By Amir Alexander

June 11, 2009

There are several reasons why the new exoplanet discovered last month orbiting the red dwarf VB 10 has set the planet-hunting community abuzz. For one thing the planet is a "cold Jupiter," similar to our giant neighbor and very different from the host of "Hot Jupiters" that make up the largest contingent of known exoplanets. For another the planet's home star is one of the smallest known stars, with a mass of only one twelfth that of the Sun. This makes it the smallest star known to possess a planet. But what makes this alien world truly exceptional to astronomers is the manner in which it was discovered: For the planet designated VB 10b is the first one ever discovered through astrometry.

As the term suggests, "astrometry" is the measurement of stars, or more specifically their precise location in the sky. In a way, "astrometry" is precisely what astronomers have always done, measuring the exact positions of celestial objects. It is therefore not altogether surprising that astrometry was also the first method used in searching for planets around faraway stars. In fact, the earliest claim by an astronomer to have discovered an exoplanet through astrometry was made as early as 1943 – more than half a century before the discovery of the first confirmed exoplanet by Michel Mayor and his colleagues.

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Image: VB 10 b, the first planet discovered by astrometry
At the time of its discovery in May, 2009, the red dwarf VB 10 was smallest star known to be home to a planet. The detection bodes well for the prevalence of planets around red dwarfs, which compose 70% of the stars in our galaxy. Credit: NASA/JPL-Caltech