Astronomers have discovered two cases of “mini-Neptune” planets that are losing their puffy atmospheres and transforming into super-Earths. The planets’ atmospheres are being stripped away by radiation from their stars, causing hot gas to escape like steam from a pot of boiling water.
“Most astronomers suspected that young, small mini-Neptunes had evaporating atmospheres,” says Michael Zhang, the lead author of both studies and a Caltech graduate student. “However, no one had ever caught one in the act until now.”
The findings are detailed in two papers published in The Astronomical Journal: one based on data from the W. M. Keck Observatory on Maunakea, Hawai’i, and the other on observations from NASA’s Hubble Space Telescope. The studies, taken together, help to paint a picture of how exotic worlds like these form and evolve.
Mini-Neptunes are a type of exoplanet that orbits stars outside our solar system. These smaller, denser versions of Neptune’s worlds are made up of large rocky cores surrounded by thick blankets of gas.
A team of astronomers led by Caltech used Keck Observatory’s Near-Infrared Spectrograph (NIRSPEC) to study one of two mini-Neptune planets in the star system TOI 560, which is located 103 light-years away, and Hubble to study two mini-Neptunes orbiting HD 63433, which is located 73 light-years away.
Most astronomers suspected that young, small mini-Neptunes had evaporating atmospheres. However, no one had ever caught one in the act until now. A planet in the gap would have enough atmospheres to puff up its radius, allowing it to intercept more stellar radiation and thus enable fast mass loss.
Michael Zhang
Their findings indicate that atmospheric gas is escaping from TOI 560’s innermost mini-Neptune, known as TOI 560.01, and HD 63433’s outermost mini-Neptune, known as HD 63433 c. Furthermore, data from Keck Observatory revealed that the gas in the vicinity of TOI 560.01 was escaping primarily toward the star.
“This was unexpected, as most models predict that the gas should flow away from the star,” says Professor of Planetary Science Heather Knutson, Zhang’s advisor and a co-author of the study. “We still have a lot to learn about how these outflows work in practice.”
Planetary Gap Explained?
Since the discovery of the first exoplanets orbiting Sun-like stars in the mid-1990s, thousands more have been discovered. Many of these are close to their stars, and the smaller, rocky ones are classified as mini-Neptunes or super-Earths. Super-Earths can be up to 1.6 times the size of Earth (and occasionally up to 1.75 times the size of Earth), while mini-Neptunes are two to four times the size of Earth. Few planets with sizes in between these two types have been discovered.
One possible explanation for this gap is that mini-Neptunes are transforming into super-Earths. The mini-Neptunes are thought to be cocooned by primordial atmospheres of hydrogen and helium. The hydrogen and helium are byproducts of the central star’s birth from gas clouds. Scientists theorize that if a mini-Neptune is small enough and close enough to its star, stellar X-rays and ultraviolet radiation can strip away its primordial atmosphere over hundreds of millions of years. This would then leave behind a rocky super-Earth with a much smaller radius, which could, in theory, retain a relatively thin atmosphere similar to that which surrounds our own planet.
“A planet in the gap would have enough atmosphere to puff up its radius, allowing it to intercept more stellar radiation and thus enable fast mass loss,” Zhang says. “However, the atmosphere is thin enough that it is quickly lost. This is why a planet would not stay in the gap for long.”
Other scenarios, according to the astronomers, could explain the gap. Smaller rocky planets, for example, may never have gathered gas envelopes in the first place, and mini-Neptunes may be water worlds rather than being enveloped in hydrogen gas. This latest discovery of two mini-Neptunes with escaping atmospheres is the first direct evidence to support the theory that mini-Neptunes are indeed transforming into super-Earths.
Signatures in the Sunlight
The astronomers were able to detect the escaping atmospheres by watching the mini-Neptunes cross in front of, or transit, their host stars. The planets cannot be seen directly but when they pass in front of their stars as seen from our point of view on Earth, telescopes can look for absorption of starlight by atoms in the planets’ atmospheres. In the case of the mini-Neptune TOI 560.01, the researchers found signatures of helium. For the star system HD 63433, the team found signatures of hydrogen in the outermost planet they studied, called HD 63433 c, but not the inner planet, HD 63433 b.
“The inner planet may have already lost its atmosphere,” Zhang explains.
The speed of the gases indicates that the atmospheres are escaping. The observed helium near TOI 560.01 is moving at a rate of up to 20 kilometers per second, while the hydrogen near HD 63433 c is moving at a rate of up to 50 kilometers per second. These mini-Neptunes gravity is insufficient to hold on to such fast-moving gas. The size of the outflows around the planets also suggests escaping atmospheres: the cocoon of gas around TOI 560.01 is at least 3.5 times the planet’s radius, and the cocoon around HD 63433 c is at least 12 times the planet’s radius.
Concerning the strange discovery that the gas lost from TOI 560.01 was flowing toward – rather than away from – its host star, future observations of other mini-Neptunes should reveal whether TOI 560.01 is an outlier or if an inward-moving atmospheric outflow is more common. “As exoplanet researchers, we’ve learned to expect the unexpected,” Knutson says. “These exotic worlds keep surprising us with new physics that go beyond what we see in our solar system.”
