A Large Planet Orbiting a Small Star

An artist's conception of the planetary system.

Astrometry, a 50 year old technique used to find exoplanets, planets around other stars, has finally succeeded. Planets outside the solar system are nearly impossible to see, so the method relies on measuring the movements of a star moving back and forth due to its planet. This technique requires numerous careful and precise measurements, and these have to be compared to many others taken over long periods of time. This is why, until now, the method has failed to provide any conclusive evidence towards exoplanets.

Using the Palomar Observatory near San Diego, fitted with an astrometry instrument, a two astronomer team from NASA's Jet Propulsion Laboratory (JPL) have conducted rigorous observations for 12 years. They have studied 30 different stars, and from that number, a new exoplanet has been discovered around one of them.

The planet, VB 10b, is very large and somewhat like the planet Jupiter in our own solar system. It has a mass six times that of Jupiter's, but generates its own internal heat, putting its temperature near the Earth's.

Compared to the planet, its star, VB 10, itself is tiny. It is a twelfth of the mass of the Sun. This is greater than its planet's mass, but the size of the star itself is about the same as its planet. The star used to be the smallest known star, and now is the smallest star known to possess a planet. The importance of the finding is that planets might be very common, even existing around smaller stars like VB 10. "This is a hint that nature likes to form planets, even around stars very different from the sun," states Wesley Traub, chief scientist for NASA's Exoplanet Exploration Program at JPL.

The star's smaller size indicates that it is in the center of a small planetary system. Rocky planets like the Earth would lie between VB 10b and VB 10. The finding indicates that astrometry is capable of finding planetary systems arranged like ours, because it locates planets similar to Jupiter.

In addition to astrometry, other methods of finding exoplanets exist. The radial velocity method analyzes the Doppler shifts in a star's light resulting from a planet tugging on it. The transit method, employed by the Kepler mission, looks at the dips of light that occur when a planet passes in front of its star.

Images: JPL-Caltech/NASA

Astronomy and Space celebrates International Year of Astronomy 2009.