For centuries, no one knew if we were alone in the Universe – or if there were other planets like ours. But thanks to new telescopes or methods in recent decades, we now know that there are thousands and thousands of planets orbiting distant stars, and they come in all shapes and sizes—big and small, rocky and gaseous, icy or wet.

A study by scientists from the University of Chicago, the University of Michigan and the University of Maryland – all in the US – suggests another list: planets with helium-rich atmospheres. Their discovery may suggest a new step in our understanding of planetary evolution.

Their simulations showed that helium is likely to accumulate in the atmospheres of certain types of exoplanets. If confirmed, this would explain a decades-long puzzle about the sizes of these exoplanets.

A new type of planets, an additional way to know their evolution

“There are many wonderful types of exoplanets out there, and this discovery not only adds a new type, but may have implications for the evolution and formation of planets in general,” said astrophysicist Leslie Rogers of the University of Chicago in the US, co-author of the paper. .

We now know that planets are incredibly common. In fact, from what can be told so far, at least half of stars like our Sun have at least one Earth- and Neptune-sized planet orbiting very close to the star. These planets are thought to have atmospheres rich in hydrogen and helium, collected when they formed from gas and dust around the star.

The Mystery of Planetary Populations

As researchers analyzed the number of these types of planets, they noticed something curious. The planets were separated into two populations. One group of planets was the size of one and a half Earths, and the other was larger than Earth.

This gap between the two populations of planets is known as the “radius valley” and is a hotly debated question in the field. Scientists believe the answer will help us understand how these planets and others like them form and evolve over time.

Some have proposed that the explanation for this gap has to do with the atmospheres of the planets. The team found that after a few billion years, hydrogen from planetary atmospheres is probably removed faster than helium.

“Hydrogen has a lower atomic mass, so it’s easier to remove,” explained the study’s first author, Isaac Malsky, a doctoral student at the University of Michigan who began working on the issue with Rogers for his undergraduate thesis. at the University of Chicago.

Over time, this leads to a build-up of helium – simulations have suggested that helium could make up 40% or more of the atmospheres mass.

Confirmations of the James Webb Space Telescope

The team suggested a way to confirm their results observationally. The recently launched James Webb Space Telescope and other powerful telescopes can read the elements in the atmosphere and their quantities. Telescopes could check if there is an unusually large amount of helium in the atmospheres of some of these planets.

If the theory is correct, these planets with helium-rich atmospheres should be especially common at the lower end of the larger-radius group, because helium accumulates as the planet begins to shrink over time, according to

None of these planets add up to the conditions necessary for the development of life, according to specialists, because they are hot, bombarded with radiation, and the atmospheres are probably at very high pressure.

The scientists explained that a better understanding of the processes that lead to the formation of planets can help us better predict what other planets exist and what they look like, as well as direct the search for planets more conducive to life.

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