The discovery of multiple exoplanets that appear to be shrinking appears to resolve a “missing link” in planetary evolution.
Four mini-Neptunes have been found very close to their stars leaking their atmospheres at a rate consistent with eventual total loss. This suggests that these worlds will eventually shrink to terrestrial planets, roughly the size of Earth, and it is their stars that will do so.
Although scientists have long thought that these two types of exoplanets were related, the pathway by which the mini-Neptunes lost their atmospheres was unknown.
Although other mechanisms could still be at play, the recently identified shrinking worlds suggest that removal by stellar irradiation is one of the main ones.
The Milky Way is a big and diverse place, and so far many types of exoplanets have been identified that are very different from those found in our own Solar System. One of them is the mini-Neptune, the most common type of world detected by the Kepler mission, but notably absent from our little corner of the galaxy.
These are worlds that are more massive than Earth and less massive than Neptune, but are still surrounded by a thick Neptune-like atmosphere of hydrogen and helium. Interestingly, these exoplanets appear to be no smaller than about twice the radius of Earth.
Super-Earths are the next category below, 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 gap in which exoplanets are extremely rare. This is known as the minor planet radius gap.
Scientists believe this gap exists because, above a certain critical limit, exoplanets have enough mass to retain a substantial primordial atmosphere that inflates their size, placing them in the class of mini-Neptunes. Super-Earths, on the other hand, don’t have enough mass, and either lost their primordial atmospheres, or never had them to begin with.
The next question then is if these exoplanets started with primordial atmospheres, how were they lost?
One potential pathway, called core-fueled mass loss, is the internal heat resulting from planetary formation, in which gravitational binding energy is converted to heat that expels the primordial atmosphere. The other is called photoevaporation, in which intense X-ray and ultraviolet radiation from the young star removes the exoplanet’s atmosphere.
Determining which of these scenarios leads to the transformation of mini-Neptunes into super-Earths requires observing leaky exoplanets and determining the rate at which they are losing 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 the rate at which these exoplanets are leaking helium into space.
These four mini-Neptunes include one called TOI 560b, which has a radius 2.8 times that of Earth, whose analysis was published by Zhang and colleagues earlier this year.
The other three are new: TOI 1430.01, with 2.1 times the size of Earth; TOI 1683.01, 2.3 times the size of Earth; and TOI 2076b, at 2.52 times the size of Earth.
The team found that all four planets had significant outflows of helium, at a rate consistent with photoevaporation, rather than core-fueled mass loss. What’s more, this loss rate is enough to wipe out the atmospheres of these exoplanets in just a few hundred million years, the team found, which is a fairly short timescale in cosmic contexts.
The team says their findings suggest that most mini-Neptunes orbiting Sun-like stars likely become super-Earths and do so through photoevaporation.
“We conclude that many, if not all, of these planets will lose their hydrogen-rich envelopes and become super-Earths,” Zhang and colleagues write in their paper, which is awaiting peer review.
“Our results demonstrate that most mini-Neptunes orbiting Sun-like stars have primordial atmospheres and that photoevaporation is an efficient mechanism for removing these atmospheres and transforming these planets into super-Earths.”
The research has been submitted to The Astronomical Journal and is available on arXiv.