Discovery: Even Tiny Stars Have Planets

By SPACE.com staff

posted: 28 May 2009
02:01 pm ET

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.