Free Astronomy Magazine May-June 2022

41 MAY-JUNE 2022 ASTRO PUBLISHING Neptune planets in the star system called TOI 560, located 103 light- years away. Their results show that atmospheric gas is escaping from the innermost mini-Neptune in TOI 560, called TOI 560.01 (also known as HD 73583b), and from the outermost mini-Nep- tune in HD 63433, called HD 63433c. This suggests that they could be turning into super-Earths. “Most astronomers suspected that young, mini-Neptunes must have evaporating atmospheres,” said Michael Zhang, lead author of both studies and a graduate student at Caltech. “But no- body had ever caught one in the process of doing so until now.” The study also found, sur- prisingly, that the gas around TOI 560.01 was es- caping predominantly to- ward the star. “This was unexpected, as most models predict that the gas should flow away from the star,” said professor of planetary science Heather Knutson of Caltech, Zhang’s advisor and a co-author of the study. “We still have a lot to learn about how these outflows work in practice.” Since the first exoplanets orbiting sun-like stars were discovered in the mid-1990s, thousands of other exo- planets have been found. Many of these orbit close to their stars, and the smaller, rocky ones generally fall into two groups: the mini-Neptunes and super-Earths. The super-Earths are as large as 1.6 times the size of Earth (and occasionally as large as 1.75 times the size of Earth), while the mini-Neptunes are between 2 and 4 times the size of Earth. Planets of these types are not found in our solar system. In fact, few planets with sizes between these two ranges have been detected around other stars. stars as seen from our point of view on Earth, telescopes can look for ab- sorption of starlight by atoms in the planets’ atmospheres. In the case of the mini-Neptune TOI 560.01, the researchers found signatures of he- lium. For the star system HD 63433, the team found signatures of hy- drogen in the outermost planet they studied, called HD 63433c, but not the inner planet, HD 63433b. “The inner planet may have already lost its at- mosphere,” explained Zhang. “The speed of the gases provides the evidence that the atmospheres are escaping. The observed helium around TOI 560.01 is moving as fast as 20 kilo- meters per second, while the hydrogen around HD 63433c is moving as fast as 50 kilometers per second. The gravity of these mini-Neptunes is not strong enough to hold on to such fast-moving gas. The extent of the outflows around the planets also indicates escaping atmospheres; the cocoon of gas around TOI 560.01 is at least 3.5 times as large as the radius of the planet, and the cocoon around HD 63433c is at least 12 times the radius of the planet.” Future observations of other mini- Neptunes should reveal if TOI 560.01 is an anomaly or whether an inward-moving atmospheric out- flow is more common. “As exoplanet scientists, we’ve learned to expect the unexpected,” Knutson said. “These exotic worlds are constantly surprising us with new physics that goes beyond what we observe in our solar system.” I n the background, an artwork of the mini-Neptune TOI 560.01, located 103 light-years away in the Hydra constellation. The planet, which orbits closely to its star, is losing its puffy atmosphere and may ultimately transform into a super-Earth. [Adam Makarenko (Keck Observatory)] One possible explanation for this size-gap is that the mini-Neptunes are transforming into the super- Earths. The mini-Neptunes are theo- rized to be cocooned by primordial atmospheres made of hydrogen and helium. The hydrogen and helium are left over from the formation of the central star, which is born out of clouds of gas. If a mini-Neptune is small enough and close enough to its star, stellar X-rays and ultraviolet radiation can strip away its primor- dial atmosphere over a period of hundreds of millions of years, scien- tists theorize. This would then leave behind a rocky super-Earth with a substantially smaller diameter (which could, in theory, still retain a relatively thin atmosphere similar to that surrounding our planet Earth). The astronomers were able to de- tect 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 !