A pair of newly found planets are the puffiest worlds ever seen. They’re both about the size of Jupiter but with less than 6 percent of its mass.

In terms of density, these planets are “like candy floss or shaving foam,” says George Dransfield. She’s an astrophysicist at the University of Oxford in England. “They’re the lightest planets that are Jupiter-sized.”

Both orbit TOI 791. That’s a sunlike star about 1,113 light-years from Earth. The TESS space telescope spotted them as they passed — or transited — in front of their star and blocked some of its light. Comparing the star when it was blocked and unblocked let Dransfield and others measure the planets’ width.

One world is about 0.993 times as wide as Jupiter. The other has about 1.155 times Jupiter’s width.

For the team to find their density, it needed to know both the planets’ size and mass. Transits only give their size. That’s where their orbits come in. The two planets periodically pass by one another as they circle their star. As they do, the planets give 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 her teammates tracked the planetary dance over several years. They used the Antarctic Search for Transiting ExoPlanets telescope, or ASTEP. It was the only telescope on Earth that could observe the planets’ entire transits. Why? Because it takes about 12 hours for each planet to pass in front of its star. It’s hard to find a place on Earth that gets 12 straight hours of darkness at a time during which to view that. But Antarctica gets three straight months of darkness each year.

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

Those measurements let her team deduce the planets’ masses. One was 9.5 times the mass of Earth. The other had the heft of 18.6 Earths. (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.

The researchers shared these findings in the July Monthly Notices of the Royal Astronomical Society.

Sizing up superpuffs

The new planets join the small group of large but lightweight worlds known as superpuffs. Astronomers aren’t sure how they form. One theory is that they’re born far from their star and move inward. Their atmospheres would heat and puff up as they neared their stars.

Some superpuffs might have giant ring systems, which could make them look bigger than they really are. Astrophysicists Anthony Piro and Shreyas Vissapragada proposed this idea in 2020. But that’s probably not the case for the two newfound planets, says Piro.

Having two ringed planets mistaken for superpuffs around the same star would be too much of a coincidence, he says. Piro works at the Carnegie Observatories in Pasadena, Calif. He did not take part in the new study.

The host star is spinning quickly, which could be a sign that it’s fairly young, Piro notes. If so, these superpuffs might still be cooling off. They might shrink as they age. “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’ chemical makeup. That 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,” Dransfield says. They also offer clues about how planets evolve and what they can end up looking like. “That also helps us [understand] Earth in the wider context of the cosmos.”

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“This discovery suggests that small icy worlds beyond Neptune may not be as inactive or unchanging as we often assumed,” says Ko Arimatsu. “Until now, Pluto was the only [object beyond Neptune] with a confirmed atmosphere.”

Arimatsu is an astronomer at the National Astronomical Observatory of Japan. It’s in Mitaka. His group shared the new finding May 4 in Nature Astronomy.

Arimatsu’s group tracked the object — known as 2002 XV₉₃ — with a network of telescopes in Japan. On January 10, 2024, those telescopes saw it move in front of a distant star. Such tiny eclipses are called occultations (Ah-kul-TAY-shuns). They can reveal whether some object in our solar system hosts an atmosphere.

If 2002 XV₉₃ were bare, the star behind it would have seemed to blink out and reappear sharply. But Arimatsu and his teammates saw the star’s light gradually fade and reappear over about 1.5 seconds. That smooth dimming is best explained by starlight passing through a layer of gas. Those data suggest this thin atmosphere has a pressure about one ten-millionth that of Earth’s.

“I was genuinely surprised,” Arimatsu says. The object is about 470 kilometers (290 miles) wide. The Grand Canyon is about that long. At such a small size, 2002 XV₉₃’s gravity should be too weak to hold on to a gas for long. So its atmosphere should have vanished long ago, unless something has added new gas to it.

Perhaps an icy body like a comet recently smashed into 2002 XV₉₃. That could have released a plume of gas around the object. Or maybe 2002 XV₉₃ releases gas regularly from icy volcanoes on its surface.

The astronomers can’t be certain what’s going on just from viewing 2002 XV₉₃ pass in front of one star. The data couldn’t show what the atmosphere is made of. So the small body might even be swaddled in dust, not gas, Arimatsu notes. Nor could they show how high above the body’s surface any atmosphere extends.

“Future observations will be important,” he says. Say its atmosphere fades over the next few years. That could mean he had detected a plume kicked up by an impact. If it sticks around or varies over time, that might make a case for surface volcanoes.

For the first time, humankind has changed a space rock’s orbit around the sun.

Four years ago, NASA rammed its DART spacecraft into the asteroid Dimorphos. The goal was to see if a space probe could change an asteroid’s motion. Why? It could be useful if a big rock were ever headed for Earth.

Soon after the crash, it was clear that DART had knocked its asteroid closer to Didymos, an even bigger rock that it orbits. Now, NASA reports that the smashup had an even bigger impact. It changed the asteroid duo’s speed as they orbit the sun together.

DART’s crash landing slowed the asteroid pair by 10 micrometers per second. That might not sound like much. But tweaking the motions of space rocks millions of kilometers (miles) from Earth is a huge deal. It could inform plans for shielding our planet from future asteroid strikes.

Researchers shared the findings March 6 in Science Advances.

DART is short for Double Asteroid Redirection Test. The initial goal had been to simply nudge Dimorphos into a tighter orbit around Didymos. Mission leaders aimed to cut 73 seconds off Dimorphos’ nearly 12-hour path around its partner. In reality, the crash shortened Dimorphos’ orbit around Didymos by 32 minutes.

Most of that change came from the impact itself. Like giving the asteroid a big shove, the spacecraft pushed Dimorphos closer to Didymos. The crash also blasted rocks off Dimorphos’ surface. The motion of that flying debris gave the asteroid an extra kick.

But some of the debris knocked off Dimorphos didn’t just leave that space rock. It escaped the gravity of the two-asteroid system altogether. That escaping debris took some momentum away from the duo. As a result, their joint motion around the sun changed, reports Rahil Makadia. He studies planetary defense at the University of Illinois in Urbana–Champaign.

Clues in tiny eclipses

To figure out how much the asteroids’ motion changed, astronomers watched the rocks pass in front of distant stars. As the asteroids passed by, they dimmed some of the stars’ light — like tiny eclipses. These blinks can be seen only from specific locations, which can fall anywhere on Earth.  

Astronomers could predict when Dimorphos and Didymos should pass in front of stars, based on their old path around the sun. By comparing those predictions to observations, they could see how much the asteroids’ path changed after DART’s hit.

Between October 2022 and March 2025, Makadia and the team gathered 22 such measurements. Amateur astronomers provided a lot of these data, Makadia says. “There was an observer who drove two days each way into the Australian outback to get these measurements.”

The asteroids’ orbit around the sun was about 150 milliseconds longer than before the DART impact, the new data show.

The European Space Agency’s Hera spacecraft will reach Didymos and Dimorphos later this year. When it does, the spacecraft could confirm the asteroids’ new orbit.

Didymos and Dimorphos were not a threat to Earth before DART. They aren’t now, either. But knowing how an impact changes one asteroid’s orbit can help figure out how to defend Earth from a dangerously big incoming space rock. That’s crucial, Makadia says, “in case we need to [deflect an asteroid] for real.”

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