What do we expect to see by using an external occulter ?

To the left and right, we see a simulation of what it is designed to do. On the left is a representation of a bright star, with the light from it glaring and spreading out over the field of view. No faint objects are visible nearby because light from the central star is so bright that it diffracts through the telescope aperture and swamps the region near the star where planets might otherwise be visible. To the right we see the light is reduced orders of magnitude by the occulter with faint objects now visible nearby.

Prevailing theory indicates stars form in dense molecular clouds--such as the Great Nebula in Orion--while planets form from accretion disks about stars. These disks are the remnants of the molecular clouds out of which the stars formed. Planets are cold bodies that radiate most of their energy in the infrared, while stars fuse hydrogen or other heavy elements and emit most of their energy in the optical and ultraviolet.

Jupiter is the most massive body in our solar system, aside from the sun, and as viewed from another star system, would be one of the easiest planets to see...if one used large enough of a telescope and it was not hampered by atmospheric effects. Taken in 1994, the HST/WFPC image at right shows the aftereffects of Comet Shoemaker-Levy/9.

Saturn is less than one third of Jupiter's mass, but it is nearly as voluminous since it has a lower average density. Because of this and its large, bright ring system, if Saturn were at the same distance from the sun as Jupiter, it would appear nearly as bright. Polar aurora are visible in an HST/STIS image shown at left.

Uranus is quite small in comparison to Jupiter and Saturn, with a mass only about 1/7th that of Saturn. It is significantly fainter given its smaller size and greater distance from the sun. In visible wavelengths, Uranus appears rather bland, but in the HST/NICMOS image shown at left, many bands and clouds are visible in addition to its own ring system and moons.

The goal of is not to take such detailed pictures of planets around other stars, which would require telescopes of far greater capability and size than can be built with reasonable sums of money today. Without effective apertures a thousand times greater than the largest telescopes available today, any detail will not be achievable for extrasolar planets. Nevertheless external occulters will allow a search for and analyze extrasolar planets in ways that cannot be done with current methods. External occulters could allow already planned telescopes to see faint dots next to the very bright stars (like in the simulation at the top of the page), tell what colors they are, possibly take spectra of them, and measure their motions around those stars.

This is significantly more than what is currently achievable. The best that can be done at present in the field of extrasolar planets is to measure the back and forth wiggles that large planets have on their parent stars, and measure light variations of stars as planets transit in front of them. At right are measures of the speed of the star 51 Pegasi along the line of sight over a few days. The back and forth wiggles betray the presence of a planet in close orbit around the star. Some sub-stellar objects may already have been imaged, but these are very special in that they are glowing in the infrared and unlike any planet in our solar system.

Furthermore, the imaging technique will allow determination of whether or not planets of a certain class exist at all around those stars. Determining what isn't there is as important for understanding how planetary systems evolve as seeing what is there.