Meet TOI-561b, the Rocky Planet Baking at 3,140°F

While everyone’s still digesting CES chip announcements, a study out this month pulled focus toward something a lot farther away: TOI-561b, a rocky exoplanet that makes Mercury look like a comfortable vacation spot.

Here’s the setup. TOI-561b orbits one of the oldest known stars in the Milky Way, which already makes it interesting — most rocky planets we’ve cataloged orbit relatively young stars, so this one’s a rare window into what planet formation looked like billions of years ago, back when the galaxy itself was still assembling its heavy elements. The planet is roughly three times the mass of Earth, putting it squarely in “super-Earth” territory, a class of world we don’t have a local example of to compare against.

But the headline number is the orbit: TOI-561b circles its star in under 12 hours. For context, Mercury — the tightest-orbiting planet in our own solar system — takes 88 days. An orbit that tight means the planet is sitting almost on top of its star, and the consequence is brutal: surface temperatures exceeding 3,140°F (1,726°C). That’s hot enough to keep rock in a molten or vaporized state on the dayside, assuming the planet is even tidally locked with a dayside/nightside distinction at all.

Why an old, hot planet matters

It’s tempting to file this under “extreme planet, cool headline” and move on, but the age angle is really the scientific payload here. Planet formation models are built mostly from what we can observe nearby and extrapolate backward. A rocky planet that formed around one of the galaxy’s oldest stars gives researchers an actual data point from that early era, rather than a simulation. If rocky, Earth-mass-ish planets could form and survive in that early galactic environment — with a different mix of heavy elements available than what our solar system had access to later — that tells us something about how common or rare Earth-like formation might be across cosmic time, not just cosmic space.

It’s also a solid reminder of how far exoplanet detection has come. We’re not just finding planets anymore — we’re getting mass estimates, orbital periods precise to fractions of a day, and temperature profiles, all from data collected by instruments watching for tiny dips in starlight. None of that existed as routine science a couple decades ago.

No word yet on atmospheric composition, and given the temperatures involved, “atmosphere” might be a strong word for whatever thin haze of vaporized rock and metal surrounds this thing, if anything sticks around at all. But between this and the black hole and multi-star system findings also making rounds this month, January’s shaping up to be a quietly strong month for people who like their science measured in stars rather than server racks.