Published: 24 November, 1999

Scientists continue search for additional solar systems

Is there another solar system besides ours?

The Sun is a rather ordinary star, one of billions in the Milky Way Galaxy and average in terms of properties like mass, size, temperature and age. Since the Sun has planets around it, it seems logical that other stars should have planets too. However, because planets are much smaller and fainter than stars, it is extremely difficult to prove that other stars really do have their own planets. Only in the last decade have astronomers been able (thanks to sophisticated instruments) to make the kinds of measurements that prove that other stars have "solar systems" too.

In fact, while today we know planets exist around two dozen or so stars, none have actually been "seen" in the usual sense. Instead, "extra solar planets" have been found by the same method used in the 1840's to discover the planet Neptune. By figuring out that a more distant but then unknown planet was responsible for oddities in the observed motion of Uranus, astronomers knew where to look for the new planet and quickly found Neptune.

Planets, like all masses in the universe, exert a gravitational pull on their parent stars, causing the stars to move slightly. Since the Sun contains more than 99% of the mass in the Solar System, it has a much greater gravitational effect on the planets than they do on it. Nonetheless, the planets do tug on the Sun and cause it to move slightly, at about 12 meters per second. In contrast, the Earth moves much faster, about 30 kilometers per second because of the Sun's pull on it. If other stars show motions similar to the Sun's, then astronomers can infer that those stars too are being tugged on by their own "planetary systems" even though we can't actually see the planets.

In 1992, two radio astronomers monitoring the motion of a very dense, compact and rapidly spinning star called a pulsar noticed that the star was making a series of tiny motions that were best explained if the star had three planets in its planetary system. Two of the planets are similar in mass to the Earth while the third one is quite a bit smaller, more like our Moon.

Since then, astronomers have been able to detect one (or sometimes more) planets around about 20 nearby Sun-like stars. Because it is much easier to detect massive planets like Jupiter, most of the newly discovered planets are much bigger than the Earth -- but we suspect that there are lots more planets that we haven't yet found. As powerful new telescopes improve our ability to "see" faint, small objects, we soon ought to get a glimpse of what those planets look like and to learn how they might be different from the nine familiar ones in our own Solar System.

You can learn more about extra solar planets at The Extrasolar Planet Encyclopaedia or exoplanets.org