Researchers have been assuming for years that young, small mini-Neptunes have evaporating atmospheres. So far, there have not been any observable examples of this and therefore only mathematical evidence. But now it seems that a team from the California Institute of Technology (Caltech) has made a breakthrough. You have two of these exoplanets envisaged arising through evaporation too super earths should develop.
Birth of a super-Earth: Hubble provides crucial data
Together with his team, Caltech doctoral student Michael Zhang published two studies in which they describe the discovery and development of a mini-Neptune. Specifically, these are the two exoplanets HD 63433c and TOI 560.01. They are about 73 and 103 light-years away from our solar system, respectively. But how exactly does a comparatively small gas planet develop into a super-Earth?
The results of the research (HD 63433c and TOI 560.01), published by the Caltech team in The Astronomical Journal, suggest that atmospheric gas is escaping from the inner reaches of the planets. The network sometimes evaluated data from the Hubble space telescope. In a press release from his institute, Zhang explains that most astronomers would assume that mini-Neptunes have evaporative atmospheres. “But nobody’s seen one doing it until now.”
However, Zhang and his colleagues found that TOI 560.01’s gas, for example, is escaping toward its star. “It was unexpected,” notes Heather Knutson. She is a professor of planetary science at Caltech. So far it has been assumed that the gas would move away from the star in such a case. “We still have a lot to learn about how these drains work in practice.”
The Missing Link of Planetary Evolution
Super-Earths are up to 1.75 times the size of our Earth. Mini Neptune planets, on the other hand, reach sizes of up to two or even four times that of Earth. So far, astronomy has discovered only a few planets that fall between these two types in size. One thesis for this missing link is that mini-Neptunes can transform into super-Earths over a period of time.
Their atmospheres are thought to be enriched with hydrogen and helium, remnants of their star’s natal phase. If a mini-Neptune is small enough and close to its star, stellar radiation could thin out its atmosphere for hundreds of millions of years. A stony super-earth would then remain.
“A planet in the gap would have enough atmosphere to inflate its radius, allowing it to capture more stellar radiation, thereby allowing for rapid mass loss. But the atmosphere is so thin that it is quickly lost. Because of this, a planet would not stay in the gap for long.”
Michael Zhang
Expect the unexpected
In the case of the exoplanet TOI 560.01, the team discovered signs of escaping helium. The star system HD 63433 hosts two planets: 63433 b, which is closer to its star, and 63433 c, which is further away. “The inner planet may have already lost its atmosphere,” Zhang explains.
“As exoplanet researchers, we have learned to expect the unexpected. These exotic worlds constantly surprise us with new physical properties that go beyond what we observe in our solar system.”
Heather Knutson
Source: “Detection of Ongoing Mass Loss from HD 63433c, a Young Mini-Neptune” (2022, The Astronomical Journal); “Escaping Helium from TOI 560.01, a Young Mini-Neptune” (2022, The Astronomical Journal); California Institute of Technology