Coronagraph - Extrasolar Planets

Extrasolar Planets

The coronagraph has recently been adapted to the challenging task of finding planets around nearby stars. While stellar and solar coronagraphs are similar in concept, they are quite different in practice because the object to be occulted differs by a factor of a million in linear apparent size. (The Sun has an apparent size of about 1900 arcseconds, while a typical nearby star might have an apparent size of 0.0005 and 0.002 arcseconds.)

A stellar coronagraph concept was studied for flight on the canceled Terrestrial Planet Finder mission. On ground-based telescopes, a stellar coronagraph can be combined with adaptive optics to search for planets around nearby stars .

This link shows an HST image of a dust disk surrounding a bright star with the star hidden by the coronagraph.

In November 2008, NASA announced that a planet was directly observed orbiting the nearby star Fomalhaut. The planet could be seen clearly on images taken by Hubble's Advanced Camera for Surveys' coronagraph in 2004 and 2006 . The dark area hidden by the coronagraph mask can be seen on the images, though a bright dot has been added to show where the star would have been.

Up until the year 2010, telescopes could only directly image exoplanets under exceptional circumstances. Specifically, it is easier to obtain images when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation. However in 2010 a team from NASAs Jet Propulsion Laboratory demonstrated that a vector vortex coronagraph could enable small telescopes to directly image planets. They did this by imaging the previously imaged HR 8799 planets using just a 1.5 m portion of the Hale Telescope.