It could be possible to link worlds apart with incredible 'shrinking' planets

A 'missing link' in planetary evolution has been resolved by the discovery of multiple exoplanets that are shrinking.

The atmospheres of four mini-Neptunes within proximity to their stars are leaking at a rate consistent with an eventual total loss. The shrinking of these worlds - and the fact that their stars are responsible for their downfall - indicates that they will eventually become terrestrial, roughly Earth-sized planets.

Mini-Neptunes have long been thought to be connected, but the pathway in which their atmospheres were lost was not known. The recently discovered shrinking worlds suggest that stellar irradiation is a leading mechanism for stripping.

It has been found that the Milky Way galaxy has many kinds of exoplanets, many of which have characteristics that differ greatly from those in the Solar System. A mini-Neptune is one of the most common kinds of worlds detected by the Kepler mission but is uncommon in our solar system.

The planets are more massive than Earth, less massive than Neptune, but still shrouded in a thick atmosphere of hydrogen and helium like Neptune. The radius of these exoplanets does not seem to get smaller than twice that of Earth.

Next on the list are super-Earths, exoplanets with an orbital radius between 1 and 1.5 times the Earth's. There is a curious gap between 1.5 and 2 Earth radii where there are very few exoplanets.

It is believed that this gap exists because, above a critical mass, exoplanets have enough mass to retain a substantial primordial atmosphere that inflates their size, placing them in the class of mini-Neptunes. On the other hand, super-Earths do not have enough mass and either lost their primordial atmospheres or never had them.

If these exoplanets were born with primordial atmospheres, how did they lose them? As a result of planetary formation, gravitational binding energy can be converted into heat to eject the primordial atmosphere, a process called core-powered mass loss. Photoevaporation occurs when the exoplanet's atmosphere is stripped away by intense X-rays and ultraviolets from the young star.

Observing leaking exoplanets and determining their mass loss rate is necessary for determining which of these scenarios leads to mini-Neptunes becoming super-Earths.

We now come to a new paper from a team of researchers led by California Institute of Technology astronomer Michael Zhang. To determine the rate at which these exoplanets are leaking helium into space, they used spectroscopy to investigate the atmospheres of four young, nearby mini-Neptunes orbiting orange dwarf stars.

According to the team, all four planets have significant helium outflows, which are likely caused by photoevaporation rather than core-driven mass loss. These exoplanets' atmospheres can be stripped in just a few hundred million years - a pretty quick timescale in cosmic terms. Scientists say most mini-Neptunes orbiting Sun-like stars probably transform into super-Earths by photoevaporating.

We conclude 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.

The results of our study demonstrate that most mini-Neptunes orbiting Sun-like stars have primordial atmospheres and that photoevaporation is an efficient process for stripping these atmospheres and transforming them into super-Earths.'