Pluto’s slow circuit around the solar system

May 1, 1997

EDITORS: Accompanying star map shows Pluto’s location in Michigan’s mid-May constellations.

ANN ARBOR—The icy planet Pluto sluggishly circles the sun in the deep freeze of space at the edge of the solar system 2.8 billion miles from the sun. This month the planet is almost directly opposite the sun in the night sky, according to University of Michigan astronomer Richard Teske. It rises in the east at sunset and sets in the west at sunrise, but is too faint to be seen without a large telescope.

“Since its discovery by Clyde Tombaugh in 1930, Pluto has only advanced one-third of the way around the sun,” Teske said. “During this same time, our Earth has completed 67 orbital revolutions.”

To appreciate Pluto’s extremely slow movement, Teske recommends you look for two constellations in Michigan’s night sky this month. The first is the constellation Gemini where Tombaugh discovered Pluto 67 years ago. The second is the constellation Ophiuchus, where Pluto is located this month. At about 10:30 p.m., the twin stars of Gemini are visible 30 degrees above the western horizon, while fainter Ophiuchus stands 30 degrees above the southeastern horizon (see attached star map).

“Since it was first discovered in 1930, Pluto has traversed only the short stretch of sky between these two constellations,” Teske said. “Another 180 of our years will pass before the frozen planet will have completed one of its own ‘years’ and returns to the constellation Gemini.” Pluto’s elliptical orbit is the most eccentric of all nine planets. It sometimes crosses inside the orbit of the next innermost planet, Neptune. When it did so in 1979, Pluto temporarily lost its title of outermost planet. By 1989 its inward-bending track brought Pluto as close to the sun as it ever gets. Since then, its orbital motion has carried it away from the sun once again. In 1999 outbound Pluto will cross Neptune’s orbit and regain the distinction of most distant of the sun’s children.

Few astronomers take a professional interest in Pluto, according to Teske. “Studying Pluto is difficult and unrewarding, because its faintness and extreme distance make it visible only through the largest telescopes,” he said. “For this reason, the planet has remained mysterious.” Recent ground-based observations and some images made with the Hubble Space Telescope, however, have begun to reveal Pluto as a real world.

Just two-thirds the size of Earth’s moon, Pluto is the smallest of the planets. Its l,440-mile diameter equals the distance between Washington, D.C., and Denver. Yet this dwarf world has its own moon. Observations of the moon’s orbital progress around Pluto have revealed the strength of Pluto’s gravity, Teske explained. A 160-pound person would weigh just eight pounds on Pluto.

“Temperatures hover around 370 degrees below zero F right now and are getting colder as Pluto draws slowly away from the sun. Under such conditions an atmosphere freezes into crystals and flakes and falls to the ground. Only a wispy bit of methane ice weakly evaporates from surface ‘snowdrifts’ to generate methane air sparser than the best vacuum that can be achieved on Earth,” Teske said.

The planet rotates slowly with 3.2 Earth days elapsing between sunrise and sunset. Nights have the same 3.2 day length. “The distant daytime sun lights up Pluto’s landscape only a bit more brightly than does the Earth’s nighttime full moon,” Teske said.

Pluto, the slow-motion planet, is still growing. Astronomers now believe that it is a world under construction. Their theories indicate the giant outer planets were born when huge numbers of icy, rocky pieces created at the birth of the solar system came together, colliding and sticking to make single, large planets.

“Ample evidence exists that great numbers of those pieces still orbit the sun in the vicinity of Pluto and beyond. Pluto is still getting bigger as some of these fragments crash down on the planet’s surface.”

Richard Teske