spaceSpace and Physics

Candidate for Closest Exoplanet Reported


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

1960 Candidate for Closest Exoplanet Reported
CfA/David Aguilar. One of the closest stars to the Sun appears to have a planet of similar mass to the one portrayed here around a similar star four times further away

A possible planet has been identified less than a dozen light-years from Earth. While its existence is yet to be confirmed, if it is real this may be the closest planet known outside the solar system. Moreover, a number of features would make this a particularly prize find among the nearly 2,000 so far announced

Gliese 15, also known as Groombridge 34, is a pair of red dwarf stars lying just 11.7 light-years away in Andromeda and visible with binoculars. The stars are 147 AU apart—almost five times the distance of Neptune from the Sun—leaving plenty of room for planets to orbit either star without being affected by the other.


The brighter of these two stars, known as Gl 15 A, has been reported in The Astrophysical Journal as being orbited by a planet around 5 Earth masses—almost certainly making it a “superearth,” a rocky planet with a mass larger than our own, rather than a gas giant like Neptune.

Before anyone gets too excited, the planet is almost certainly not habitable. With an orbital period of 11 days, even around such a dim star, the temperature is thought to be above 100°C. Moreover, Gl 15A is classified as a variable star—even if the average temperature was more comfortable, frequent flares would likely sterilize the surface. Still there is excitement at having a planet close enough to examine in detail. 

Suspected exoplanets have been reported around several of the nearest stars including Alpha Centauri B, pretty much as close as a planet outside the solar system can be. However, these remain unconfirmed, and the evidence is often considered less reliable than in this case. So it is at least possible that Gl 15Ab, as the new find has been dubbed, is the closest known planet outside the solar system. It is highly unlikely, however, to be the closest that exists. Gl 15 is the 16th closest star system to our own. Given what we are learning about how common planets are, it's a safe bet there are quite a few planets waiting to be found around the closer objects.

It might be expected that the first planets we would find are those around the nearest stars. However, early attempts at planet finding using the radial velocity method skipped over red dwarfs, and most of the more recent finds have depended on the planet's orbit intercepting our direct line to its star, excluding a large portion of planets near and far.


Gl 15Ab was identified by a team led by Dr. Andrew Howard of the University of Hawaii who are using the Keck Observatory to conduct a census of planets around stars within 80 light-years, including many neglected in previous studies. They're using radial velocity, watching for wobbles in the motion of the parent star caused by the gravity of its planet.

Planets in binary systems conjure up images of Tatooine's twin sunsets, but even if Gl 15Ab was suitable for life its inhabitants would barely know their star had a companion, with GL 15b being a quarter as bright as an Iridium flare.


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