Planetary scientists are getting excited about the planet Pluto -- even though it never gets closer than four billion kilometers from Earth. A few years ago, the International Astronomical Union in Paris had to squelch talk about downgrading our solar system's tiniest planet to mere asteroid status. Now there's a new buzz about Pluto because next year's planned New Horizons space probe to our most distant planet survived attempts to kill it and remains in President Bush's 2006 budget.

But don't pace the floor waiting for the results of the visit to this far-off hunk of rock, which meanders around the sun for 248 years in a single orbit. If the New Horizons probe is launched, it will take 11 years to reach Pluto.

"Sounds like there'd be no rush, but there actually is a rush, scientifically," says Bruce Betts, a scientist at the Planetary Society, a space-advocacy group based in California.

That's because Pluto has made the turn at one pointy end of its almond-shaped orbit and is now heading away from the sun. At some point -- and it's not exactly clear when -- Pluto's atmosphere is likely to freeze out on the surface. So, if you want to see the planet while it actually has an atmosphere -- and also see whatever surface there is below what would freeze out -- you need to get a probe to Pluto in the next couple of decades.

Pluto has its idiosyncrasies. Like rings drawn on a table, our sun's planets orbit on the same, mostly flat plane. But Pluto's orbit tilts a bit. It even intersects Neptune's orbit for 20 years, briefly making Neptune our farthest planet.

Nevertheless, we don't have to worry about a monumental collision between Neptune and Pluto. When the two orbits intersect, "they're pretty much on opposite sides of the sun," says Russ Poch, a physics professor at Howard Community College in Maryland, who often writes about planets on the college's website. "So it's more likely you're going to have a comet hitting a planet before that."

Pluto was first spotted 75 years ago by a young American astronomer, Clyde Tombaugh, who named the dark and distant planet for the Roman god of the underworld. Pluto remained mysterious until 1990, when the Hubble telescope was launched into orbit 600 kilometers above the Earth. Hubble provided clear photos and a spectral analysis of Pluto and its lone, curious moon, Charon -- curious because Charon is half as big as Pluto itself. Charon wasn't found until 1978, hiding among the so-called Kuiper Belt of objects, including Pluto, which traverse the edges of our solar system beyond Neptune.

The Planetary Society's Bruce Betts says the region is still yielding astonishing new information about our solar system. "There have been a couple of other discoveries," he says. "The most notable, in the past year or so, is a body called Sedna, which is still much smaller than Pluto but is a good-sized body. Sedna has a really strange orbit. It's very, very elliptical. It goes much, much farther out than Pluto."

Scientists are itching to get a close look at Pluto because, like the four inner planets -- Mercury, Venus, Earth, and Mars -- it's some form of rock, though ice-covered most of the time. It's not a giant gaseous sphere like the other four outer planets -- Jupiter, Saturn, Uranus, and Neptune. Pluto also has some unexplained bright markings that fascinate scientists.

Howard Community College's Russ Poch says even far-off Pluto may help us understand our world, just as researchers are gleaning information from the recent Cassini probe to Titan, one of the moons of Saturn. "It was found to have some gases that were very similar to what our early Earth's atmosphere had," Professor Poch explains. "People always want to hypothesize, 'Well, if this is there, could life exist?' We're based on carbon-based life, but are there other kinds of life that might be based on other types of materials? So it would be interesting to see if there were chemicals out there that primitive life could evolve from."

A more familiar Pluto -- Mickey Mouse's pet dog in Walt Disney cartoons -- made his first appearance in 1930, the same year Clyde Tombaugh named the new and distant planet. But the dog did not get its name until the following year. Whether or not Mr. Disney named the floppy-eared canine after the planet, he never said.



Pluto makes the grade as a planet rather than an obscure trans-Neptunian object, out in the distant Kuiper Belt, because it orbits the sun, has a moon of its own, and is of sufficient diameter. Pluto is estimated to be 2,300 kilometers across.

Pluto's elliptical orbit fascinates scientists because -- unlike other planets that travel at a relatively constant speed through a circular orbit -- Pluto moves faster as it draws closer to the gravitational pull of the sun. Then, heading away from the sun -- as it has recently begun to do on its almost 250-year journey through a single orbit -- Pluto gradually slows, then almost stops and is drawn back yet again by the sun's gravity.

The planet's symbol, PL, is not inspired only by Pluto's name. It also honors American astronomer Percival Lowell, whose observations of the solar system beyond Neptune -- long before Pluto was actually discovered in 1930 -- led him to speculate that another planet would be found.

Pluto is a cold, dark place. 40 times farther from the sun than the Earth, it gets 1/1600 as much light -- and incredibly less warmth -- than we do. How cold is cold? About minus-235 degrees Celsius on a typical Plutonian day.

Like Pluto, its moon, Charon has an underworld twist. It, too, takes its name from a mythological character -- Greek instead of Roman: Charon was the boatman who piloted unfortunate souls across the River Styx into Hades' domain.]

1999 was the last of 20 straight years in which Pluto's orbit most recently crossed Neptune's. It won't happen again for 222 years.

Planets like Uranus, Neptune and Pluto are so far away that they are never seen by the naked eye.

The distinction between a planet and a star is often thought to be quite simple: Many people perceive that in the night sky, a star twinkles because it's a sun?while a planet does not twinkle. But even though explosions on stars are indeed giving off great heat and flashes of light, while planets are comparatively calm, the twinkling has little to do with what's going on in space. It is caused by disturbances (waves) in our atmosphere. So planets do sometimes appear to twinkle. But stars more often twinkle because they are bright points of light, whereas the closer planets, with their discernible, disc-like shapes, are less vivid.

Planets, by definition, circle a sun. Therefore, objects like comets that zing through space, or moons that circle planets -- no matter how large they may be -- cannot be planets.