QUESTION:
We were wondering why we don't hear anything
more about the two planets that were discovered about a month ago in
a different galaxy.  Can the Hubble "see" them?

ANSWER from Megan Donahue on March 21, 1996:
Planets around other stars are very difficult to see, and if you try to
imagine the following model, you'll realize why.

Stars are extremely bright, hot balls of gas that generate their own
energy by nuclear fusion in the cores.  Planets, on the other hand,
are very cold in comparison. We can see the other planets around our
own Sun because they reflect the Sun's light. Planets around other stars
also reflect light, but since that light is so faint in comparison to
the light being generated by the star, it is very hard to see. Think
about what it's like to look directly at a very bright streetlight.
If there is a tiny little firefly close to the light, we aren't able
to see it because our eyes are being blasted by the light from the street
light.

Another reason that planets around other stars are so difficult to see
is that they are extremely small and close to their parent stars. One way
to visualize this is to take our own solar system as an example, and divide
all the sizes and distances in it by 10 billion. (This is called "scaling
down", and it helps when we're trying to think about the relative sizes of
things.) So if we imagine that we could shrink everything in our solar system
by a factor of 10 billion, the Sun would be the size of a large grapefruit.
The Earth would be extremely tiny, about the size of a grain of sand, and it
would be about 15 meters away from the Sun.

Now imagine that we are at the distance of a nearby star, about 40 light-years
away, looking back on the Sun system. [A light year is the distance light
can travel in one year = about 10^13 km (10,000,000,000,000 km). Thought
question:
if you know the speed of light is 300,000 km/sec, how would you figure out
how far light can travel in a year? Hint: How many seconds are in one year?]

The Earth (if we could see it) would appear to be very very close to the
Sun, 0.08 arcseconds to be quantitative about it. Because of the Earth's
atmosphere, stars aren't precise points in the sky: as the light passes
through the atmosphere the atmosphere shifts a bit this way and that,
bending the path the light takes. [Thought question: does light travel as
fast through the atmosphere as it does through the near-vacuum conditions
in space?] Because the atmosphere shifts the light path around, the image
formed by the star is a little smeared out. In general, this smearing is
around 1 arcsecond in size, so all the stars appear to have disks of 1
arcsecond from the ground.

In orbit, there isn't any atmosphere to smear the light around, but there is a
fundamental limitation on how small stars appear (even with perfect telescopes)
because light is a wave. Because waves have finite wavelengths, the smaller the
wavelength the smaller image it can make. Also, the larger the telescope, the
smaller this limit is. The limit for the Hubble Space Telescope, then, at say
optical wavelengths of 4000 Angstroms in size, is about 0.04 arcseconds, just
barely smaller than the Earth-Sun angular distance when at a distance of 40
light years.

This means that even if HST looked at a star using an instrument with the
tiniest pixels (in this case, the Faint Object Camera or the Planetary
Camera), there would not be ANY pixels (picture elements) between a star
and its companion planet. If the planets were as bright as the star, there
might be some hope of separating the two, but as we said earlier, planets
are much much fainter than stars. Even Jupiter is about 100-150 million
times fainter than the Sun at optical wavelengths - very difficult to
see!!! At other wavelengths, say radio or the near infrared, the planet to
sun brightness difference is not so huge, and it is easier to "see" planets
at those wavelengths.

[But those are longer wavelengths, so to make the physical limit on image size
small enough, you need telescopes larger than HST! But you don't
necessarily need to put them in orbit. For example, at radio wavelengths,
the atmosphere doesn't much affect the light path.]

That is a very long answer to say that unless a planet is much brighter (more
reflective) than Jupiter and farther away from its parent star, there is not
much chance for HST to see it.

But NASA is now considering some new missions that may be launched by the
times you guys are in college that will be specially designed to be able to
detect planets around our local star neighbors. And maybe by the time your
kids are in college we'll be able to tell whether, say, the atmospheres of
Earth-like stars are suitable for life, and we'll start picking the best
places to visit!

Talk about long term planning. ;)

One more thought question for you (after the lengthy explanation):
Why can we see many of the planets in our own Solar System (Jupiter, Mars,
etc.) quite easily? What planets are the most difficult to see and why?