Earth-Sized Planet Discovered in Star System Closest to Our Own

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Published on: October 17, 2012

Following in the heels of the discovery of an extraordinary star system with a planet orbiting a quaternary star, comes the discovery of a near twin of Earth orbiting a star in the trinary star system of Alpha Centauri, the closest system to our own, at a distance of just 4.3 light years away.  The lightest planet, outside the Sol System, yet discovered around a Sun-like star, orbits Alpha Centauri B, a type K1 V star, slightly smaller and redder than our star, Sol.
This artist's concept shows the newfound alien planet Alpha Centauri Bb, found in a three-star system just 4.3 light-years from Earth.

“This result represents a major step towards the detection of Earth twins in the immediate vicinity of the Sun.” the team that discovered teh planet in the Alpha Centauri system wrote in their paper.

“This is the first planet with a mass similar to Earth ever found around a star like the Sun. Its orbit is very close to its star and it must be much too hot for life as we know it,” said Stéphane Udry from the Geneva Observatory, a co-author of the paper that will be published in Nature on Oct. 17, and member of the team that used the HARPS instrument to find the planet. “But it may well be just one planet in a system of several. Our other HARPS results, and new findings from Kepler, both show clearly that the majority of low-mass planets are found in such systems.”

The planet is called Alpha Centauri Bb and it whips around its star every 3.2 days, orbiting at a distance of just 6 million kilometers (3.6 million miles), closer than Mercury’s orbit around the Sun. By comparison, Earth orbits at a comfortable 150 million kilometers (93 million miles) from the Sun.  Given its distance from its parent star it is likely very hot, at around 1,227§ Celsius (2,240§ Fahrenheit) and covered with molten rock, the researchers say.

“At this temperature, there is a lot of chance that the surface — if it’s made of rock, for example — it’s not solid, but it’s more like lava,” Dumusque told reporters Tuesday.

The Alpha Centauri system has long thought by many astronomers to be a perfect candidate to host Earth-sized worlds. In 2008 a team of astronomers ran computer simulations of the system’s first 200 million years, and in each instance, despite different parameters, multiple terrestrial planets formed around the star. In every case, at least one planet turned up similar in size to the Earth, and in many cases this planet fell within the star’s habitable zone.

But while astronomers have looked for years, previous searches of planets in the Alpha Centauri system came up empty.

Until now.

“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,” says Xavier Dumusque (Geneva Observatory, Switzerland and Centro de Astrofisica da Universidade do Porto, Portugal), lead author of the paper. “It’s an extraordinary discovery and it has pushed our technique to the limit!”

HARPS is part of the European Southern Observatory’s 11.8-foot (3.6 meters) telescope at the La Silla Observatory in Chile. The instrument allows astronomers to pick up the tiny gravitational wobbles an orbiting planet induces in its parent star.

The European team detected the planet by using the radial velocity method — by picking up the tiny wobbles in the motion of the star Alpha Centauri B created by the gravitational pull of the orbiting planet. The effect is extremely small, as it causes the star to move back and forth by no more than 51 centimeters per second (1.8 km/hour). The team said this is the highest precision ever achieved using this method.

This wide-field view of the sky around the bright star Alpha Centauri was created from photographic images forming part of the Digitized Sky Survey 2. The star appears so big just because of the scattering of light by the telescope's optics as well as in the photographic emulsion. Alpha Centauri is the closest star system to the Solar System.
Credit: ESO

The detection, to be published Wednesday in the journal Nature, was so difficult that some astronomers aren’t yet convinced that Alpha Centauri Bb exists.

For example, Artie Hatzes of the Thuringian State Observatory in Germany lauded the discoverers’ technical achievement but said he believes the jury is still out.

“As the American astronomer Carl Sagan once said, ‘Extraordinary claims require extraordinary evidence,'” Hatzes wrote in a commentary piece in the same issue of Nature. “Although a planetlike signal is present in the data, the discov­ery does not quite provide the ‘extraordinary evidence.’ It is a weak signal in the presence of a larger, more complicated signal. In my opin­ion, the matter is still open to debate.”

Udry, however, said that the team’s statistical analyses show a “false alarm probability” of just one in 1,000 — meaning there’s a 99.9 percent chance that the planet exists.

And some experts don’t agree with Hatzes that Alpha Centauri Bb requires extraordinary supporting evidence.

“The reason why this seems to be an extraordinary claim is because everyone has heard of Alpha Centauri B; it’s a household name,” said Greg Laughlin of the University of California, Santa Cruz, who was not part of the discovery team. “It’s extraordinary not so much in terms of the robustness of the result, but rather just in terms of the fact that’s it’s a well-known nearby star.”

Even though it resides in a three-star system — consisting of close-orbiting Alpha Centauri A and Alpha Centauri B, along with the more distant Proxima Centauri — the newfound world’s orbit is stable over the long haul, Laughlin said. So are orbits in Alpha Centauri B’s habitable zone, he added.

It’s possible that Alpha Centauri A and Proxima Centauri may host planets as well, Udry said. The system will likely be the subject of newly intense scientific scrutiny, as astronomers seek to confirm the existence of Alpha Centauri Bb, learn more about it (such as whether or not it has an atmosphere) and hunt for additional nearby alien worlds.

“If you want to envision exploring this system, then it’s almost twice as easy to get there as anywhere else,” Laughlin said. “This is our backyard, and to find out that planet formation did occur there is just extraordinarily exciting.”

Alpha Centauri is one of the brightest stars in the southern skies and is actually a triple star — a system consisting of two stars similar to the Sun orbiting close to each other, designated Alpha Centauri A and B, and a more distant and faint red component known as Proxima Centauri.
A comparison of the relative sizes of our Sun and the stars in the Alpha Centauri system
Alpha Centauri B is very similar to the Sun but slightly smaller and less bright. The orbit of Alpha Centauri A is hundreds of times further away from the planet, but it would still be a very brilliant object in the planet’s skies.

Read the team’s paper


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