|image: University of Warwick/Mark Garlic|
Most of what we know about planetary cores is conjecture, on account of the fact that we live on top of ours and have extremely limited access to all of the others. Since none of the planets whizzing around our own star are daring enough to expose their own, astronomers have been forced to use telescopes and planetary surveys, peering out into the void for a glimpse of cosmic libertinity.
More seriously, we’ve found what’s believed to be the first exposed planetary core — and it’s a highly unusual planet to boot, known as a Hot Neptune. So-called “Hot Jupiters” are planets of Jupiter size and mass, approximately 318x that of Earth. Hot Neptunes are significantly smaller, at 20-40x Earth mass. Uranus and Neptune are both about 17x the mass of Earth, while Saturn is 95x. Statistically, Hot Neptunes are very unusual, while Hot Jupiters are common, for reasons that aren’t entirely clear.
TOI-849b, however, appears to have begun life as one such planet, before spiraling in towards its current position. TOI-849b is approximately 39 Earth masses, with a radius of 3.44 Earth radius and a mean density of 5.2g cm3, which the authors note is the densest Neptune-sized planet ever discovered. The planet is so close to its primary, its year is ~18 hours long. The surface temperature, meanwhile, is a balmy 1800K.
So how do we know it’s an exposed planetary core? Objects the density and size of this one are expected to attract gaseous envelopes around themselves as part of the formation process. If TOI-849b orbited farther from its star, we would expect it to look more like our own ice giants Neptune and Uranus. The envelope of hydrogen and helium we should see around the planet is entirely absent. And even given the certainty of photoevaporation, the astronomers who conducted the observations would expect to see something.
That leaves two possibilities: Either TOI-849b is a failed gas giant that accreted enough solid material to become one but failed to gather a gas layer, or it originally possessed such an atmosphere but lost it somehow. This wouldn’t be the first time we’ve picked up evidence of a planet being torn apart by its primary — additional examples include 47 Ursae Majoris, WD 1145+017b, and RZ Piscium, each of which shows evidence of planetary disruption.
Lead author Dr. David Armstrong from the University of Warwick Department of Physics said, “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.
“This is the first time that we’ve discovered an intact exposed core of a gas giant around a star.”
So, what’s it made of? We aren’t sure yet. That’ll be the work of other telescopes and future survey missions, now that the TOI-849b has been identified as a likely target.