Today, scientists (and along with you and me) know about the existence of more than 4300 exoplanets – worlds that revolve around alien stars. Is it any wonder that sometimes there are very strange objects among them. So, recently an international team of astronomers announced the discovery of a planet that should not exist at all. Gas giant WASP-107b, located 212 light-years from Earth, looks more like a cotton candy bunch than a planet because of its mass, which is much less than the mass required to form a shell of gas that envelops worlds like Jupiter and Saturn. The exoplanet was first discovered in 2017 and even then seemed strange to astronomers. Judge for yourself – a planet comparable in size to Jupiter is for some reason 10 times lighter than him, besides, it rotates very close to its star – at a distance of only 9 million kilometers. The authors of the study note that the new discovery could change modern ideas about the appearance of giant planets.
How and where do hot Jupiters form?
Astronomers today study many extraterrestrial systems, most of which are very different from ours. However, determining exactly how these solar systems formed is not easy. According to the dominant theories of planetary formation, gas giants form where our Jupiter and Saturn are, that is, at a distance several times greater than the distance between the Earth and the Sun. In order to get hot Jupiter under such conditions, the giant planets must migrate into the system. For example, one of the leading theories of the formation of the solar system suggests that Jupiter and Saturn formed far from the Sun, migrated inward, and then returned back to their present locations.
Hot Jupiters – these are giant planets that orbit very close to their host star and are found in about 1% of systems. Astronomers believe that hot Jupiters form through a process called core accretion. A rocky core the size of the Earth or larger forms in the protoplanetary disk. Once the mass of the core reaches a critical mass (about 10 Earth masses or more), it goes through an extremely rapid accretion phase, pulling hydrogen and helium into the atmosphere and eventually gaining enough gas to become a gas giant. Note that the cores of gas giants can be strange. Jupiter, for example, has a fuzzy core, which I talked about in more detail in this article.
Protoplanetary disks that form planets around young stars have a hole in the middle created by the star’s magnetic field. New research has shown that the inner disc boundary can form gas giant planets without requiring them to form further and migrate inward. So, according to a 2019 paper published in the Astrophysical Journal Letters, hot Jupiters can form in a place or in a place close to their stars and stay there throughout their life.
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In mid-January, an international team of astronomers announced the discovery of an exoplanet unlike any other planet known to science. Its strangeness lies in the fact that the mass of its core is much less than the mass required, it is believed, to create a huge shell of gas. So, according to work published in the Astronomical Journal, WASP-107b is about 30 times more massive than Earth (for comparison – the mass of our Jupiter is 300 times the mass of the Earth).
It turns out that the exoplanet WASP-107b has the size of Jupiter, but only one-tenth of its mass. Surprisingly, this combination gives the planet roughly the same density as a marshmallow. The findings from the study also mean that this is actually not hot Jupiter, but the real hot Neptune.
To estimate the mass of the strange exoplanet, scientists first used data from the Keck Observatory in Hawaii. The researchers applied the radial velocity method, which allows not only to determine the mass of the planet, but also to observe the oscillatory motion of its star, associated with gravitational attraction. Calculations have shown that
WASP-107b has a mass of about one-tenth the mass of Jupiter and about 30 times the mass of Earth.
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Astronomers then analyzed to determine the most likely internal structure of the planet, and came to the surprising conclusion: with such a low density, the planet should have a solid core, no more than four times the mass of the Earth. This means that over 85% of the exoplanet’s mass is contained in a thick layer of gas that surrounds the core. Let me remind you that until now it was believed that without a massive core, gas giants cannot overcome the critical threshold required to create and maintain their large gas envelopes. However, for WASP-107b, a different scenario is the most plausible.
Scientists have suggested that this planet formed far from its star, where the gas in the disk was cold enough. Therefore, gas accretion could occur very quickly. Later, the planet moved to its current position, and this happened either due to interaction with the disk, or as a result of interaction with other planets of this system.