Exoplanet Guide

Scienceline’s Guide to the Exoplanets: The Boiling Egg

K2-137 b: Where a year passes in an afternoon

May 25, 2020
A fake planet with "Scienceline's Guide to Exoplanets" labeled across its equator. Other planets zoom past behind it.
Your favorite planets, and you didn't even know they existed. [Credit: Curtis Segarra | CC BY-NC-ND 2.0]

The Boiling Egg
K2-137 b
Discovered by: K2
Discovered: 2017
Distance from Earth: ~310 light-years
Size: ~0.9 Earth radii
Surface climate: Probably bad
Habitability for humans: Probably worse

Imagine if Earth made a full orbit of the Sun — a full year — in the course of an afternoon.

In fact, we know of an exoplanet, not terribly unlike Earth in size and mass, that does exactly that: K2-137 b. With an orbital period of about 4 hours and 20 minutes, over half a decade on K2-137 b’s surface would pass in the course of a 24-hour Earth day.

It isn’t the shortest; there are known planets with even shorter years, as short as 49 minutes: orbiting pulsars, which are physically much smaller (and much denser) than main-sequence stars — even stars like the red dwarf that K2-137 b orbits. But K2-137 b is fascinating because, were it further away from its star, it might even be Earthlike, with liquid water.

As you might imagine, for K2-137 b to have that short of an orbit, it must be extremely close to its star. We’ve looked at many planets that tightly round their stars, but K2-137 b orbits at another level: 0.006 AU, or just over half a million miles, about twice the distance from the Earth to the Moon.

That might not seem that close; it’s not like the Moon is at particular risk of falling back down and crashing into Earth, let alone a Moon that’s twice as far away. But K2-137’s star isn’t just a large, picturesque red object in its sky. Remember that K2-137 b’s star, even if it’s a red dwarf and a fraction of the size of the Sun, is still 150,000 times more massive than Earth.

Even small stars like K2-137 b’s red dwarf will play havoc on planets at such a short distance, thanks to radiation and tidal forces. Some astronomers believe that an unconfirmed exoplanet, KOI 1843.03 — which, if it exists, is similarly close to its star, with a similarly four-odd-hour-long year — is literally being pulled into an oblong, rugby-ball-shaped object by its star’s gravity.

Scientists haven’t been able to observe K2-137 b quite yet, but it’s not inconceivable that K2-137 b might find itself in a similar situation. So if you’re looking to travel there to experience a year that lasts just over four hours, you might have to brave a planet that is being pulled apart into a shape that’s more egg than orb.

That’s not to mention the temperatures no doubt hot enough to turn the surface into a molten shell, or the fact that any planet so close to its star would be continually sterilized by bone-chilling deluges of ionizing radiation.

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