Pluto was discovered 75 years ago this week, and astronomers still don't know what to make of the small, frigid world.
They aren't sure exactly what Pluto's made of, how it formed, or why it orbits so oddly compared to the other eight planets. The intriguing questions extend outward. Pluto might have other moons that haven't been found yet. Out beyond Pluto, it's possible a 10th planet -- as big or larger than Pluto -- awaits discovery.
"Do other Plutos remain to be discovered, or is it one-of-a-kind?" wonders Will Grundy, an associate astronomer at the Lowell Observatory, from where Pluto was found.
Meanwhile, scientists can't even agree if Pluto is really a planet.
The hunt for Pluto began in 1905 when Percival Lowell (of Martian Canal infamy) hypothesized about the possibility of a Planet X in the outer solar system.
Lowell died before Pluto was discovered. Clyde Tombaugh found it on Feb. 18, 1930 in a concerted scan of the sky. Tombaugh compared two photographs taken at the Lowell Observatory and noted the object's movement against the background of stars.
Most of Pluto's orbit is out beyond that of Neptune. But the path is oblong, so Pluto spends part of its 248-year orbit -- the time it takes to make one circle around the Sun -- inside the track of Neptune.
Pluto's path is also extremely inclined, by 17.1 degrees, to the main plane of the solar system where the other planets travel.
Asteroids also circle the Sun in the solar system's main plane. So do some comets. But many comets, like Pluto, have highly inclined orbits. This similarity, plus Pluto's small size -- smaller than Earth's Moon -- has led many astronomers to conclude that Pluto has been improperly classified all along. It is not a planet, they say, but rather a Kuiper Belt Object, a member of a swarm of comet-like objects beyond Neptune.
"You start to see where Pluto fits in better with Kuiper objects,'' says Hal Weaver of the Johns Hopkins University Applied Physics Laboratory and project scientist on NASA's New Horizons mission, planned to launch next year en route to the ninth planet.
Being far away and small, Pluto is a mere point of light in most telescopes. So it gives up its secrets grudgingly. But high-powered telescopes have in recent years probed some of Pluto's mysteries.
|All About Pluto|
Studies in 2003 showed that despite an almost nonexistent atmosphere, Pluto has wind and seasons and appears to have recently gone through a phase of global warming.
Among the most significant developments related to Pluto was the discovery of the Kuiper Belt, Grundy told SPACE.com. Since the first Kuiper Belt Object was found in 1992, more than 1,000 have been spotted, some roughly half as big as Pluto. Studies of the variety of objects out there "can tell us so much about the compositional, collisional, and dynamical environment Pluto inhabits today and when it was forming."
The leading theory for the formation of Pluto and its moon, Charon, is a wild one: A nascent Pluto was struck by another Pluto-sized object.
Observational evidence for this collision theory remain thin, and it was just last month that a computer model was finally generated to describe how the scenario could have played out. Imagine a glancing blow and a lot of cosmic Silly Putty getting stretched and repacked into new spheres with new rotations.
Charon is much bigger than any other moon in relation to the size of its host planet, further muddying Pluto's status. Some astronomers think of the setup as a double planet.
Observations are "slowly chipping away" at the mysteries of Pluto, Grundy said, "but the really big breakthroughs probably won't come until we see Pluto up close from a spacecraft flyby."
That's expected to occur in 2015, when New Horizons finally reaches the distant world after a nine-year journey.
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Eccentricity: Pluto's distance from the Sun varies by 25 percent. In 1999, Pluto crossed the orbital path of Neptune to become, once again, the most distant planet in the solar system.
Orbit: 248 Earth years
Time to rotate: 6.4 Earth days
Mass: 0.2% of Earth's
Diameter: 1,430 miles (2,300 km), or 18 percent of Earth's
Distance from Sun: 39.5 times as far as Earth, on average
What's In A Name?
Pluto is also the name of the Greek god of the underworld. It was suggested by many people, but credit was given to an 11-year-old girl from England. Rejected names included: Minerva, the goddess of knowledge, because it was already in use, and Constance, proposed by Constance Lowell - the widow of Percival Lowell, who first hypothesized Planet X. "That suggestion was quietly ignored," says Kevin Schindler of the Lowell Observatory.
Mickey Mouse's Dog?
Mickey's dog, though yet unnamed, made his debut in "The Chain Gang" in 1930 - the same year the planet made its debut to earthlings. Pluto, the Disney character, was named the following year, which leads Disney archivists to assume the dog took the name of the planet dominating the news at the time, said Disney archives director Dave Smith.
A Big Secret
Clyde Tombaugh said he knew right away the specks he was looking at were evidence of Planet X, but the observatory director thought they should be cautious. So Tombaugh went to dinner and waited for nightfall on Feb. 18, 1930. But it was cloudy, so even though he killed another couple of hours at the local theater watching "The Virginian," he was not be able to get more proof of Pluto that night. The discovery was not announced to the public until Percival Lowell's birth date, March 13, 1930.
Not a Planet?
Since Tombaugh's death in 1997, many astronomers have increasingly urged the International Astronomical Union, which names celestial objects, to strip Pluto of its status as a planet. After a news report generated a flurry of irate e-mails about the possible change, officials assured the world that Pluto would remain a planet. Most astronomers have settled on calling Pluto a Kuiper Belt Object, too, and allowing a sort of dual citizenship as far as the public is concerned. Neil deGrasse Tyson, director of New York's Hayden Planetarium at the American Museum of Natural History, removed Pluto from his exhibit of planets five years ago. "I still have folders of hate mail from third-graders,'' Tyson says.
SOURCE: Associated Press and SPACE.com reporting