Astronomers have taken the measure of an unusual exoplanet, and it appears to be dark, hot, and airless.

And we knew it was weird even before this.

Gliese 367, aka GJ 367, is a small, cool red dwarf star only about 30 light-years from Earth. A planet candidate was detected by the Transiting Exoplanet Survey Satellite in 2019, which was later confirmed to be real. The planet, called GJ 367b, is small, just 60% the diameter of Earth, but it’s extremely dense, about 10 grams per cubic centimeter, nearly twice as dense as Earth… and our home world is the densest in the solar system! This is also significantly denser than iron, so this planet is really beefy despite its small size.

It orbits its host star in only a million kilometers away, so it’s scorched, with a surface temperature of around 1,400°C. Yikes. It orbits the star in about 7.7 hours. Yes, hours. Its year is less than a third of one Earth day.

We’d expect the planet to have no atmosphere; that density means it’s likely a solid ball, and being that hot the air there would’ve boiled off long ago. Astronomers pointed JWST at it recently thought to see what they could find [link to journal paper].

The JWST observations followed it for more than one orbit. As the planet circles its star we see its dark side (when it’s between us and the star), then half an orbit later we see its day side (when it’s on the other side of the star from us). We also see all the phases in between (like the changing phases of the moon over the course of the month). By looking at how the planet’s brightness changed over its orbit, astronomers could figure out a lot about it.

For example, a planet like this will be tidally locked to its star, such that its year will equal its day. That means one face always points toward the star and is brutally hot, with the other half always pointing away and therefore very cold. If the planet had air, the change in temperature would cause the atmosphere to flow around, taking away heat from the star-facing side and warming the other side, then cooling off and flowing back to the star-facing side, cooling it. No such temperature mitigation was seen, indicating the planet lacks a thick atmosphere. At best, its air is 1/10^th as thick as Earth’s atmosphere, but the data are consistent with it being airless.

By comparing how bright the day and night sides are, they can also see the planet is extremely dark, far less reflective than, say, Earth. It probably glows a dull red on the day side, but the night side is essentially black. This plus the lack of atmosphere means the planet has probably lost all its volatiles, materials that boil away easily.

And that density… it’s hard to make a planet that dense all by itself. A far more likely scenario is that it was once bigger, and had a dense metallic core. Then a catastrophic but grazing collision with a planet-sized body stripped off its lower-density outer layers, leaving behind the far denser core. This would’ve ripped away its atmosphere too, maybe giving it a head start to becoming the airless world it is now.

What an alien place! The day side is possibly molten, with the star looming hugely in the sky, seventy times larger than the Sun as seen from Earth. The dark side is much cooler, but dark indeed, and even though it’s smaller than Earth the surface gravity is about 30% higher. 

And this planet is only 30 light-years away, in a galaxy 120,000 light-years across! That implies such terrible worlds are common, and are strewn throughout the Milky Way. What wonders still await us, as we peer even more deeply into our nearby Universe?

• Astronomers have discovered another exoplanet using astrometry! This method uses extremely accurate measurements of a star’s position in the sky. If an exoplanet is there, and it’s massive enough, as it orbits the star its gravity tugs on the star, and star makes a little circle as well. Over time, this motion can be measured with astrometric observatories like Gaia. Astronomers looked at Gaia observations of the nearby young Sun-like star AF Leporis and found it moved in this way, so they then turned to the Keck telescope, and saw the planet near the star in the images! [link to research paper] This also makes AF Leporis b one of the few that have been directly imaged, too. It has a mass about three times that of Jupiter, making it one of the least massive planets actually seen in images, too.

• Where does the Sun’s magnetic field originate? The usual thought is that it begins deep inside the Sun, over 200,000 km deep (the Sun’s radius is ~700,000 km). However, a new idea based on simulations of the Sun’s interior indicates it may be much shallower: Just 30,000 km down [link to journal paper]. There’s a known motion of ionized gas (called plasma) inside the Sun that has an 11-year cycle, and it’s been thought to be tied to the Sun’s 11-year magnetic cycle, but it’s not been clear where this oscillation happens. The new sims indicate the shallower origin fits observations better. That’s a pretty big change to previous thinking, and if true will have a big impact on how the Sun is modeled, and our understanding of what powers the magnetic cycle. This same cycle is what causes solar storms, like the powerful ones in the spring of 2024 that created aurorae seen all over the world.

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