Illustration by Mark Garlick, University of Warwick
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The planet TOI-849b is almost as wide as Neptune, but it's an incredibly dense, rocky world that orbits close to its host star.

Illustration by Mark Garlick, University of Warwick

Bizarre new planet is largest known rocky world, 40 times as massive as Earth

The bulky object—possibly the core of a failed gas giant—challenges what astronomers think about how planets form.

About 730 light-years away, not far on the scale of our galaxy, an utterly bizarre planet orbits a sun-like star. Big, dense, and tightly tethered to its home star, the planet is unlike anything astronomers have yet seen—either in our own solar system or afar.

The roasted world known as TOI-849b is the most massive rocky planet ever observed, with as much as 40 Earths’ worth of material crammed inside. Perplexingly, TOI-849b’s tremendous bulk suggests that it should be a giant, gassy world like Jupiter, yet it has almost no atmosphere. Explaining how such a world emerged challenges what scientists understand about how planets grow.

“It’s very difficult to make a planet as massive and dense as TOI-849b without it becoming a gas giant,” David Armstrong, an exoplanet researcher at the University of Warwick and lead author of a study reporting the new planet today in the journal Nature, says in an email. “Something in that standard process went wrong.” Instead, Armstrong and his colleagues think the world is the exposed, airless core of a giant planet that should have outgrown Jupiter.

“These are the objects that push theory further, and make the fields of exoplanets and planetary science so exciting,” study co-author Ravit Helled of the University of Zurich says in an email.

“It’s definitely weird!” adds Jonathan Fortney, director of the Other Worlds Laboratory at the University of California Santa Cruz, who wasn’t involved in the observations. “But I’m not sure what it’s telling us.”

A weirdo among wild things

Over the last decade, planet-hunters have spotted thousands of far-flung worlds among the galaxy’s star fields. Most are decidedly unearthly, falling into categories such as “hot Jupiters”—big, gassy planets on tight orbits—or “super-Earths,” rocky planets bigger than ours but smaller than Neptune. TOI-849b, however, defies classification.

The world was spotted by NASA’s planet-hunting space telescope TESS—the Transiting Exoplanet Survey Satellite—which is searching 200,000 of the nearest, brightest stars. The planet betrayed its presence by crossing the face of its star and briefly blotting out a smidgen of starlight. Those fleeting, shadowy transits revealed that the alien world circles its star every 18 hours, meaning that its surface temperature is a sweltering 2800°F.

TESS observations also showed that the planet is about 3.4 times as wide as Earth, or 85 percent as wide as Neptune—making it a world of unusual size for its position so close to its star. Up until now, astronomers have primarily observed hot Jupiters or much smaller super-Earths in such tight orbits, and nothing has populated what’s known as the hot-Neptune desert.

“There really are no planets of that mass there,” Fortney says. TOI-849b is the right radius to be a hot Neptune, but its mass is two to three times larger.

Further observations of the host star’s gravitational wobble, made with the HARPS instrument at the La Silla Observatory in Chile, determined that while TOI-849b is roughly as wide as Neptune, it is at least twice as massive. All that bulk means TOI-849b is extremely dense. The rocky planet might have a thin veneer of atmosphere, probably composed of hydrogen and helium—but not nearly as much gas as a world that hefty should hold on to.

“We think it has a mixture of metals, silicates, water, and possibly a (very) small atmosphere,” Helled says.

A relic from the distant past?

The odd properties of the planet suggest to Armstrong and his colleagues that the mystery world is probably the core of a gas giant planet—something that should have grown more massive than Jupiter. The gas giants in our solar system likely have dense cores of rock and exotic materials, although neither core is thought to come close to TOI-849b

“Our best estimates for how massive that part of Jupiter’s core is are surprisingly uncertain,” Armstrong says. “But some recent work suggested an upper limit of about 25 Earth masses. TOI-849b is even more massive than that.”

Current theories of planet formation describe worlds as growing from small seeds of rock and ice planted in the swirling disks of gas and dust that surround newborn stars. Some planets, like Earth, amass a smattering of material and stay small, while others gather gases and balloon into inflated worlds with gargantuan atmospheres, like Jupiter and Saturn.

Once a planet grows to roughly 10 Earth masses, Armstrong says, it begins a process called runaway gas accretion, and the planet’s gravity rapidly hauls in surrounding hydrogen and helium. A seed with 40 Earth masses should, in all likelihood, accumulate an absolutely astounding amount of gas—but that’s not what TOI-849b looks like today.

“We can already say that planets like TOI-849b are rare, but nevertheless they do exist, and now we have to find out how, and why,” Helled says.

One possibility is that TOI-849b cleared a gap in the gas around its star, ran out of material to collect, and stalled. Another possibility is that TOI-849b is the husk of a formerly huge planet that somehow lost its atmosphere, perhaps because of its proximity to the star—although if this is the case, planetary scientists don’t know how the planet could have dumped several hundred Earth masses in gas over a few billion years.

A third scenario is that cataclysms during the planet’s early years—such as collisions with other similarly sized planetary seeds—both bulked up the rocky planet core and stripped away its atmosphere.

“I think the key clue is that TOI-849b lies in the Neptunian desert, which hints at a rare history,” Armstrong says. “That suggests, to me, one of the more unusual pathways.”