Audiences flocked to see James Cameron’s  epic movie Avatar, the story of a disabled human who gets a chance to live a new life as a 3m tall blue-skinned humanoid alien from a world called Pandora. In the movie Pandora is a lush jungle-covered moon of a gas giant planet called Polythemis in the Alpha Centauri system just 4.36 light years (1.3 parsecs) from our own Solar System. The Centauri system is of course, real and comprises our Sun’s nearest stellar neighbours, while Polythemis and Pandora are fiction. Theses fantasies are suddenly more plausible as astronomers have announced the discovery of a roughly Earth-sized planet in the Alpha Centauri system.  Alas the planet is a scorched cinder of a planet, more hostile even than our Solar System’s Mercury, but there may be more planets to be found in our closest neighbouring star system.


image of centauri system

This artist’s impression shows the planet orbiting the star Alpha Centauri B. Alpha Centauri B is the most brilliant object in the sky and the other dazzling object is Alpha Centauri A. Our own Sun is visible to the upper right.(Image credit:ESO/L.Calçada/N.Risinger (


(UPDATE: by November 2015 it looked as though this reported planet was not real after all, see Ghost in the time series: no planet for Alpha Cen B by Vinesh Rajpaul, Suzanne Aigrain and Stephen J. Roberts.)

The newly discovered planet orbits about six million km from the star Alpha Centauri B, much closer than Mercury is to the Sun in our Solar System.The planet has a mass of a little more than that of the Earth making it one of the smallest exoplanets known.Without a doubt it is a rocky planet, with a barren and cratered surface, on its sunlit side the surface  may reach a temperature of about 1200C.Exoplanets are not given names as such so the planet is known as Alpha Centauri Bb.It is the closest known exoplanet to Earth, the previous record holder is the Jupiter-like Epsilon Eridani a which is about ten light years away.

The new planet was detected using the HARPS instrument on the 3.6m telescope at the European Southern Observatory’s La Silla Observatory in Chile.HARPS measures the movements of a star with extraordinary precision.The European astronomers did not actually see the planet, rather they detected its presence by observations of the tiny regular oscillations in the motion of the star Alpha Centauri B created by the gravitational pull of the orbiting planet .This effect is minute and finding the planet taxed the HARPS instrument to the limit.The discovery was officially announced on 17 October 2012.

The constellation of Centarus the centaur lies too far south on the celestial sphere to be visible from the UK and Ireland.In ancient Babylon this constellation was seen as a great bull, but classical times it was associated with Chiron, the wise centaur who tutored Achilles and Hercules.Later, Hercules shot Chiron with a poisoned arrow while in the midst of one of his drunken binges.The god, Zeus, in an atypically kindly act placed the agonized centaur into the sky where his suffering would cease. Sometimes called Rigil Kentaurus (Foot of the Centaur), Alpha Centauri appears as the brightest star in the constellation, and at magnitude -0.01 is fifth brightest star seen from Earth.

Alpha Centauri is in fact not a single star but a system of three stars.The primary, Alpha Centauri A, is a yellow G2V star amazingly similar to our Sun.Its secondary, Alpha Centauri B, is a slightly smaller and cooler K1 star and is the parent star of the newfound planet.Both stars are calculated to be somewhat older than the Sun, having existed for 4.85 billion years (some sources quotes figures as high as 8 billion years) compared the Sun’s 4.6 billion.Centauri A is about 10% larger than the Sun and half as bright again.The last and definitely least of the trio is the tiny M5 red dwarf Proxima Centauri.

Until now it was unclear if there could there be planets orbiting the stars of Alpha Centauri. Astronomers have long debated whether planets can exist in stable orbits around multiple star systems.Some doubted that planets cannot exist at all in such systems.The  argument went that the gravitational tug of additional stars would prevent planets from maintaining relatively circular orbits around their parent star, or even in the distant past have prevented material from accreting together to form worlds.If this had been correct, multiple star systems would have no planets, but presumable awesome belts of asteroids.In recent years we have discovered numerous planets in multi-star systems, so it was clear that there could indeed be planets in the Centauri system.While the jury was still out, some astronomers had carried out elaborate simulations suggesting planets ought to be able to exist in such star systems.In 1997 one of these simulations of the Centauri system, showed that there was a stable zone some 3 AU from Centauri A and Centauri B. If correct, what does this mean?


image of sun from centauri system

From the Alpha Centauri system, the Sun (left) would appear as an extra star in Cassiopeia.(Image credit:Colin Johnston/Armagh Planetarium)


