The Incredible ‘Shrinking’ Planet Could Be a Missing Link Between Worlds
The discovery of several exoplanets that seem to shrink seems to solve a “missing link” in planetary evolution.
Four mini-Neptunes close to their stars were found fleeing their atmospheres at a rate consistent with eventual total loss. This suggests that these worlds will eventually shrink into terrestrial planets, roughly the size of Earth – and moreover, that it’s the fault of their stars.
Although scientists have long thought these two types of exoplanets are related, the pathway by which mini-Neptune lost its atmosphere was unknown.
While other mechanisms may still be at play, the newly identified shrinking world suggests that stellar radiation stripping is a major factor.
The Milky Way galaxy is a large and diverse place, and there are many types of exoplanets that have been identified to date that are very different from what we find in our own solar system. One of them is mini-Neptune – the most common type of world detected by the Kepler mission, but notably absent from our little corner of the Milky Way.
They are worlds more massive than Earth and less massive than Neptune, but still immersed in a hydrogen and helium atmosphere similar to that of Neptune. Interestingly, these exoplanets do not appear to be smaller than about twice the radius of Earth.
Super-Earths are at the bottom of the next category, exoplanets that are between 1 and 1.5 times the radius of Earth. Between about 1.5 and 2 Earth radii there is a curious difference in which exoplanets are extremely rare. This is called the small planetary radius deviation.
Scientists believe that this difference exists because, beyond a certain critical threshold, exoplanets have sufficient mass to maintain an early atmosphere which increases their size, placing them in the category of mini-Neptunes. canned. Super-Earths, on the other hand, don’t have enough mass and have either lost their original atmosphere or never had one to begin with.
The next question is, if these exoplanets started with primitive atmospheres, how did they get lost?
One possible pathway, called core-fed mass loss, is internal heat generated by planetary formation, in which gravitational binding energy is converted to heat that ejects the initial atmosphere. The second is called photoevaporation, in which intense X-rays and ultraviolet radiation from the young star destroy the atmosphere of the exoplanet.
To determine which of these scenarios leads to the transformation of mini-Neptunes into super-Earths, one must examine the escapes of exoplanets and determine the rate at which they lose mass.
This brings us back to a new paper from a team of researchers led by astronomer Michael Zhang of the California Institute of Technology (Caltech). They used spectroscopy to study the atmospheres of four nearby young mini-Neptunes orbiting orange dwarf stars, to determine if these exoplanets were leaking helium into space.
These four mini-Neptunes include one called TOI 560b, which has a radius 2.8 times that of Earth, an analysis of which was published earlier this year by Zhang and his colleagues.
The other three are new: TOI 1430.01, 2.1 times the size of Earth; TOI 1683.01, 2.3 times the size of Earth; and TOI 2076b, 2.52 times the size of Earth.
The team found that all four planets exhibited significant helium outflow, at a rate consistent with photo-evaporation, rather than core-induced mass loss. What’s more, this rate of loss is enough to strip the atmospheres of these exoplanets in just a few hundred million years, the team found — a very short time in cosmic terms.
The team says their findings suggest that most mini-Neptunes orbiting Sun-like stars are likely to transform into super-Earths, and do so through photo-evaporation.
“We conclude that many, if not all, of these planets will shed their hydrogen-rich envelopes and become super-Earths,” Zhang and his colleagues wrote in their paper.
“Our results demonstrate that most mini-Neptunes orbiting Sun-like stars have primitive atmospheres, and that photovaporation is an effective mechanism for removing these atmospheres and turning these planets into super-Earths.”
research was submitted to astronomical diaryand is available on arXiv.