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.

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.

• Mission to Pluto
• Hot Deal! Pluto, the Last Oasis for Life
• Pluto Hit By Twin to Create Moon, Study Suggests
• Puzzling Seasons and Signs of Wind Found on Pluto
• Solar System Surprise: A New View of What's Out There

Map Pluto from Home! Starry Night software brings the universe to your desktop. Map the sky from your location, or just sit back and let the cosmos come to you. LEARN MORE!