▁ ▁Plan ets , ▁astero ids , ▁and ▁com ets ▁Plan ets , ▁astero ids , ▁and ▁com ets ▁all ▁reflect ▁light ▁from ▁their ▁parent ▁stars ▁and ▁emit ▁their ▁own ▁light . ▁ ▁For ▁cool er ▁objects , ▁including ▁solar - system ▁plan ets ▁and ▁astero ids , ▁most ▁of ▁the ▁emission ▁is ▁at ▁inf ra red ▁w avel ength s ▁we ▁cannot ▁see , ▁but ▁that ▁are ▁rout in ely ▁measured ▁with ▁spect rom eters . ▁ ▁For ▁objects ▁surrounded ▁by ▁gas , ▁such ▁as ▁com ets ▁and ▁plan ets ▁with ▁atmos pher es , ▁further ▁emission ▁and ▁absor ption ▁happens ▁at ▁specific ▁w avel ength s ▁in ▁the ▁gas , ▁im print ing ▁the ▁spectrum ▁of ▁the ▁gas ▁on ▁that ▁of ▁the ▁solid ▁object . ▁ ▁In ▁the ▁case ▁of ▁world s ▁with ▁thick ▁atmos pher es ▁or ▁complete ▁cloud ▁cover ▁( such ▁as ▁the ▁gas ▁gi ants , ▁Ven us , ▁and ▁Sat urn ' s ▁satellite ▁T itan ▁( mo on )), ▁the ▁spectrum ▁is ▁mostly ▁or ▁completely ▁due ▁to ▁the ▁atmosphere ▁alone . ▁ ▁Plan ets ▁The ▁reflected ▁light ▁of ▁a ▁planet ▁contains ▁absor ption ▁bands ▁due ▁to ▁min er als ▁in ▁the ▁rocks ▁present ▁for ▁rock y ▁bodies , ▁or ▁due ▁to ▁the ▁elements ▁and ▁mole cules ▁present ▁in ▁the ▁atmosphere . ▁To ▁date ▁over ▁ 3 , 5 0 0 ▁ex op lan ets ▁have ▁been ▁discovered . ▁ ▁These ▁include ▁so - called ▁Hot ▁Jup it ers , ▁as ▁well ▁as ▁Earth - like ▁plan