View of a planetary transformation process: astronomers report two “mini-Neptunes” whose central star is apparently just “cooking off” their thick layers of atmosphere. In the end, it is possible that only their solid core will remain. Accordingly, the two gas worlds could turn into super-earths, the researchers explain. The results thus contribute to the knowledge about the different possible development histories of exoplanets. They also provide an explanation for a noticeable gap in the size distribution of planets in space, say the scientists.
From Mercury to Neptune – the eight planets in our solar system have very different masses, characteristics and formation histories. However, the diversity appears even greater when looking into the distance of space: the analytical view of many worlds outside of our cosmic homeland has made it clear in recent years that there are planets with properties that do not occur here. These include gas planets called mini Neptunes. These are smaller, denser versions of our planet Neptune, between 2 and 4 times the size of Earth. They consist of thick layers of gas – but inside there is probably a large rock core. Another category of exoplanets that do not exist in the solar system are the so-called super-Earths: rocky planets that have several times the mass of Earth.
There is apparently an interesting connection between these two planet types, the results of the researchers led by Michael Zhang from the California Institute of Technology in Pasadena now suggest. They present them in two separate publications. The focus of NASA’s Hubble Space Telescope was a mini Neptune orbiting the star HD 63433, 73 light-years away. The second study presents the findings of a mini-Neptune in the TOI 560 star system, 103 light-years away, which the researchers have been studying with the WM Keck Observatory in Hawaii.
Two steaming mini Neptunes in sight
As they explain, these planets are not directly visible, but they make themselves felt as they pass in front of their parent star. Starlight also shimmers through the atmospheres of the planets as they pass by – or through the matter that escapes from them. Conclusions about the elements contained are also possible from signatures in the light spectrum.
In the case of the mini-Neptune TOI 560.01, the researchers found signs of helium escaping the planet. They also found a loss in HD 63433c – in this case they found the signature of hydrogen. “The speed of the gases provided the indication of the atmospheric shrinkage,” says Zhang. The observed helium around TOI 560.01 is moving at up to 20 kilometers per second, while the hydrogen at HD 63433c is moving at up to 50 kilometers per second.
As the scientists explain, the parent star causes the two mini-Neptunes to outgas: its radiation mobilizes matter and then causes the hot gas to escape like steam from a pot of boiling water. The detected extent of the outflows indicates that the atmospheres will be largely lost. “The gravity of these mini-Neptunes isn’t strong enough to bind the fast-moving gas,” Zhang explains. “It has already been suggested that young mini-Neptunes exhibit outgassing atmospheres. But until now nobody had observed such a planet,” emphasizes the researcher.
Transformation explains a gap
The results now provide direct evidence to support the theory that super-Earths can evolve from mini-Neptunes. Because at the end of the outgassing process, the rock cores that are stuck in the planets that were originally dominated by gas could be exposed. Such remnants with a possibly thin residual atmosphere would then be referred to as a super-Earth. In this context, in the case of the HD 63433 system, the researchers also report a second planet closer to the star, where this process could already be advanced. Because they could not detect any outgassing from him. “HD 63433b may already have lost its atmosphere,” says Zhang.
According to the scientists, the results also shed new light on why only a few exoplanets with sizes between mini-Neptunes and super-Earths have been discovered. The explanation would be that these gaseous planets often transform into super-Earths relatively quickly: if a mini-Neptune is small enough and close to its star, X-ray and ultraviolet radiation could erode its original atmosphere over a period of just hundreds of millions of years . “Because of this, a planet would not stay in the gap for long,” Zhang said.
However, many questions remain unanswered about the characteristics and origins of the mini-Neptunes and super-Earths – and the current investigations have also raised new ones: Surprisingly, the data showed that the gas around TOI 560.01 flows mainly in the direction of the star. “This was unexpected as most models predict gas should flow away from the star,” says co-author Heather Knutson of the California Institute of Technology. Future observations of other mini-Neptunes should therefore now show whether the effect at TOI 560.01 is an anomaly or whether inward atmospheric outflow is more common. “We still have a lot to learn about how these outflows work,” says Knutson. “These exotic worlds keep surprising us with new physical properties that go beyond what we observe in our solar system,” says the astronomer.
Source: California Institute of Technology, WM Keck Observatory, Space Telescope Science Institute, Article: The Astronomical Journal doi: 10.3847/1538-3881/ac3f3b and doi: 10.3847/1538-3881/ac3fa7