Tom Keith, age 15, of Madison, Alabama, for his question:

Can a new planet be detected by its gravity?

A few months ago, many of us heard that a tenth planet had been discovered in our Solar System. But as so often happens, the news was premature. Scientists have found slight gravitational signs that just might indicate the presence of a tenth planet, out beyond the orbit of Pluto. However, much more evidence is needed before we can be sure.

Let's suppose, just suppose that our Solar System has a tenth planet, out there beyond the orbit of Pluto. Nobody will ever see it, for Pluto and even Neptune are well beyond the range of human vision. Uranus is much closer to us, but it is barely visible only on very rare occasions. Obviously other means must be used to detect a still more remote tenth planet.

Without a doubt, its presence will be detected first by the antics of gravity. This same method was used to detect the presence of Neptune and Pluto. It revealed just where these invisible planets were located    so that powerful telescopes could be trained on exactly the right spot in the infinite sky.

The discovery of Neptune was a classic example of this type of astronomical detective work. Uranus was discovered in 1781, simply by scanning the skies. Later, unexpected wobbles were detected in the big planet's orbital path. These so called perturbations indicated that the path of Uranus is influenced by another invisible planet, still farther from the sun.

We tend to assume that orbiting planets are governed by the sun's gravity. This is only partly true, for gravity is a two way tug. The sun pulls at the planets and the planets pull back. What's more, every planet exerts a mild pull on all the others. So thought the great French mathematician Leverrier, when he set about tracing the invisible planet that caused the wobbling perturbations in the orbit of Uranus.

This was in the 1800s. Leverrier had no modern computers and the tedious figuring took several years. In 1846, he told a Berlin astronomer named Galle: "Direct your telescope to a point on the ecliptic in the constellation Aquarius in longitude 326 degrees and you will find within a degree of that place a new planet, looking like a star of 9th magnitude having a perceptible disc." Galle obeyed these precise instructions and in half an hour his telescope revealed the planet Neptune for the first time. It was one degree from the spot Leverrier had indicated.

Later, the location of Pluto was spotted, on the basis of gravitational perturbations in the orbit of Neptune. Naturally this tried and true method will help to locate a 10th planet, if one exists. It will be traced by the effects of its gravity so that telescopes can be directed to a tiny pinpoint. Photographic evidence would be visual proof that a 10th planet exists.