730 light years away from Earth is a planet that has captured the attention of the astronomical community since its discovery in 2020: TOI 849b. It orbits a star similar to the Sun at a very close distance, so close that a year on this planet would last for only 18 hours.
So far, nothing special, since hundreds of planets are known to orbit very close to their own star. What is surprising, however, are its physical characteristics. The planet has a mass of about 40 Earth masses, about a tenth of that of Jupiter, while its size is similar to that of Neptune. This means that TOI 849b has a very similar density to Earth, about 5.2 g/cm3. Such a value is quite normal for a small, rocky planet like Earth or Mercury, but it is completely unexpected for a planet as large as Neptune.
According to planetary formation theories, such a massive and large planet should be able to capture hydrogen and helium from the protoplanetary disk, form a large atmosphere, and become a gas giant like Jupiter and Saturn.
Something must have happened during the formation of TOI 849b that prevented it from retaining an atmosphere or that removed it at a later time.
One hypothesis is that the planet was once a gas giant far from its star, but that it was thrown into its present orbit by an interaction with another planet. In this position, the temperatures are so high and the radiation so intense that their combined effect could have completely obliterated TOI 849b's atmosphere, leaving only its rocky core exposed.
It is just as likely that the planet instead formed in a gas-poor region of the protoplanetary disk, and thus was never able to accumulate hydrogen by forming an atmosphere.
Whatever the reality, the important point is that we are observing the core of a gas giant similar to what might be inside Jupiter. Learning more about this object could therefore tell us a lot about the formation of gas giants in our solar system.
Credit: University of Warwick/Mark Garlick.
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