“What is that?” Scientists using NASA’s James Webb Space Telescope couldn’t believe what they saw 3,800 light years from Earth: a rare exoplanet, a world outside our solar system, with … an exotic atmosphere never seen before. Its composition, dominated by helium and carbon, is so strange that researchers do not understand how the body could have formed. And the most curious thing: it has a strange lemon shape. And the diamonds at its heart.
“It was an absolute surprise. “It’s extremely different from what we expected,” says Peter Gao, of the Carnegie Earth and Planetary Laboratory in Washington, of the team’s reaction. The study has been accepted for publication in “The Astrophysical Journal Letters”.
This Jupiter-mass object, called PSR J2322-2650b, orbits a pulsar, a neutron star “the mass of the Sun, but the size of a city” that rotates rapidly and emits beams of electromagnetic radiation at regular intervals. These pulsing beams, which last between milliseconds and seconds, can only be seen when pointed directly at Earth, like the beams of a lighthouse. This double possibility, of seeing or not the planet lit by its star, made it possible to study it in more detail.
In this way, the researchers realized that the exoplanet has “a new type of atmosphere that no one had seen before. Instead of finding the usual molecules we expect to find on an exoplanet, like water, methane, and carbon dioxide, we saw molecular carbon, specifically C₃ and C₂,” says lead researcher Michael Zhang of the University of Chicago.
Temperatures on the planet vary between 650ºC in the coldest points on the night side and 2,037ºC in the hottest points on the dayside. Molecular carbon is very unusual because, at these temperatures, if there are other types of atoms in the atmosphere, carbon will bond to them. Molecular carbon only predominates when there is almost no oxygen or nitrogen. Of the approximately 150 planets studied by astronomers inside and outside the solar system, no others have detectable molecular carbon. Clouds of soot are likely to float in the air from this object. At depth, these clouds can condense and form diamonds.
A year of less than eight hours
PSR J2322-2650b is located extraordinarily close to its star, just 1,600,000 kilometers away. In contrast, the distance between Earth and the Sun is approximately 257 million kilometers. Due to its extremely narrow orbit, the exoplanet’s full year – the time it takes to orbit its star – is only 7.8 hours. Gravitational forces from the much heavier pulsar pull the planet into this strange lemon shape.
Together, the star and exoplanet could be considered a “black widow” system, although it is not a typical example. Black Widow systems are a rare type of dual system in which a rapidly rotating pulsar is paired with a small, low-mass stellar companion. In the past, companion material flowed into the pulsar, causing it to spin faster over time, generating a strong wind. This wind and radiation bombarded and evaporated the smaller, less massive companion. Like the spider for which it was named, the pulsar slowly consumed its unfortunate companion. But in this case, the companion is officially considered an exoplanet.
Among the 6,000 known exoplanets, it is the only one reminiscent of a gas giant (with a mass, radius and temperature similar to a hot Jupiter) orbiting a pulsar. Only a few pulsars have planets.
“Did this object form like a normal planet? No, because their composition is completely different,” explains Zhang. “Was it formed by stripping a star of its exterior, like ‘normal’ black widow systems do? Probably not, because nuclear physics does not produce pure carbon. It is very difficult to imagine how this extremely carbon-rich composition is obtained. “This appears to rule out any known formation mechanism.”
Study co-author Roger Romani, of Stanford University and the Kavli Institute for Particle Astrophysics and Cosmology, offers an explanation: “As the companion cools, the mixture of carbon and oxygen inside begins to crystallize. The pure carbon crystals float to the surface and mix with the helium, and that’s what we observe. But then something must happen to push the oxygen away and nitrogen. And therein lies the mystery.
“But it’s good not to know everything,” Romani thinks. “I really want to know more about the particularity of this atmosphere. “It’s great to have a riddle to solve.”
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