Astronomers have now found over 500 planets orbiting other stars. Virtually all of these known "exoplanets" were discovered indirectly by inferring the presence of a planet by the gravitational tug and pull its mass imposes on the star during its orbit. This tug and pull can be observed as a periodic variation in the velocity of the star and can be observed from earth using an instrument called a "spectrograph" attached to a telescope. Most of the planets discovered outside our own solar system are "gas giants" at least as massive as Jupiter and have been found in close-in orbits to their stars and are not likely to be capable of supporting life. But it is very likely that there are a large number of planets that could do so. It is thought that such planets are likely to be rocky like earth and to orbit their stars within the "habitable zone" where conditions would allow and support the existence of liquid water. These orbits are far enough from the star that the temperature is not too hot due to the star's radiation but they are not so far away that any water would be permanently frozen. Because these planets must necessarily orbit at some distance for their stars to be found in the habitable zone and they are likely to be smaller rocky planets (like earth), the tug and pull they effect on their stars' velocities are considerably smaller than those of the large gas giants close to their stars. In order to find planets in the habitable zone it is necessary to use a highly precise and stable spectrograph that can very accurately measure tiny changes in the velocities of stars. Dr. Suvrath Mahadevan of Pennsylvania State University has designed such a spectrograph and will build it and put in on the 10-meter aperture Hobby-Eberly Telescope in Texas. This spectrograph is carefully designed to be extremely stable and to have a very precise calibration capability that will be able to determine small velocity changes. Dr. Mahadevan's "Habitable Zone Planet Finder" spectrograph is funded by NSF's Major Research Instrumentation program through the Division of Astronomical Sciences.
