These planets are as big as Jupiter and as dense as cotton candy
Two newly discovered planets are the most bloated planets ever seen. The planets are each roughly the size of Jupiter but have less than 6 percent of its massreport the astronomers in the July report Monthly Notices of the Royal Astronomical Society.
“We compare its density to something like cotton candy or shaving cream,” says astrophysicist George Dransfield of the University of Oxford. “These are the lightest Jupiter-sized planets.”
Both planets orbit TOI 791, a Sun-like star located about 1,113 light-years from Earth. The planet-hunting TESS space telescope spotted 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, Dransfield and his colleagues measured the radii of the planets: 0.993 and 1.155 times that of Jupiter.
To determine the density of the planets, the team needed to know both their size and their mass. But transits only give size. This is where the orbits of the planets come into play. They are configured in such a way that the planets pass each other periodically, giving each other a little gravitational tug.
“They participate in this fun little dance,” Dransfield says. “We were able to measure the mass of the planets based on their intensity in each other’s orbits.”
Dransfield and his colleagues monitored the planetary pas de deux for several years using the Search in Antarctica for transiting exoplanets telescope, or ASTEP. Because Antarctica experiences three months of continuous darkness each year, ASTEP was the only telescope on Earth capable of observing the entire 12-hour transits of the planets, Dransfield says.
“Without this telescope, this discovery would not have happened,” she says.
These measurements allowed him and his colleagues to deduce the masses of the planets: 9.5 and 18.6 times that of Earth. (Jupiter has 317 times the mass of Earth.) This means their densities are 0.038 and 0.047 grams per cubic centimeter. For comparison, the density of cotton candy is typically around 0.05 grams per cubic centimeter.
Sizing Super Puffs
known as super puffs. Astronomers don’t know exactly how they form. One theory is that they are born far from their star and migrate inward, their atmosphere warming and expanding as they go.Some super puffs might have giant ring systems that make them appear larger 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, California. Having two masquerades around the same star would be too much of a coincidence, he said.
The host star turns quickly, which could be a sign that she’s relatively young, Piro notes, although measuring a star’s age is notoriously difficult. If so, these super-puffs could further cool and contract as they age.
“We could catch these planets at an intermediate stage,” Piro says.
The next step will be to use the James Webb Space Telescope to probe the composition of the planets, 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,” says Dransfield. “It also helps us contextualize Earth within the broader context of the cosmos.”
