Pluto Is Puny, But It’s No Asteroid, JPL Team Reports

Pluto, the most distant planet in the solar system, has a significant atmosphere and thus cannot be an asteroid masquerading as a planet, as some scientists have suggested, according to a report published today by scientists at the Jet Propulsion Laboratory in Pasadena.

The finding, in the new issue of the British journal Nature, represents a victory for Pluto over those who have suggested that its planetary status be yanked.

Using NASA’s Infrared Astronomical Satellite, JPL scientists also came up with the best measurement yet of Pluto’s size. It is about 1,370 miles in diameter, give or take 100 miles, and that makes it slightly smaller than the Earth’s moon, which measures 1,538 miles in diameter.

Pluto is so far away and so small that scientists have struggled for years to come up with a precise estimate of its size.

“Since its discovery in 1930, Pluto has been an enigmatic object,” the scientists wrote. The research was conducted over a three-year period by JPL astronomers Edward F. Tedesco, Glenn J. Veeder and R. Scott Dunbar, and Larry A. Lebofsky of the University of Arizona. The study is to continue through 1990.

Scientists once estimated that Pluto could be more than six times the mass of the Earth, based on its effect on the orbit of Neptune, an effect that led ultimately to Pluto’s discovery. The estimate of its mass shrank as scientists learned more about the planet, but when its satellite, Charon, was discovered in 1978, scientists realized that their estimates had been too large by a factor of 50, the astronomers wrote.

Further research continued to nip away at the estimated size of the planet, leading up to today’s announcement that Pluto is puny indeed, although much larger than the largest asteroids.

Bizarre Orbit

Like Earth, Pluto’s moon is disproportionately large compared to the host planet. Charon is about 800 miles in diameter, more than half the diameter of the planet itself, according to the Nature report.

Pluto’s bizarre orbit keeps the planet an average distance of 4 billion miles from the sun, but the orbit is sharply tilted, carrying the planet far above the plane in which all the other planets travel. The orbit is so out of kilter with the other planets that right now Pluto is actually closer to the sun than Neptune is, but by 1999 it will again be the ninth planet from the sun.

Its small size, icy composition and peculiar orbit have led some scientists to suggest that Pluto may actually be an asteroid, and there has been a semi-serious effort to yank Pluto’s status as a planet.

The latest findings, including the fact that Pluto has a methane-rich atmosphere, show that Pluto’s “properties differ significantly from those of asteroids,” the astronomers reported.

Asteroids, the largest of which measure a few hundred miles in diameter, have no atmosphere.