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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

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

In particular, there is information on pulsar planets at


Padi Boyd
for the Ask an Astrophysicist

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