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Oct. 18, 2012

Around a Stellar Neighbor, an Earth-Sized World

by Charles Bergquist

Click to enlarge images
The stellar neighborhood just got more crowded.  A team of European astronomers say they've detected what appears to be a planet about the same mass as Earth orbiting the star Alpha Centauri B, one of the closest neighboring stars.
 
Though the planet may be like ours in size, it lies much closer to its star, making a 'year' on the planet (still just named ‘Alpha Centauri Bb’)  last 3.2 Earth days. The closeness to the star also means that temperatures on the planet’s surface would likely be around 1,200 degrees Celsius (about 2,200 degrees Fahrenheit), the astronomers say. That would make it likely to have a lava surface too hot to support life as we know it. The find raises the possibility, however, that other Earth-sized planets could be found in more favorable conditions around neighboring stars.
 
The new report, by Xavier Dumusque of Switzerland's Geneva Observatory and colleagues, was published this week in the journal Nature.   Dumusque and colleagues detected the exoplanet by analyzing over four years worth of data  collected by the European Space Organization's HARPS (High Accuracy Radial velocity Planet Searcher) instrument, based at the ESO La Silla 3.6m telescope in Chile's Atacama desert.   The HARPS instrument looks for tiny shifts in the color of the light spectrum coming from a star as it is tugged back and forth by the gravity of an orbiting planet.  Other planet-hunting instruments, such as NASA's Kepler spacecraft, look instead for periodic dimming in the light from a star as a planet moves between the star and Earth.
 
The researchers said the Alpha Centauri system, long a favorite home solar system for the alien species of science-fiction authors, was 'a very special case' for planet hunters. "It's our next door neighbor," said Stephane Udry, one of the co-authors of the paper. "Even if the discovery just stands perfectly normally in the series of discoveries we've made up to now, it's a landmark discovery, because it's very low mass, and it's our closest neighbor," Udry said. He added the closeness of the star and its brightness might make it easier to do further observations of the system using future instruments.
 
"This result represents a major step towards the detection of Earth twins in the immediate vicinity of the Sun, " Dumusque said.  "It's an extraordinary discovery and it has pushed our technique to the limit."  He called the result a validation of the HARPS technique, saying that "the small amplitude of the signal shows that the radial-velocity technique is capable of reaching the precision needed to detect habitable super-Earth planets around stars similar to our Sun, or even habitable Earths around cooler stars."   
 
Not everyone is totally convinced of the find, however. Artie Hatzes, an astronomer at the Thuringian State Observatory in Tautenburg, Germany, writing in a commentary accompanying the paper in Nature, cautioned that a lot of complex signal filtering and data analysis had to be performed for the Geneva team to make their find. "It is a weak signal in the presence of a larger, more complicated signal. In my opinion, the matter is still open to debate," he wrote. "Other analytical tools, using alternative ways of filtering out the stellar variability, might arrive at different conclusions on the basis of the same data."   However Dumusque called the signal from the instrument "tiny, but real," saying that there is less than one chance in 10000 that the signal was false.
 
Astronomers have plans to look more at the system using a variety of instruments, including the planned James Webb Space Telescope. Calls for any exploratory mission to visit Alpha Centauri may have to wait, however, as the system is 'close' only in astronomical terms. It's over 4.3 light years from Earth, or 2.6 × 1013 miles away. The recently retired space shuttle, operating at around 17,500 miles per hour, would take about 165,000 years to get there using conventional rocket power. A spacecraft similar to Voyager 1 would require some 80,000 years to arrive -- meaning that some alternate propulsion system would need to be used to have a practical chance of paying Alpha Centauri a visit.
About Charles Bergquist

Charles reminds Ira when it's time to stop talking, helps wrangle this Web site, and produces segments for the radio program. His favorite stories involve chemistry, inventions, nanotechnology, and shiny things with blinking lights.

The views expressed are those of the author and are not necessarily those of Science Friday.

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