KATON KARO ZAI IYA CETON SARARIN ISKAR DUNIYA

Ka auna girman ƙananan duniyoyin wajen tsarin rana (exoplanets) sai ka ga sun kasu gida biyu: ƙoli ɗaya a kusan ninki 1.3 na radius ɗin Duniya, wani kuma a kusan ninki 2.4, da kwari a 1.75. Ana tunanin ƙananan duniyoyi ne na dutse tsirara — manyan Duniya (super-Earths). Manyan, ƙananan Neptune (sub-Neptunes), za su kasance duniyoyin dutse da har yanzu ke lulluɓe cikin sararin iskarsu na asali (primordial atmosphere), mai yawan hydrogen. Don haka dole ne wasu duniyoyi sun rasa hydrogen mai yawa. Amma waɗanne ne, kuma ta yaya?

Wanda aka saba zargi

Ana tsammanin duniyoyin dutse masu girman Duniya ko fiye za su fuskanci manyan karo (giant impacts) ɗaya ko fiye a ƙarshen samuwarsu, wasu daga cikinsu bayan faifan iskar gas da ke kewaye da sabon tauraron ya watse. Wani shahararren bayani game da rukunan biyu yana ɗora laifi a kan waɗannan karo na ƙarshe: girgizar karon tana ɗumama mantle na duniyar, saman da ke ƙuna yana sa iskar gas ta tsere, sai duniya mai sararin iska ta koma babbar Duniya (super-Earth).

Darius Modirrousta-Galian, na Shanghai Jiao Tong University, da Jun Korenaga, na Yale University, suna jayayya cewa wannan hoto ya bar rabin lissafin. An karɓi bincikensu don bugawa a mujallar Nature Astronomy.

Ɗaya ɓangaren lissafin

Karo ba ɗumama duniya kawai yake yi ba. Yana kuma ƙara nauyi, kuma dukkan wannan nauyi yana zurfafa rijiyar janyewa (gravitational well) da ke riƙe sararin iskar. A gefe guda, wani ɓangare ne kawai na makamashin karon ke ɗumama mantle a zahiri:

• tsakiyar ƙarfe ta jikin da ya yi karon tana nutsewa ta cikin mantle, tana sakin ɗan makamashi kaɗan a hanya, kuma ta ƙare can cikin zurfi, har yanzu da zafi fiye da kima;
• dutse mai narkewa yana shanye makamashi a matsayin zafin ɓoye (latent heat), ba tare da ɗaga zafin jiki ba;
• zafin da ya kai saman duniyar yana dusashewa can sama a cikin sararin iska. Zafin layukan da ke sarrafa tserewar iskar hasken tauraro ne ke ƙayyade shi, ba ƙasa mai zafi da ke ƙarƙashinsu ba.

Ga duniyar da tsakiyarta ta ƙarfe ta kai 30% na nauyinta, kusan 84% na makamashin karon yana shiga mantle, daidai da cikakkun kwaikwayon karo. Iskar gas da jikin da ya yi karon ke yagewa kai tsaye ƙanƙanta ce; raƙumin girgizar (shock wave) yawanci yana cire kusan 10 zuwa 20% na sararin iskar. Gaba ɗaya, marubutan sun gano cewa ƙarfin riƙon janyewar duniyar yana ƙaruwa fiye da yadda mantle ɗinta ke ɗumama: bayan karon, ana riƙe sararin iskar da ƙarfi fiye da da.

Duniyoyi miliyan ɗari

Don gwada wannan a kan dukkan al’umma (population), ƙungiyar ta kwaikwayi duniyoyi miliyan 100 masu bambance-bambancen nauyi, zafi, tsakiyar ƙarfe da sararin iska irin na zahiri. A kowane zagayen kwaikwayo, kowace duniya tana da damar 52, 63 ko 74%, ya danganta da zagayen, ta samun babban karo bayan faifan iskar gas ya ɓace — ƙimomin da aka ɗauko daga kwaikwayon samuwar duniyoyi — da jikin da ya yi karon da ya kai har rabin nauyin ƙarshe. Daga nan samfurin (model) yana bin narkakken mantle yayin da yake hucewa da sararin iskar yayin da yake zubewa, tsawon shekaru biliyan 5.

Sakamakon yana sake samar da ƙolukan biyu da aka lura, a kusan radius ɗin Duniya 1.3 da 2.4 da kwari a 1.75, ga kowane girke-girken karo da aka gwada. Gwaje-gwajen kwatanci biyu (control runs) sun nuna dalili. Ba tare da kowane karo na ƙarshe ba, duniyoyi da yawa fiye da kima suna ƙarewa tsirara. Da karo ga kowace duniya, da yawa fiye da kima suna riƙe hydrogen ɗinsu. Karon ne ke daidaita ma’auni ta hanyar tura duniyoyi su wuce wani iyakar nauyi (mass threshold) wanda sama da shi hydrogen yana zama a wurinsa.

Girman karo, ƙarin aminci ga sararin iska

Damar rasa dukkan sararin iska tana raguwa yayin da nauyin farko na duniyar da kason jikin da ya yi karon ke ƙaruwa. Marubutan sun daidaita wani lanƙwasa mai sauƙi ga kason jikin da ya yi karon wanda a gare shi kashi 1% kawai na duniyoyi ke rasa komai: χ = 0.58 − 0.0188 (M/M⊕)^1.794, inda χ shi ne nauyin jikin da ya yi karon raba nauyin ƙarshe, M kuma nauyin farko a ma’aunin nauyin Duniya. Ga duniyar da ta fara da nauyin Duniya 5, wannan yana nufin jikin da ya yi karo mai kusan kwata na nauyin ƙarshe; sama da lanƙwasar, kusan ko da yaushe duniyoyi suna riƙe sararin iskarsu na asali.

Zukata masu nauyi ga ƙananan Neptune

Idan karo yana taimaka wa duniyoyi su riƙe hydrogen ɗinsu, to wataƙila ƙananan Neptune sun girma ta hanyar ƙarin karo kuma suna da cikin dutse mafi nauyi fiye da manyan Duniya. Duniyoyin da aka auna sun riga sun nuna wannan hanya: ƙananan Neptune sun fi kusan nauyin Duniya 4 zuwa 5, manyan Duniya kuma ƙasa da haka. Marubutan sun lissafa na’urorin da za su iya gwada ra’ayin — JWST, Ariel da Twinkle da za su nemi hydrogen a cikin sararin iskar manyan duniyoyin, PLATO da JUST da za su auna nauyinsu, da Nancy Grace Roman Space Telescope da za ta kai ga ƙananan duniyoyi masu nisa.