(ORDO NEWS) — The discovery of several exoplanets that appear to be collapsing appears to eliminate the “missing link” in planetary evolution.
Four mini-Neptunes in close proximity to their stars are found to be leaking their atmospheres at a rate corresponding to a possible total loss. This suggests that these worlds will eventually shrink into terrestrial planets roughly the size of Earth – and what’s more, their stars are to blame.
While scientists have long believed that the two types of exoplanets were related, the route by which the mini-Neptunes lost their atmospheres was unknown.
While other mechanisms may still be at work, newly identified shrinking worlds suggest that removal by stellar radiation is leading. .
The Milky Way Galaxy is a large and diverse place, and many types of exoplanets have been identified to date that are very different from those we find in our own solar system.
One of them is a mini-Neptune, the most common type of world discovered by the Kepler mission, but conspicuously absent from our small corner of the galaxy.
These are worlds that are more massive than Earth, and less massive than Neptune, but still shrouded in a dense Neptune-like atmosphere of hydrogen and helium. Interestingly, these exoplanets seem to be no less than about twice the radius of the Earth.
Super-Earths are the next category, exoplanets, which are 1-1.5 times the radius of the Earth. Approximately between 1.5 and 2 Earth radii there is a curious gap in which there are very few exoplanets. This is known as a gap in the radius of the minor planets.
Scientists believe this gap exists because, beyond a certain critical limit, exoplanets have enough mass to retain a substantial primordial atmosphere, which increases their size, placing them in the mini-Neptune class.
On the other hand, super-Earths do not have enough mass and have either lost their primordial atmospheres or never had them to begin with.
The next question is, did these exoplanets originate with primordial atmospheres, how could they get lost?
One potential pathway, called core mass loss, is internal heat from planetary formation, in which gravitational binding energy is converted into heat that expels the primordial atmosphere.
The other is called photoevaporation, in which intense X-ray and ultraviolet radiation from a young star destroys an exoplanet’s atmosphere.
Determining which of these scenarios leads to the transformation of mini-Neptunes into super-Earths requires observation. leaking exoplanets and determining the rate at which they are losing mass.
This brings us back to a new paper by a team of researchers led by Caltech astronomer Michael Zhang (Caltech). They used spectroscopy to study the atmospheres of four young mini-Neptunes orbiting orange dwarfs to determine the rate at which these exoplanets are spewing 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 by Zhang and colleagues earlier this year.
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.
All four planets had significant helium emissions, the team found, at a rate consistent with photoevaporation rather than core mass loss.
In addition, this rate of loss is sufficient to de-atmosphere these exoplanets in just a few hundred million years, the team found, a fairly short amount of time in a cosmic context.
The team says their results show that most mini-Neptunes orbiting sun-like stars likely turn into super-Earths and do so through photoevaporation.
“We concluded that many, if not all, of these planets will lose their hydrogen-rich shells and become super-Earths,” Zhang and colleagues write in their paper, which is awaiting peer review.
“Our results show that most mini-Neptunes orbiting Sun-like stars have a primordial atmosphere, and that photoevaporation is an effective mechanism to remove these atmospheres and turn these planets into super-Earths.”
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