NASA Webb Telescope Discovers Lemon-Shaped Planet, the ‘Stretchiest’ Ever Seen

An unusual object orbiting a rapidly spinning star might be a new phenomenon in the universe.

Earth isn’t a perfect sphere. The rotation of our planet causes it to bulge ever so slightly at the equator, making it about 0.3 percent wider there than from pole to pole.

But that’s nothing compared with PSR J2322-2650b, an object the mass of Jupiter studied recently by the James Webb Space Telescope. This planet’s equatorial diameter is about 38 percent wider than its polar diameter, giving the world the odd appearance of a lemon, and a very strange atmosphere.

“It’s the stretchiest planet that we’ve confirmed the stretchiness of,” said Michael Zhang, an exoplanet scientist at the University of Chicago and the lead author of a paper describing the planet published Tuesday in The Astrophysical Journal Letters.

PSR J2322-2650b was discovered in 2011 by the Parkes radio telescope in Australia. It is more than 2,000 light-years from Earth. The world was immediately interesting for being a Jupiter-size gas giant that orbits a pulsar, a dense and rapidly spinning star left over after a supernova. Pulsars are so named because they shoots out jets of radiation from their poles. The planet is only one million miles from the star, completing an orbit in about eight hours. It is the only gas giant known to be orbiting a pulsar.

This proximity gives the planet its unusual shape as the star’s gravity pulls on it. “It’s close enough that things are actually being funneled from the object to the pulsar,” said Peter Gao, an exoplanet scientist at the Carnegie Institution for Science in Washington, D.C., and an author of the paper. “You have a literal tip, like a point, where material actually comes out of the planet and spirals in.”

Using the Webb telescope’s infrared capabilities, the team was able to study the atmosphere of the planet, the first time this has been done for a planet orbiting a pulsar. Those observations hinted at the world’s odd shape. And they also revealed something extremely bizarre: The planet is devoid of hydrogen, oxygen and nitrogen, elements common on other planets, including gas giants. Instead, it is made mostly of helium and molecular carbon. “A helium- and carbon-dominated world is something we’ve never seen before,” Dr. Gao said.

Its carbon atmosphere might give it “clouds made out of graphite,” Dr. Zhang said, and diamonds at its core. Bands of storms would trace the world’s lemon-like exterior in the shape of a W, while it most likely has a red color because of dust and soot-like particles formed by the carbon.

“It’s a wacky, weird thing,” said Emily Rauscher, a theoretical astrophysicist at the University of Michigan, who was not involved in the paper. “It’s not formed in a way like any normal planet.”

PSR J2322-2650b’s strange properties might mean it is not actually a planet at all, but instead the remnant of a star that once orbited the pulsar and has slowly been eaten away. “We favor the star scenario,” said Dr. Gao, possibly making this a type of system known as a black widow pulsar, where we see a star being eaten by a pulsar.

This might be the very last moments of such a system, with PSR J2322-2650b on the cusp of being entirely consumed. “It would have lost 99.9 percent of its mass, and we just happened to catch it right at the very end,” Dr. Gao said.

Alternatively, Dr. Zhang said it could also be “an entirely new type of object that we don’t have a name for,” with PSR J2322-2650b staying in a stable orbit around its pulsar for billions of years, rather than being imminently eaten. He hopes to look for more such worlds in future to find out.

“I hope we have a sibling to compare this object to,” he said. “If it’s continuously losing mass, we had to be really lucky to see it in its last breath before it disappears.”

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