Not Just Snowball After All, Pluto’s Density a Surprise

Pluto, the solar system’s smallest planet and the target of intense scrutiny these days, is turning out to be far more baffling than scientists had expected.

Astronomers reported at a meeting of the American Astronomical Society here Tuesday that they have discovered that Pluto is far too dense to be the loosely compacted snowball they had thought it was.

The tiny planet, which measures only about 1,444 miles in diameter, turns out to be about twice the density of water, astronomers reported.

“It’s very unusual for bodies in the outer solar system to have that high a density,” said Richard P. Binzel of the Planetary Science Institute in Tucson.

Binzel said the surprising conclusion is that Pluto has a lot of rock in addition to the methane and water ice that scientists had identified previously. That puzzles them because the other outer planets, such as Neptune and Uranus, are huge, gaseous balls of very low density. No one knows why Pluto should be different.

The discovery of Pluto’s high density is one of a series of insights growing out of an intense study of the planet during an opportunity that only comes around once every 124 years. Pluto and the orbit of its moon, Charon, are in direct alignment with the Earth right now; so, as Charon orbits Pluto, the two take turns eclipsing each other.

By studying changes in such things as luminosity when Charon is hidden by Pluto, scientists are able to learn much about the size, density and composition of the two bodies.

The mutual eclipses will continue through next year, giving scientists an extraordinary chance to study Pluto and its moon.

Alan Stern of the University of Colorado said the high density, coupled with Pluto’s bizarre orbit and nearby moon, which is half as big as the planet, “suggests that something really unusual is up at Pluto.”

Stern and a fellow astronomer, Larry Trafton of the University of Texas, have concluded that Pluto’s thin atmosphere, consisting mostly of methane gas, sometimes reaches so far out that it envelopes Charon completely, a unique relationship between a planet and its moon. No other planet is known to share its atmosphere with its moon.

Atmosphere Often Lacking

But Stern speculated that for most of the time, Pluto does not have an atmosphere. Normally, Pluto is the most distant planet from the sun, but its erratic orbit also carries it inside the orbit of Neptune, where it is now. During its closer approach to the sun, Stern speculated, warmer temperatures cause the methane ice on its surface to melt, creating a thin atmosphere. But after a few decades, the surface cools and refreezes as the planet moves farther from the sun, turning the atmosphere into dew on the surface of the planet.

If that theory is correct, Pluto should get a lot brighter within the next two or three decades when the planet wraps itself in a frozen blanket of white ice and settles down for a long winter, sans atmosphere.

Stern and Trafton plan to use two telescopes--one in Colorado and one in Texas--over the next few months to see if Pluto has a few other surprises.

The two scientists have concluded that Pluto could have many other moons--like the other outer planets--just waiting to be discovered.

Using sophisticated new techniques which capture nearly all of the light received from a distant object, Stern and Trafton think they should be able to find such moons, if they are at least as large as 60 miles in diameter.

Importance Noted

The search is important, Sterns said, because in a sense Pluto offers astronomers their last chance to redeem themselves. Much of what they thought they knew in the days before the Space Age has been disproved by space probes, which now have visited all but two planets.

The Voyager spacecraft is due to reach Neptune next year, and scientists expect to have to rewrite their textbooks again after it gets there.

But there are no plans to send any probes to far off Pluto in the foreseeable future, so what can be learned must be learned from the ground.

“Pluto is the last astronomer’s planet,” Sterns said in reference to the space probes that have proved them wrong so often at all the other planets. “Can we get the last one right?” he asked.