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Dedicated star-trackers turn up a big planet around a tiny star

exoplanet, extrasolar planet, astrometryAstronomers looking for planets encircling stars outside our solar system—known as extrasolar planets, or exoplanets—are claiming the first planetary detection from a technique known as astrometry, which tracks the location of stars in the sky with great precision.

Astrometric observations of a nearby star called VB 10 revealed a telltale motion in the star's position caused by the gravitational pull of its companion planet, dubbed VB 10b. VB 10 and its planet are an astronomical odd couple: the star is one of the least massive known, whereas the planet itself is some six times the mass of Jupiter. In contrast to the solar system, where the sun is some 10 times the diameter of the largest planet, VB 10 and VB 10b have roughly the same width.

Most of the 300-plus known extrasolar planets have been found by tracking changes in a star's light output over time. The most prolific approach, the radial velocity method, looks for shifts in that light caused by the Doppler effect as the tug of an orbiting planet pulls the star nearer and more distant to us along our line of sight. The other approach, the transit method, tracks the periodic dimming of a star caused by a planet passing in front of it. Astrometry has been on the table for decades but had yet to uncover any convincing evidence for new planets.

For the new discovery, set to be published in the Astrophysical Journal, Steven Pravdo and Stuart Shaklan of the NASA Jet Propulsion Laboratory in Pasadena, Calif., took astrometric observations of 30 stars over 12 years at Palomar Observatory in southern California. They finally bagged their quarry around an ideal target for astrometry: the star VB 10 is relatively close by, about 20 light-years away, and not very massive, less than 10 percent the mass of our sun. The lighter the star relative to its planet, the more the planet's pull will perturb the star's position, and the closer the star to Earth, the more that motion will be apparent in the sky.

In addition, the size of the exoplanet and its relatively large distance from its star aided its astrometric discovery. "With astrometry, you can picture it like a teeter-totter," David Charbonneau, an astronomer at the Harvard-Smithsonian Center for Astrophysics, told the New Scientist. "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."

Image credit: NASA/JPL-Caltech

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