Astrometry nabs an exoplanet

Long-proposed search method finally finds what it’s been looking for

Researchers for half a century have tried — and failed — to use the motion of stars moving across the sky to discover planets that lie beyond the solar system. Now a team has finally used the method, known as astrometry, to find one of these orbs. The newfound extrasolar planet, six times heavier than Jupiter, orbits the low-mass star VB 10 some 20 light-years from Earth, report Steven Pravdo and Stuart Shaklan of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., in an upcoming Astrophysical Journal.

The traditional method of identifying extrasolar planets, which now number more than 350, relies on tracking the velocity of a parent star along the line of sight to Earth — rather than across the sky. Because an orbiting planet pulls its parent star ever so slightly to and fro, the star’s line-of-sight motion speeds up and slows down periodically, revealed by telltale shifts in the color of starlight recorded from Earth. This technique, known as the wobble or Doppler shift method, detects heavyweights that lie close to their star most easily, since such planets produce the largest changes in that star’s speed along the line of sight to Earth. But the method can only reveal the minimum mass of a planet.

In contrast, the astrometric method, which records the positions of stars as they slowly drift across the sky, pins down the exact mass of a planet. The method favors massive planets that lie far from a parent star, since such planets cause a star to move by the largest amount across the sky. Researchers have previously used astrometry to measure the precise mass of planets found with the wobble method, but this is the first time the technique has been used to discover an extrasolar planet, says Alan Boss of the Carnegie Institution for Science in Washington, D.C.

The planet found by Pravdo and Shaklan lies about as far from its star as Mercury, the solar system’s innermost planet, lies from the sun. Yet the newly discovered body, unlike Mercury, is relatively cool since its parent star is only one-twelfth the mass of the sun and therefore produces much less radiation and heat.

The researchers say that because the astrometric technique finds massive planets that orbit far from parent stars, the method could identify stars that might be examined in follow-up studies for smaller, Earth-mass planets with tighter orbits. A search for those close-in bodies might best be examined with the wobble method. Should VB 10 harbor a rocky planet that lies much closer in, it might be habitable — or at least contain liquid water.

Planet hunter David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., says VB 10 is on the list of stars to be examined with his suite of eight tiny telescopes, known as the MEarth project (SN: 12/20/08, p. 16). The telescopes look for habitable planets, as small as twice Earth’s mass, that pass directly in front of their low-mass parent star as seen from Earth, creating a minieclipse. The periodic dip in starlight reveals the planet.

Charbonneau cautions that it’s unlikely that any Earthlike planet that might orbit VB 10 has the right alignment to produce a minieclipse observable from Earth.

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