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Planet found in nearest star system to Earth

Charles Black, Founder and CEO of Sen
Oct 17, 2012, 7:00 UTC

Sen—A planet with a mass similar to Earth has been detected in our nearest star system, Alpha Centauri. Although the planet has about the same mass as Earth there is no chance of life as it orbits too close to its star. The discovery was made using a telescope in Chile operated by the European Southern Observatory (ESO).

Alpha Centauri is actually a 'triple' star. Two of the stars, similar in size to the Sun, orbit close to each other and are designated Alpha Centauri A and B. A is the brightest of the three. The third star, known as Proxima Centauri, is the closest star to Earth. Alpha Centauri is the nearest star system to our Solar System - only 4.3 light years away.

The newly discovered planet orbits Alpha Centauri B at a distance of just 6 million kilometres, much closer than Mercury to the Sun. This means the planet is extremely hot and outside the "habitable zone" - the area where the temperature is just right for liquid water to exist and thus for life to flourish. The planet wizzes around its star once every 3.2 days.

The planet was discovered by astronomers using ESO's HARPS (High Accuracy Radial velocity Planet Searcher) instrument on its 3.6-metre telescope at the La Silla Observatory in Chile. The new world was detected from the tiny wobble of the star caused by the gravitational pull of the orbiting planet. The gravitational effect of the planet's orbit is extremely small, causing the star to wobble by no more than 51cm per second (1.8km per hour).

The findings are published in the journal NatureThe paper's lead author, Xavier Dumusque, said: "Our observations extended over more than four years using the HARPS instrument and have revealed a tiny, but real, signal from a planet orbiting Alpha Centauri B every 3.2 days. It’s an extraordinary discovery and it has pushed our technique to the limit! This result represents a major step towards the detection of a twin Earth in the immediate vicinity of the Sun. We live in exciting times!"

Professor Isabelle Baraffe, Head of the Astrophysics group, CEMPS at the University of Exeter said: "This is an absolutely fantastic result. The Holy Grail is to find the twin of the earth which means having a planet with the mass of the earth orbiting a sun and in a habitatble zone which would favour the presence of liquid water and therefore we think would favour the presence of life. The discovery of this planet is the first step towards this Holy Grail. Of course this is not in the habitable zone as it's very close to its star so the conditions are burning at the surface of the planet. It does tell us, however, that the Alpha Centauri system has other planets. As it is unthinkable that there would be one planet in isolation, this star is a gold mine for the detection of planets. What is great is that because the star is so close and so bright, it will be possible to make follow up observations."

Since 1995 over 800 planets have been discovered orbiting distant suns - but scientists are excited to find a planet in our own backyard. The discovery supports previous findings and research on the abundance of planets in our galaxy and the Universe. As Professor Don Polloco from the University of Warwick observed: "This discovery shows that, if we look hard enough, we're likely to find planets everywhere."

ESO illusatration of exoplanet in the Alpha Centauri star system

Artist's illustration shows the planet orbiting the star Alpha Centauri B, a member of the triple star system that is the closest to Earth. Credit: ESO/L. Calçada/N. Risinger

There are several techniques for detecting planets outside our Solar System. Using the gravitational wobble has enabled scientists to work out the mass of the planet, but not its diameter, which is found using the transit method of planet detection. Just as Venus and Mercury can sometimes be seen passing in front of the Sun’s disc, exoplanets also pass in front of their parent stars and as they do so there is a dip in the light from the star. NASA's Kepler mission is using the transit method to discover exoplanets.

Andrew Cameron, Professor of Astronomy at the University of St Andrews said: “This is yet another demonstration of the exquisite capability of the HARPS and HARPS-North spectrographs to measure the masses of very small planets orbiting distant stars. In the future, it will be important to combine measurements of this kind with the transit method, so we can measure their diameters as well as their masses. If we can determine their bulk composition, we will learn something about their formation history, water content and atmospheres”.

Suzanne Aigrain from the University of Oxford said: "This is a very exciting result, because the star is so nearby and the planetary signal is so small. It represents a major step forwards in understanding and mitigating the effects of stellar activity, which will be crucial for the detection of increasingly Earth-like planets."

The La Silla Observatory is one of three major sites managed by ESO in Chile, the other two being Paranal, which is home to the Very Large Telescope (VLT) and Chajnantor where ALMA is producing cutting-edge observations of the Universe.