Source: PHL at UPR Arecibo

Source: PHL at UPR Arecibo

By Dirk Schulze-Makuch

Accord­ing to the Exo­plan­et Cat­a­log main­tained by the Plan­e­tary Hab­it­abil­i­ty Lab­o­ra­to­ry (PHL) of the Uni­ver­si­ty of Puer­to Rico at Areci­bo, the num­ber of con­firmed plan­ets out­side our own solar sys­tem is approach­ing 1,000, while anoth­er 3,500 exoplanets—most of them detect­ed by NASA’s Kepler mission—are yet to be con­firmed. We’re not talk­ing only about Jupiter- or Nep­tune-like gas giants, but also Super-Earths (ter­res­tri­al plan­ets sev­er­al times the mass of Earth) and Earth-size planets.

From this grow­ing list, the PHL, direct­ed by Abel Mén­dez, has iden­ti­fied the top 12 poten­tial­ly hab­it­able exo­plan­ets based on an Earth Sim­i­lar­i­ty Index (ESI). Their top choice is Kepler 62e, with an ESI val­ue of 0.83 (an ESI of 1.0 would be a 100 % match with Earth in terms of astro­nom­i­cal para­me­ters). Kepler 62e is a Super-Earth in the Con­stel­la­tion Lyra, with an esti­mat­ed mass of 3.6 Earth mass­es and an esti­mat­ed radius 1.6 times that of Earth. Its sur­face tem­per­a­ture is esti­mat­ed at 31oC (88oF) and it is 7 bil­lion years old, sig­nif­i­cant­ly old­er than Earth. It’s also very far away—1,200 light years, mean­ing that we won’t be vis­it­ing it any time soon.

How much like our own world is Kepler 62e, real­ly? We should be care­ful to dis­tin­guish Earth sim­i­lar­i­ty from plan­e­tary hab­it­abil­i­ty. In many respects the Moon has very sim­i­lar astro­nom­i­cal val­ues to Earth, yet we know it’s a dead rock. On the oth­er hand there’s Saturn’s moon Titan, a top can­di­date for find­ing prim­i­tive extrater­res­tri­al life in our solar sys­tem. But Titan couldn’t be more dif­fer­ent from Earth—an icy moon with liq­uid methane/ethane lakes on its sur­face, a nitro­gen-methane atmos­phere, and tem­per­a­tures well below Earth’s arc­tic regions.

My own favorite can­di­date for a hab­it­able exo­plan­et is Gliese 581d. A mere 20 light years from us, it’s #12 on the PHL list. The plan­et has an esti­mat­ed mass of about sev­en Earth mass­es, with a radius about dou­ble Earth’s. Gliese 581d orbits a red dwarf star with an orbital peri­od of 67 days, which is impor­tant. Why? A red dwarf has less ener­gy out­put than our (yel­low dwarf) sun, and an orbital peri­od of 67 days would put the plan­et in a Mars-like orbit in terms of tem­per­a­ture. In our own solar sys­tem, Mars is cold and dry today because it was too small to retain a thick atmos­phere, inter­nal heat­ing, and a mag­net­ic field. How­ev­er, if Mars had been a Super-Earth like Gliese 581d, it would sure­ly still be heat­ed from inside, have kept its mag­net­ic field and thick atmos­phere, and would like­ly still have liq­uid oceans (and per­haps even life!) on its surface.

Gliese 581d’s esti­mat­ed sur­face tem­per­a­ture of ‑37oC (-35oF) should not con­cern us too much. If aliens were to observe Earth from afar, based just on the amount of incom­ing solar radi­a­tion they would esti­mate our sur­face tem­per­a­ture as ‑18oC (0oF). It’s only due to the green­house effect that Earth’s aver­age tem­per­a­ture is actu­al­ly a benign +15oC (59oF; yes, a lim­it­ed green­house effect can be a good thing!). On a Super-Earth plan­et such as Gliese 581d, I would expect the effect to be quite a bit stronger than on Earth. And one last thing in this planet’s favor: Gliese 581d is near­ly twice as old as Earth, which could have giv­en evo­lu­tion plen­ty of time to devel­op advanced, per­haps even tech­no­log­i­cal­ly advanced, life forms.

Dirk Schulze-Makuch is a pro­fes­sor of astro­bi­ol­o­gy at Wash­ing­ton State Uni­ver­si­ty, and has authored or co-authored sev­en books relat­ed to the pos­si­bil­i­ty of life in the solar sys­tem and beyond.