Helium could make up almost half the mass of the atmosphere of giants exoplanets who migrated close to their starexplaining why there is a mysterious size gap in the scale of these worlds.
About 5,200 exoplanets have already been confirmed, and many of them are larger worlds that orbit close to their star, in some cases with orbital periods lasting just a few days. However, the first transit observations made by NASA Kepler Space Telescope and now for TESSthe Transiting Exoplanet Survey Satellite, found a puzzling paucity of planets with radii between 1.4 and 2.4 times that of Earth. Astronomers call this the ‘lightning valley’, and while it appears to be telling us something fundamental about the nature, formation and evolution of planets, scientists have yet to determine what that something is.
Now, a new view of the lightning valley from a team led by PhD student Isaac Malsky of the University of Michigan and Leslie Rogers of the University of Chicago suggests that it could signal an increasing abundance of helium gas in the atmosphere of worlds 2 ,4 times larger. than Earth. Worlds of this scale are often described as mini-Neptuneand if they have a rocky core, it’s well under a thick layer of atmosphere.
Related: Exoplanets: Worlds beyond our solar system
At the beginning of their lives, while they were still forming within the protoplanetary disk of gas and dust, planets that formed farther from their star can migrate inward. The closer they move to their star, the more they are impacted by the star’s heat and radiation, a mixture of stellar winds It is flags which can gradually remove an atmosphere of a planet in the firing line. When this happens, a planet can develop a comet-like tail as the gas is stripped to leave a rocky core bare.
The atmosphere on these worlds is made largely of hydrogen and helium. Jupiterin our solar system, is a good example of this atmospheric composition, being 90% hydrogen and 10% helium. However, hydrogen is lighter than helium and can escape into space more easily.
Malsky and Roger’s team designed a computer model that simulated 70,000 exoplanets of different sizes, orbiting different stars and at different temperatures, to see what effect their nearby star’s heat would have on its atmosphere. They found that, in fact, the hydrogen was removed faster than the helium, resulting in a decrease in the abundance of hydrogen relative to the amount of helium present.
In the most extreme circumstances, some of the planets they simulated had atmospheres with more than 40% helium by mass. These helium worlds would occupy the lower end of the larger size range, about 2.4 times larger than Earth in radius – whether they have a hydrogen- or helium-rich atmosphere, the heat from their nearby star would still cause the atmosphere to heat up. become swollen and expand. , increasing the radius of the planet.
Smaller worlds on the other side of the radius valley, with radii 1.4 times that of Earth or less, would have lost all their hydrogen and helium and would have no significant atmosphere, limiting their radius to just that of their rocky core. It is possible that, having lost their primordial atmosphere, these planets could then shed a new thinner atmosphere, similar to Earth’s. But if they’re much closer to their star than Earth is to the sun, they’ll face a battle to maintain that new atmosphere as well.
“There are so many weird and wonderful types of exoplanets out there, and this discovery not only adds a new type, but could have implications for understanding evolution and planet formation in general,” Rogers said in a paper. declaration (opens in new tab). “Getting a better understanding of this population can tell us a lot about the origins and evolution of Neptune-sized planets, which are clearly a common result of the planetary formation process.”
These new findings support previous search which postulates that not only do planets lose their thick primordial atmosphere as they migrate closer to their star, but this inward migration of multiple planets in a system can trap worlds in gravitationally resonant chains, like ‘peas in a pod’.
Helium, despite being the second most common element in the universe, was only detected on an exoplanet for the first time in 2018. However, with the launch of NASA James Webb Space Telescope (JWST), astronomers have a new instrument in their toolkit for detecting atmospheric gases on exoplanets. If the radius trough is indeed the result of planets wandering too close to their star and having their atmospheres heated by shock in space, then the JWST could provide observational evidence by performing spectroscopy of hot planets with radii of about 2.4 times that of the Sun. Earth to determine the abundance of helium in its atmosphere.
The results were published in Astronomy of Nature (opens in new tab).
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