Pluto

[General] [History] [Charon] [Unknowns]

PHYSICAL DATA Sidereal period:   247.7 Rotation period:     6 days, 9 hrs, 12 min Mean orbital velocity: 4.7 km/s Diameter: 1,137 km Mean Surface Temp: -230°C Distance from Sun: 1,347 to 1,507 million km

    Whether or not Pluto is really a planet is a topic that gets astronomers in very heated discussion. This body is so small that some people would rather classify it as a large asteroid. However, because it has been traditionally taught that there are nine planets in our solar system, and because the debate is still continuing, we will discuss this body along with the other planets.

General
Pluto is a strange planet. For one thing, 8% of the time its orbit actually brings it in past Neptune. And then, of course, there is the fact that it is so small (smaller than Earth's moon). It does have a satellite (Charon), but this pair is sometimes considered to be better classified as a double asteroid.

Some people think that Pluto is the largest of the Kuiper Belt (a disk-shaped region beyond Neptune that is presumed to hold many ice objects ranging in size from very small to very large). If Pluto was the largest of these objects, it could have been influenced by Neptune and pushed into its rather strange and lengthy (249 year) orbit around the Sun.

When you think of Pluto, think small. Not only is it smaller than Earth's moon, it is actually smaller than six other of the Solar System's moons: Io, Europa, Ganymede, Callisto, Titan, and Triton are all bigger.With a radius of about 1100 km, it's about as wide as Greenland is long.

Pluto also is strange in that it rotates in the opposite direction from most of the other planets. Also Pluto and its moon rotate together, always showing the same face to each other. And Pluto's incline is thought to be about 50°. (see image at left).

Although Pluto's orbit comes in closer than Neptune, they are on different planes and will not collide. From Earth, Pluto has a yellow hue but HST images indicate it is more salmon-colored with what might be patches of light (frozen water) and dark (frozen methane) frost. The planet appears to be a combination of rock (70%) and ice (30%). Pluto has a northern ice cap but little is known of much of the planet. The atmosphere of Pluto probably consists primarily of oxygen but it is likely frozen for the majority of Pluto's year.

History
It took 25 years of hunting to find Pluto. In 1905, Percival Lowell (1855-1916) began a search for Planet X, a hypothetical planet that helped account for the relationships between the other planets. After Lowell's death, his colleagues at the Flagstaff Arizona Observatory continued the systematic search of the sky. On February 18, 1930, they found Pluto but at first believed it to be too small to call a planet.

Some people think Pluto was named after the Disney cartoon character but is was, of course, the other way around. When Pluto was being named, Lowell's widow suggested Zeus, the New York Times liked Minerva. and some suggested Lowell as its name. In Greek mythology, Pluto is the god of the dark and evil underworld. This ball of rock and ice may not be evil but it is dark and hard to see. Even the Hubble (HST) can only see Pluto as a fuzzy disk (left). Pluto is so far from the Sun that it is in virtually perpetual darkness.

So far Pluto holds another record: it is the only planet that has not been visited by any spacecraft.  

Charon
In 1978, Pluto's satellite Charon was discovered. When seen with Pluto (right), Charon can be seen to be about half the size of Pluto. It's unusual for a planet to have a moon nearly half its size. That's a big moon.

Pluto and Charon are so far away from the Sun and little light reaches it. Some suggest that they were formerly together but split into lighter (Charon) and heavier (Pluto) segments. The HST images of Pluto and Charon (left) resemble abstract art seen through a fuzzy lens.

Unknowns

1. What does Pluto look like close up?
2. Are there seasons on Pluto?
3. Why do Pluto and Churn always face each other?
4. What are the dark areas on Pluto made of?
5. Why is Pluto's orbit so unusual?

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