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Astronomy: Hot, rocky exoplanets are the scorched cores of former gas giants

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The planets are nestled close to their stars, where stellar winds may have blown ancient atmospheres away

Science News said:
Earth may not provide the best blueprint for how rocky planets are born.

An analysis of planets outside the solar system suggests that most hot, rocky exoplanets started out more like gassy Neptunes. Such planets are rocky now because their stars blew their thick atmospheres away, leaving nothing but an inhospitable core, researchers report in a paper posted online October 15 at arXiv.org. That could mean these planets are not as representative of Earth as scientists thought, and using them to estimate the frequency of potentially life-hosting worlds is misleading.

“One of the big discoveries is that Earth-sized, likely rocky planets are incredibly common, at least on hotter orbits,” says planetary scientist Eric Lopez of NASA’s Goddard Space Flight Center in Greenbelt, Md., who wasn’t involved in the study. “The big question is, are those hot exoplanets telling us anything about the frequency of Earthlike planets? This suggests that they might not be.”

Observations so far suggest that worlds about Earth’s size probably cluster into two categories: rocky super-Earths and gaseous mini-Neptunes (SN Online: 6/19/17). Super-Earths are between one and 1.5 times as wide as Earth; mini-Neptunes are between 2.5 and four times Earth’s size. Earlier work showed that there’s a clear gap between these planet sizes.

Because planets that are close to their stars are easier for telescopes to see, most of the rocky super-Earths discovered so far have close-in orbits — with years lasting between about two to 100 Earth days — making the worlds way too hot to host life as we know it. But because they are rocky like Earth, scientists include these worlds with their cooler brethren when estimating how many habitable planets might be out there.

If hot super-Earths start out rocky, perhaps it is because the worlds form later than their puffy mini-Neptune companions, when there’s less gas left in the growing planetary system to build an atmosphere. Or, conversely, such planets, along with mini-Neptunes, may start with thick atmospheres. These rocky worlds may have had their atmospheres stripped away by stellar winds.

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The biggest problem with our discoveries of exoplanets is that we really don't have a good way (yet) for discovering planets of Earth-type in stellar systems similar to our own. What is now easy for us to find are systems with short-period closely-orbiting planets aligned in an orientation that allows us to see eclipses. Those systems are almost certainly a minority of the ones out there. A system like our own would be nearly impossible for us to discover via our current technology. That will change, almost certainly, as our technology advances, which it is doing at a rapid pace.

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True, the tools we have now are definitely biased to find planets unlike ours. Will be interesting to see if Earth-like planets are really uncommon or just hidden at the moment.