NASA Mission Reveals Possible Reason Behind Shrinking Exoplanets

Some exoplanets seem to be losing their atmospheres and shrinking. In a new study, astronomers report evidence of a possible cause: The cores of these planets are pushing away their atmospheres from the inside out.

Exoplanets (planets outside our solar system) come in a variety of sizes, from small, rocky terrestrial planets to colossal gas giants. In the middle lie rocky super-Earths and larger sub-Neptunes, the latter of which have puffy atmospheres. But there is a conspicuous absence: a "size gap" of planets that fall between 1.5 to 2 times the size of Earth, or right between super-Earths and sub-Neptunes.

"Scientists have now confirmed the detection of over 5,000 exoplanets, but there are fewer planets than expected with a diameter between 1.5 and 2 times that of Earth," says Jessie Christiansen, a staff scientist at Caltech's IPAC astronomy center, science lead for the NASA Exoplanet Archive, and lead author of the new study, publishing in The Astronomical Journal. "Exoplanet scientists have enough data now to say that this gap is not a fluke. There's something going on that prevents planets from reaching and/or staying at this size."

The scientists think this gap could be caused when sub-Neptunes lose their atmospheres over time. This loss would happen if the planet doesn't have enough mass, and thus gravitational force, to hold onto its atmosphere. Sub-Neptunes that aren't massive enough would shrink to about the size of super-Earths, leaving the gap between the two sizes of planets. A separate Caltech study, led by Professor of Planetary Science Heather Knutson, finds direct evidence of sub-Neptunes losing their atmospheres.

However, exactly how these atmospheres are destroyed has remained a mystery. The new study, which utilized data from NASA's K2, an extended mission of the Kepler Space Telescope, reports evidence for a theory called "core-powered mass loss." In this scenario, radiation emitted from a planet's hot core pushes the atmosphere away from the planet over time, "and that radiation is pushing on the atmosphere from underneath," Christiansen says.

While this research reveals the most likely mechanism explaining how these exoplanet atmospheres are stripped away, the work is far from complete, she adds. The findings will likely be put to the test by future studies before the mystery of the planetary size gap is solved once and for all.

Read the full story from the Jet Propulsion Laboratory, which is managed by Caltech for NASA.