Astronomers have identified two different cases of “mini-Neptune” planets losing their puffy atmospheres and likely transforming into super-Earths. Radiation from planets’ stars strips their atmospheres, causing hot gases to escape like steam from a pot of boiling water.
“Most astronomers suspected that young, small mini-Neptunes must have evaporating atmospheres,” says Michael Zhang, lead author of both studies and a graduate student at Caltech. “But no one had ever caught one doing it until now.”
The results are published in two separate articles in The Astronomical Journal: one is based on data from the WM Keck Observatory in Maunakea, Hawaiʻi and the other paper involves observations from NASA’s Hubble Space Telescope. Together, the studies help paint a picture of how alien worlds like these form and evolve.
Mini-Neptunes are a class of exoplanets, which are planets orbiting stars outside our solar system. These worlds, which are smaller and denser versions of the planet Neptune, are made up of large rocky cores surrounded by thick blankets of gas.
In the new studies, a team of astronomers led by Caltech used the Keck Observatory’s Near Infrared Spectrograph (NIRSPEC) to study one of the star system’s two mini-Neptune planets called TOI 560, located 103 years- light ; and they used Hubble to observe two mini-Neptunes orbiting HD 63433, located 73 light-years away.
Their results show that atmospheric gas is escaping from the innermost mini-Neptune in TOI 560, called TOI 560.01, and from the outermost mini-Neptune in HD 63433, called HD 63433 c.
Additionally, data from the Keck Observatory surprisingly showed that the gas around TOI 560.01 was mainly escaping towards the star.
“This was unexpected, because most models predict that the gas should move away from the star,” says planetary science professor Heather Knutson, Zhang’s adviser and co-author of the study. “We still have a lot to learn about how these outputs work in practice.”
The 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 them orbit close to their stars, and the smaller, rockier ones generally fall into two groups: mini-Neptunes and super-Earths. Super-Earths are as large as 1.6 times the size of Earth (and sometimes as large as 1.75 times the size of Earth), while mini-Neptunes are between two and four times the size of Earth. Earth. Few planets with sizes between these two types of planets have been detected.
One possible explanation for this discrepancy is that mini-Neptunes transform into super-Earths. Mini-Neptunes are theorized to be cocooned by primordial atmospheres made of hydrogen and helium. The hydrogen and helium are leftovers from the formation of the central star, which arose from the clouds of gas. If a mini-Neptune is small enough and close enough to its star, stellar X-rays and ultraviolet radiation can strip its primordial atmosphere over a period of hundreds of millions of years, scientists theorize. This would then leave behind a rocky super-Earth with a noticeably smaller radius, which could, in theory, still retain a relatively thin atmosphere similar to that which surrounds our own planet.
“A planet in space would have enough atmosphere to inflate its radius, causing it to intercept more stellar radiation and thus allowing rapid mass loss,” Zhang says. “But the atmosphere is thin enough that it gets lost quickly. That’s why a planet wouldn’t stay in the gap for long.
Other scenarios could explain the discrepancy, astronomers say. For example, smaller rocky planets may never have gathered gas envelopes, and mini-Neptunes could be watery worlds and not enveloped in hydrogen gas. This latest discovery of two mini-Neptunes with escaping atmospheres represents the first direct evidence supporting the theory that mini-Neptunes do indeed transform into super-Earths.
Signatures in the sun
Astronomers have been able to detect escaping atmospheres by watching mini-Neptunes cross or transit in front of their host stars. Planets cannot be seen directly but as they pass in front of their stars as seen from our vantage point on Earth, telescopes can look for the absorption of starlight by atoms in the atmospheres of the planets. In the case of the mini-Neptune TOI 560.01, the researchers found helium signatures. For star system HD 63433, the team found hydrogen signatures 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,” says Zhang.
Gas velocity provides evidence that atmospheres are escaping. The helium observed around TOI 560.01 moves at a speed of 20 kilometers per second, while the hydrogen around HD 63433 c moves at a speed of 50 kilometers per second. The gravity of these mini-Neptunes is not strong enough to hold such fast-moving gas. The magnitude of the outflows around the planets also indicates escaping atmospheres: the gas cocoon around TOI 560.01 is at least 3.5 times larger than the radius of the planet, and the cocoon around HD 63433 c is at least 12 times the radius of the planet.
As for the strange discovery that waste gas from TOI 560.01 was flowing towards – instead of away from – its host star, future observations of other mini-Neptunes should reveal whether TOI 560.01 is an anomaly or a Inward atmospheric flow is more common.
“As exoplanet scientists, we’ve learned to expect the unexpected,” Knutson says. “These alien worlds constantly surprise us with new physics that go beyond what we observe in our solar system.”