Is it possible that one type of planet can become another? Can a mini-Neptune lose its atmosphere and become a super-Earth? The discovery of two examples of mini-Neptunes transitioning to super-Earths might help explain the size distribution of exoplanets.
We classify exoplanets in ways related to our Solar System. The Hot terms Jupiters, mini-Neptunes, and Super-Earths help us quickly recognize what they are talking about. Why do we find so many planets in other solar systems than in our Solar System?
The question highlights the size gap in exoplanets.
Two new studies look at a pair of SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA SALVAGEDATA The discovery provides examples of a phenomenon that scientists have speculated about, but it might also help explain the gap in the sizes of exoplanets we find.
HD 63433c and TOI 560.01 are the mini-Neptunes. There are separate papers about some of the same authors. The lead author of both papers is Michael Zhang.
The Hubble Space Telescope and the W.M. Keck Observatory were used by the researchers. Both of these mini-Neptunes are undergoing similar changes. Their atmospheres are stripped away by the radiation from their stars. The planets are moving towards super-Earths.
Mini-Neptunes are not as big as Neptune. They have similar atmospheres to Neptune. There are layers of ice, rock, and liquid oceans. They are between 1.7 and 3.9 Earth radii and have rocky cores. Mini-Neptunes are called gas dwarfs.
Our Solar System's ice giants, Uranus and Neptune, are larger than Earth. Super-Earths have a larger diameter than Earth. The term "super-Earth" does not indicate a planet's potential habitability, only its size.
Mini-Neptunes are planets on the upper end of the scale for super-Earths.
As we found more and more exoplanets, these terms became common. Sometimes they are used interchangeably. There are 4933 confirmed exoplanet discoveries in NASA's exoplanet catalogue.
There are examples of mini-Neptunes losing their atmospheres and sliding down the size scale to become super-Earths in the new papers. Astronomers predicted this phenomenon long ago, but it was difficult to observe it in action.
Michael Zhang, lead author of both studies and a graduate student at Caltech, said that most astronomer suspected that young, mini-Neptunes must have evaporating atmospheres.
HD 63433c is a mini-Neptunes. It is 2.67 times Earth's radius and 73 light-years away. It is on an 18.8-day journey around a star similar to our Sun, but younger at about 450 million years old. HD 53433c has lost most of its primordial atmosphere according to the paper.
HD 73583b is the second mini-Neptune undergoing mass-loss. Its radius is larger than the Earth's. The young star is a 600 million-year-old K-dwarf. The period is 6.4 days.
There is a noticeable size gap in the range of exoplanets discovered so far. The range of exoplanets is not evenly populated. Super-Earths can be as large as 1.6 times Earth's size. Mini-Neptunes are between 2 and 4 times the size of Earth. There are very few exoplanets with sizes between 1.6 and 2 Earth radii. The small planet radius gap is more of a size than a gap, according to the few that have been found. Whatever we want to call it, it is there.
Why?
The low number of planets between 1.5 and 2 times Earth's diameter is called the photoevaporation valley by exoplanet scientists. The evidence for the photoevaporation has been covered by many papers over the years, but the two new papers are the first to present direct observational evidence supporting it.
The photoevaporation valley is explained by the fact that both mini-Neptunes are in young solar systems. The planets are shrouded in an atmosphere of hydrogen and helium after their star formed. Our own Neptune is far from the Sun, but if a small Neptune was closer to its star, it could take away the hydrogen and helium. The studies show that the atmospheric stripping is happening in young solar systems. The photoevaporation can leave behind the rocky core of the mini-Neptune. The atmosphere of the super-Earth could be affected by the characteristics of the star, the planet and their separation. The observed photoevaporation valley is explained by the fact that it happens in only hundreds of millions of years.
The atmosphere is thin enough that it gets lost, so a planet in the size gap can intercept more stellar radiation. This is the reason a planet wouldn't stay in the gap for long.
Studying the photoevaporative removal of a planet's atmosphere in a solar system takes away some of the impact. The atmosphere is being stripped away by forces that are epic. The observed helium is moving at a rate of 20 kilometres per second. 72,000 kilometres per hour is equal to 20 km/s. The International Space Station travels at a speed of over 30,000 km/h.
The hydrogen escaping from HD 63433c moves at speeds up to 50 kilometres per second. That is 180,000 kilometres per hour.
The mini-Neptunes are not strong enough to hold on to the fast moving gas.
The measured speed of the hydrogen and helium around these planets is not the only indicator of an escaping atmosphere.
The cocoon of gas around TOI 560.01 is at least 3.5 times the size of the planet, and the one around HD 63433c is at least 12 times the size.
What would this look like in our Solar System? The Earth has been battered by the stellar wind of the Sun for billions of years. Earth's magnetosphere protects the atmosphere from the Sun's radiation and we are only here because of it. Earth's atmosphere is not the same as remnant hydrogen and helium from the solar nebula. Mini-Neptunes have a massive gaseous envelope.
There are two mini-Neptunes in the papers that have only a short orbital period. They are very close to their stars, which makes them great observational targets. What would happen if one of these mini-Neptunes lost its atmosphere?
Imagine a planet several times larger than Earth and closer to the Sun than Mercury is. It would transit in front of the sun on a weekly or monthly basis. The Earth-Moon system could travel through a stream of hydrogen/helium escaping from its atmosphere occasionally. It would be something to behold and would have shaped all of humanity's early myths. The inner Solar System's history could be completely changed by a massive planet in that location.
If they are adolescents, they are not the type of adolescents that can become more like Earth. The atmosphere of Earth has a high weight. Its atmosphere isn't like a mini-Neptune because it isn't remnant hydrogen and helium. Earth's atmosphere is likely to be caused by volcanic out-gassing, loss of lighter atmospheric components to space, plate tectonics, and magma ocean crystallization. Finding mini-Neptunes in distant solar systems doesn't tell us much about how many Earth-like planets are out there.
observational evidence supports a long-standing theory. It also has a surprise.
The gas escaping from TOI 560.01 moves closer to its star. Is that not normal? Many parts of the field are not sure what normal is. They've found many weird planets that don't look like anything in our Solar System. Only observations of other planets can explain if the mini-Neptune's atmosphere is an anomaly or not.
The gas should flow away from the star, according to most models, but this was unexpected.
As exoplanet scientists, we have learned to expect the unexpected.
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