by Mike Wall
16 October 2012
Around Alpha Centauri B
This artist's concept shows the newfound alien planet Alpha Centauri
found in a three-star
system just 4.3 light-years from Earth.
CREDIT: ESO/L. Calçada
The star system closest to our own sun
hosts a planet with roughly Earth's mass and may harbor other alien
worlds as well, a new study reports.
Astronomers detected the alien planet around the sunlike star Alpha
Centauri B, which is part of a three-star system just 4.3
light-years away from us.
The newfound world is about as massive as
Earth, but it's no Earth twin; its heat-blasted surface may be
covered with molten rock, researchers said.
The mere existence of the planet, known as
Alpha Centauri Bb,
suggests that undiscovered worlds may lurk farther away from its
star - perhaps
in the habitable zone, that just-right range of
distances where liquid water can exist.
"Most of the low-mass planets are in
systems of two, three to six or seven planets, out to the
habitable zone," study co-author Stephane Udry, of the Geneva
Observatory, told reporters today (Oct. 16).
So the discovery,
"opens really good prospects for
detecting planets in the habitable zone in a system that is very
close to us," Udry added. "In that sense, this system
Alpha Centauri Bb zips around its star
every 3.2 days, orbiting at a distance of just 3.6 million miles (6
million kilometers). For comparison, Earth orbits about 93 million
miles, or 150 million km, from the sun.
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
as well as in the
photographic emulsion. Alpha Centauri
is the closest star
system to the Solar System.
Image released Oct.
CREDIT: ESO/Digitized Sky Survey 2
The research team, led by Xavier Dumusque of Geneva
Observatory and the University of Porto in Portugal, spotted Alpha
Centauri Bb using an instrument called the High Accuracy Radial
velocity Planet Searcher, or
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.
In the case of Alpha Centauri Bb, these wobbles are very tiny
indeed; the planet causes its star to move back and forth at no more
than 1.1 mph (1.8 kph).
It took more than 450 HARPS measurements
spread out over four years of observing to detect the planet's
signal, Dumusque said.
"It’s an extraordinary discovery,
and it has pushed our technique to the limit," he said in a
The detection, to be published tomorrow
(Oct. 17) 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
'Extraordinary claims require
extraordinary evidence,'" Hatzes wrote in a commentary piece
in the same issue of Nature.
"Although a planet-like 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
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
"It's extraordinary not so much in
terms of the robustness of the result, but rather just in terms
of the fact that it's a well-known nearby star."
A lava world?
Dumusque and his colleagues determined that Alpha Centauri Bb is
about 13 percent more massive than Earth, suggesting it's a rocky
In addition to being the closest known
exoplanet, it's also the first planet with a mass similar to Earth
ever found around a sunlike star, researchers said. (Alpha
Centauri Stars and Planet Explained)
Alpha Centauri Bb's extreme closeness to its parent star probably
gives the planet a surface temperature around 2,240 degrees
Fahrenheit (1,227 degrees Celsius), making it unsuitable for life,
"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
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
"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
Astronomers have now discovered more
exoplanets, but thousands more -
including 2,300 detected by NASA's planet-hunting Kepler Space
Telescope to date - await confirmation by follow-up investigations.
Work so far suggests that small, rocky
planets such as Earth are quite common throughout our Milky Way