Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered. Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun.
Radius: 1,187 km
Discovered: February 18, 1930
Gravity: 0.62 m/s²
Discoverer: Clyde Tombaugh
Moons: Charon, Hydra, Nix, Kerberos, Styx
History of Pluto
In Roman mythology, Pluto is the god of the underworld. The planet received this name (after many other suggestions) perhaps because it’s so far from the Sun that it is in perpetual darkness and perhaps because “PL” are the initials of Percival Lowell.
Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.
After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no Planet X. But that doesn’t mean there aren’t other objects out there, only that there isn’t a relatively large and close one like Planet X was assumed to be. In fact, we now know that there are a very large number of small objects in the Kuiper Belt beyond the orbit of Neptune, some roughly the same size as Pluto.
Until 2015 even the Hubble Space Telescope was able to resolve only the largest features on its surface. On 14 July 2015, the New Horizons spacecraft did a flyby of Pluto after being launched.
|Mass (kg)||1.29 x 1022|
|Mean density (kg/m3)||2030|
|Escape velocity (m/s)||1100|
|Average distance from Sun||39.48 AU (5,906,376,200 km)|
|Rotation period (length of day in Earth days)||6.39 (retrograde)|
|Revolution period (length of year in Earth years)||247.92|
|Obliquity (tilt of axis degrees)||122.5|
|Orbit inclination (degrees)||17.15|
|Orbit eccentricity (deviation from circular)||0.248|
|Mean temperature (K)||37|
|Visual geometric albedo (reflectivity)||about 0.5|
|Atmospheric components||perhaps methane and nitrogen|
|Surface materials||perhaps methane ice|
Structure of Pluto
Pluto lies in the darkest reaches of our solar system, taking 248 years for it to orbit the sun, and it is 2.8 billion miles away from the sun when at its closest approach. It was discovered by accident in the 1930s when calculations, which later turned out to be in error, had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing the error Tombaugh did a careful sky survey which turned up Pluto anyway. At the moment it is the furthest planet from the sun, but as its orbit is so elongated it is sometimes nearer to the sun than Neptune, as it was from January 1979until February 1999. There are also further oddities in that Pluto’s orbital path is also markedly tilted from the plane in which the other planets orbit, and as with Uranus, Pluto orbits on its side.
Pluto is the only planet in our solar system not to be visited by a spacecraft, making our knowledge of it relatively sketchy. However, a spacecraft called New Horizons was launched in January 2006, which should reach Pluto in 2015. We do know that its diameter makes it the smallest planet in our solar system, smaller even than our moon. Pluto’s mass is only one fifth of our earth, with a rock ice composition similar to that of Neptune’s moon Triton. It is estimated that it is probably 70% rock and 30% water ice. Pluto’s atmosphere may contain mostly nitrogen with some carbon monoxide and methane. It is extremely tenuous however and may exist as a gas only when Pluto is closest to the sun. The planet’s surface temperature varies between approximately –390F and 346F.
The unusual nature of the orbits of Pluto and of Triton and the similarities of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune’s, but this now seems unlikely. A more popular idea is that Triton, like Pluto, once moved into an independent orbit around the sun, and was captured by Neptune. Perhaps Triton, Pluto, and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Oort cloud. Like the earth’s moon Charon may be the result of a collision between Pluto and another body.
Since Pluto is so far from Earth, little was known about the dwarf planet’s size or surface conditions until 2015, when NASA’s New Horizons space probe made a close flyby of Pluto. New Horizons showed that Pluto has a diameter of 1,473 miles (2,370 km), less than one-fifth the diameter of Earth, and only about two-thirds as wide as Earth’s moon.
Observations of Pluto’s surface by the New Horizons spacecraft revealed a variety of surface features, including mountains that reach as high as 11,000 feet (3,500 meters), comparable to the Rocky Mountains on Earth. While methane and nitrogen ice cover much of the surface of Pluto, these materials are not strong enough to support such enormous peaks, so scientists suspect that the mountains are formed on a bedrock of water ice.
Pluto’s surface is also covered in an abundance of methane ice, but New Horizons scientists have observed significant differences in the way the ice reflects light across the dwarf planet’s surface.
Another distinct feature on Pluto’s surface is a large heart-shaped region known unofficially as Tombaugh Reggio. The left side of the reggioCis covered in carbon monoxide ice. Other variations in the composition of surface materials have been identified within the “heart” of Pluto.
In the center left of Tombaugh Reggio is a very smooth region unofficially known by the New Horizons team as “Sputnik Planum,” after Earth’s first artificial satellite, Sputnik. This region of Pluto’s surface lacks craters caused by meteorite impacts, suggesting that the area is, on a geologic timescale, very young — no more than 100 million years old. It’s possible that this region is still being shaped and changed by geologic processes.
These icy plains also display dark streaks that are a few miles long and aligned in the same direction. It’s possible the lines are created by harsh winds blowing across the dwarf planet’s surface.
NASA’s Hubble Space Telescope has also revealed evidence that Pluto’s crust could contain complex organic molecules.
