A strange heavyweight in their sights: Astronomers have discovered an exoplanet that they interpret as the exposed core of a gas giant: Either it has lost its shell over the course of its history or, under certain circumstances, it was unable to create a massive gas atmosphere. Further investigations of the “naked” celestial body could thus enable conclusions to be drawn about the composition of the nuclei of gas planets like Jupiter, say the scientists.
The planet TOI 849 b went online by David Armstrong’s international team of astronomers from the University of Warwick using the so-called transit method. It was the focus of NASA’s “TESS” satellite, which can detect the slight decrease in brightness in the light of a star that is generated by a planet passing in front of it. According to the results, the planet TOI 849 b orbits a star about 730 light-years away from the earth, which is very similar to our sun. However, some unusual features of the planet attracted the special attention of the astronomers and so they focused more and more closely on TOI 849 b.
Amazingly close to the star and close
According to the data, it is about the size of Neptune in our solar system, but orbits its parent star very closely: in just 18 hours it whizzes around the center of this system. Its surface temperature is around 1500 degrees Celsius. As the scientists explain, TOI 849 b is surprisingly located in the so-called “Neptune Desert”. This term is used to describe the region near stars, in which there are hardly any planets with the mass of Neptune. The mass estimation of the exoplanet also resulted in a special surprise. The researchers used the Doppler effect: Inferences about the mass are possible by measuring the “wobbling” of the parent star of the exoplanet, which is caused by its gravitational force during orbit.
It was shown: TOI 849 b is about 40 times heavier than the earth, but its radius is only 3.4 earth radii. This celestial body must therefore have an unusually high density. “The planet is strangely close to its star when you look at this mass. In other words: there are no known planets with this mass that have such a short orbital period around their star, ”says Armstrong. According to the researchers, the extreme density of the planet can only be explained by the fact that TOI 849 b consists primarily of iron, rock and water, but only a very small proportion of hydrogen and helium.
“For such a massive planet, such a high density, or such a small proportion of hydrogen and helium, is extremely astonishing,” emphasizes co-author Christoph Mordasini from the University of Bern. “With such a mass, one would expect that the planet would have attracted a lot of hydrogen and helium during its formation in the protoplanetary disk,” says the scientist. The most plausible explanation is therefore: “The fact that these gases are not present suggests that TOI 849 b is an exposed planetary core,” says Armstrong.
Gas bowl: lost or never formed
The scientists then ran model simulations to study how the planet might have evolved. “We combined findings on the diverse processes that take place in the formation and development of planets,” says Mordasini. In this way, the researchers came up with possible explanations as to why TOI 849 b does not have a thick gas envelope but rather represents an exposed planetary core. “It is possible that the exoplanet was once similar to Jupiter, but then lost almost all of its outer gas due to various influences,” says Mordasini. This could have been triggered by tidal forces as a result of the extremely close orbit around the star or a collision with another planet. Evaporation from the atmosphere could also have played a role – but it cannot alone be responsible for all of the gas lost, the scientists say.
According to them, TOI 849 b could also be a “failed” gas giant: “Once the core had formed, something could have gone completely different than normal, and the core never formed a massive atmosphere, as is usually the case “, Says Mordasini. As he explains, this could have happened if a gap formed in the gas during formation in the protoplanetary disk. “Or if the material in the disc ran out at the time when gas accretion normally takes place,” says Mordasini.
Whatever the cause, the study is now showing for the first time that planets exist and are identifiable, which are exposed nuclei, say the astronomers. “We now have the opportunity to look at the core of a planet in a way that is not possible in our own solar system,” says Armstrong. “For example, there are still big open questions about the characteristics of the Jupiter core,” emphasizes the scientist.
So far there is no information about the chemical composition of TOI 849 b, but new telescopes could soon change this. Clues could emerge by examining the remains of the planet’s atmosphere, which can be analyzed using the translucent starlight. “Since TOI 849 b is so close to its star, the remaining atmosphere around the planet has to be constantly refilled from the core. So if we can study this atmosphere, we can get some insight into the composition of the core, ”says Armstrong.
Source: University of Warwick, University of Bern, specialist article: Nature, doi: 10.1038 / s41586-020-2421-7