Meet the Milky Way’s puffiest planets

A pair of newly discovered planets are the puffiest planets ever seen. The planets are each about the size of Jupiter but have less than 6 percent of its mass, astronomers report in the July Monthly Notices of the Royal Astronomical Society.

“We liken it in density to something like candy floss or shaving foam,” says astrophysicist George Dransfield of the University of Oxford. “They’re the lightest planets that are Jupiter-sized.”

Both planets orbit TOI 791, a sunlike star about 1,113 light-years from Earth. The planet-hunting TESS space telescope spotted the planets as they passed, or transited, in front of their host star and blocked some of its light. Comparing the blocked star to the unblocked star let Dransfield and colleagues measure the planets’ radii: 0.993 and 1.155 times that of Jupiter.

To find out how dense the planets are, the team needed to know both their size and mass. But transits only give the size. That’s where the planets’ orbits come in. They are configured in such a way that the planets periodically pass by one another, giving each other a little gravitational tug.

“They’re in this fun little dance,” Dransfield says. “We were able to measure the masses of the planets by how much they kick each other’s orbits.”

Dransfield and colleagues monitored the planetary pas de deux over several years using the Antarctic Search for Transiting ExoPlanets telescope, or ASTEP. Because Antarctica gets three months of continuous darkness each year, ASTEP was the only telescope on Earth that could observe the planets’ entire 12-hour-long transits, Dransfield says.

“Without this telescope, this discovery would not have happened,” she says.

Those measurements let her and colleagues deduce the planets’ masses: 9.5 and 18.6 times that of Earth. (Jupiter has 317 times Earth’s mass.) That means their densities are 0.038 and 0.047 grams per cubic centimeter. For comparison, cotton candy’s density is typically about 0.05 grams per cubic centimeter.

Sizing up superpuffs

The new planets join a small cadre of large but light worlds known as superpuffs. Astronomers aren’t sure how they form. One theory is that they are born far from their star and migrate inward, their atmospheres heating and puffing up as they go.

Some superpuffs might have giant ring systems that make them look bigger than they really are, theoretical astrophysicist Anthony Piro and colleagues proposed in 2020. That’s probably not the explanation for these two planets, says Piro, of the Carnegie Observatories in Pasadena, Calif. Having two masqueraders around the same star would be too much of a coincidence, he says.

The host star is spinning quickly, which could be a sign that it’s relatively young, Piro notes, although measuring a star’s age is notoriously difficult. If that’s the case, then these superpuffs might still be cooling off and could contract as they get older.

“We might be catching these planets at an intermediate stage,” Piro says.

The next step is to use the James Webb Space Telescope to probe the planets’ composition, which could reveal more about their origins, Dransfield says.

“Finding things like superpuffs, which are incredibly rare, means we can learn more about how planets form, how planets evolve and what outcomes are possible,” Dransfield says. “That also helps us contextualize Earth in the wider context of the cosmos.”