PLUTO

The Dwarf Planet

Many people were shocked in 2006 when the International Astronomical Union announced that Pluto was no longer a planet. Instead, it was henceforth to be known as a dwarf planet, along with other outer solar system worlds such as Quaoar, Makemake, Eris, and Sedna. This didn’t change the fact that Pluto has been the “King of the Kuiper Belt” for decades, but in the eyes of the public, the demotion of Pluto was sad news. The media got involved and there were dueling scientists on TV talk shows, all over an attempt to define planets more rigorously than in the past.

Pluto is still the same world it was before the announcement. The change in status from planet to dwarf planet didn’t affect its physical parameters one bit. But it does allow better classification of the worlds of the outer solar system, and that’s important. First, Pluto lies some 5 billion kilometers (3.1 billion miles) from the Sun, and it takes 248 years to make one orbit around the Sun. Pluto was discovered in 1930 by Clyde Tombaugh (1906–1997), and it hasn’t even made one complete orbit since its discovery.

Pluto Facts

  1. Closest point to Sun: 4.4 billion kilometers (2.7 billion miles)
  2. Most distant point from Sun: 7.3 billion kilometers (4.5 billion miles)
  3. Length of year: 247.8 Earth years
  4. Length of day: 6.39 Earth days (retrograde)
  5. Tilt of axis: 123 degrees
  6. Gravity: 0.9 Earth’s gravity

Pluto orbits the Sun in a region of the solar system called the Kuiper Belt. It stretches out from the orbit of Neptune and probably contains many worlds the size of Pluto (or even larger). Like many of those other objects, Pluto is made of rock and ice, with a surface made almost entirely of nitrogen ice mixed with small amounts of carbon dioxide and methane. Pluto probably has a rocky core surrounded by a water ice mantle. There may be some kind of heating going on inside, since patchy surface features seem to indicate that some kind of cryovolcanic activity is forcing icy material out from beneath the surface.

Pluto’s Moons

From its frozen icy surface, the Sun would look very much like a large, bright star. If you could stand on Pluto, you’d see its largest moon Charon off in the distance. There are also four other moons, all recently discovered in observations made by the Hubble Space Telescope. Charon is actually classified as a dwarf double planet with Pluto. They’re locked together in what’s called a tidal resonance, which means that they each present the same face to each other as they orbit around their common center of gravity.

The moons Nix and Hydra were discovered in 2005. They have bright and dark patches on their surfaces, which suggests the existence of ice deposits. Two other very small moons temporarily named P4 and P5, have been found. One was discovered as part of a search for hazards in the Pluto system that might affect the upcoming New Horizons mission to the outer solar system.

What Are Dwarf Planets?

When the International Astronomical Union debated the definition of planet and came up with a set of criteria that excluded Pluto, Sedna, Eris, and other newly discovered objects in the Kuiper Belt, the members also devised a new definition for those worlds. They are dwarf planets—worlds that orbit the Sun, and are rounded by their self-gravity, but have not yet cleared their orbits of other planetesimals. They’re also not satellites of other worlds. Currently there are five recognized dwarf planets:

1. Pluto (with its companion Charon)

2. Ceres

3. Haumea

4. Makemake

5. Eris

These last three were discovered in 2004 and 2005, and their discovery played a large role in the discussion about redefining planets and dwarf planets. The issues are still being debated, and there may be further clarifications coming.

Where Did Pluto Come From?

Pluto has an eccentric orbit, which means that it follows a very elliptical path around the Sun. Other planets’ orbits are more circular while Pluto’s path is highly inclined. Most of the other worlds orbit near the plane of the solar system, but Pluto’s orbit takes it above and below the plane. That eccentricity and high inclination tell an interesting tale of Pluto’s origins. First, it’s probably a planetesimal left over from the formation of the solar system some 4.5 billion years ago. It never coalesced with other pieces to form a planet. At one time, it was thought that perhaps Pluto had been captured from outside the solar system, since that would explain its strange orbital characteristics. However, early in solar system history, the gas giants likely formed much closer to the Sun. At some point, they migrated to their current positions in the outer solar system. Neptune’s migration likely swept Pluto and other similarly sized Kuiper Belt objects into their current positions and orbits.

Pluto’s Discovery

Pluto is tough to spot. It can’t be seen without a good-sized telescope and a lot of patience. In the late nineteenth century, astronomers speculated about whether or not there even was another planet out beyond Neptune, the latter having been discovered because of its effect on the orbit of Uranus. Further observations of Uranus showed that there were still some effects not accounted for by Neptune’s presence. The idea of a Planet X spurred Percival Lowell (1855–1916), who founded Lowell Observatory in Flagstaff, Arizona, to mount a search for a ninth planet. As it turns out, Lowell and others did photograph Pluto, but they didn’t know what they had. It took a determined search by a young man named Clyde Tombaugh to find this elusive object. He made photographic plates of the sky where Pluto was predicted to be and then compared them to find an object that seemed to move. When he found his quarry on February 18, 1930, the news galvanized the world. It was the first planet discovered in the new century, and it was found by an American. At the suggestion of an English schoolgirl named Venetia Burney, Tombaugh named his discovery Pluto and noted that the first two letters were also a salute to Percival Lowell, who had died some years earlier. Later, it turned out that the extra perturbations in Uranus’s and Neptune’s orbits were the result of a mathematical error, and not due to Pluto’s influence.

Exploring the Outer Frontier

The exploration of Pluto has, for many years, come through ground-based observatories and orbiting instruments such as the Hubble Space Telescope. That will change in 2015 when the New Horizons mission will fly past Pluto and its moons and give the first up-close views of this distant world. After that, the probe will move farther out through the Kuiper Belt, examining other objects in this new frontier of the solar system.

The Kuiper Belt is a disk-shaped region in the outer solar system that stretches out beyond Neptune. It lies inside the Oort Cloud, which is shaped more like a sphere. The Kuiper Belt is filled with small icy worlds and comet nuclei. The Oort Cloud also contains countless cometary nuclei.