Science Gazette

Planets that shed their atmospheres become super-Earths

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The discovery is the first time that air loss has been detected in the United States.

Two occurrences of “mini-Neptune” planets shedding their puffy atmospheres and presumably converting into super-Earths have been discovered by astronomers. The planets’ atmospheres are being stripped away by radiation from their stars, causing hot gas to escape like steam from a kettle of boiling water.

“Most astronomers assumed that young, tiny mini-Neptunes must have evaporating atmospheres,” explains Michael Zhang, a PhD student at Caltech and principal author of both investigations. “However, until recently, no one had ever captured one in the act.”

The conclusions are detailed in two studies published in The Astronomical Journal: one is based on data from the W. M. Keck Observatory on Maunakea, Hawaii, and the other is based on Hubble Space Telescope observations. The studies serve to provide a picture of how strange worlds like these arise and evolve when taken together.

Exoplanets are planets that orbit stars beyond our solar system. Mini-Neptunes are a kind of exoplanet. Large rocky cores are covered by thick blankets of gas on these planets, which are smaller, denser counterparts of Neptune.

A team of scientists headed by Caltech utilized the Near-Infrared Spectrograph (NIRSPEC) at Keck Observatory to examine one of two mini-Neptune planets in the star system TOI 560, which is 103 light-years distant, and Hubble to analyze two mini-Neptune planets circling HD 63433, which is 73 light-years away.

Their findings indicate that atmospheric gas is fleeing from TOI 560’s innermost mini-Neptune, TOI 560.01, and HD 63433’s outermost mini-Neptune, HD 63433 c.

Furthermore, observations from the Keck Observatory revealed that the gas surrounding TOI 560.01 was fleeing mostly toward the star.

Professor of Planetary Science Heather Knutson, Zhang’s adviser and a co-author of the work, said, “This was surprising since most models suggest that the gas should move away from the star.” “There’s still a lot to learn about how these fluxes function in reality,” says the researcher.

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How Is the Planetary Gap Explained?

Since the discovery of the first exoplanets circling Sun-like stars in the mid-1990s, hundreds more have been identified. Many of them circle their stars close to the point where they collide, and the smaller, rocky ones are divided into two categories: mini-Neptunes and super-Earths. The super-Earths may be up to 1.6 times the size of Earth (and up to 1.75 times the size of Earth on rare occasions), while the mini-Neptunes are between two and four times the size of Earth. There have been few planets discovered in the size range between these two planet types.

One theory is that mini-Neptunes are transitioning into super-Earths, causing the gap. The mini-Neptunes are thought to be encased in primeval hydrogen and helium atmospheres. The hydrogen and helium are byproducts of the center star’s development, which is made up of gas clouds. Scientists predict that if a mini-Neptune is tiny and near enough to its star, stellar X-rays and ultraviolet radiation may take away its primordial atmosphere over hundreds of millions of years. This would leave behind a rocky super-Earth with a much smaller radius, but one that might theoretically maintain a thin atmosphere comparable to that which surrounds our own planet.

“A planet in the gap would have enough atmosphere to blow out its radius, allowing it to collect more solar radiation and allowing for rapid mass loss,” Zhang explains. “However, the atmosphere is thin enough that it swiftly dissipates. This is why a planet would not be able to remain in the void for very long.”

According to the astronomers, other situations might explain the discrepancy. Smaller stony planets, for example, may never have accumulated gas envelopes in the first place, and mini-Neptunes may be water worlds without hydrogen gas. The recent finding of two mini-Neptunes with escaping atmospheres is the first concrete proof that mini-Neptunes are really transforming into super-Earths.

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In the Sunlight Signatures

By observing the mini-Neptunes pass in front of, or transit, their host stars, the scientists were able to identify the departing atmospheres. The planets cannot be seen directly, but telescopes can check for absorption of starlight by atoms in the planets’ atmospheres as they pass in front of their stars as viewed from Earth. The researchers discovered helium signs on the mini-Neptune TOI 560.01. The researchers discovered hydrogen signs in the outermost planet they analyzed, HD 63433 c, but not in the inner planet, HD 63433 b, in the star system HD 63433.

“It’s possible that the inner planet’s atmosphere has already been gone,” Zhang says.

The pace with which the gases move indicates that the atmospheres are fleeing. The hydrogen surrounding HD 63433 c is travelling at a rate of 50 kilometers per second, whereas the helium around TOI 560.01 is moving at a rate of 20 kilometers per second. These mini-Neptunes’ gravity is insufficient to hang on to such fast-moving gas. The size of the outflows surrounding the planets also suggests that atmospheres are escaping: the gas cocoon around TOI 560.01 is at least 3.5 times the planet’s radius, while the cocoon around HD 63433 c is at least 12 times the planet’s radius.

Future investigations of additional mini-Neptunes should indicate whether TOI 560.01 is an exception or whether inward-moving atmospheric outflows are more widespread.

“We’ve learnt to anticipate the unexpected as exoplanet scientists,” adds Knutson. “New physics that goes beyond what we see in our solar system is continuously shocking us on these alien planets.”

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