A lemon-shaped exoplanet, discovered with the James Webb Space Telescope, defies everything we thought we knew about worlds beyond our solar system: What on earth is this?
Since scientists first confirmed the existence of an extrasolar planet in the mid-1990s, exoplanets have continued to astonish us with their bizarre properties. A recent finding using JWST may be the oddest yet, boasting an atmosphere unlike any seen on another planet. The team behind the discovery admits they can’t fully explain how such a planet came into being.
That in itself wouldn’t be unprecedented. The earliest confirmed exoplanets—Poltergeist (PSR B1257+12 B) and Phobetor (PSR B1257+12 C), identified in 1992—also orbit pulsars, a very young and rapidly spinning class of neutron star. What makes PSR J2322-2650b truly remarkable is its ellipsoid, lemon-like shape and an atmosphere that appears unlike anything observed before.
“This was a total surprise,” said Peter Gao of the Carnegie Earth and Planets Laboratory, a member of the research team. “After we pulled the data in, our collective reaction was, ‘What the heck is this?’ It’s far from what we anticipated.”
The planet’s atmosphere is dominated by helium and carbon and may host clouds of carbon soot that condense into diamond-like grains that rain down onto the surface.
In terms of scale, PSR J2322-2650b orbits its pulsar host at roughly 1 million miles (about 1.6 million kilometers), completing an orbit in around eight hours. The lemon shape arises from tidal forces exerted by the incredibly strong gravity of the nearby dead star.
“Here we’re looking at a completely new kind of planetary atmosphere,” explained Michael Zhang from the University of Chicago. “Instead of the usual molecules we expect to find on exoplanets—water, methane, carbon dioxide—we’re seeing molecular carbon, specifically carbon-3 and carbon-2.”
PSR J2322-2650b is tidally locked to its star, so one face—its dayside—constantly faces the pulsar, while the opposite side remains in perpetual darkness. The dayside reaches up to about 3,700°F (2,040°C), whereas the nightside can dip to around 1,200°F (650°C).
At these extreme temperatures, carbon would typically bind with other elements to form various compounds, but in this atmosphere, oxygen and nitrogen appear to be in very short supply. Among roughly 150 exoplanet atmospheres studied to date, no other world has shown detectable molecular carbon.
Did this planet form like a typical planet, or did it emerge from a very different process? That remains unclear. It’s unlikely to be the result of simply stripping material from a star, as seen in some exotic binary systems, because ordinary nuclear physics doesn’t readily produce pure carbon in such quantities. The team suspects that the planet’s unusual carbon-rich makeup challenges our standard formation theories.
One potential formation scenario involves a unique, atmosphere-driven process inside the planet. As the companion cools, carbon and oxygen inside might crystallize, with pure carbon crystals rising toward the surface and mixing into the helium layer. Yet a crucial piece of the puzzle is keeping oxygen and nitrogen out, which is exactly the mystery the researchers are eager to solve.
This discovery is an invitation to explore the strange and to ask tough questions. The team is eager to learn more about this strange atmosphere and what it could reveal about planetary diversity in the universe.
Robert Leach is a science journalist in the United Kingdom whose work has appeared in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek, and ZME Science. He also covers science communication for Elsevier and the European Journal of Physics. He earned a Bachelor of Science in physics and astronomy from the UK Open University. You can follow him on Twitter @sciencef1rst.
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