Discovering exoplanets, which are planets orbiting other stars, is extremely difficult because small faint planets are extremely close to much larger and brighter host stars. The problem is analogous to observing a raging forest fire from several miles away and trying to see one of the firefighters lighting a match.
Astronomers discovered most of the roughly 400 currently known extrasolar planets by indirect observational techniques. Direct images of exoplanets are much more emotionally satisfying to both astronomers and the general public, but very few direct images of exoplanets exist. Thanks to a new observational technique that situation may soon change.
Optical Vortex Coronagraph and Adaptive Optics
In a paper published in the April 15, 2010 issue of Nature, a team of astronomers led by E. Serabyn at the Jet Propulsion Laboratory (JPL) announced a demonstration of a new technique for making direct images of exoplanets. Serabyn's team demonstrated their technique by directly imaging previously discovered exoplanets around the star, HR 8799.
The JPL team's technique uses an optical vortex coronagraph combined with adaptive optics to block the light from the host star and reveal the exoplanets. Adaptive optics techniques produce very sharp images by rapidly adjusting the telescope optics to correct for distortions from Earth's atmospheric turbulence. Stargazers see these distortions as twinkling when they look at stars. Telescopes equipped with adaptive optics can produce images nearly as sharp as if they were in space.
What is an Optical Vortex Coronagraph?
A traditional coronagraph uses a small opaque disk placed at the proper point in the telescope's light path to block the light from the host star. A telescope and coronagraph with the high quality optics needed to directly image exoplanets will produce a series of alternating bright and dark rings, called a diffraction pattern, that will mask images of exoplanets.
An optical vortex coronagraph nearly eliminates the diffraction pattern and allows direct images of extrasolar planets.
As described in the April 15, 2010 Nature paper and its references, a vortex coronagraph replaces the opaque disk with a glass disk. The glass disk however has a spiral pattern etched in it. The etched glass disk produces a dark destructive interference region that effectively blocks all the light from the central star without producing the surrounding diffraction pattern.
Destructive interference occurs when two light beams have the maxima of one wave directly line up with the minima of the other wave so that the two waves cancel each other out.
The adaptive optics makes the star's image very sharp. The vortex coronagraph blocks the star's image. Hence the central star is obscured and there is nothing to mask the direct image of surrounding planets.
HR 8799
The JPL team demonstrated the potential of optical vortex coronagraphs using the Hale telescope on Mount Palomar. They stopped down the telescope's aperture from 5 meters to 1.5 meters to use the most optically perfect portion of the telescope's mirror.
The JPL team then directly imaged HR 8799 and its planets. These exoplanets had been previously imaged with the much larger Keck 10 meter telescope, but the new images of the HR 8799 exoplanets with a smaller telescope demonstrate a more powerful observing technique.
The JPL team demonstrated that telescopes equipped with adaptive optics and optical vortex coronagraphs can directly image extrasolar planets. As astronomers begin to apply the technique they will directly discover more exoplanets.
Further Reading
Discovering the Extrasolar Planet CoRoT-9b
Discovery of the Super Earth Planet GJ 1214b
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
