Mike . Grimm, age 14, of Peoria, Illinois, for his question:

How did they calculate the existence of Pluto

Astronomers tackled this exciting conundrum in the 1700s. The problem was presented by perturbation, which happend to be a very disturbing problem in its own right. Heavenly bodies exert gravitational forces according to their masses. Since they rarely if ever move in perfect harmony, their masses affect each other. Such perturbances cause slight changes in the paths or all the members of our closely knit solar system.

The planetary orbits are ellipses and their orbital speeds are slightly faster when each planet comes its closest to the sun because during this phase of the yearly orbit, the mighty sun exerts stronger gravitational force. Though the sun's mighty gravity governs almost all the motions of the planets, to a small degree their own lesser gravities also affect each other.

Traveling in their separate lanes, the inner planets are always catching up and passing those in the wider orbits outside them. Hence, in the merry go round of the solar system, the planets are forever coming closer to each other and then moving farther apart. Since the force of gravity diminishes with distance, the gravitational force.between the planets diminishes and increases. These variations, plus the variations of the sun's distance cause perturbances or slight wobbles in planetary paths.. For example, the changing positions of Jupiter and Saturn cause a minor perturbance every 60 years or so and a major one every 900 years.

Uranus was discovered in 1781 and a few observations enabled astronomers to compute its orbit. However, repeated observations revealed something amiss. Slight orbital wobbles suggested Uranus was influenced by another invisible planet, farther out in the solar system. These perturbances were computed along with the distance of the sun to pinpoint the possible source of the disturbances. This led to the discovery of Neptune in 1846.

Then of all things, the old story of perturbances repeated itself. Detailed observations revealed slight wobbles in the path computed for Neptune. But Neptune moves so slowly,that tracing the source seemed almost impossible. However, it turned  out that Uranus also was influenced by this still invisible planet. This time the distance of the sun was computed with the perturbances of both Uranus and Neptune to pinpoint the likely part of the sky. Finally, little Pluto was located in 1930  ¬just about where all those patient calculations indicated that it should be.

The computed orbits of Neptune and Uranus, compared with their observed perturbations, revealed just about where Pluto should be. Numerous telescopic plates of this region were studied in fine detail. They were crowded with dots of starry light    but only time could show which was a planet. On January 30, 1930, it was proved that one small dot was moving very slowly against the background of fixed stars. This one had to be an orbiting planet.