A
Jupiter-like planet has been discovered orbiting one of the smallest stars
known, suggesting that planets could be more common than previously thought.
This exoplanet
finding is the first discovery for a long-proposed tool for hunting
planets, called astrometry.
"This
is an exciting discovery because it shows that planets can be found around
extremely lightweight stars," said Wesley Traub, the chief scientist for
NASA's Exoplanet Exploration Program at Jet Propulsion Laboratory in Pasadena, Calif. "This is a hint that nature likes to form planets, even around stars
very different from the sun."
Long-awaited
success
Astrometry was
first attempted 50 years ago to search for planets outside our solar system, but
the method requires very precise measurements over long periods of time, and
until now, has failed to turn up any exoplanets.
The
technique involves measuring
the precise motions of a star on the sky as an unseen planet tugs the star
back and forth.
A team of
two astronomers from JPL has, for the past 12 years, been mounting an
astrometry instrument to a telescope at the Palomar Observatory near San Diego. After careful, intermittent observations of 30 stars, the team has identified a
new exoplanet around one of them.
"This
method is optimal for finding solar-system configurations like ours that might
harbor other Earths," said team member Steven Pravdo of JPL. "We
found a Jupiter-like planet at around the same relative place as our Jupiter,
only around a much smaller star. It's possible this star also has inner rocky
planets. And since more than seven out of 10 stars are small like this one,
this could mean planets are more common than we thought."
The
discovery will be detailed in the Astrophysical Journal.
The
newfound exoplanet, called VB 10b, is about 20 light-years away in the
constellation Aquila (a light-year is the distance that light travels in a
year, about 6 trillion miles or 10 trillion kilometers). It is a gas giant,
with a mass six times that of Jupiter, and an orbit far enough away from its
star to be labeled a "cold Jupiter" similar to our own.
In reality,
though, the planet's own internal heat would give it an Earth-like temperature.
The
planet's star, called VB 10, is tiny. It is what's known as an M-dwarf and is
only one-twelfth the mass of our sun, just barely big enough to fuse atoms at
its core and shine with starlight.
Mini
solar system
For years,
VB 10 was the smallest star known — now it has a new title: the smallest star
known to host a planet. In fact, though the star is more massive than the
newfound planet, the two bodies would have a similar girth.
Because the
star is so small, its planetary system would be a miniature, scaled-down
version of our own.
For
example, VB 10b, though considered a cold Jupiter, is located about as far from
its star as Mercury is from the sun. (In contrast, so-called "hot
Jupiters" orbit so close to their stars that their surfaces are
thought to be scorching.) Any rocky Earth-size planets that might happen to be
in the neighborhood would lie even closer in.
"Other
known exoplanets around larger M-dwarf stars are also similar to our Jupiter,
making the stars fertile ground for future Earth searches," said team
member Stuart Shaklan, also of JPL. "Astrometry is best suited to find
cold Jupiters around all kinds of stars, and thus to find more planetary
systems arranged like our home."
Tiny
movements
Two to six
times a year, for the past 12 years, Pravdo and Shaklan have bolted their
Stellar Planet Survey instrument onto Palomar's five-meter Hale telescope to
search for planets.
The
instrument, which has a 16 megapixel charge-coupled device, or CCD, can detect
very minute changes in the positions of stars. The VB 10b planet, for instance,
causes its star to wobble a small fraction of a degree. Detecting this wobble
is equivalent to measuring the width of a human hair from about 1.8 miles, or three
kilometers, away.
Other
ground-based planet-hunting techniques in wide use include radial velocity and
the transit method. These methods have discovered more than 300 extrasolar
planets to date.
Like
astrometry, radial velocity detects the wobble of a star, but it measures
Doppler shifts in the star's light caused by motion toward and away from us.
The transit
method looks for dips in a star's brightness as orbiting planets pass by and
block the light. NASA's space-based Kepler mission, which began searching for
planets on May 12, will use the transit method to look for Earth-like worlds
around stars similar to the sun.