Many planets may have helium-rich atmospheres, study finds

For centuries no one knew if we were alone in the universe or if there were even other planets like ours.

But thanks to new telescopes and methods in recent decades, we now know that there are thousands and thousands of planets around distant stars, and they come in all sorts of shapes and sizes – large and small, rocky and carbonated, cloudy or icy. or wet.

A study by scientists from the University of Chicago, University of Michigan, and University of Maryland suggests another for the list: planets with helium atmospheres. Moreover, the discovery may suggest a new step in our understanding of the planet’s evolution.

Their simulations revealed that helium is likely to accumulate in the atmospheres of certain types of exoplanets over time. If confirmed, it would explain a decades-long puzzle about the size of these exoplanets.

“There are so many weird and wonderful types of exoplanets, and this discovery not only adds a new type, but may have implications for understanding planetary evolution and formation in general,” the researcher said. astrophysicist from the University of Chicago, Leslie Rogers, co-author of the new article published in natural astronomy.

Ray Valley Mystery

It’s taken us so long to find distant planets because even the biggest ones are dwarfed by the stars they orbit. Scientists have therefore found an ingenious way to spot them: by looking for the trough in the light of a star when a planet passes in front of it. This tells you the size of the planet.

We now know that planets are incredibly common. In fact, from what we can tell so far, at least half of all stars like our sun have at least one planet between the size of Earth and Neptune orbiting very close to the star. These planets are believed to have atmospheres with lots of hydrogen and helium, collected when the planets first formed from gas and dust around the star.

But as the scientists examined the number of these types of planets, they noticed something curious: the planets were separated into two populations. One cluster was about the size of an Earth and a half and one cluster was twice the size of the Earth or more, but there were almost none in between.

This discrepancy between the two populations of planets is known as the “radius valley”, and it is a hotly debated issue in the field. Scientists believe the answer will help us understand how these and other planets form and change over time.

Some have proposed that the explanation for this discrepancy may be related to planetary atmospheres. It’s hard to be a planet close to its star; you’re constantly bombarded with X-rays and UV light, which could strip your atmosphere.

“For example, perhaps the smallest set of planets have lost their atmosphere entirely and exist only as rocky cores,” said the study’s first author, Ph.D. Isaac Malsky. student at the University of Michigan who began exploring the issue with Rogers for his undergraduate thesis at the University of Chicago.

A team including Rogers and Malsky decided to take a closer look at this phenomenon, known as atmospheric escape.

They created models based on the data we have about planets and the rules of physics, to better understand how heat and radiation would affect planetary atmospheres. Then they created 70,000 simulated planets — varying the size of the planets, the type of stars orbiting them, and the temperature of the atmosphere — and modeled what would happen to them over time.

The team found that after billions of years, hydrogen in planetary atmospheres likely escapes faster than helium. “Hydrogen has a lower atomic mass, so it’s easier to get rid of,” Malsky explained.

Over time, this results in a buildup of helium – simulations suggest that helium could make up 40% or more of the mass of atmospheres.

Telescope confirmations

The team suggested a way to confirm their findings through observation. The recently launched James Webb Space Telescope and other powerful telescopes can get a reading of elements in the atmosphere and their amounts. Telescopes could check if there is an unusually high amount of helium in the atmospheres of some of these planets.

If the theory is correct, those planets with helium-rich atmospheres should be especially common at the lower end of the larger-radius group, as helium builds up as the planet begins to shrink over time at as its atmosphere gradually detaches.

The two distinct planet-size groups are created because even a small amount of helium and hydrogen creates a very puffy atmosphere that can significantly inflate the planet’s radius, Malsky explained. If they have any atmosphere left, they will be in the larger radius group; if there are no more, they will be in the smaller radius group.

None of these planets are considered good candidates for supporting life – they are hot, bombarded with radiation, and the atmospheres are probably very high pressure.

But the scientists explained that improving our understanding of the processes that lead to the formation of planets can help us better predict what other planets are and what they look like, as well as guide the search for more hospitable planets.

“A better understanding of this population could tell us a lot about the origins and evolution of sub-Neptune-sized planets, which are clearly a common result of the planet-forming process,” Rogers said.