A newfound exoplanet may be the exposed core of a gas giant

A planet about 734 light-years away appears to be missing most of its atmosphere

Satellite and Earth-based telescope observations show that the newly discovered exoplanet has a radius nearly 3.5 times Earth’s and a mass about 39 times as big. Those dimensions combined point to a density roughly the same as Earth’s, suggesting that the exoplanet is mostly rock. Unlike other massive planets, this world, called TOI 849b, has a barely there atmosphere, making up 4 percent of its mass at most, a new study suggests.

That atmosphere is “absolutely minuscule for a planet of its size,” says astronomer David Armstrong of the University of Warwick in Coventry, England. “This one is almost entirely an exposed rocky ball.”

The planet’s large mass and near lack of an atmosphere suggest that TOI 849b may be the remnant core of a gas giant, Armstrong and colleagues report July 1 in Nature.  It might be the first exposed gas giant core ever found.  

Using NASA’s Transiting Exoplanet Survey Satellite, or TESS, the team spotted TOI 849b as it passed in front of a sunlike star about 734 light-years away. Follow-up observations with the European Southern Observatory’s La Silla Observatory in Chile revealed the planet’s mass, which is more than twice that of Neptune. Combining those observations with the planet’s inferred volume showed that TOI 849b is the densest planet of its size discovered so far.

The exoplanet whips around the star once every 18 hours, orbiting so close that its surface sizzles around 1500° Celsius. That puts it in a rare class: Most planets that lie so close to their stars are Jupiter-sized and larger, or Earth-sized and smaller. Only a handful of these “hot Neptunes” have been spotted before (SN: 7/30/19).

In the standard theory of how planets form, any ball of rock that reaches about 10 Earth masses or more should gobble up gas insatiably from the disk of gas and dust in which it formed (SN: 5/11/18). “Beyond that mass, it’s very hard to stop it turning into a gas giant,” Armstrong says. “You get this huge infall of gas that overwhelms the formation process.”

At 39 Earth masses, TOI 849b should have that thick atmosphere, so where is it? There are two main possibilities for the missing gas, the researchers report. One is that the planet opened a gap in the protoplanetary disk as it was forming, and so had a more meager buffet to eat from (SN: 5/20/20). That could have stalled the planet’s growth before it became a full-blown gas giant, leaving just a core.

The other option is that TOI 849b was a gas giant but lost its atmosphere somehow. Energy from the exoplanet’s star could have heated the atmosphere enough that it blew or boiled away, or collisions with other planets could have tossed out the gas atmosphere but left the rocky core.

It’s “a little premature” to say that TOI 849b is definitely a remnant core of a gas giant, says Elisabeth Adams, an astronomer at the Planetary Science Institute who is based in Somerville, Mass. There are still other possibilities, like rocky planets merging after most of the protoplanetary disk had dissipated.

But if TOI 849b is a former giant, studying this planet and others like it will help astronomers learn about the centers of planets like Jupiter and other gas giants. Those cores are otherwise hard to study hidden in their thick, gassy cocoons.

“We don’t even know how big Jupiter’s core is, and we’ve sent spacecraft to Jupiter,” says Adams, who was not involved in the work.

The exoplanet’s thin atmosphere might be gases released from inside the planet itself. Looking at the spectrum of starlight filtering through that atmosphere with future space telescopes could reveal what the planet is made of, Adams says.