Products You May Like
A computer illustration of HD 20794d.
Credit: NASA
We might take it for granted that Earth is habitable year-round. Researchers have discovered an Earth-like planet with such an eccentric orbit that it swoops in and out of its habitable zone throughout its year. Though the super-Earth could theoretically be friendly to life when it’s close to its star, the rest of its orbit passes through sub-freezing temperatures, where liquid water is no longer fair game.
That planet is called HD 20794d, and its mass is 6.6 times greater than Earth’s. Its name indicates that it’s the fourth planet to have been discovered orbiting HD 20794, a yellow dwarf that can be seen in the constellation Eridanus on a clear night. Technically, the “d” is a misnomer: Astronomers thought they found a planet orbiting HD 20794 back in 2011, but they were mistaken. The name HD 20794a now belongs to a planet that doesn’t actually exist, while exoplanets HD 20794b, HD 20794c, and HD 20794d actually circle their star.
Credit: Gabriel Pérez Díaz, SMM (IAC)
But HD 20794d doesn’t quite circle after all, because its orbit is an oval. This isn’t unusual—most planets’ orbits are sort of egg-shaped, or “eccentric”—but HD 20794d’s eccentricity is particularly elongated. That means its distance from HD 20794 varies: Sometimes the planet is just 0.75 astronomical units (69.7 million miles) from its star, and sometimes it’s as far as 2 AU (186 million miles) away. For part of its 647-day year, HD 20794d is flush with HD 20794’s radiation. At other times, it’s so devoid of its star’s warmth that its water, if it has any, would freeze solid.
That’s disappointing for anyone who might have hoped HD20794d could harbor life. But this strange super-Earth still has plenty of secrets to share. Because it’s much closer to Earth than most of the 5,000 other confirmed exoplanets, HD20794d is certain to offer scientists the types of research opportunities only a handful of fellow super-Earths could provide. Its periodic proximity to its star will also make it easier for astronomers to study its atmosphere.
“HD 20794 is not an ordinary star,” said Swiss astronomer and study co-author Xavier Dumusque. “Its luminosity and proximity makes it an ideal candidate for future telescopes whose mission will be to observe the atmospheres of exoplanets directly.”