QUESTION:
What information could the Hubble give us about a new planet?

ANSWER form Megan Donahue on April 23, 1996:
You wouldn't use the Hubble Telescope to try to find other planets. It's
pretty easy to understand why, if you know what I mean by "field of view."
If you go to a big sporting event, like a soccer game in a big stadium,
you might bring along a pair of binoculars. You wouldn't watch the entire
game through them, however! By looking through the binoculars, you can
only see a small part of the field at one time. You might try to keep
them focussed on the ball-handler, but it would be a very difficult task
to try to keep up with the ball through the whole game. You wouldn't
see the other players get into position for the plays, and you might
lose track of the ball. You would probably choose to spend most of the
time watching without the aid of binoculars!
 
To find a planet, or other Solar System object, like a comet or an asteroid,
astronomers must look at rather wide areas of the sky. There are special
telescopes called "Schmidt telescopes" which are designed for this. A single
Schmidt image might cover an area of the sky spanned by several Moons.
 
By comparison, the Wide Field Planetary Camera (2), the largest imager on
the Hubble Telescope, has a very small field of view. If you held a pin by
its head at arms length and looked at the area of sky covered by the TIP
of the pin, that is approximately the field of view spanned by the Wide
Field Camera. It's pretty small! It would take much longer to cover
the same area of sky than a ground-based Schmidt telescope would.
 
On the other hand, once a planet, comet, or asteroid has been discovered,
astronomers know exactly where it is on the sky and where its headed. They
can predict where it will be at a certain time and point the Hubble Telescope
at it and get an excellent view, unobstructed and unblurred by the Earth's
atmosphere.
 
Depending on what instrument is used to study the planet, comet or asteroid,
astronomers might learn about the composition of the gasses surrounding it,
its shape and rotation, the colors of features on its surface (if it has
one). One task astronomers have set out to do with the Hubble Telescope is
to detect moons around planets that we already know about. Some moons are
quite interesting - like Jupiter's moon, Io, whose poor body is squished about
by strong gravitational forces during its orbit around Jupiter, causing 
its surface to have a constant roller-coaster ride of earthquakes and 
volcanic activity. You would definitely rather live in southern California.
 
I hope this answers your questions.
 
Megan