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Hot Rocks

February 20, 2022

Here on Earth, a birthday comes once every 365 days — once a year. On planets in some other star systems, though, the birthdays would be fast and furious, because a year on these planets lasts less than one Earth day.

Astronomers have discovered several of these “ultra-short-period” planets. None of the planets is more than a few million miles from its star. Most of them probably are “locked” so the same hemisphere always faces their star, just as the same hemisphere of the Moon always faces Earth.

One example is Gliese 367b. It was discovered by a space telescope in 2019, and astronomers reported its details late last year.

The planet orbits a small, cool star once every eight hours. It’s about two and a half times as far from the star as the Moon is from Earth. The system is 30 light-years away, in Vela, the sail, which just peeks above the southern horizon in late evening.

Gliese 367b is smaller than Earth, and about half as massive. That makes it almost as dense as iron. In fact, most of the planet probably is iron, wrapped in a thin layer of rock. Radiation and winds from the nearby star may have stripped away most of its outer layers. Or perhaps they were blasted away by collisions with other planets.

Gliese 367b is so close to its star that its surface may be molten. If so, then gases from the hot rock may form a thin atmosphere — a blanket of iron-rich air for a world where the birthdays never stop.

Script by Damond Benningfield

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