Worlds Around Other Stars Can Change Their Classification
If our solar system were a hobbyist kit, it would come boxed up with four rocky terrestrial planets, like Earth; and four gas giant planets like Jupiter.
What astronomers have discovered around some other stars is a rare type of planet not found in our solar system. It is not too big or too small but fits between the radius of Earth and the radius of Neptune. Toward the bottom of this range are dense "super-Earths" (no, not the home of the comic book hero, Superman) that are slightly bigger than Earth. Toward the top of the range are the puffier so-called "mini-Neptunes," which have a fraction of the radius of the planet Neptune.
Astronomers are assembling a complex picture of how these sorts of "missing link" planets come to be. Apparently, the mini-Neptunes start out as smaller, denser versions of the planet Neptune. But radiation from a planet's host star heats their hydrogen/helium atmosphere which then escapes into space like a hot air balloon. The planet goes onto sort of a crash-diet, losing a lot of mass until all that's left is a dense, rocky core that is still bigger than Earth and may retain a thin skin of an atmosphere.
Astronomers using the Hubble Space Telescope and the Keck Observatory have identified two different cases of "mini-Neptune" planets that are losing their puffy atmospheres and possibly transforming into super-Earths. This is further evidence for the diversity – if not unpredictability – of how planets form and evolve around other stars.
Find additional articles, images, and videos at HubbleSite.org
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