b
NASA Goddard Space Center (video/2:03 11 September 2019)

Further studies using the Hubble Space Telescope were performed, corroborating the results of the Kepler in Spitzer observations and allowing additional measurements of the planet's atmosphere. Two separate analyses by researchers at Université de Montréal and University College London (UCL) of the Hubble data were published in 2019. Both examined spectra of starlight passing through the planet's atmosphere during transits, finding that K2-18b has a hydrogen–helium atmosphere with a high concentration of water vapor, which could range from between 0.01% to 12.5%, up to between 20% and 50%, depending on what other gaseous species are present in the atmosphere. At the upper concentration levels, the water vapor would be sufficiently high to form clouds. [7] [8] [16] The UCL-led study was published on 11 September 2019 in the journal Astronomija narave the study led from the Université de Montréal was posted one day earlier on the preprint server arXiv.org and later published in Časopis Astrofizični časopis. [13] The UCL-led analysis detected water with a statistical significance of 3.6 standard deviations, equivalent to a confidence level of 99.97%. [8]

This was the first super-Earth exoplanet within a star's habitable zone whose atmosphere was detected, [8] and the first discovery of water in a habitable-zone exoplanet. [6] [7] Water had previously been detected in the atmospheres of non-habitable-zone exoplanets such as HD 209458 b, XO-1b, WASP-12b, WASP-17b, and WASP-19b. [17] [18] [19]

Astronomers emphasised that the discovery of water in the atmosphere of K2-18b does not mean the planet can support life or is even habitable, as it probably lacks any solid surface or an atmosphere that can support life. [6] Nevertheless, finding water in a habitable zone exoplanet helps understand how planets are formed. [6] A study led by astronomers from the University of Cambridge considered the interior structure of the planet and found a range of possible solutions, from a rocky core with a thick hydrogen envelope to a planet primarily made up of water with a thinner atmosphere. A subset of these solutions could allow for liquid water on the surface of the planet, albeit at temperatures and pressures higher than STP. [20] K2-18b is now expected to be observed with the James Webb Space Telescope, due to launch in 2021, and the ARIEL space telescope, due to launch in 2029. Both will carry instruments designed to determine the composition of exoplanet atmospheres. [7]

The detailed simulation of planetary spectrum in 2020 has indicated the 1.4um absorption band attributed previously to water may actually be due to methane. The water vapor spectral signatures would not be dominant for cool (below 600 K) planets. [21] [22]


Poglej si posnetek: Shtohen planetet ku mund te jetohet (December 2022).