The Atlas of the Solar System THE DISCOVERY OF PLUTO


(Patrick Moore and Garry Hunt, “The Atlas of the Solar System,” Crescent Books, New York, 1990, p. 396.)

* Because a slight perturbation remained in the orbits of the known planets in 1846, it was surmised that another planet might exist beyond Neptune.

* Percival Lowell made a rough calculation of the likely location of the speculated planet. He used a telescope located in Arizona to search for the unknown planet. But by the time of Lowell’s death, in the year 1916, he had not found his Planet “X.”

* In the year 1929, astronomers at the Lowell Observatory in Flagstaff, Arizona decided to once more initiate Lowell’s search for Planet “X.” Using an involved approach which considered star-field’s, Clyde Tombaugh, an amateur astronomer who used the new telescope at the Lowell Observatory was able to photograph Pluto in the year 1930. The planet Tombaugh found was named Pluto. Pluto became the outermost known planet of the solar system.


(Bevan M. French and Stephen P. Maran, eds., “A Meeting with the Universe,” NASA EP-177, U.S. Government Printing Office, 1981.)

*The previous card is an image of Pluto and its moon Charon taken by the U.S. Naval Observatory.

* Pluto is a planetary oddball, a strange world that has baffled scientists ever since it was discovered in 1930. It is not the large gas giant that one might expect to find in the outer reaches of the solar system. Instead, it is a small world, much smaller than the Earth, and in fact roughly as large as our Moon. It is probably composed of a mixture of rock and ice. It even has been suggested that Pluto is not a genuine planet, but simply a moon that somehow escaped from Neptune.

* Pluto is usually the farthest known planet from the Sun, its mean distance almost 6 billion kilometers (almost 4 billion miles) out. It takes 248 years for Pluto to complete one orbit around the Sun, but the orbit is so elongated that it actually spends about 20 years of this time inside the orbit of Nepture. (In fact, Pluto is inside Neptune’s orbit now, and will be until 1999, so that Neptune is temporarily the furthest planet from the Sun.)

* Despite Pluto’s distance and the extreme difficulty of observing it, our view of the faraway planet has changed greatly in the last few years. As we have looked more carefully, Pluto has become an even smaller and brighter object than we thought it was.

* It seems to have a bright layer of frozen methane (“marsh gas,” chemically CH4) on its surface. Even more surprising, reexamination of old photographs revealed that Pluto is not alone; it has a moon. Pluto now seems to be about 3,000 to 3,500 kilometers (1,900 to 2,200 miles) in diameter. Pluto’s moon, Charon, is large by comparison, about 1,200 to 1,500 kilometers (750 to 930 miles) in diameter.

* Pluto will hold its secrets for a long time yet. It is simply too far away for our current spacecraft to reach it in a reasonable length of time. It will be many years before any machines or humans see Pluto up close, dimly lit by a Sun so distant that it seems like just a rather bright star in the blackness of space.

* The last planet? Tiny, mysterious Pluto is so far from the Sun that it appears only as a tiny speck of light that moves slowly against the background of the fixed stars. So inconspicuous that it was not discovered until 1930, Pluto is not a gas giant planet like all the others in the outer solar system. Instead it is a small, rocky world about the size of Earth’s Moon. Recent examinations (1981) of old photographs, combined with new observations, indicate that Pluto itself has a moon.

(The following is from “The Solar System,” NASA/ASEP, 1989, p. 12.)

* Pluto is named for the Greek god of wealth.

* Pluto is the ninth planet from the Sun.

* Pluto has nothing in common with gas giants.

* One day on Pluto equals 6 Earth days, 9 hours, and 18 minutes.

* The diameter of Pluto is 2,170 miles.

* Pluto’s surface gravity is .05 of Earth’s gravity

* Pluto’s surface temperature is -369° F.

* The atmosphere on Pluto has not been detected.

* Pluto, the smallest known planet, is about the size of Earth’s moon.

* This planet’s orbit takes it inside the orbit of Neptune.

* Pluto’s one moon, Charon, was discovered in 1978 and is about half the size of Pluto.

* By the end of this century, Pluto will be the only planet not visited by a space probe.

* Is Pluto really a planet?


(NASA, Jet Propulsion Laboratory, “Our Solar System at a Glance,” NASA Information Summaries, PMS 010-A (JPL), June 1991.)

Pluto is the most distant of the planets, yet the eccentricity of its orbit periodically carries it inside Neptune’s orbit, where it has been since 1979 and where it will remain until March 1999. Pluto’s orbit is also highly inclined, tilted 17 degrees to the orbital plane of the other planets.

Discovered in 1930, Pluto appears to be little more than a celestial snowball. The planet’s diameter is calculated to be approximately 2,300 kilometers (1,430 miles), only two-thirds the size of our Moon. Ground-based observations indicate that Pluto’s surface is covered with methane ice and that there is a thin atmosphere that may freeze and fall to the surface as the planet moves away from the Sun. Observations also show that Pluto’s spin axis is tipped by 122 degrees.

The planet has one known satellite, Charon, discovered in 1978. Charon’s surface composition is different from Pluto’s; the moon appears to be covered with water-ice rather than methane ice. Its orbit is gravitationally locked with Pluto, so both bodies always keep the same hemisphere facing each other. Pluto’s and Charon’s rotational period and Charon’s period of revolution are all 6.4 Earth days.

Although no spacecraft have ever visited Pluto, NASA is currently exploring the possibility of such a mission.