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PLUTO
Pluto is usually the furthest planet from our Sun in the solar system, residing at an average distance of 3.6 billion miles. "Usually" is the key word, in that Pluto exhibits a highly elliptical and tilted orbit, thereby making Neptune the furthest planet from the Sun for "brief" periods of time. Pluto rotates on its axis every 6.39 days and takes 247.7 years to revolve around the Sun. It has one known satellite named Charon, which is about half the size of Pluto. Charon orbits Pluto every 6.4 days and also rotates once during this time frame, matching Pluto's rotation. Therefore, Pluto and Charon are acting like a dumbbell in space, rotating around each other in a near perfect lock-step. This is the closest thing to a double-planet system that we know about. Pluto is by far the smallest planet in the solar system. In fact, Pluto is smaller than seven of the solar system's moons, including the Earth's.
Atmosphere and Weather: Since there have been no spacecraft probes to fly-by Pluto, not much is known about this icy planet. Much of what scientists know are observations made from Pluto and Charon eclipsing each other and Pluto occulting (covering up) a star. When Pluto occulted a star, a haze was revealed suggesting that Pluto likely has an extremely thin atmosphere. This atmosphere is probably made up of a blend of nitrogen, carbon monoxide and methane. These elements may exist as a gas when Pluto is closest to the Sun (at perihelion), but would then freeze as Pluto moves further away. The surface of Pluto is frozen methane, possibly formed from methane which fell from the atmosphere. There is some evidence that Pluto's tenuous atmosphere may be slowly escaping into space, possibly moving to Charon. When Pluto occulted the above mentioned star, the haze layer was also theorized to indicate that Pluto has a temperature inversion, possibly indicating that Pluto has some weather. The surface temperature of Pluto has been estimated in a range from -378 to -396 degrees F.
Like Uranus, Pluto is highly tilted at 98.3 degrees. This would give one side of the planet extremely long periods of darkness or light, depending on that side's orientation to the Sun. However it is likely that the planet's temperature is very uniform around the globe. This is due to the fact that the Sun casts such feeble light at that distance and Pluto's atmosphere is almost non-existent.
A side note: There has been considerable debate recently whether to classify Pluto as a planet, or a minor planet. Some astronomers think that it may be part of the Kuiper Belt. This is a belt of asteroids (minor planets) and/or large comet-like objects which lie at the outer reaches of our solar system just beyond Neptune. Due to Pluto's small size and highly eccentric and tilted orbit, there may be some merit to this argument. But the International Astronomical Union left the classification of Pluto as a planet.
A planet has three general characteristics. First, it must directly orbit a star. Second, it must be small enough that it has not been undergoing internal nuclear fusion (like a star). Lastly, it must be large enough that its gravity gives it the general shape of a sphere. Pluto has all three of these characteristics plus a moon and an atmosphere. Out of the tens of thousands of known asteroids in our solar system, none exhibit all three qualities of having an atmosphere, a moon, and a gravitational pull strong enough to shape the rock as a sphere. Ceres is the largest asteroid in our solar system and resides at an average distance of 257 million miles from the Sun, in the great asteroid belt between Mars and Jupiter. It has a diameter of 584 miles. Approximately 200 asteroids have diameters of 60 miles or greater. But most asteroids are small, less than a mile in diameter. In fact, if you combined all the mass of the asteroids in our solar system, their mass would still not equal the mass of the Earth's Moon. Some astronomers argue, that instead of "demoting" Pluto from a planet to a minor planet, that Ceres should be "promoted" to a planet. Their argument is that Ceres displays the three general characteristics of a planet in that it orbits our Sun, has no internal nuclear fusion and has enough gravity that its shape is a sphere. Nearly all asteroids are irregular in shape and are essentially nothing more than rocks in space. (For a sampling of what asteroids are shaped like, refer to the pictures of the two Martian moons, Deimos and Phobos in the Mars page).
Pluto is the only planet that has not been visited by a spacecraft from Earth. This mysterious planet is really a double planet system, since its moon, Charon, is very close to Pluto and about the same size.
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It may seem hard to believe that Pluto could have an atmosphere because it is so cold at 39 AU, where Pluto resides, but it does. Because there are times when Pluto is closer to the sun than is Neptune (making it the 8th planet for roughly 20 years at a time), ices on Pluto's surface evaporate and form an atmosphere. It is continually produced and lost again as long as Pluto is inside Neptune's orbit.
The air is made mostly of nitrogen gas, just like that of the Earth and Saturn's moon Titan, with the addition of carbon monoxide (CO - what comes out of your car) and methane (CH4).
The atmosphere is also similar to that of Neptune's moon Triton.. On Triton there are seasons and motions within the atmosphere. Because Pluto has a heavier atmosphere than Triton, there may even be clouds, winds, and storms. However, seeing these clouds and winds on Pluto is difficult.
It is also possible that the presence of nearby Charon draws material escaping from Pluto's atmosphere to recondense on the surface of Charon. So this binary planet may exchange atmospheric molecules.
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No one knows whether or not Pluto has a magnetosphere. Scientists were very surprised to find that Jupiter's icy moon Ganymede had a magnetosphere because it is hard to explain how an icy body can develop a magnetic field. Nevertheless, Pluto may well have a magnetic field, being warmed by tidal forces stemming from its dual orbit with its moon Charon.
On the other hand, Pluto may have a magnetic interaction with the interplanetary magnetic field similar to that of a comet. Only exploration of this unique system will give us the answer.
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After the discovery of Neptune in 1846, mathematical theory suggested that there still might be a ninth planet. Scientists set out to discover it, and it was finally identified in 1930 by Clyde Tombaugh after a careful search of the sky.
Finding Pluto was difficult. It had to be done by noticing its motion against the background of stars. Because Pluto is so small, it is also very dim in the sky. At 39 Astronomical Units from the sun, and with 248 years to complete its orbit around the sun, Pluto also moves very slowly. So it was many years before the 9th planet could be identified by its motion.
Pluto is named after the Roman god of the underworld. It has one moon named Charon. The two objects act more like a double planetary system. They orbit each other, as if they were in a standoff, waiting for the other to turn their back. Some people say that Pluto isn't a planet at all, but rather a satellite that escaped Neptune's gravitational pull.
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Planet
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Diameter: 2,301 km (1,430 miles)
Mass: .013x10^24 kilograms (0.002 x Earth's)
Density: 1,750 kg/m^3
Minimum Distance from Sun: 4.34 billion km
Maximum Distance from Sun: 7.4 billion km
Minimum Distance from Earth: 4.34 billion km
Name in Roman/Greek Mythology: Pluto/Hades
Rotation Period around Axis: 6.4 days
Revolution Period: 248 years
Tilt of Axis: 122 degrees
Surface Gravity: 0.58 m/s^2 (0.06 x Earth's)
Average Surface Temperature: 44K
Number of Satellites: 1 known
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