Nearby alien star hosts what could be the oldest potentially-habitable exoplanet ever found

Credit: Planetary Habitability Laboratory/University of Puerto Rico at Arecibo

Wednesday, June 4, 2014, 12:47 AM - There is an alien in our midst and it is not alone. Kapteyn's Star, a red dwarf star right in our interstellar neighbourhood, is not from here. It is an ancient visitor to our galaxy, and orbiting around it are two planets, one of which has now been identified as the oldest potentially-habitable planet we've found so far.

Discovered back in 1898, Kapteyn's Star was already remarkable at the time because, relative to our solar system, it was the fastest-moving star ever found. It was overtaken by Bernard's Star less than two decades later, but in the years since, it has found several ways of distinguishing itself from the rest of the billions of red dwarfs in our galaxy.

For one, it's something of a rebel, orbiting the galactic centre in the opposite direction to the vast majority of the objects in the Milky Way. It also runs with a pack, as there are several other stars travelling along with it, known (somewhat appropriately) as the Kapteyn moving group. Also, it is one of the oldest stars we know of. Red dwarf stars are long-lived as a rule, but Kapteyn's Star is so stable and steady-burning that it's been difficult for astronomers to pin an upper limit on its age. The current estimates flag it as around 11.5 billion years old. To put that in relative terms, the Universe itself is only a little over 2 billion years older than that, at 13.7 billion years old.

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Based on all this, as well as the chemical composition of the star, it is believed that Kapteyn's Star is an alien visitor to our galaxy, arrived here along with billions of others as the Milky Way swallowed up an ancient dwarf galaxy (the remnant of which might be the globular cluster known as Omega Centauri). This video animation, produced by Victor Robles, James Bullock and Miguel Rocha from the University of California – Irvine amd Joel Primack from the University of California - Santa Cruz, shows how this dwarf galaxy was slowly digested by the Milky Way over hundreds of millions of years.

Now only 13 light years away from us, this cool, dim red dwarf star, which is less than one-third the size and mass of our Sun, has been an object of scrutiny by astronomers, and recent observations have revealed an amazing discovery - it plays host to two super-Earth planets!

An international team of astronomers, using the specialized instruments on three different observatories (two in Chile and one in Hawaii), were the ones to find these worlds. Observing Kapteyn's Star as it speeds through our galaxy, they detected tiny perturbations in the star's motion, caused as the gravity exerted on the star by its planetary companions pulled it back and forth. Using this data, they were able to identify the mass of the two worlds and how long it takes for them to make one orbit.

Kapteyn b, the closer in of the two planets, is roughly 5 times as massive as Earth, and it circles the star once every 45 days. Even though it is much closer to its star than Mercury is to our Sun, Kapteyn b orbits within the habitable zone - the region of space around the star where conditions are just right (not too hot or too cold) for liquid water to exist on a planet's surface. The second planet, Kapteyn c, is even bigger, and on an orbit beyond the habitable zone, taking 121 days to circle the star.

"Finding a stable planetary system with a potentially habitable planet orbiting one of the very nearest stars in the sky is mind blowing. This is one more piece of evidence that nearly all stars have planets, and that potentially habitable planets in our Galaxy are as common as grains of sand on a beach," said Pamela Arriagada, one of the study's authors from the Carnegie Institution for Science, according to a press release.

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Credit: Planetary Habitability Laboratory/University of Puerto Rico at Arecibo

Although the orbit of Kapteyn b  (shown to the right) is more elliptical than Earth's orbit, the fact that it stays within the star's habitable zone for its entire orbit brings up some intriguing possibilities - specifically the potential for there to be liquid water on the planet's surface, and also for life to have developed there.

Although there have been several times throughout Earth's history that life has had to nearly start over from scratch, it's had over 3.5 billion years to develop into the abundance that we see around us. Our own species has had the last 200,000 years to develop to what we are today. However, Kapteyn b has around 8 billion years on us, so if life actually developed there, what would it have developed into, and if intelligent life developed, how much more advanced would they be over us by now?

It will take some time for researchers to examine the planets of this system, to see if Kapteyn b really is capable of supporting life. The upcoming James Webb Space Telescope might deliver this information to us when it goes active in 2018, but since Kapteyn's Star is only 13 light years away, that's close enough that other, ground-based telescopes may be the ones that crack the case.

A fun fact about Kapteyn's Star is that it has already been featured in several works of fiction as either having planets, being home to sentient beings, and even hosting a "billion-year-old extragalactic super-civilization." A particularly fun short story, Sad Kapteyn, by Alastair Reynolds, tells the tale of an intelligent probe sent by humanity to investigate the system for us. Check it out here.

(H/T to Prof. Abel Méndez of the Planetary Habitability Laboratory for info on Kapteyn's Star and for the link to Sad Kapteyn)