Does our solar system represent the rule or the exception? Do similar collections of worlds surround other stars in the galaxy, or is the sun peculiar? Although this is one of the fundamental questions driving modern astronomy, the answer remains elusive. Over the past nine years, astronomers have discovered at least 111 planets around sunlike stars by looking for the slight back-and-forth motion that these bodies impart to their parent suns. Yet this technique detects only the most massive and tightly orbiting objects. If extraterrestrial astronomers applied the same method to our solar system, they might manage to identify Jupiter, and maybe Saturn, but they would completely miss the smaller bodies that make the sun's family so rich and varied: asteroids, comets and the terrestrial planets.

How can astronomers detect those smaller bodies and paint a more complete picture of the diversity of planetary systems? A clue appears in the western sky in the spring, right after sunset. If you watch closely, you might see the zodiacal light, a faint triangle of light extending up from the horizon. The zodiacal light is produced by sunlight bouncing off interplanetary dust particles in our solar system. The triangle of light stretches along the sun's path in the sky, indicating that the dust forms a disk in the plane of Earth's orbit. What makes the dust interesting is that it should not be there. The individual dust particles are so small--about 20 to 200 microns across, judging from the color of the zodiacal light--that sunlight quickly causes them to spiral into the sun and burn up. Dust particles that are even smaller are quickly blown away from the solar system by radiation pressure. Therefore, for dust to be present it must be replenished continuously.

This article was originally published with the title The Hidden Members of Planetary Systems.

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