Incredible "shrinking" planets could be a missing link between worlds

Credit image: pixabay images
Credit image: pixabay images

Article by: Andacs Robert Eugen, on 06  August 2022, at 02:04 am Los Angeles time

Four Neptune-like mini-planets near their stars have been discovered losing their atmospheres at a rate consistent with a cycle leading to an eventual total loss. This suggests that these worlds will eventually shrink into Earth-sized terrestrial planets-and it is the fault of their stars that they will do so.

Although scientists have long believed that these two types of exoplanets are related, the reason why the mini-planets lost their atmosphere was unknown.

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 the ones we find in our solar system. One of these is the mini-Neptune - the most common type of world detected by the Kepler mission, but which is notably absent from our little corner of the galaxy.

These are worlds more massive than Earth and less massive than Neptune, but which are still enveloped in a thick atmosphere of hydrogen and helium similar to that of Neptune. Interestingly, these exoplanets do not appear to be smaller than about twice the radius of Earth.

Super-Earths are the next category, exoplanets that have a radius between 1 and 1.5 times that of Earth. Between about 1.5 and 2 Earth radii, there is a curious interval where exoplanets are extremely rare. This is known as the minor planet radius difference.

Scientists believe this gap exists because, above a certain critical limit, exoplanets have enough mass to retain a substantial primordial atmosphere that increases their size, placing them in the class of mini-Neptune planets. Super-Earths, on the other hand, do not have enough mass and have either lost their primordial atmosphere or never had one.

The next question is: if these exoplanets had primordial atmospheres to begin with, how were they lost?

One potential pathway, called core-fueled mass loss, is internal heat from planet formation, where gravitational energy is converted to heat that repels the primordial atmosphere. The other pathway is called photoevaporation, where intense X-ray and ultraviolet radiation from the young star removes the exoplanet's atmosphere.

To determine which of these scenarios leads to the transformation of mini-Neptunes into super-Earths, it is necessary to observe the exoplanets that undergo this process and determine the rate at which they lose mass.

This brings us back to a new paper by 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 releasing helium into space.

Among these four mini-Neptunes is one called TOI 560b, which has a radius 2.8 times that of Earth, the observation 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, with a size 2.52 times that of Earth.

The team says their findings suggest that most mini-Neptunes orbiting Sun-like stars are likely turning into super-Earths and doing 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 to remove these atmospheres and turn these planets into super-Earths."

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