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PLUTO
GENERAL INFORMATION
Pluto is a dwarf planet (or plutoid) that usually orbits past the orbit of Neptune. It was classified as a
dwarf planet in 2006; before that it was considered to be a planet, the smallest planet in our solar
system. There are many other dwarf planets in our Solar System. Pluto is smaller than a lot of the other
planets' moons, including our moon. Pluto has not been visited by spacecraft yet; we only have blurry pictures
of its surface; even the Hubble Space Telescope orbiting the Earth can only get grainy photos because Pluto is
so far from us. In 2015, a spacecraft called New Horizons (launched by NASA in 2006) will visit Pluto.
SIZE
Pluto is about 1,413 miles
(2274 km) in diameter.
This is about 1/5 the
diameter of the Earth.
Pluto is smaller than the 8
planets in our Solar
System.
MASS AND GRAVITY
Pluto's mass is about 1.29 x 1022 kg. This is about 1/500th of the mass of the Earth. The gravity on Pluto is 8%
of the gravity on Earth.
Pluto is the least massive planet in our Solar System (and is now classified as a dwarf planet).
A 100 pound person on Pluto would weigh only 8 pounds.
LENGTH OF A DAY AND YEAR ON PLUTO
ach day on Pluto takes 6.39 Earth days. Each year on Pluto takes 247.7 Earth years (that is, it takes 247.7 Earth
years for Pluto to orbit the Sun once).
PLUTO'S ORBIT
Pluto is 39 times farther from than the sun than the Earth is.
Pluto ranges from 2.8 to 4.6 billion miles (4.447 billion to
7.38 billion km) from the Sun. From Pluto, the sun would look
like a tiny dot in the sky.
Occasionally, Neptune's orbit is actually outside that of Pluto;
this is because of Pluto's highly eccentric (non-circular) orbit.
During this time (20 years out of every 248 Earth years),
Neptune is actually the farthest planet from the Sun (and not
Pluto). From January 21, 1979 until February 11, 1999, Pluto
was inside the orbit of Neptune. Now and until September
2226, Pluto is outside the orbit of Neptune.
Orbital Eccentricity
Pluto has a very eccentric orbit; that means that its distance from the sun varies a
lot during its orbit around the sun. Sometimes it is even closer to the Sun than the
planet Neptune (it was that way from January 1979 to February 11, 1999)! Pluto
also rotates about its axis in the opposite direction from most of the other planets.
Orbital Inclination
Pluto's orbit is tilted from the
plane of the ecliptic. This angle,
its orbital inclination, is 17.15°.
This is a much larger inclination
of those of the eight planets.
TEMPERATURE ON PLUTO
Pluto is VERY, VERY cold. Its temperature may range from between -396°F to -378°F (-238°C to -228°C, or
35 K to 45 K). The average temperature is -393°F (-236°C = 37 K).
PLANETARY COMPOSITION
Pluto's composition is unknown. It is probably made up of about 70% rock and 30% water. This is determined
from density calculations; Pluto's density is about 2,000 kg/m3. There may be methane ice together with frozen
nitrogen and carbon dioxide on the cold, rocky surface.
ATMOSPHERE
Not much is known about Pluto's atmosphere. It is probably mostly nitrogen with a little carbon
monoxide and methane - definitely not breatheable by humans. The atmospheric pressure is
probably very low. The atmosphere forms when Pluto is closest to the Sun and the frozen
methane is vaporized by the solar heat. When it is farther from the Sun, the methane freezes again. From Pluto,
the sky would appear black, even when the Sun (the size of a star) is up.
PLUTO'S MOONS
Pluto has one large moon, named Charon; two minscule moons were discovered in 2005. The tiny moons are
called.
Although Charon is small, about 1,172 km (728 miles) in diameter, it about half of the size of Pluto itself.
Charon orbits about 19,640 km from Pluto on average. It may be covered by water ice and probably has no
atmosphere. Charon is in a synchronous orbit around Pluto. That is, Charon is always over the same spot on
Pluto; Charon's orbit takes exactly one Pluto day.
Charon was discovered by Jim Christy in 1978. Charon was named after the mythological demon who ferried
people across the mythological river Styx into Hades.
The two tiny moons, Nix and Hydra, are from 30 and 100 miles (45 to 160 km) in diameter, and orbit Pluto
about 27,000 miles (44,000 km) from Pluto, more than twice as far as the orbit of Charon.
