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Earth-sized planet found around star next door to sun

Posted: October 16, 2012

Our sun's neighbor harbors a blistering world about the size of Earth, astronomers announced Wednesday, raising hopes a life-supporting planet could be found close to home.

This artist's impression shows the planet orbiting the star Alpha Centauri B, a member of the triple star system that is the closest to Earth. 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. Credit: ESO/L. Calcada
The planet orbits a star in the Alpha Centauri system, a stellar triplet about 4.3 light-years from the sun.

Using a European Southern Observatory facility in Chile, astronomers discovered the planet by detecting tiny wobbles in the motion of the star Alpha Centauri B, which is similar to the sun but slightly smaller and less bright, according to scientists.

The oscillations in the motion of of Alpha Centauri B are caused by the tug of gravity from the newly-discovered planet, which is named Alpha Centauri Bb.

The planet has a mass slightly more than that of the Earth, and it whips around its parent star every 3.2 days at a distance of less than 4 million miles.

Researchers say the planet's surface is heated up to 2,200 degrees Fahrenheit, and its molten surface is too hot to sustain life or liquid water.

The discovery leads some scientists to wonder if Alpha Centauri could harbor a planet like Earth in the habitable zone, the cosmic sweet spot where scientists believe conditions could support life. Planets close to their parent star are too hot, causing water to evaporate and starving life of vital nutrients. Colder planets lie further away from the star.

Studies show star systems with small-mass planets often have more than one planet, meaning there may be undiscovered worlds lying farther from Alpha Centauri B or its larger companion - Alpha Centauri A.

"This result represents a major step towards the detection of a twin Earth in the immediate vicinity of the sun," said Xavier Dumusque of the Geneva Observatory in Switzerland and the Center for Astrophysics of the University of Porto in Portugal.

Dumusque is lead author of a paper on the discovery appearing in the Oct. 17 issue of the journal Nature.

The Alpha Centauri system also includes a third star named Proxima Centauri, which is smaller and cooler than the sun.

File photo of La Silla Observatory in Chile. Credit: ESO/Iztok Boncina
Astronomers use two principal methods for finding planets around other stars. The radial velocity method, used to detect Alpha Centauri Bb, relies on measuring the effect of a planet's gravity on the parent star. The transit method, employed by other telescopes including NASA's Kepler observatory in space, records dips in a star's brightness as a planet passes between the observer and the star.

The transit method allows astronomers to extract more information about an exoplanet's size and potential composition, but it only works if a planet's orbit is perfectly aligned with the observer. Only a small fraction of extrasolar planets can be detected by transit observations.

Data from the High Accuracy Radial Velocity Planet Searcher, or HARPS, spectrograph uncovered the planet in Alpha Centauri. HARPS is located at the 11.8-foot telescope at La Silla Observatory in Chile.

"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," Dumusque said. "It's an extraordinary discovery and it has pushed our technique to the limit."

The detection of a planet in the habitable zone around Alpha Centauri B would require a more sensitive instrument.

"This is the first planet with a mass similar to Earth ever found around a star like the sun," said Stephane Udry of the Geneva Observatory, co-author of the paper in Nature. "Its orbit is very close to its star and it must be much too hot for life as we know it, 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."