Pluto’s surface is one of the coldest places in the solar system, at roughly minus 375 degrees Fahrenheit. When compared with past images, pictures of Pluto taken by the Hubble Space Telescope revealed that the dwarf planet had apparently grown redder over time, apparently due to seasonal changes.
How did Pluto and its moons get their names?
The Romans named the five planets closest to the Sun after their most important gods. These were the only planets that were bright enough for them to see. Later, when telescopes were used, other planets were discovered. Astronomers decided to continue naming the planets after Roman gods. At the time of Pluto’s discovery, it was considered to be a planet (it is now classified as a dwarf planet). Being very cold and the farthest from the Sun, Pluto was named after the Roman god of death. According to Roman myth, when someone died, they traveled down to the Underworld. First, they had to cross the River of the Dead, called the river Styx. Everyone was buried with a coin, to pay the ferryman, Charon, who would carry the dead across the river Styx in his boat. Pluto’s moon is named Charon after this ferryman of the Underworld. The naming of Pluto’s other moons follows this same basic scheme Nix is the Greek goddess of darkness and night and mother of Charon, Hydra is the nine-headed serpent which battled Hercules, Kerberos is many-headed dog that guarded the entrance to the underworld in Greek mythology, and Styx is the river that souls had to cross over to get to Hades, or the underworld, and the goddess who ruled over it. The International Astronomical Union (IAU) is the final arbiter in naming celestial objects.
Pluto’s rings would be too faint and too small to see directly. But occasionally, Pluto passes in front of a distant star, and that lets us study it in exquisite detail,” Throop said. “As Pluto passes in front of the star, the star’s light blinks out, like a moth blocking out the beam from a flashlight. We searched through the observations to try to find any hint that the starlight was being blocked by rings of Pluto.”
So far, they haven’t found any rings. But Throop will keep looking. He is working with NASA’s New Horizons mission, which is sending a spacecraft to Pluto, to arrive in 2015. When it passes by Pluto, one of New Horizons’ goals will be to conduct a search for rings, at much greater sensitivities than can be done from the Earth.
And ironically, Throop’s search now will actually help plan the encounter in 2015. “Rings are made of tiny dust grains, and we want to be sure that New Horizons will not collide with anything at Pluto,” he said. “By knowing where there aren’t rings, we help assure a safe path where the spacecraft will fly.”
When New Horizons reaches the Pluto system, the spacecraft will provide a wealth of new data about this mysterious region of the Solar System. Studying worlds like Pluto can teach astrobiologists about how dwarf planets form and evolve. This information can ultimately help us determine the types of planets that could exist throughout the Universe. Scientists are still unsure of what we will find at Pluto. Some research suggests that deposits of primordial organic matter might lie on the tiny world’s surface – and liquid water may exist a hundred miles below ground.
What Is Pluto-Like?
Pluto is very, very cold. The temperature on Pluto is 375 to 400 degrees Fahrenheit below zero. Pluto is so far away from Earth that scientists know very little about what it is like. Pictures from NASA’s New Horizons spacecraft show that Pluto has mountains, valleys, plains, craters, and maybe glaciers.
Pluto has about one-fifteenth the gravity of Earth. A person who weighs 100 pounds on Earth would weigh only 7 pounds on Pluto.
Most planets orbit the sun in a near-circle. The sun is in the center of the circle. But Pluto does not orbit in a circle! The orbit of Pluto is shaped like an oval. And the sun is not in the center. Pluto’s orbit is also tilted.
Orbit & Rotation
Pluto’s rotation is retrograde compared to the solar systems’ other worlds; it spins backward, from east to west.
Average distance from the sun: 3,670,050,000 miles (5,906,380,000 km) — 39.482 times that of Earth
Perihelion (closest approach to the sun): 2,756,902,000 miles (4,436,820,000 km) — 30.171 times that of Earth
Aphelion (farthest distance from the sun): 4,583,190,000 miles (7,375,930,000 km) — 48.481 times that of Earth
Pluto’s entire moon system is believed to have formed by a collision between the dwarf planet and another planet-sized body early in the history of the solar system. The smashup flung material that coalesced into the family of satellites observed around Pluto.
“The moons form a series of neatly nested orbits, a bit like Russian dolls,” said team lead Mark Showalter of the SETI Institute in Mountain View, Calif.
The known moons of Pluto are
Charon: Discovered in 1978, this small moon is almost half the size of Pluto. It is so big Pluto and Charon are sometimes referred to as a double planet system.
Nix and *Hydra: These small moons were found in 2005 by a Hubble Space Telescope team studying the Pluto system.
Kerberos: Discovered in 2011, this tiny moon is located between the orbits of Nix and Hydra.
Styx: Discovered in 2012, this little moon was found by a team of scientists searching for potential hazards to the New Horizons spacecraft flyby in 2015.
Scientists are searching for a possible moon orbiting Pluto and also signs of a possible debris field generated by the theoretical impact billions of years ago.
New Horizons is the first spacecraft to explore Pluto, its moons, and the icy bodies in the Kuiper Belt. Its mission is to map the composition and surface features of Pluto and its moons and to search for new moons and even rings.
