A cosmic sprinter on a short track: that's how you might describe planet GJ 367 b. It whips around its star in just 7.7 hours. This world is slightly smaller than Earth but has a personality: it was once thought to be nearly a solid ball of iron.

New observations from the TESS telescope and the ESPRESSO instrument on the Very Large Telescope (VLT) have refined the planet's portrait. It turns out its radius is 0.736 times that of Earth, and its mass is 0.503 times Earth's. That gives a density of 6.9 grams per cubic centimeter. For comparison, Earth's density is 5.5, and pure iron is about 7.9. The planet isn't an iron record-holder; it's more of an average Joe.

Scientists built a model of its internal structure, accounting for tidal distortions. They found that the planet's iron fraction is between 50 and 70%. That's close to Mercury, whose huge core makes up about 60% of its mass. The mystery of the "iron planet" is solved: it doesn't violate the laws of planet formation; it's just similar to a neighbor in our solar system.