Our planet, Earth, orbits 1 AU (astronomical unit, about 150 million km or 93 million miles) from the Sun.To enjoy an Earth-like climate a planet orbiting Centauri A, which is a little brighter and hotter than the Sun, would need to orbit about 20% further out, this would mean its year would be longer, 470 of our days or more.This would be inside the zone of stability.However if this hypothesis is correct, the Centauri system will have no equivalents of Jupiter and the other giant planets.In fact, a search by astronomers at the ESO in 2006 failed to find any super-jovian planets (that planets larger than Jupiter) in wide (100-300 AU) orbits of Alpha Centauri.

A terrestrial visitor to an inhabitable planet in the Alpha Centauri system would soon realize this was an alien world when the second sun rose in the sky.Centauri B takes almost 80 years to complete its eccentric orbit around its primary.This orbit takes it from as close as 11.2 AU to Centauri A to a distant 36 AU.Having two suns may suggest any hypothetical habitable planet would experience an arid climate but its distance and relative dimness means in fact that Centauri B’s warmth would be imperceptible on our hypothetical Earth-like world.However its light would dominate the night sky.The star would appear as a dazzling disc which would fluctuate in brightness and move across the sky as the star and planet moved in their orbits around the Centauri A. At its dimmest when furthest from the planet Centauri B would be more than 100 times as bright as the full Moon, at maximum it would be a stunning 14 000 times as bright as the full Moon.The planet would as the years went by at times see days with two suns and nights illuminated by a tiny distant sun, both utterly outside Terrestrial experience.

You have noticed that the third member of the trio has not been mentioned much.Proxima, like most of the stars in the Universe, is a small and dim M class red dwarf.It contains about an eighth of the amount of matter making up our Sun and its diameter is about 15% that of the Sun (so Proxima is just about half again as wide as Jupiter). This little star is so faint from Earth that it was only discovered in 1915. It is the closest member of the Centauri system to us, and is separated from its sisters by a distance, huge in human terms, of about 12 000 AU (about 0.2 light years).Its orbital period about the other two must be about half a million years! So unimpressive is Proxima, that it would be relatively hard to see from our hypothetical planet orbiting Centauri A; appearing as a fourth magnitude object in the night sky.As it is so feeble, Proxima is spending its nuclear fuel slowly, so it will outlive its sister stars.A few billion years from now, Centauri A and B will have expanded into red giants before collapsing into tiny white dwarfs, yet Proxima will look virtually the same as it does now.

Alpha Centauri is often thought of as the obvious first target when we come to explore the stars. Could we send a spacecraft there? Well we could, but we would need to be patient. The New Horizon probe, currently speeding across the Solar System to rendezvous with Pluto in 2015 before flying into the interstellar void is humanity’s fastest space vehicle to date. Its course will not take it in the direction of Alpha Centauri, but if it had been aimed there it would take about 63 000 years to span the gulf of four light years. In future centuries we may develop new advanced techniques such as nuclear pulse propulsion or beam powered lightsails which could cut this journey time down to decades. However, long before travel to Alpha Centauri is feasible, we know today that there is a planet to visit there. New telescopes and observatories currently on the drawing board could potentially detect large terrestrial worlds orbiting our neighbouring stars. Only slightly more advanced technology instruments could detect the atmospheres of such worlds. Were such an observation to detect an atmosphere on a Centaurian planet containing oxygen (a gas so reactive that it must be continuously regenerated by living organisms on Earth) it be almost certain that life exists there.  We would know that we have neighbours, whether they are blue humanoids or blue-green algae, just 4.3 light

years away.
Find out about the nearby Alpha Centauri star system and its newly-discovered Earth-size planet, in this infographic.
Source All about our solar system, outer space and exploration
(Article by Colin Johnston, Science Communicator)


V.U. North · October 18, 2012 at 17:53

Alpha Centauri is visible much further north than you say in your article.

You can’t see it anywhere in Europe, not even in Southern Europe, but it can be seen in the northern hemisphere, south of 29 degrees north, low on the horizon during the Spring and Summer.

The Medieval Muslim astronomers in Arabia could see it and that’s why its name in English is a corruption of the original Arabian name-

رجل القنطورس Rijl Qanṭūris, from the phrase Ar-Rijl al-Qanṭūris “the foot of the Centaur.

South of 29 degrees south Alpha Centauri never sets in the sky and is visible all year round.

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