Along with providing our first close-up views of this distant world, New Horizons will also study Pluto’s fascinating atmosphere. After it flies past Pluto, it may continue on to an encounter with a Kuiper Belt object.
Launched in 2006, New Horizons is the first in the series of NASA’s New Frontiers missions.
A full-scale model of New Horizons is on display in the Museum in Washington, DC. The Johns Hopkins University Applied Physics Laboratory donated the model to the Museum in 2008.
Horizons and its Instruments
New Horizons has seven instruments for gathering images and other data. Because it travels too far from the Sun to rely on solar power, the spacecraft, and its instruments are nuclear powered by a radioisotope thermoelectric generator (RTG).
LORRI (Long Range Reconnaissance Imager)—Telescopic camera that takes high-resolution images.
Ralph—Imaging system that returns color and composition maps.
Alice—Ultraviolet imaging spectrometer for determining atmospheric composition and structure.
SWAP (Solar Wind Around Pluto) and PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation)—Used to study Pluto’s escaping atmosphere.
REX (Radio Science Experiment)—Gathers data on the pressure and temperature of Pluto’s atmosphere.
SDC (Student Dust Counter)—Counts and measures dust particles encountered by the spacecraft.
New Horizons’ 9 1/2-year journeys to Pluto began in 2006 with its launch from Cape Canaveral, Florida. Its Atlas V rocket-propelled it to about 58,000 kilometers (36,000 miles) per hour, making it the fastest-traveling spacecraft ever launched. It passed the orbit of the Moon in only nine hours (Apollo spacecraft took three days) and passed Mars in less than three months. A close encounter with Jupiter provided a gravity assist and a further boost in speed, so it could reach Pluto in July 2015.
Important Dates about Pluto
1930 Clyde Tombaugh discovers Pluto.
1977-1999 Pluto is closer to the sun than Neptune. It will be another 230 years before Pluto is again closer to the sun.
1978 American astronomers James Christy and Robert Harrington discover Pluto’s largest moon, Charon.
1988 Astronomers discover that Pluto has an atmosphere.
2005 Two additional moons are discovered: Nix and Hydra. Astronomers believe they may have formed at the same time Charon did, perhaps in the same large impact event.
2006 NASA’s New Horizon mission is launched with a mission to explore Pluto and the Kuiper Belt. It is scheduled to arrive in 2015.
2011 Pluto’s fourth moon is discovered.
Facts About Pluto
- Pluto is named after the Greek god of the underworld.
- This is a later name for the more well known Hades and was proposed by Venetia Burney an eleven-year-old schoolgirl from Oxford, England.
- Pluto was reclassified from a planet to a dwarf planet in 2006.
- This is when the IAU formalised the definition of a planet as “A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.”
- Pluto was discovered on February 18th, 1930 by the Lowell Observatory.
- For the 76 years between Pluto being discovered and the time it was reclassified as a dwarf planet it completed under a third of its orbit around the Sun.
- Pluto has five known moons.
- The moons are Charon (discovered in 1978,), Hydra and Nix (both discovered in 2005), Kerberos originally P4 (discovered 2011) and Styx originally P5 (discovered 2012) official designations S/2011 (134340) 1 and S/2012 (134340) 1.
- Pluto is the largest dwarf planet.
- At one point it was thought this could be Eris. Currently, the most accurate measurements give Eris an average diameter of 2,326km with a margin of error of 12km, while Pluto’s diameter is 2,372km with a 2km margin of error.
- Pluto is one-third water.
- This is in the form of water ice which is more than 3 times as much water as in all the Earth’s oceans, the remaining two-thirds are rock. Pluto’s surface is covered with ices and has several mountain ranges, light and dark regions, and a scattering of craters.
- Pluto is smaller than a number of moons.
- These are Ganymede, Titan, Callisto, Io, Europa, Triton, and the Earth’s moon. Pluto has 66% of the diameter of the Earth’s moon and 18% of its mass. While it is now confirmed that Pluto is the largest dwarf planet for around 10 years it was thought that this was Eris.
- Pluto has an eccentric and inclined orbit.
- This takes it between 4.4 and 7.3 billion km from the Sun meaning Pluto is periodically closer to the Sun than Neptune.
- Pluto has been visited by one spacecraft.
- The New Horizons spacecraft, which was launched in 2006, flew by Pluto on the 14th of July 2015 and took a series of images and other measurements. New Horizons is now on its way to the Kuiper Belt to explore even more distant objects.
- Pluto’s location was predicted by Percival Lowell in 1915.
- The prediction came from deviations he initially observed in 1905 in the orbits of Uranus and Neptune.
- Pluto sometimes has an atmosphere.
- When Pluto elliptical orbit takes it closer to the Sun, its surface ice thaws and forms a thin atmosphere primarily of nitrogen which slowly escapes the planet. It also has a methane haze that overs about 161 kilometers above the surface. The methane is dissociated by sunlight into hydrocarbons that fall to the surface and coat the ice with a dark covering. When Pluto travels away from the Sun the atmosphere then freezes back to its solid state.
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