Credit: NASA/ESA/CSA/Ralf Crawford (STScI)
Key Takeaways:
- Exoplanet PSR J2322-2650b, orbiting a pulsar, possesses an unprecedented helium-and-carbon-dominated atmosphere, characterized by molecular carbon (C3 and C2) and soot clouds, a composition unlike any previously observed.
- This exoplanet, with a mass comparable to Jupiter, completes an orbit every 7.8 hours and is gravitationally distorted into a lemon shape by its pulsar host; theoretical models suggest that intense internal pressure could condense its carbon into diamonds.
- Observations utilizing the James Webb Space Telescope were uniquely facilitated by the host pulsar's gamma-ray emission, enabling detailed spectroscopic analysis of the exoplanet's atmosphere without interference from the star's luminosity.
- The highly carbon-enriched composition of PSR J2322-2650b challenges existing planetary formation mechanisms, as its characteristics do not align with known processes for typical planet formation or "black widow" system evolution.
OK, first, let’s get the name out of the way. Scientists have cataloged this exoplanet as PSR J2322-2650b. It’s called that because it orbits a pulsar (a rapidly rotating neutron star) designated PSR J2322-2650, which is located in the constellation Sculptor.
Using the James Webb Space Telescope (JWST), the team has determined that the exoplanet has an exotic helium-and-carbon-dominated atmosphere unlike any ever seen before. It has a mass about the same as Jupiter, but soot clouds float through the air — and deep within the planet, these carbon clouds might condense to form diamonds. It orbits the pulsar every 7.8 hours at a distance of only 1 million miles (1.6 million kilometers), barely more than 1 percent Earth’s distance from the Sun.
Also, computer modeling of the planet’s brightness variations along its orbit revealed that gravitational forces from the much heavier pulsar are distorting PSR J2322-2650b into the shape of a lemon.
A unique atmosphere
“The planet orbits a star that’s completely bizarre — the mass of the Sun, but the size of a city,” explained the University of Chicago’s Michael Zhang, the principal investigator on this study, which is accepted for publication in The Astrophysical Journal Letters. “This is a new type of planet atmosphere that nobody has ever seen before.”
“This was an absolute surprise,” said team member Peter Gao of the Carnegie Earth and Planets Laboratory in Washington, D.C. “I remember after we got the data down, our collective reaction was ‘What the heck is this?’”
An easy discovery
Like all pulsars, PSR J2322-2650b emits beams of radiation from its poles. Because it spins, those beams sweep across our field of view and make the star appear to pulse at regular intervals, in this case just milliseconds apart.
Luckily, the star is emitting mostly gamma rays and other high-energy particles, which are invisible to JWST’s infrared sensors. This means scientists can study the planet in detail across its whole orbit. That’s usually a tough task, because planets are generally millions of times fainter than the stars they 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,” explained Maya Beleznay, a graduate student at Stanford University 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 better study this system in more detail than normal exoplanets.”
Taking stock of the planet
“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 C2,” said Zhang.
At the core of the planet, subjected to intense pressure, it’s possible this carbon could be squeezed into diamonds. But to the scientists, the larger question is how such a planet could have formed at all.
“It’s very hard to imagine how you get this extremely carbon-enriched composition,” said Zhang. “It seems to rule out every known formation mechanism.”
A black widow?
Some astronomers are classifying this pair of objects a “black widow” system. Black widows pair a rapidly spinning pulsar with a small, low-mass companion star. Usually, material from the companion streams onto the pulsar, causing it to spin faster, which powers a strong wind. That wind coupled with the pulsar’s radiation evaporate the smaller and less massive star. But PSR J2322-2650b is an exoplanet, not a star.
“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.”
Team member Roger Romani, of Stanford and the Kavli Institute for Particle Astrophysics and Cosmology Institute, is one of the world’s preeminent experts on black widow systems. He 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,” Romani theorized. “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 there’s controversy.”
“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.”
