Astronomers have reported the discovery of what may be the first exposed core of an exoplanet. TOI-849b, located 730 light-years away, is a peculiar world believed to be the remnant core of a much larger planet, whose atmosphere was likely blown away by its star. The findings are reported in Nature.
The world is truly peculiar. The planet, discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS), packs over twice the mass of Neptune in the same size. Despite being the size of a giant planet it has roughly the same density as Earth, and if an atmosphere is present is only a small fraction of its mass. It also orbits very close to its star, completing a full revolution in just 18 hours. It's this, combined with its unusual size, that makes this world quite special; planets of this size are a rarity that close to a star.
In fact, astronomers call this region the hot-Neptune desert. We have found Earth-size planets very close to their stars, and planets the size of Jupiter or bigger, the so-called hot-Jupiters, but Neptune-sized worlds very close to their stars are uncommon.
“TOI 849 b is the most massive terrestrial planet – that has an Earth-like density – discovered. We would expect a planet this massive to have accreted large quantities of hydrogen and helium when it formed, growing into something similar to Jupiter. The fact that we don’t see those gases lets us know this is an exposed planetary core,” lead author Dr David Armstrong from the University of Warwick Department of Physics said in a statement.
“This is the first time that we’ve discovered an intact exposed core of a gas giant around a star.”
Their argument that this strange exoplanet is a leftover core is sound. Neptune-like planets might not have enough gravity to keep their atmospheres very close to their stars. They might form and then lose their atmosphere. Or perhaps never even reach the stage of a gas giant, stopping prematurely. These worlds are in a class known as Chthonian planets, from the Greek word for deities from the infernal underground. Some possible Chthonian worlds have been found before but none as extreme as TOI-849b.
“One way or another, TOI 849 b either used to be a gas giant or is a ‘failed’ gas giant,” Dr Armstrong said. “It’s a first, telling us that planets like this exist and can be found. We have the opportunity to look at the core of a planet in a way that we can’t do in our own Solar System. There are still big open questions about the nature of Jupiter’s core, for example, so strange and unusual exoplanets like this give us a window into planet formation that we have no other way to explore."
There are many unknowns regarding this world but some mysteries might be solved in the very near future. Researchers believe that it is losing its atmosphere into space, which gives astronomer an exciting opportunity to study its chemical composition. Hubble and upcoming telescopes such as the James Webb Space Telescope may be able to tell us more about this world.