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“Super-Earths” may be dead worlds

Feb. 26, 2014
Courtesy of the Royal Astronomical Society
and World Science staff

Many plan­ets that look like big ver­sions of Earth through our tele­scopes are probably un­suit­a­ble for life, a study con­cludes.

The re­search pro­poses that many of these so-called “Super-Earths” are really more like “Mini-Nep­tunes,” with at­mo­spheres too thick and temp­er­a­tures too high for life to ex­ist.

The mass or "weight" of the in­i­tial rocky co­re de­ter­mines wheth­er the fi­nal plan­et is po­ten­tially hab­it­a­ble. On the top row of the di­a­gram, the co­re has a mass of more than 1.5 times that of the Earth. The re­sult is that it holds on to a thick at­mos­phere of hy­dro­gen (H), deu­ter­i­um (H2) and he­li­um (He). The low­er row shows the ev­o­lu­tion of a smaller mass co­re, be­tween 0.5 and 1.5 times the mass of the Earth. It holds on to far less of the light­er gas­es, mak­ing it much more like­ly to de­vel­op an at­mos­phere suit­a­ble for life. (Cred­it: NA­SA / H. Lam­mer)


In the last 20 years the search for Earth-like plan­ets around oth­er stars has ac­cel­er­ated, with the launch of mis­sions like the Kep­ler space tele­scope. 

As­tro­no­mers have found many worlds with ap­par­ent si­m­i­lar­i­ties to ours, though some­what larger, as small­er ones are ex­treme­ly hard to de­tect. These are called “Super Earths.” A few are even in a star’s “hab­it­able zone” where tem­per­a­tures are suit­a­ble for wa­ter to be liq­uid, mak­ing them prime tar­gets in the search for al­ien life.

Plan­e­tary sys­tems, in­clud­ing our own So­lar sys­tem, are thought to form from hy­dro­gen, he­li­um and heav­i­er el­e­ments that or­bit their par­ent stars in a so-called pro­to­plan­e­tary disk. Dust and rocky ma­te­ri­al is thought to clump to­geth­er over time, eventually form­ing rocky cores that go on to be plan­ets. The gra­vity of these cores at­tracts hy­dro­gen from the disk around them, some of which is stripped away by the ul­tra­vi­o­let light of the young star they or­bit.

The new stu­dy, led by Hel­mut Lam­mer of the Space Re­search In­sti­tute of the Aus­tri­an Acad­e­my of Sci­ences, ap­pears in the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty.

Lam­mer and his team sim­u­lat­ed the bal­ance of cap­ture vs. re­mov­al of hy­dro­gen for plan­e­tary cores weigh­ing the equiv­a­lent of be­tween 0.1 and 5 Earths, in the hab­it­a­ble zone of a Sun-like star. 

They found that fledg­ling plan­ets less than half the weight and size of Earth won’t cap­ture much gas. Earth-sized ones can cap­ture or lose their hy­dro­gen, de­pend­ing on the disk and as­sum­ing that the young star is much brighter in ul­tra­vi­o­let light than the Sun is to­day. But the weight­i­est plan­e­tary cores, si­m­i­lar to the “su­per Earths” found around many stars, hold on to al­most all their hy­dro­gen and end up as “mini Nep­tunes” with thick at­mo­spheres.

The re­sults sug­gest that for some of the re­cently disco­vered super-Earths, such as Kep­ler-62e and -62f, be­ing in the hab­it­a­ble zone is not enough to make them habi­tats, the re­search­ers said. “With such thick at­mo­spheres, the pres­sure on the sur­faces will be huge, mak­ing it al­most im­pos­si­ble for life to ex­ist,” said Lam­mer.


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Many planets that look like big versions of Earth through our telescopes are probably unsuitable for life, even if they have liquid water, a study concludes. The research proposes that many of these so-called “Super-Earths” are really more like “Mini-Neptunes,” with atmospheres far too thick for life to exist. In the last 20 years the search for Earth-like planets around other stars has accelerated, with the launch of missions like the Kepler space telescope. Astronomers have found many worlds with apparent similarities with the Earth. A few are even in a star’s “habitable zone” where temperatures are suitable for water to be liquid, making them prime targets in the search for alien life. Planetary systems, including our own Solar system, are thought to form from hydrogen, helium and heavier elements that orbit their parent stars in a so-called protoplanetary disk. Dust and rocky material is thought to clump together over time, eventually forming rocky cores that go on to be planets. The gravity of these cores attracts hydrogen from the disk around them, some of which is stripped away by the ultraviolet light of the young star they orbit. The new study, led by Helmut Lammer of the Space Research Institute of the Austrian Academy of Sciences, appears in the journal Monthly Notices of the Royal Astronomical Society. Lammer and his team simulated the balance of capture vs. removal of hydrogen for planetary cores weighing the equivalent of between 0.1 and 5 Earths, in the habitable zone of a Sun-like star. They found that fledgling planets less than half the weight and size of Earth won’t capture much gas. Earth-sized ones can capture or lose their hydrogen, depending on the disk and assuming that the young star is much brighter in ultraviolet light than the Sun is today. But the weightiest planetary cores, similar to the “super Earths” found around many stars, hold on to almost all their hydrogen and end up as “mini Neptunes” with thick atmospheres. The results suggest that for some of the recently discovered super-Earths, such as Kepler-62e and -62f, being in the habitable zone is not enough to make them habitats, the researchers said. “With such thick atmospheres, the pressure on the surfaces will be huge, making it almost impossible for life to exist,” said Lammer.