An artist's rendition shows an Earth-like planet, right, orbiting… (European Southern Observatory )
Scientists have spent years hunting for Earth-like planets orbiting stars elsewhere in the galaxy. Now they have found one that's a relative stone's-throw away.
The so-called exoplanet — a planet beyond our solar system — is circling Alpha Centauri B, just 4.37 light-years from Earth. A mere 3.7 million miles from the star's surface, it is far too hot to support life. But there's a chance that another planetary sibling may reside in the system's so-called habitable zone, where liquid water could exist, scientists say.
The discovery, described in Thursday's edition of the journal Nature, has further whet planetary scientists' appetite for finding nearby Earth-like planets — ones close enough for humans to potentially visit one day.
"It's the most reasonable star to think about going to, and now we know there's a planet around it," said Nick Gautier, deputy project scientist for NASA's planet-hunting Kepler space telescope, who wasn't involved in the discovery. "So that's pretty exciting."
Astronomers have confirmed the existence of more than 800 exoplanets orbiting sun-like stars since the first was discovered in 1995. Since its launch in 2009, Kepler has picked out more than 2,300 potential candidates by staring at roughly 150,000 stars and waiting for them to blink — a telltale sign that a planet is crossing in front of a star and casting a tiny shadow of sorts that the Kepler telescope can measure.
Many of these far-away planets are large, easy-to-spot gas giants, like Jupiter or Saturn. Scientists have to wait to see them transit across a star to detect them, because they're otherwise far too small and dim to be spotted with a telescope.
But planets do exert a tiny gravitational pull on the star, causing it to wobble. That wobble can be measured by tracking how much its motion squeezes and stretches the wavelengths of light that scientists see coming from the star. The relatively weak signal can be obscured by "noise," such as the light coming from a companion star or idiosyncrasies of the star's own behavior.
A European team of planetary scientists using the La Silla Observatory in Chile were measuring the wobble in the light coming from Alpha Centauri B between February 2008 and July 2011. The scientists hadn't even started out expecting to find a planet — they were looking to figure out how best to filter out all the "noise" when looking for planets. But after sifting through hundreds of measurements, the planet's signal emerged.
This new planet orbits so close to its star — it's 25 times closer to its star than the Earth is to the sun — that its effect on the star's wobble was easier to pick up than Earth's would be. That wobble was clocked at 51 centimeters per second, roughly five times stronger than the Earth's effect on the sun.
This planet is too close to its star for comfort — it takes just three days, five hours and 40 minutes to orbit its star, and its surface is roughly 2,200 degrees Fahrenheit, according to study leader Xavier Dumusque, a planetary scientist at the University of Geneva in Sauverny, Switzerland.
But at an estimated minimum of 1.13 Earth masses, it's probably a rocky body, researchers said. And since low-mass planets are thought to form in multi-planet systems, Gautier said, there might very well be another Earth-sized planet in the habitable zone.
Such a planet wouldn't even need to be as far out as Earth, he added — Alpha Centauri B is smaller and cooler than the sun, so a planet could perhaps be habitable in a tighter orbit than Earth's.