JWST finds planet with all-carbon atmosphere orbiting 'black widow' star

Science advances through data that don't fit our current understanding. At least that was Thomas Kuhn's theory in his famous On the Structure of Scientific Revolutions. So scientists should welcome new data that challenges their understanding of how the universe works. A recent paper, available in pre-print on arXiv, using data from the James Webb Space Telescope (JWST) might just had found some data that can do that. It looked at an exoplanet around a millisecond pulsar and found its atmosphere is made up of almost entirely pure carbon.

This type of pulsar, PSR J2322-2650, is known as a "black widow" system, as it powers its high energy outbursts by stealing material from a neighboring star. In this case, that neighboring star has likely been degraded to a "hot Jupiter" companion planet that orbits its parent neutron star every 7.8 hours. A typical "black widow" formation process has two steps - one where the neutron star (which in this case is also a pulsar) steals the material, and a second step where it blasts its companion with high energy gamma radiation, ripping off most of the companion star's outer layers and resulting in a Jupiter-sized exoplanet composed mainly of helium.

The exoplanet around PSR J2322-2650, known as PSR J2322-2650b, does fit the description of a Jupiter-sized planet that seems to have the same density as what would be expected if it was made up primarily of helium. However, its atmosphere is unlike any other black widow companion ever seen. According to the spectrographic reports from JWST, its atmosphere is composed mainly of elemental carbon, taking the form of tricarbon (C3) or dicarbon (C2).

Usually those types of elements are found in the tails of comets, or in actual flames here on Earth. Their presence in a planet's atmosphere, especially in such abundant quantities, is new to science.

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Another interesting thing about the planet's atmosphere is the difference between the day and night side. On the dayside, which is always facing the pulsar since the planet is tidally locked, temperatures can reach above 2000 C and there are very clear chemical signatures. However, on the night side, there were almost no features at all, suggesting that side of the planet is covered in soot or something similar that doesn't have any distinct features.

To further prove how strange this planet's atmosphere is, the researchers calculated the ratios between carbon and oxygen as well as carbon and nitrogen. The C/O ratio was over 100, while the C/N ratio was over 10,000. In comparison, the Earth has a C/O ratio of .01 and a C/N ratio of 40. Obviously, there's a lot of carbon on this planet.

And that doesn't fit well with models of how scientists thought the planet should form. As part of the "black widow" process, the outer layers of the planet should have been either siphoned up by the companion star or burned away by that star's radiation. The fact that such a rich carbon atmosphere still exists remains a mystery. There are processes that can create such an atmosphere, such as a white-dwarf merger between who "carbon stars", but even that falls short of explaining how the planet's C/O ratio got so high.

Other aspects of the planet align with general theory though. Circulation models predict that rapidly rotating planets, like PSR J2322-2650b, would have strong westerly winds, which is different from the typical easterly winds on other tidally locked hot Jupiters. The JWST data show that the hottest part of the planet is about 12 degrees west of center, providing the first ever observational evidence of this western wind phenomena.

In other words, PSR J2322-2650b is contradictory. It's the right size and shape for a typical black widow pulsar system. Its window circulation also fits well with our best models. But its atmosphere is something else entirely, and scientists will have to go back to the theory to try to find a way to make it make sense with the new data. While they're busy doing that, JWST will continue scanning the sky for more anomalies that could drive the next scientific revolution.