For centuries, no one knew if we were alone in the universe – or if there were even other planets like ours.
But thanks to the new telescopes and methods of the past decades, we now know that there are thousands upon thousands of planets orbiting distant stars, and they come in all kinds of shapes and sizes — big and small, rocky and gaseous, cloudy and icy or wet.
A study by scientists from the University of Chicago, the University of Michigan and the University of Maryland suggests another study to the list: planets with helium atmospheres. Moreover, the discovery may mark a new step in our understanding of planetary evolution.
Their simulations found that the atmospheres of certain types of exoplanets are likely to build up helium over time. If confirmed, it would explain the decades-old mystery about the sizes of these exoplanets.
University of Chicago astrophysicist Leslie Rogers, a fellow author of the new paper published in the journal natural astronomy.
Secret Valley Radius
It has taken us a long time to find distant planets because the largest ones far outnumber the stars that orbit them. So scientists came up with an ingenious method for their discovery: by looking for a dip in a star’s light 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 close to the star. It is assumed that these planets have atmospheres with a lot of hydrogen and helium, which were collected when the planets first formed from the gas and dust around the star.
But when scientists looked at the numbers of these types of planets, they noticed something curious — the planets were divided into two groups. One group was about the size of a planet and a half Earth, and one group was twice the size of Earth or larger, but there was almost nothing between them.
This gap between the two groups of planets is known as the “Valley of the Radius,” and it’s a hotly debated issue in the field. Scientists believe the answer will help us understand how these and other planets formed and evolved over time.
Some have suggested that the explanation for this gap may have something to do with the planets’ atmospheres. It’s hard to be a planet close to your star; You are constantly bombarded with X-rays and UV rays, which can strip the atmosphere away from you.
“For example, the smaller group of planets may have lost their atmospheres completely and exist only as rocky cores,” said study first author Isaac Malsky, Ph.D. A student at the University of Michigan who first began exploring the question with Rogers on his undergraduate thesis at the University of Chicago.
A team including Rogers and Malsky decided to look more closely at this phenomenon known as atmospheric escape.
They created models based on the data we have about planets and the rules of physics, in order to fully understand how heat and radiation affect a planet’s atmosphere. Then they created 70,000 simulated planets — varying in planetary size, type of star they orbit, and atmospheric temperature — and modeled what would happen to them over time.
The team found that, after several billion years, hydrogen would likely escape into planetary atmospheres faster than helium. “Hydrogen has a lower atomic mass, so it’s easier to strip it away,” Malsky explained.
Over time, this results in a buildup of helium – simulations have suggested that helium could make up 40% or more of the atmosphere’s mass.
The team proposed a way to confirm their findings with observation. The recently launched James Webb Space Telescope and other powerful telescopes can get a readout of atmospheric elements and their magnitudes. Telescopes can check to see if there is an unusually large amount of helium in the atmospheres of some of these planets.
If the theory is correct, then these planets with helium-rich atmospheres should be especially common at the lower end of the largest-radius group, because helium builds up when a planet begins to shrink over time as its atmosphere is gradually stripped away.
Malsky explained that the two distinct planet-size combinations were created because even a small amount of helium and hydrogen creates an extremely bloated atmosphere that can greatly inflate the planet’s radius. If they had any atmospheres at all, they would be in the group with the largest radius; If it disappears, they will be in the group with the smallest radius.
None of these planets are thought to be good candidates for harboring life — they’re extremely hot, they’re bombarded with radiation, and their atmospheres are likely to be under very high pressure.
But scientists have shown that improving our understanding of the processes that lead to planet formation can help us better predict what other planets are and what they look like, as well as guide the search for more host planets.
“Getting a better understanding of this group can tell us a lot about the origins and evolution of the smaller Neptune-sized planets, which are obviously a combined outcome of the planet formation process,” Rogers said.
Isaac Malsky et al., Helium-enhanced planets along the upper edge of the Valley of Radius, natural astronomy (2022). DOI: 10.1038/s41550-022-01823-8
Provided by the University of Chicago
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