The Closest Known Potentially Habitable Planet Is 13 Light-Years Away

Astronomers have discovered two new super-Earths orbiting an ancient 11.5 billion year-old star a "mere" 13 light-years from here. One planet is in the habitable zone, prompting a researcher to wonder what kind of life could have evolved over such a long period.

For comparison, these exoplanets are 2.5 times older than Earth and only two billion years younger than the universe itself, which is about 13.7 billion years-old. If there's life on one of these planets — and that's a big if — it's been there for a potentially very long time.

And amazingly, it's "only" 13 light-years away. That makes it the closest confirmed potentially habitable exoplanet to Earth, not including Tau Ceti e, an unconfirmed planet located 11.9 light-years away. The next best bet after that is Gliese 581-d, which is 20.2 light-years away. It's also worth noting that Alpha Centauri, the closest star to our own — just 4.3 light-years away — hosts a planet, but it's parked way to close to the sun to be habitable (its year is a mere three days long).

O Kapteyn! My Kapteyn!

So, a bit about this discovery. These planets orbit Kapteyn's Star, a halo red dwarf that was discovered at the end of the 19th century by Dutch astronomer Jacobus Kapteyn. It's the second fastest moving star in the sky and the 25th closest star to our solar system. With a magnitude of nine it can be seen through a telescope or with a pair of binoculars. It has a third of the mass of the sun and can be seen in the southern constellation of Pictor.

New data analyzed by astronomers at the Queen Mary School of Physics now shows that Kapteyn is not alone; it's orbited by at least two super-Earths, Kapteyn-b and Kapteyn-c. The astronomers were looking at data collected from the HARPS spectrometer at the ESO's La Silla observatory in Chile. The findings were corroborated by data from HIRES at Keck Observatory and PFS at Magellan/Las Campanas Observatory. The new planets were found using the Doppler Effect, which shifts the star's light spectrum depending on its velocity. This technique allows astronomers to determine several properties of extrasolar planets, including their masses and orbital periods.

A Temperate Super-Earth

Kapteyn-b has a mass that's nearly five times that of Earth's. It may be able to sustain liquid water at its surface; Kapteyn-b orbits every 48 days, which places it in the circumstellar habitable zone. That might sound close — and it is — but keep in mind that red dwarfs are not as powerful as G-type main sequence stars like our own. The astronomers, a team led by Guillem Anglada-Escude, say it's the oldest potentially habitable planet known to date. And by my calculations, it's the closest known and confirmed potentially habitable planet to Earth.

The other planet, Kapteyn-c, is less promising in terms of habitability. It's a massive super-Earth that's seven times heavier than Earth, requiring about 121 days to complete an orbit. Astronomers think it's too cold to support liquid water.

The atmospheric composition of the planets is not known.

Related: A planet so big they're calling it Godzilla | Red dwarfs may sterilize habitable planets

Strange Beginnings

Kapteyn's Star has an interesting history. It was born in a dwarf galaxy that was absorbed and disrupted by the ancient Milky Way. This galactic event put the star in a rapid halo orbit. The likely remnant core of the original dwarf galaxy is thought to be Omega Centauri — a globular cluster located about 16,000 light-years from Earth which contains hundreds of thousands of similarly old suns.

This video simulation shows the merging and formation of the characteristic tidal streams of stars resulting from such a galactic merging event.

The study is set to appear in an upcoming issue ofMonthly Notices of the Royal Astronomical Society, but you can read it here. Supplementary source: Queen Mary School of Physics and Astronomy.

The image is courtesy of Victor Robles, James Bullock, and Miguel Rocha at University of California Irvine and Joel Primack at University of California Santa Cruz.
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