Wildernees Underfoot: The solar system gets more complicated

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The more we learn about our planetary neighborhood, the less we know…

Here’s a challenge from your school days: try naming the nine acknowledged planets in our solar system. Even more of a challenge, put them in order based on their distance from the sun.

If you already struggle to remember all nine, imagine how hard it would be if the number of planets were doubled. Or tripled. It might just happen; many astronomers believe we’re on the verge of discovering a whole new set of planets in the outer reaches of our solar system, about 1,000 to 10,000 astronomical units away from the sun. (An astronomical unit is roughly the distance between the sun and the Earth—about 93 million miles.) In this outlying zone, sunlight is so faint that planets and other large objects don’t reflect enough light for our telescopes to detect.

You had it easy in school, back when scientists generally agreed on a somewhat less messy layout of the solar system—a sun orbited by Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Memorize these, and a few moons, meteors, asteroids, and comets, and you thought you had it all figured out.

But our picture of the solar system grew more complex as we built more powerful telescopes and then sent exploratory spacecraft to the planets themselves. Right away, we began to discover lots and lots of moons. These moons were often more fascinating than the pockmarked rock that orbits our planet. They had such features as volcanoes, gas atmospheres, storms—and most thrilling—ice. Ice meant water, and water meant the possibility of simple life. Over the last few decades, we have added dozens of new moons to this picture, such as Jupiter’s volcanic moon Io and Saturn’s gas-covered Titan.

Questions about the actual number of planets in our solar system actually began with an argument against including tiny Pluto. Some astronomers argued that the outermost planet was simply too small to be considered a true planet. Estimates of its diameter shrunk even more after it was discovered that it had a previously unknown moon, Charon, which blurred its size in telescopes and made it appear larger. Those in favor of keeping Pluto in the planetary lineup prevailed, however.

Astronomers always anticipated that we might find similar small planets farther out from the sun. But new computer models of the solar system suggest that these planets might not be so small. It is entirely likely that there are planets larger than Mars, or Earth, in the outlying zone. Perhaps as many as a dozen or more.

From the Sapt. 7-13, 2005, issue

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