Under certain circumstances and with the right instruments. The planet could not be seen via its causing the star it orbits to 'wiggle'. This takes very high precision astrometry over many months to years to detect. Planets could be detected, as they apparently have already been done, by monitoring the frequencies of many spectral lines from the star to a doppler precision of 10 meters per second. This, however, requires a very high resolution spectrometer and the light gathering capability of the Hubble is probably not large enough for velocity changes to be detected with high statistical confidence. Ground based telescopes like the Keck and the Gemini telescopes, among others, will be the champions of this method with their enormous apertures in excess of 5 meters.
Planets could be detected by their reflected light by the Hubble, because the optics and 'seeing' conditions on orbit are superior to anything on the ground looking up through the turbulent atmosphere. The Space Telescope Imaging Spectrometer ( STIS) will be installed in the 1998 servicing mission, and will have a special occulting mechanism that will allow searches for Jupiter-sized planets around nearby stars to be attempted. A Jupiter-sized planet orbiting a star 3 light years away at the distance of Jupiter from the sun, will be seen about 5 arcseconds from the star. Only HST's perfect optics and no distortion will allow astronomers to search for faint objects within a few arc seconds from a nearby star. Stay tuned. In another few years, we may know of more planets outside our solar system, that there are planets within our solar system. This will be an historic moment in human exploration. And it's all starting to happen right now! But for Hubble, the best it can do is look for details that are a few tens of AU across in the images of distant young stars. This may be enough to see traces of planets sweeping out rings in protoplanetary disks...only time and good fortune will tell!
So far, HST has conducted research on proto-planetary disks around young stars:
HST has studied details in the disk of AB Auriga which may indicate planets.
HST has also studied the destruction of Proto-planetary disks in the Orion Nebula.
Copyright 1997 Dr. Sten Odenwald
Return to Ask the Astronomer.