Webb Spots Exoplanet with Unexplained Composition

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Scientists using NASA's James Webb Space Telescope have observed a rare type of exoplanet, or planet outside our solar system, whose atmospheric composition challenges our understanding of how it formed.

Officially named PSR J2322-2650b, this Jupiter-mass object appears to have an exotic helium-and-carbon-dominated atmosphere unlike any ever seen before. Soot clouds likely float through the air, and deep within the planet, these carbon clouds can condense and form diamonds. How the planet came to be is a mystery. The paper appears Tuesday in The Astrophysical Journal Letters.

"This was an absolute surprise," said study co-author Peter Gao of the Carnegie Earth and Planets Laboratory in Washington. "I remember after we got the data down, our collective reaction was 'What the heck is this?' It's extremely different from what we expected."

Image A: Exoplanet PSR J2322-2650b and Pulsar (Artist's Concept)

This planet-mass object was known to orbit a pulsar, a rapidly spinning neutron star. A pulsar emits beams of electromagnetic radiation at regular intervals typically ranging from milliseconds to seconds. These pulsing beams can only be seen when they are pointing directly toward Earth, much like beams from a lighthouse.

This millisecond pulsar is expected to be emitting mostly gamma rays and other high energy particles, which are invisible to Webb's infrared vision. Without a bright star in the way, scientists can study the planet in intricate detail across its whole orbit.

"This system is unique because we are able to view the planet illuminated by its host star, but not see the host star at all," said Maya Beleznay, a third-year PhD candidate at Stanford University in California who worked on modeling the shape of the planet and the geometry of its orbit. "So we get a really pristine spectrum. And we can study this system in more detail than normal exoplanets."

"The planet orbits a star that's completely bizarre - the mass of the Sun, but the size of a city," said the University of Chicago's Michael Zhang, the principal investigator on this study. "This is a new type of planet atmosphere that nobody has ever seen before. Instead of finding the normal molecules we expect to see on an exoplanet - like water, methane, and carbon dioxide - we saw molecular carbon, specifically C₃ and C_2."

Molecular carbon is very unusual because at these temperatures, if there are any other types of atoms in the atmosphere, carbon will bind to them. (Temperatures on the planet range from 1,200 degrees Fahrenheit at the coldest points of the night side to 3,700 degrees Fahrenheit at the hottest points of the day side.) Molecular carbon is only dominant if there's almost no oxygen or nitrogen. Out of the approximately 150 planets that astronomers have studied inside and outside the solar system, no others have any detectable molecular carbon.

PSR J2322-2650b is extraordinarily close to its star, just 1 million miles away. In contrast, Earth's distance from the Sun is about 100 million miles. Because of its extremely tight orbit, the exoplanet's entire year - the time it takes to go around its star - is just 7.8 hours. Gravitational forces from the much heavier pulsar are pulling the Jupiter-mass planet into a bizarre lemon shape.

Image B: Exoplanet PSR J2322-2650b (Artist's Concept)

Together, the star and exoplanet may be considered a "black widow" system, though not a typical example. Black widow systems are a rare type of double system where a rapidly spinning pulsar is paired with a small, low-mass stellar companion. In the past, material from the companion streamed onto the pulsar, causing the pulsar to spin faster over time, which powers a strong wind. That wind and radiation then bombard and evaporate the smaller and less massive companion. Like the spider for which it is named, the pulsar slowly consumes its unfortunate partner.

But in this case, the companion is officially considered an exoplanet, not a star. The International Astronomical Union defines an exoplanet as a celestial body below 13 Jupiter masses that orbits a star, brown dwarf, or stellar remnant, such as a pulsar.

Of the 6,000 known exoplanets, this is the only one reminiscent of a gas giant (with mass, radius, and temperature similar to a hot Jupiter) orbiting a pulsar. Only a handful of pulsars are known to have planets.

"Did this thing form like a normal planet? No, because the composition is entirely different," said Zhang. "Did it form by stripping the outside of a star, like 'normal' black widow systems are formed? Probably not, because nuclear physics does not make pure carbon. It's very hard to imagine how you get this extremely carbon-enriched composition. It seems to rule out every known formation mechanism."

Study co-author Roger Romani, of Stanford University and the Kavli Institute for Particle Astrophysics and Cosmology Institute, proposes one evocative phenomenon that could occur in the unique atmosphere. "As the companion cools down, the mixture of carbon and oxygen in the interior starts to crystallize," said Romani. "Pure carbon crystals float to the top and get mixed into the helium, and that's what we see. But then something has to happen to keep the oxygen and nitrogen away. And that's where the mystery come in.

"But it's nice to not know everything," said Romani. "I'm looking forward to learning more about the weirdness of this atmosphere. It's great to have a puzzle to go after."

Video A: Exoplanet PSR J2322-2650b and Pulsar (Artist's Concept)

This animation shows an exotic exoplanet orbiting a distant pulsar, or rapidly rotating neutron star with radio pulses. The planet, which orbits about 1 million miles away from the pulsar, is stretched into a lemon shape by the pulsar's strong gravitational tides. NASA&rsqu…

Animation: NASA, ESA, CSA, Ralf Crawford (STScI)

With its infrared vision and exquisite sensitivity, this is a discovery only the Webb telescope could make. Its perch a million miles from Earth and its huge sunshield keep the instruments very cold, which is necessary for these observations. It is not possible to conduct this study from the ground.

The James Webb Space Telescope is the world's premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).