In the first photographs ever of the only unexplored planet in the solar system, NASA researchers Thursday confirmed what they suspected about Pluto all along: that the "icy little dwarf" is a misfit among the family of planets.
Photographs of the ninth rock from the sun taken by the Hubble Space Telescope reveal sharp contrasts between light and dark areas about the size of continents and oceans of Earth. But researchers stress that they aren't yet seeing geological features--only differences in brightness.
Still, these contrasts set Pluto apart from its mostly monochrome giant neighbors Neptune and Uranus. "The most exciting thing is the blotchiness," said space telescope scientist Bruce Morgan. "Pluto never fails to surprise us."
Even with the relatively clear eye of the space telescope, Pluto appears in vague shadows more than a formal portrait. Still, researchers are thrilled to finally lay eyes on the most remote outpost in the solar system.
"Even in these raw images we can see fantastic detail," said Alan Stern, a planetary scientist with the Southwest Research Institute, pointing to what he said is a polar cap spreading over half of Pluto's image. "It's rewarded us beyond expectation. It's got more features than any object in the outer solar system."
At 3 billion-odd miles away, Pluto teeters on the frontier of our planetary system. And while some researchers consider Pluto the puniest of planets, others say it belongs more naturally to the comet family. They point out its proximity to the recently discovered Kuiper belt just beyond the planets, which is composed of 30,000 to 40,000 dirty iceballs--mostly mountain-size--that orbit the sun.
Stern described the environment of Pluto as a "shooting gallery," where collisions between objects large enough to wipe out the dinosaurs on Earth are fairly common.
In some ways, Pluto has more in common with the comets that occasionally stray from the Kuiper belt into Earth's area than it does with the other planets. It is smaller than the moon, and its orbit is strangely askew--dipping inside the orbit of Neptune from time to time like the loose brim of a floppy hat.
And like a comet, Pluto grows a halo of reflective gases as frozen nitrogen and methane escape when the planet approaches the warmth of the sun. As Pluto veers off again, this temporary atmosphere freezes, and rains back to the surface as ice. "There's probably real weather," Stern said.
Pluto has eluded astronomers thus far simply because it's so small, and so far away. From Pluto's perspective, the sun might look much like any other star. If you could drive there at 65 mph, you would arrive in 7,000 years, Morgan said.
Pluto is now only 2.8 billion miles from Earth. The last time it was this close was one Plutonian year ago--about the time George Washington was chopping down his father's cherry tree.
It was only in 1978 that astronomers discovered that Pluto has a constant companion, a moon half its size, called Charon. Pluto and its moon always face each other, like Western square dancers.
Researchers hope that understanding Pluto will shed light on our own planetary origins. For example, the fact that Pluto spins on its side suggests that it was knocked about harshly eons ago.
Planetary scientists now believe that the creation of the solar system was a messy and rather violent process. "Star formation is very inefficient," said planetary scientist Richard Terrile of the Jet Propulsion Laboratory in Pasadena. When the sun formed, he explained, "it left behind the debris we call Earth and Jupiter and the other planets." When the planets formed, the leftover debris turned into moons and rings.
Pluto and its cousins in the Kuiper belt are chunks of space junk that didn't get incorporated into something else, or were knocked out of the solar system altogether.
As to whether the solar system's black sheep earns the official appellation "planet," Stern and colleagues are adamant. "It's round. It has a satellite. It has an atmosphere," said Stern. Thus, it is a planet, he said.
It qualifies as a planet, Lowell Observatory astronomer Marc Buie points out, because its atmosphere is stuck gravitationally to its surface. A comet's atmosphere gets blown off as it approaches the sun, never to return. The new photographs, says Buie, tell scientists that Pluto's "got a lot more contrast than anything else" in its vicinity. "It's going to be an interesting place."
If all goes well, the National Aeronautics and Space Administration should have two small probes on the way to Pluto by 2001, and there is the potential for a couple of Russian probes to descend directly into the atmosphere.