Closest-Ever Exoplanet Discovered In The Alpha Centauri Star System

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Lawrence LeBlond for redOrbit.com – Your Universe Online

Astronomers have discovered a planet similar in mass to the Earth orbiting a star in the Alpha Centauri system — the nearest to Earth. Surpisingly, according to the astronomers, this planet which is just 4 light years away is also the lightest ever discovered around a star like our Sun.

While the planet itself may be more similar in size and mass than Venus is to Earth, that’s where the good news ends. The planet, a resident of the Alpha Centauri system, is orbiting a host star much closer than Mercury’s orbit around the Sun, meaning it is too hot to support life as we know it. Despite this, the discovery is significant, and offers hope that other similar planets could be out there, some possibly even circling the same star, that are just right for life–what astronomers refer to as the ‘Goldilocks Zone‘.

Planets in the Goldilocks Zone are Earth-like planets that are not too hot or too cold to support life as we know it. Astronomers are quick to point out that a star system with one planet is likely to have several more, giving researchers more reason to believe a Goldilocks planet could be sleeping in an Alpha Centauri bed that’s just right.

The European team of astronomers, led by Xavier Dumusque, of the Geneva Observatory in Switzerland and Centro de Astrofisica da Universidade do Porto, Portugal, said the planet has a similar mass of the Earth and is also the lightest exoplanet ever discovered around a star like that of our Sun. They unearthed the planet using the HARPS instrument on the 3.6-meter telescope at European Southern Observatory’s (ESO) La Silla Observatory in Chile. A report on the findings will appear in the journal Nature today.

Alpha Centauri, the planet’s host system, is one of the brightest stars in the southern skies and is the nearest star system to our very own Solar System–just 4.3 light years away. There are actually three stars in the Alpha Centauri system–two stars similar in size to our Sun orbiting close to each other, named Alpha Centauri A and B, and a third more distant faint red star known as Proxima Centauri.

Astronomers have been speculating and hypothesizing about the possibility of planets orbiting this star system for centuries. But, previous stellar studies have never been able to find the proverbial needle in the haystack–that is–until now.

What delayed the discovery for so long is the fact that Alpha Centauri is a very complicated system. The authors explain that, because the stars are orbiting so close to one another, the effect of a comparatively tiny planet is very difficult to detect, even for some of the most precise planet hunting instruments available.

But astronomers have finally been able to uncover such a planet in this star system, one that orbits Alpha Centauri B, and does so in a very tight orbit of just 3.6 days. It also has a surface temperature approaching 2,200 degrees Fahrenheit, making it far too hot for life to thrive. However, astronomers speculate that the planet is “tidally locked,” meaning the same side always faces its host star; further investigations will be required to prove this, but the team thinks its possible the planet could have an atmosphere.

Other than the planet’s close proximity to Earth, astronomers say it is rather unremarkable. It is only one of some 840 confirmed exoplanets that have been discovered since the early 1990s, and, like the rest, hasn’t offered a plausible life-bearing scenario.

Still, this star system is “a very special case,” remarked Stephane Udry of the Geneva Observatory, the senior author on the paper. “It’s our next door neighbor.”

“So even if the discovery just stands perfectly normally in the discoveries we have had up to now, it’s a landmark discovery, because it’s very low-mass and it’s our closest neighbor,” he added.

Marek Kukula, Public Astronomer at the Royal Observatory Greenwich, said the very existence of this planet makes for a tantalizing scenario.

“Everything that we’ve discovered in the last few years tells us that where we find one small, rocky planet there are likely to be others,” Kukula told Jason Palmer at BBC News. “I think the odds are very good that there may well be other planets in this system a little further out, perhaps a little more comfortable temperatures – so I think the hunt is on.”

This “may well be just one planet in a system of several,” Udry also speculated, adding that, with further results from HARPS and new observations from the Kepler Space Telescope, there exists a high likelihood that more low-mass planets could be found in Alpha Centauri

The team detected the planet by picking up tiny wobbles in the motion of Alpha Centauri B created by the gravitational pull of the planet. The effect, however, is so minute, that picking up on it was likely a miracle. The pull causes the star to wobble back and forth by no more than 1.1 miles per hour, or about the speed of a baby crawling. This was the highest precision ever achieved in planet hunting, the team said.

Now astronomers, who have been awe-struck by the discovery, are scheming up missions that could potentially be geared toward studying the star system closer. Some have even gone as far as pressuring NASA and the ESA to come up with missions to send satellites and probes to the system, which, although relatively close in astronomical terms, is some 25 trillion miles away.

To put such a mission into perspective, one first has to look at other similar projects. NASA’s Voyager 1, for example, has traveled some 11 billion miles since its launch in 1977. Even at speeds of 35,000 mph, it would take that probe another 3,000 years to travel just 1 trillion miles, and more than 72,000 years before it could even reach Alpha Centauri’s doorstep.

Geoff Marcy, an astronomer at the University of California, Berkeley, who leads another ground-based search effort, said that the discovery should at least strengthen the needs for a space-based telescope that could track down planets even better than the ones we have now, namely Kepler. Such a telescope could also glean clues about a planet’s atmosphere, if it so has one.

Currently, Kepler is the number 1 exoplanet hunter in space. It has so far detected more planets in its short lifespan than all other observatories have combined. Kepler’s main focus is hunting down planets in the Milky Way that are within the habitable zone of their host stars. Of the more than 2,000 exoplanet candidates picked up by Kepler, about 207 are similar in size to Earth.

While a space-based planet imager is still largely a dream, one planetary scientist has received funding to realize just that.

Ralph McNutt, of Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, has received NASA study funding to design an “innovative interstellar explorer.” Once designed, built, and launched–atop the most powerful rockets technology can offer–such a probe would still take 28,000 years to reach Alpha Centauri.

While it is likely to be many lifetimes before anyone can get a good clear picture of what the yet unnamed planet and its environment are like, it doesn’t hurt to dream.

Dumusque, who dares to dream, described what it might be like on this Earth-like planet.

“Its closest star is so near that it would always hang huge in the sky. And whichever side of the planet faced the star would be broiling hot, with the other side icy cold. Because of the mass of the planet, it’s likely a rocky surface like Earth. But the rocks would be more like lava, like a lava planet,” said Dumusque, as cited by Seth Borenstein of the Associated Press.

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