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Are super-Earths really mini-Neptunes?

Feb. 6, 2013
Courtesy of the Royal Astronomical Society
and World Science staff

Many re­cently dis­cov­ered plan­ets orig­i­nally judged to be some­what like gi­ant Earths are really more like small Nep­tunes, ac­cord­ing to a new stu­dy.

In the last two dec­ades as­tro­no­mers have found hun­dreds of plan­ets in or­bit around oth­er stars. One type of these so-called “exoplan­ets” is the super-Earths, thought to be largely rocky and a good deal big­ger than our own world. Smaller ones are ex­tremely hard to de­tect.

Diagram of a mini-Neptune. (Courtesy of RAS, H. Lammer et al.)


The new work sug­gests that these Super-Earths are ac­tu­ally cov­ered by a vast, hy­dro­gen-rich shroud of gas and are un­likely to ev­er be­come Earth-like.

The sci­en­tists,
led by Hel­mut Lam­mer of the Space Re­search In­sti­tute of the Aus­tri­an Acad­e­my of Sci­ences, pub­lish their work in the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty.

Lam­mer and col­leagues looked at the im­pact of radia­t­ion on the up­per at­mo­spheres of “super-Earths” or­biting the stars Kepler-11, Gliese 1214 and 55 Can­cri. These plan­ets are all es­ti­mat­ed to be a few times heav­i­er and slightly larg­er than the Earth, and to cir­cle their host stars very close­ly. The way in which the weight of plan­ets scales with their sizes sug­gests that they have sol­id cores sur­rounded by hy­dro­gen or hy­dro­gen-rich at­mo­spheres, ac­cord­ing to sci­en­tists, probably cap­tured from the clouds of gas and dust from which the plan­ets formed.

The new mod­el sug­gests that the host stars’ ex­treme ul­tra­vi­o­let light, or light much “bluer” than blue light vis­i­ble with our eyes, heats up the gas­e­ous en­velopes of these worlds. This makes them puff up, up to sev­eral times the width of each plan­et, lead­ing gas to es­cape from them fairly quick­ly. None­the­less most of the at­mos­phere re­mains in place.

Rath­er than be­com­ing more like the Earth, the super-Earths may more closely re­sem­ble Nep­tune, which to­geth­er with Ura­nus, is a smaller “gas gi­ant” in our So­lar sys­tem, Lam­mer said. If the find­ings are right, then super-Earths fur­ther out from their stars in the “hab­it­a­ble zone,” where the tem­per­a­ture would al­low liq­uid wa­ter to ex­ist, would hold on to their at­mo­spheres even more ef­fec­tive­ly. If that hap­pens, they would be much less likely to be hab­it­a­ble.

The team’s find­ings will be put to the test in 2017 when the Eu­ro­pe­an Space Agen­cy launches a mis­sion called CHE­OPS, for CHar­ac­ter­is­tic Ex­O­Plan­ets Sat­el­lite. This will study super-Earths in more de­tail and should be able to tell wheth­er some of these ex­ot­ic worlds could one day be more like our own, Lam­mer said.


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Many recently discovered planets originally judged to be somewhat like giant Earths are really more like small Neptunes, according to a new study. In the last two decades astronomers have found hundreds of planets in orbit around other stars. One type of these so-called “exoplanets” is the super-Earths, thought to be largely rocky and a good deal bigger than our own world. Smaller ones are extremely hard to detect. The new work, led by Helmut Lammer of the Space Research Institute of the Austrian Academy of Sciences, suggests that these Super-earths are actually enshrouded by a vast, hydrogen-rich fog and are unlikely to ever become Earth-like. The scientists publish their work in the journal Monthly Notices of the Royal Astronomical Society. Lammer and colleagues looked at the impact of radiation on the upper atmospheres of “super-Earths” orbiting the stars Kepler-11, Gliese 1214 and 55 Cancri. These planets are all estimated to be a few times heavier and slightly larger than the Earth, and to circle their host stars very closely. The way in which the weight of planets scales with their sizes suggests that they have solid cores surrounded by hydrogen or hydrogen-rich atmospheres, according to scientists, probably captured from the clouds of gas and dust from which the planets formed. The new model suggests that the host stars’ extreme ultraviolet light, or light much “bluer” than blue light visible with our eyes, heats up the gaseous envelopes of these worlds. This makes them puff up, up to several times the width of each planet, leading gas to escape from them fairly quickly. Nonetheless most of the atmosphere remains in place. Rather than becoming more like the Earth, the super-Earths may more closely resemble Neptune, which together with Uranus, is a smaller “gas giant” in our Solar system, Lammer said. If the findings are right, then super-Earths further out from their stars in the “habitable zone,” where the temperature would allow liquid water to exist, would hold on to their atmospheres even more effectively. If that happens, they would be much less likely to be habitable. The team’s findings will be put to the test in 2017 when the European Space Agency launches a mission called CHEOPS, for CHaracterising ExOPlanets Satellite. This will study super-Earths in more detail and should be able to tell whether some of these exotic worlds could one day be more like our own, Lammer said.