A planet six times as massive as Jupiter (left) has been found around the red dwarf star VB 10 (right). The star is the lightest known to host a planet (Illustration: NASA/JPL-Caltech)

An extrasolar planet has been found by observing subtle changes in a star’s position in the sky for the first time. The technique, called astrometry, is best suited to finding planets at great distances from their stars, complementing more common techniques, which tend to turn up planets orbiting their stars at close range.

The planet’s star is also the lightest known to host a planet, and researchers hope other such discoveries will shed light on how common planets are around low-mass stars, which far outnumber their higher-mass cousins.

Most of the 350 or so known exoplanets have been discovered by the so-called radial velocity technique. Stars and planets actually orbit each other around their common centre of mass, and this method looks for telltale shifts in a star’s light spectrum that indicate the star is wobbling due to an orbiting exoplanet.

Another popular technique looks for periodic dips in a star’s brightness that occur when an orbiting planet passes in front of, or transits, its star.


Both of those techniques are most sensitive to planets that orbit their stars at close distances. That’s because massive, close-in planets cause the most dramatic changes in a star’s velocity, measured in the radial velocity technique, and planets on tight orbits transit their stars more often than do planets orbiting farther away, making detections easier to confirm.

Now, astronomers led by Steven Pravdo of NASA’s Jet Propulsion Laboratory have discovered the first planet found using the astrometry technique, which was first used to hunt for extrasolar planets 50 years ago. The method searches for tiny changes in the position of a star in the sky as it moves due to an orbiting exoplanet.

‘Like a teeter-totter’

The technique is sensitive to finding planets far away from their stars. “With astrometry, you can picture it like a teeter-totter,” says David Charbonneau, a planet hunter at the Harvard-Smithsonian Center for Astrophysics. “The star is very massive and the planet is low mass, so the way you get a big wobble is by putting a planet way out on the other end of the balance beam.”

“The star is going to execute a circle on the sky because it’s do-si-do-ing with an unseen planet,” he continues. “The size of that circle is bigger if the planet is farther out, so it’s easier to measure.”

Indeed, even though the newly found planet lies relatively far away from its star, measuring changes in the star’s position was difficult.

Several times a year for the past 12 years, Pravdo and JPL colleague Stuart Shaklan bolted an instrument with a 16 megapixel CCD onto the 5-metre Hale Telescope at California’s Palomar Observatory. After observing the positions of 30 stars over that time, they spotted tiny changes in the position of one star that showed it had a planet. The detection was equivalent to measuring the width of a human hair from a distance of about 3 kilometres.

Small and cool

The newly found planet, which weighs about six times as much as Jupiter, orbits a type of low-mass star called a red dwarf.

Dubbed VB 10, the star lies about 20 light years away from Earth and weighs about one-twelfth the mass of the sun. It is so small and cool that the newfound planet, called VB 10b, gets very little heat from the star, even though it lies about as far away from the star as Mercury lies from the sun.

“We found a Jupiter-like planet at around the same relative place as our Jupiter, only around a much smaller star,” said Pravdo in a statement. “It’s possible this star also has inner rocky planets.”

Red dwarf planets

Pravdo says astronomers may be able to find any such planets, adding that Europe’s Very Large Telescope may be able to detect them with the radial velocity technique.

He adds that this discovery will shed light on how common planets are around low-mass red dwarf stars, which are the most abundant type of star in the galaxy. “The statistics are not as well known as for the solar-type systems,” Pravdo told New Scientist, adding that fewer than 10 red dwarfs have so far been found to host planetary systems.

If planets do turn out to be common around red dwarfs, “that would be really exciting”, he says. “Since more than seven out of 10 stars are small like this one, this could mean planets are more common than we thought.”

Proposed space missions, including NASA’s Space Interferometry Mission and Europe’s Gaia probe, would use the astrometry technique, with SIM hunting for planets slightly larger than Earth.

Journal reference: Astrophysical Journal (forthcoming)