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The Question
(Submitted September 22, 1997)
How common are planetary systems around other stars?
The Answer
The evidence is mounting that planets are quite common around other stars.
Because the mass of any planet around it's parent star is much less than
the star itself, it is difficult for us to observe the effects of the
planets from Earth, and it really isn't possible right now to make an
intelligent estimate of the percentage of other stars that have planets.
However, a number of different observational studies have results that when
combined imply that planets are the rule and not the exception. For
example, theorists believe that planets form from a disk of material
circling a star when it is young. Observations of young stars with the
Hubble Space Telescope and other instruments have directly imaged such
circumstellar material. Very young stars are also found to show evidence
of jets coming out of the poles---jets are a very strong indicator of a
disk structure.
Researchers who study the precise timing of pulsars have found that
some pulsars show a wobble in the period of the pulsar. That is,
instead of the pulsar always having the same period, sometimes it is
slightly faster, and other times slightly slower. This is strong
evidence for orbital motion of the pulsar about the center of mass of a
system. From careful analysis of the pulsars' period changes, orbits
have been deduced that suggest planets circle the pulsars.
Within the
last two years or so, a number of research collaborations, including
ones in the US, have found compelling evidence for Jupiter-sized
planets around relatively nearby Sun-like stars. These systems are
mush more similar to the Sun and our planets than are the systems
around pulsars. These teams use very precise measurements of the
stars' radial velocity, that is, the speed at which it is moving toward
or away from us. If the star has planets a very small wobble will be
seen in the radial velocity. They show that many stars may host
Jupiter-like planets, some in very close, or very eccentric orbits.
The hunt is on for more planets around Sun-like stars, now that the
technique has proven fruitful!
So, while I can not give you an exact numerical answer to your question,
I hope I have given you an idea of why astronomers have recently begun to
suspect planets around many stars.
For more information, check out
http://www.obspm.fr/department/darc/planets/sites.html
There is a section on extrasolar planets in WebStars with links to sites
elsewhere on the subject. This, and many other topics, are discussed in
WebStars at
http://heasarc.gsfc.nasa.gov/docs/www_info/webstars.html
In particular, there is information on pulsar planets at
http://www.astro.psu.edu/users/alex/pulsar_planets.htm
Cheers,
Padi Boyd
for the Ask an Astrophysicist
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