Incredible “shrinking” planets may be a missing link between worlds

The discovery of several seemingly shrinking exoplanets appears to solve the “missing link” in planetary evolution.

Four small Neptunes were found in close proximity to their stars leaking out of their atmosphere at a rate consistent with the final total loss. This suggests that these worlds will eventually shrink down to terrestrial planets roughly the size of Earth – moreover, it is their stars’ fault that they will.

Although scientists have long believed that these two types of exoplanets are related, the path by which miniature Neptune lost its atmosphere was unknown.

While other mechanisms may still be at play, the newly identified shrinking worlds suggest that abstraction by stellar radiation is one of the leading approaches.

The Milky Way is a large and diverse place, and there are many types of exoplanets identified so far that are very different from what we find in our solar system. One of them is the small planet Neptune – the most common type of world discovered by the Kepler mission, but it is noticeably absent from our little corner of the galaxy.

These are worlds larger than Earth, less massive than Neptune, but still surrounded by a dense, Neptune-like atmosphere of hydrogen and helium. Interestingly enough, these exoplanets appear to be at least twice the radius of Earth.

Superplanets are the next category, exoplanets with a radius of between 1 and 1.5 times that of Earth. Between about 1.5 and 2 Earth radii, there is a strange gap where exoplanets are extremely rare. This is known as the planet’s small radius gap.

Scientists believe this gap exists because, above a certain critical limit, exoplanets have enough mass to hold a large primordial atmosphere that amplifies their size, putting them in the small Neptune class. On the other hand, the super-terrestrial planets do not have enough mass, and either they lost their primordial atmosphere, or they never started with it.

The next question is if these exoplanets started with primitive atmospheres, how were they lost?

One potential pathway, called core-supported mass loss, is endogenous heat from planet formation, where gravitational binding energy is converted into heat that ejected the primordial atmosphere. The other is called photoevaporation, in which intense X-rays and ultraviolet rays from the young star remove the atmospheres of the exoplanets.

Determining which of these scenarios leads to mini-Neptune turning into super-terrestrial planets requires observing the leaking exoplanets, and determining the rate at which they are losing mass.

That 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 atmospheric-study spectroscopy of four young Neptune close-ups orbiting orange dwarf stars, to determine the rate at which these exoplanets are leaking helium into space.

These four miniature Neptunes include one called TOI 560b, which is 2.8 times the diameter of Earth, whose analysis Zhang and colleagues published 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.

The team found that all four planets had large outflows of helium, at a rate consistent with photo-evaporation, rather than a loss of mass with primary energy. Additionally, this rate of loss is enough to strip the atmosphere of these exoplanets in just a few hundred million years, the team found — that’s a very short time scale in cosmic contexts.

The team says their findings suggest that most young Neptunes orbiting Sun-like stars probably turn out to be super-Earths, and they do so by photoevaporation.

“We conclude that many, if not all, of these planets will lose their hydrogen-rich atmospheres and become super-Earths,” Zhang and colleagues write in their paper awaiting peer review.

“Our results show that most young Neptunes orbiting Sun-like stars have primitive atmospheres, and that photoevaporation is an effective mechanism for stripping these atmospheres and transforming these planets into super-Earths.”

The search was submitted to Astronomical Journalavailable at arXiv.