Pluto

Pluto

Astronomers soon found that the gravitational effect of Neptune failed to account for all the irregularities in the motion of Uranus. Two American astronomers, Percival Lowell and William H. Pickering, undertook the formidable task of calculating the likely position of a hypothetical planet, and in 1930 Clyde Tombaugh, guided by Pickering’s predictions, discovered Pluto, the most distant of the Sun’s family of planets.

Pluto’s average distance from the Sun is nearly forty times that of the Earth. Yet because its orbit is quite elliptical – more so, in fact, than that of any other planet, it can come closer to the Sun than can Neptune. The two planets could not collide, however, since the planes of their orbits are inclined by about 15 degrees to each other. In fact, the inclination of the plane of Pluto’s orbit to the plane of the earth’s orbit (and therefore to the general plane of the Solar System) is just over 17 degrees. Pluto, unlike the other planets, can therefore wander outside the zodiac. Its orbital period is 248 years, and it will next be closest to the sun (and hence to the Earth) in 2237. Its distance will then be about 2,800 million miles, or less than that of Neptune.

 Orbit of Pluto

Very little is known about Pluto itself, but this is not surprising in view of its immense distance. Its diameter is thought to be less than 4,225 miles. If we assume for the planet an average density of four times that of water, its mass would be less than one-tenth that of the earth. At all events, it must be a barren and frigid world, the coldest and darkest planet in the solar system.