DISCOVERY OF PLUTO
Pluto was discovered after the 8 planets and was originally considered a planet itself (until 2006). In the early
1900s, Planet "X" was the temporary name given to the then-unknown planet beyond Neptune that disturbed the
orbits of Uranus and Neptune. Percival Lowell calculated the rough location of Planet "X's" orbit, but died in
1916 before it was found. This planet was eventually found by the American astronomer Clyde W. Tombaugh
in 1930 and named Pluto. He did his observations at the Lowell Observatory in Arizona.
PLUTO-EARTH COMPARISON
PLUTO'S NAME
Pluto was named after the Roman god of the underworld, Pluto.
Its symbol is the combined letters "P" and "L," either for Percival Lowell or for
Pluto.
This is the symbol of Pluto.
The name Pluto was suggested by Venetia Burney of England, who was 11 years old at the time. She suggested
the name to her grandfather, who was Librarian at Oxford. He passed her idea to the astronomers who were
trying to name the newly-discovered planet.
PLUTO: PLANET, ASTEROID, OR TNO
Pluto's unusual orbit makes some scientists think that Pluto is not a regular planet, but a "dwarf planet," "minor
planet" or a Trans Neptunian Object (TNO) [Kuiper Belt objects left over from the formation of the solar
system]. In 2006 Pluto was re-classified as a dwarf planet. Alternatively, Pluto could have been be listed as an
asteroid (it would have been given the asteroid number 10,000). It also may be the first TNO - it will also still
be considered a dwarf planet.
Pluto
1. Pluto is now considered to be a dwarf planet. Is Pluto always the farthest "planet" from the
Sun?_________________
2. Is Pluto rocky or gaseous?_______________________
3. Is Pluto bigger than the Earth, smaller than the Earth, or about the same size as the
Earth?_______________________
4. How long does it take Pluto to revolve around the Sun once (how long is one year on
Pluto)?_______________________
5. Is Pluto's orbit close to being circular or quite eccentric? _____________________
6. How many degrees is the orbit of Pluto tilted from the plane of the ecliptic? _______________________
7. Does Pluto have a moon? ______________________
8. When was Pluto discovered and by whom? In _________ by ___________________
9. Has a spacecraft from Earth ever visited Pluto? _____________
10. From Pluto, would the sky appear blue, red, or black? ______________________
Fill in the blanks below.
Word Bank:
Tombaugh
farthest
dot
moon
spacecraft
mass
gravity
8
1978
ninth
direction
Sun
orbit
Pluto
asteroid
Pluto is the ________________________ and usually the ________________________ planet (a dwarf planet) from the
Sun in our Solar System; it is also the smallest planet in our Solar System. This cold, rocky planet was the last planet to be
discovered (Pluto was considered to be a planet from its discovery until 2006, when it was reclassified as a dwarf planet).
Clyde W. ________________________ discovered Pluto in 1930. Unlike the 8 larger planets in our Solar System, Pluto
not been visited by our ________________________ yet. We only have blurry pictures of its surface. From Pluto, the Sun
would look like a tiny ________________________ in the sky.
Pluto is about 1,413 miles (2274 km) in diameter. This is about 1/5 the diameter of the Earth. It is smaller than a lot of the
other planets' moons, including our ________________________.
Pluto's ________________________ is about 1.29 x 1022 kg; it is the least massive planet in our Solar System. Pluto is
about 1/500th of the mass of the Earth. The ________________________ on Pluto is only 8% of the gravity on Earth. A
100-pound person on Pluto would weigh only ________________________ pounds.
Each day on Pluto takes 6.39 Earth days. Each year on Pluto takes 247.7 Earth years (that is, it takes 247.7 Earth years for
Pluto to orbit the ________________________ once).
Pluto has a very eccentric ________________________; that means that its distance from the sun varies a lot during its
orbit around the sun. Pluto also rotates about its axis in the opposite ________________________ from most of the other
planets. Pluto's orbit is tilted from the plane of the ecliptic. This angle, its orbital inclination, is 17.15°. This is the largest
inclination of any of the planets. Pluto's unusual orbit makes some scientists think that Pluto is not a regular planet, but
perhaps an ________________________.
Pluto has one large moon, named Charon, that is almost as big as ________________________ itself. Two minuscule
moons were discovered in 2005. Charon was discovered by Jim Christy in ________________________. Charon was
named after the mythological demon who ferried people across the mythological river Styx into Hades.