Bizarre Exposed Core of Gas Giant Planet Discovered for First Time

An international team of researchers has discovered an extremely unusual, dense object, around 730 light-years from Earth, which they say is the exposed remnant core of a gas giant planet.

The object is the first exposed core of a gas giant ever found, as well as the most massive known planet with an Earth-like density.

The planet, dubbed TOI-849b, has a diameter around 3.4 times that of the Earth, making it slightly smaller than Neptune. Its mass is 39 times that of our planet, according to a study published in the journal Nature.

TOI-849b orbits a star very similar to our sun within the so-called "Neptune Desert." This term refers to a region surrounding stars in which it is rare to find planets with a mass close to, or larger than, Neptune's.

In fact, TOI-849b orbits so close to its host star that one year on the planet is only around 18 hours. The proximity also means the planet is blasted with intense radiation, leading to surface temperatures of around 2,700 F.

Because the planet has such a high density, the researchers said it must consist mostly of iron, rock and water. But usually, planets that are this massive would be surrounded by large quantities of hydrogen and helium gas, which is not the case with TOI-849b.

The fact that the scientists didn't observe those gases around the object suggests that it is likely an exposed planetary core.

"This is because dense objects like this, once they become more massive than a 'critical' mass—around 10 Earth masses—rapidly build up hydrogen and other light gases to become a planet like Jupiter," David Armstrong, lead author of the study from the Department of Physics at the University of Warwick, England, told Newsweek.

For a planet as massive as TOI-849b to form without any hydrogen atmosphere is very difficult. So the researchers propose two possible explanations. The first is that TOI-849b was once a gas giant that lost its outer atmosphere—through a planetary collision or other process—revealing the exposed core.

Alternatively, the planet could have become "stuck" partway through formation and never built up the atmosphere it should have—in other words, it is a "failed" gas giant.

"This could have occurred if there was a gap in the disk of dust and gas that the planet formed from due to gravitational interaction with the planet, or if the disk ran out of material right at the very moment when gas accretion normally follows," Christoph Mordasini, another author of the study from the University of Bern Physics Institute in Switzerland, said in a statement.

The researchers found TOI-849b using NASA's TESS mission, which is a satellite observing millions of stars looking for the signatures of planets.

exoplanet
Artist's illustration of a Neptune-sized planet orbiting close to its host star. © University of Warwick/Mark Garlick

"We were looking for objects like this [but] TOI-849b is far more massive than anything we expected to find, so was still a surprise," Armstrong said. "It's incredible to be able to make these kinds of discoveries."

"When the data started coming in for this one, we actually thought it was going to be an error. With the unusually high mass the first few data points led us to believe it was a false positive signal. But when we carried on observing the star to be sure we realized we were looking at something special."

The latest discovery provides an unprecedented glimpse into the interior of a gas giant planet, while demonstrating that exposed planetary cores exist and that they can be found very close to their host stars.

"With more work, we can build up a population of cores, which will be a huge step in our understanding of the interiors of planets," Armstrong said.

"Investigating planetary cores is very challenging, with large uncertainties still present even for the planets in our own solar system. TOI-849b itself is much more massive than we expect even gas giant cores to be, and this might imply new planet formation or evolution pathways which we don't yet fully understand."