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Astronomers have discovered what seems to be the core of a once-mighty gas giant, devoid of the thick shroud of gas that once surrounded it. This huge but decrepit world, the first we鈥檝e seen of its kind, could help us learn about how planets are formed.

David Armstrong at the University of Warwick, UK, and his colleagues found this strange planet, called TOI-849b, using NASA鈥檚 Transiting Exoplanet Survey Satellite (TESS). TESS watches distant stars for periodic dips in their light that occur when a planet passes in front of it. They then confirmed it with measurements from several other observatories.

While TOI-849b is smaller than Neptune, it is 40 times as massive as Earth, with roughly the same density as Earth. That means that it is probably a rocky world, not a gaseous one like Neptune 鈥� the largest rocky world we have ever seen. Its surface could contain about 12 copies of Earth鈥檚 surface.

鈥淲e don鈥檛 see any other planets that are this size and this density,鈥� says Armstrong. Usually, a rocky world this big would be expected to build up a thick atmosphere and become a gas giant similar to Jupiter. We don鈥檛 know why it didn鈥檛.

One idea is that it could have been starved of gas as it formed, so that it could build a core but not an atmosphere, but it isn鈥檛 clear what would cause that. Perhaps more likely is the idea that it formed like a regular gas giant but somehow lost its gas, either because it got too close to its star and the atmosphere was ripped away or through a catastrophic collision with another giant planet.

The location of the planet is also unexpected. It orbits its star once every 18.4 hours, an orbit so close that the planet鈥檚 temperature is about 1500掳 C. Usually, Neptune-sized planets so close to their stars either get ripped apart or partially vapourised by the star鈥檚 powerful radiation, leading astronomers to nickname the area in which TOI-849b orbits the 鈥渉ot Neptunian desert鈥�.

鈥淭his planet is really bizarre, compared with the planets in our solar system but also compared with the other 4000 planets we know of,鈥� says Carole Haswell at the Open University, UK.

鈥淏ecause it鈥檚 unique, it has potential to challenge our ideas of how planet formation works because it doesn鈥檛 follow the evolutionary path of the vast majority of planets we know of,鈥� she says.

If TOI-849b did form like a normal gas giant before some event tore away its atmosphere, it could also help us learn about planets closer to home. 鈥淚t鈥檚 very, very hard to study the cores of planets,鈥� says Armstrong. 鈥淓ven in the solar system, we don鈥檛 know that much about Jupiter鈥檚 core, or Saturn or Neptune or Uranus, because there鈥檚 all that atmosphere in the way.鈥�

This planet may have a very thin atmosphere that forms as heat from the star vaporises rocks and dust on the planet鈥檚 surface. If so, the next generation of powerful telescopes may be able to use that atmosphere to learn about the chemical composition of TOI-849b, which could help us understand giant planets more generally.

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