Exoplanet Guide

Scienceline’s Guide to the Exoplanets: The Abyssal Watcher II

Kapetyn b: 100-billion-year-old ice is more than just a dream

July 27, 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 Abyssal Watcher II
Kapetyn b
Discovered by: High Accuracy Radial Velocity Planet Searcher
Discovered: 2014
Mass: ~4.8 Earth masses
Surface climate: Ancient
Habitability for humans: …Possibly

Just over twelve light-years from the Earth lies a very dim red dwarf called Kapetyn’s Star.

Kapetyn’s Star is eye-catching for a number of reasons, most of which have little to do with exoplanets in themselves. For one, it’s the closest known star that belongs not to the Milky Way’s disc, but to the halo: the bubble around it. Indeed, some astronomers believe that, along with a few of its neighbors, Kapetyn’s Star may have once formed outside of the Milky Way. Kapetyn’s Star may be an exile from an ancient dwarf galaxy that merged with ours.

It’s also interesting because, of all the stars presently known to host exoplanets, Kapetyn’s Star has the lowest metallicity.

To astronomers, every element heavier than helium is a metal; yes, that includes things like nitrogen and oxygen and neon. The vast majority of such elements didn’t exist in the early universe until they were formed, either as elements fuse within stars or in supernovae, when stars violently die. So, logically, as billions of years pass and as more stars live out their lives, the more heavy elements — the more metals — exist in the universe.

Therefore, looking at how many metals are in a star is one way of looking at a star’s age. And Kapetyn’s Star is old — as are the two known exoplanets around it. Not quite as old as at least one planet we’ve looked at, but still an estimated 11 billion years old: over twice as old as the Sun.

Kapetyn’s Star has two known exoplanets. It’s the inner of those two, Kapetyn b, that excites astronomers the most, and for a very straightforward reason: it lies within its star’s habitable zone.

When it was discovered, Kapetyn b was actually the closest known planet in its habitable zone. In cosmic terms, twelve light-years is virtually just over the road. But Kapetyn b has since been superseded by even closer planets — perhaps for the best, because it may not be as habitable as anyone quite hoped for.

Kapetyn b is in the habitable zone, yes, and it is a super-Earth: most likely terrestrial, and only a few Earth masses. But some scientists believe that its average temperature is around -68 degrees Celsius, or -91 degrees Fahrenheit. Unless the planet has atmosphere or something else to hold in heat, the surface couldn’t sustain liquid water; instead, it might be frozen over.

If 11 billion years seems very old, it’s barely the beginning. Kapetyn’s Star, burning through its hydrogen fuel much slower than the Sun ever could, has the potential to live for much longer: well into the hundreds of billions of years.

Kapetyn b might not even be out of the first tenth of its life.

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