The incredible case of the waning planets

How do you go from being a big, gaseous planet to a small, solid one? The answer may lie in four newly discovered exoplanets, four ‘incredible shrinking worlds’ that are rapidly shrinking in size under the watchful eyes of astronomers. These are four ‘mini Neptunes’, all of them very close to their parent stars and all leaking their dense atmospheres into space at a considerable rate, which is causing them to rapidly reduce in size. In a study just published on the arXiv server, a team of researchers led by Michael Zang , from the Department of Astronomy at the California Institute of Technology (Caltech), suggests that these worlds could eventually shrink to terrestrial planets of a size similar to that of Earth. And the ‘fault’ that they do, say the scientists, lies with their stars. Mini-Neptunes, worlds smaller than Neptune with rocky cores surrounded by an atmosphere of very hot gas or water, have long been suspected of being related in some way to the terrestrial planets, but how these ‘ gas dwarfs’ lost their atmospheres. The Caltech astronomers’ study appears to have found stellar irradiation to be the main mechanism, although other factors could also be at play. A motley ‘planetary zoo’ Until now in the Milky Way, the galaxy in which we live, many types of planets have been identified that are very different from those of the Solar System. And among the most abundant are, precisely, the mini Neptunes. More massive than Earth but less massive than Neptune, from which they take their name, these planets are often enveloped in a dense atmosphere of hydrogen and helium. The smallest discovered so far are twice the size of the Earth. And in between, there is the category of ‘super-Earths’, exoplanets that have between 1.5 and twice the radius of the Earth. But, interestingly, between 1.5 and 2 Earth radii hardly any planets are known, a lack that is known as ‘the small planet radius gap’. Scientists believe this size gap exists because, above a certain critical limit, exoplanets have enough mass to retain a primordial atmosphere that inflates their size, placing them in the class of mini-Neptunes. Super-Earths, however, do not have enough mass and have lost their primordial atmospheres, or may never have had them at all. But if the mini Neptunes began their existence with atmospheres, how did they lose them? One possible pathway, called ‘core-driven mass loss’, points to internal heat resulting from planet formation, with gravitational energy converting to heat and driving off the primordial atmosphere. The other mechanism is called ‘photo evaporation’, and it posits that the intense X-ray and ultraviolet radiation from stars strips away the atmosphere of their exoplanets. Determining which of these scenarios makes it possible for mini-Neptunes to transform into super-Earths requires finding intermediate exoplanets, that is, still with atmospheres but in the process of losing them, and determining the rate at which these worlds are ‘shrinking’. Which brings us back to the article by Zhang and his colleagues. Why those planets are shrinking The researchers used spectroscopy to study the atmospheres of four nearby young mini-Neptunes, all orbiting orange dwarfs, and determine how fast they were leaking helium into space. One of them, TOI 560b, with a radius 2.8 times that of Earth, was already analyzed by Zhang and his team in a paper published earlier this year. The other three, TOI 1430.01, TOI 1683.03, and TOI 2076 b, are new, and their radii range from 2.1 to 2.52 times that of Earth. But what all four have in common is a significant ‘leak’ of helium into space, at a rate that is also consistent with the idea of ​​photoevaporation. According to the study, the atmospheric evaporation rate of these four exoplanets is sufficient to lose them completely in just a few hundred million years, a fairly short time on a planetary scale. LEARN MORE James Webb photographs Eärendel, the farthest star ever observed Tonga’s eruption released enough water into the atmosphere to raise Earth’s surface temperature “We conclude,” the researchers write, “that many, if not all of these planets will lose their hydrogen-rich envelopes and become super-Earths. Our results demonstrate that most mini-Neptunes orbiting Sun-like stars have primordial atmospheres, and that photoevaporation is an efficient mechanism to strip atmospheres and transform these planets into super-Earths.”