The Furthest Planets: Uranus, Neptune, and Pluto

Pluto Mission Comets Kuiper Belt Solar System Asteroids
Sun Saturn Jupiter Galaxies Furthest Planets

They are distant cousins of planet Earth, heavenly bodies which are a mighty distance from the Sun. But while they may not get the publicity of Mars and Saturn, they are destined for further scientific exploration before the end of the decade. Scientists are just beginning to unravel the mysteries of Uranus, Neptune, and Pluto, also known as the seventh, eighth, and ninth planets. Our knowledge of these planets is arguably limited, but is expected to grow significantly in the years ahead.

Uranus is popularly known as the third largest of the gas giants, yet it is less than half the diameter of Saturn. Five main satellites orbit in the plane of its equator. The largest, Oberon, has a diameter of 1630 kilometers. The other satellites include Titania (1600 km), Ariel (1330 km), Umbriel (1110 km), and Miranda (a few hundred km). Like Saturn, Uranus possesses a ring system, but the rings differ in color and width. The rings of Uranus appear darker and thinner than those of Saturn. Minor moons can be detected on each side of Uranus’ outermost rings. Because of Uranus’ distance from the sun, and the fact that there’s little internal heating there, the mean temperature of the planet is extremely low. In fact, Uranus’ atmosphere is so cold that methane (CH4) forms above the clouds.

While Neptune is generally the eighth planet from the sun, it is, on occasion, farther away than Pluto because of the nature of its elliptical orbit. While Neptune is smaller than Uranus, it actually contains more mass. Faint, dark rings are perceptible around Neptune, but the rings are less significant than those of Saturn or Uranus. Neptune has two main moons-one, nearly 3,000 km in diameter, the other, only a few hundred km across. Still, despite being far away from the sun, Neptune still has considerable internal heating because of its mass. Neptune’s atmosphere is made up mainly of hydrogen (85 percent), helium (13 percent), and methane, which can be seen as white, cirrus-like clouds high in the atmosphere. So far, the only spacecraft that has visited Neptune was Voyager 2 in 1989.

Pluto holds the distinction of being the farthest planet from the sun for the most part, although, as mentioned earlier, Neptune is, on occasion, farther away. Because of Pluto’s distance from the sun, its surface temperature is as low as -230 degrees C. Pluto is also unique in that it is the smallest of the planets, measuring only half the size of Mercury. Its landscape is known for being incredibly rocky. In 1994, the Hubble Space Telescope revealed the nature of Pluto’s surface, including the northern polar cap. Pluto possesses only one known satellite, Charon, with a diameter of 1199 km, nearly half the size of the planet itself. Ironically, Charon orbits Pluto at precisely the same speed as Pluto rotates. Pluto is the only planet in our solar system that has never been paid a visit by a spacecraft from earth-but that will change in a few years. NASA has plans for a launch to Pluto in the year 2006. The voyage is designed to reveal information about Pluto’s surface, geology, and atmosphere. And, while exploration of Pluto, Neptune, and Uranus is still in its infancy, space-watchers are hopeful that the three planets will receive more attention in the years ahead, as astronomers probe the outer reaches of the galaxy in the quest for additional information about space.


pluto home page