Pluto orbits beyond Neptune and is classified as a dwarf planet. It is much smaller than Earth's moon. Pluto has a diameter of 2,274 km and its orbit averages 39.5 AU from the sun. Pluto was discovered in 1930 and named after the Roman god of the underworld. It has a large moon called Charon about half its size. The discovery of additional Kuiper belt objects and debate over its classification led to Pluto being reclassified as a dwarf planet in 2006.

1. Pluto orbits beyond the orbit of Neptune (usually). It is much smaller than any of
the official planets and now classified as a "dwarf planet". Pluto is smaller than seven
of the solar system's moons
(the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).
orbit: 5,913,520,000 km (39.5 AU) from the Sun (average)
diameter: 2274 km
mass: 1.27e22 kg
In Roman mythology, Pluto (Greek: Hades) 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.
Pluto has not yet been visited by a spacecraft. Even the Hubble Space
Telescope can resolve only the largest features on its surface (left and
above). A spacecraft called New Horizons was launched in January
2006. If all goes well it should reach Pluto in 2015.
Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered
(in 1978) just before its orbital plane moved edge-on toward the inner solar system. It
was therefore possible to observe many transits of Pluto over Charon and vice versa.
By carefully calculating which portions of which body would be covered at what
times, and watching brightness curves, astronomers were able to construct a rough
map of light and dark areas on both bodies.

2. In late 2005, a team using the Hubble Space Telescope
discovered two additional tiny moons orbiting Pluto.
Provisionally designated S/2005 P1 and S/2005 P2, they are now
known as Nix and Hydra. They are estimated to be between 50
and 60 kilometers in diameter.
Pluto's radius is not well known. JPL's value of 1137 is given
with an error of +/-8, almost one percent.
Though the sum of the masses of Pluto and Charon is known pretty well (it can be
determined from careful measurements of the period and radius of Charon's orbit and
basic physics) the individual masses of Pluto and Charon are difficult to determine
because that requires determining their mutual motions around the center of mass of
the system which requires much finer measurements -- they're so small and far away
that even HST has difficulty. The ratio of their masses is probably somewhere
between 0.084 and 0.157; more observations are underway but we won't get really
accurate data until a spacecraft is sent.
Pluto is the second most contrasty body in the Solar System (after Iapetus).
There has recently been considerable controversy about the classification of Pluto.
It was classified as the ninth planet shortly after its discovery and remained so for 75
years. But on 2006 Aug 24 the IAU decided on a new definition of "planet" which
does not include Pluto. Pluto is now classified as a "dwarf planet", a class distinct
from "planet". While this may be controversial at first (and certainly causes confusion
for the name of this website) it is my hope that this ends the essentially empty debate
about Pluto's status so that we can get on with the real science of figuring out its
physical nature and history.
Pluto has been assigned number 134340 in the minor planet catalog.
Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it
was from January 1979 thru February 11 1999). Pluto rotates in the opposite direction
from most of the other planets.
Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly
1.5 times longer than Neptune's. Its orbital inclination is also much higher than the
other planets'. Thus though it appears that Pluto's orbit crosses Neptune's, it really
doesn't and they will never collide. (Here is a more detailed explanation.)

3. Like Uranus, the plane of Pluto's equator is at almost right angles to the
plane of its orbit.
The surface temperature on Pluto varies between about -235 and -210
C (38 to 63 K). The "warmer" regions roughly correspond to the regions
that appear darker in optical wavelengths.
Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is
probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas
of the surface seem to be covered with ices of nitrogen with smaller amounts of
(solid) methane, ethane and carbon monoxide. The composition of the darker areas of
Pluto's surface is unknown but may be due to primordial organic material or
photochemical reactions driven by cosmic rays.
Little is known about Pluto's atmosphere, but it probably consists primarily of
nitrogen with some carbon monoxide and methane. It is extremely tenuous, the
surface pressure being only a few microbars. Pluto's atmosphere may exist as a gas
only when Pluto is near its perihelion; for the majority of Pluto's long year, the
atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the
atmosphere escapes to space perhaps even interacting with Charon. NASA mission
planners want to arrive at Pluto while the atmosphere is still unfrozen.
The unusual nature of the orbits of Pluto and of Triton and the similarity 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 in
an independent orbit around the Sun and was later 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.
Pluto can be seen with an amateur telescope but it is not easy. There are
several Web sites that show the current position of Pluto (and the other planets) in the
sky, but much more detailed charts and careful observations over several days will be
required to reliably find it. Suitable charts can be created with many planetarium
programs.
Charon

4. Charon ( "KAIR en" ) is Pluto's largest satellite:
orbit: 19,640 km from Pluto
diameter: 1206 km
mass: 1.52e21 kg
Charon is named for the mythological figure who ferried the dead across the River
Acheron into Hades (the underworld).
(Though officially named for the mythological figure, Charon's discoverer was also
naming it in honor of his wife, Charlene. Thus, those in the know pronounce it with
the first syllable sounding like 'shard' ("SHAHR en").
Charon was discovered in 1978 by Jim Christy. Prior to that it was thought that
Pluto was much larger since the images of Charon and Pluto were blurred together.
Charon is unusual in that it is the largest moon with respect to its primary planet in
the Solar System (a distinction once held by Earth's Moon). Some prefer to think of
Pluto/Charon as a double planet rather than a planet and a moon.
Charon's radius is not well known. JPL's value of 586 has an error margin of +/-13,
more than two percent. Its mass and density are also poorly known.
Pluto and Charon are also unique in that not only does Charon
rotate synchronously but Pluto does, too: they both keep the
same face toward one another. (This makes the phases of
Charon as seen from Pluto very interesting.)
Charon's composition is unknown, but its low density (about 2
gm/cm3) indicates that it may be similar to Saturn's icy moons
(i.e. Rhea). Its surface seems to be covered with water ice. Interestingly, this is quite
different from Pluto.
Unlike Pluto, Charon does not have large albedo features, though it may have
smaller ones that have not been resolved.

5. It has been proposed that Charon was formed by a giant impact similar to the one
that formed Earth's Moon.
It is doubtful that Charon has a significant atmosphere.
Read more about Pluto l Pluto facts, pictures and information. by nineplanets.org
What are the requirements for being a planet?
I was always under the impression that for an object to be a planet it had to have a satellite
orbiting around it, a moon, that is why Pluto can be called a planet even though it is so
small. My question then is, why are Mercury and Venus planets and what are the
parameters required for planet status.
Not all planets have moons (you've pointed out that Venus and Mercury do not), and it's not a
requirement.
The definition of planet is mostly a historical distinction. Planets must be orbiting the Sun (or
another star), and must be "large," whatever that means. Beyond that, there are no parameters --
after all, there are only 9! Historically, things have been called planets, and we stick to that.
Some astronomers think Pluto should not be called a planet because it doesn't qualify in their
minds as "large" and it has a weird orbit. Really, though, it's just a label.
August 2006 Update by KLM: this month the International Astronomical Union (IAU) voted to
update the definition of what makes a planet. According to their decision a planet must satisfy
the following three criteria:
It must be an object which independently orbits the Sun
It must have enough mass so that gravity pulls it into a roughly speroidal
shape
It must be large enough to "dominate" its orbit (ie. its mass must be much
larger than anything else which crosses its orbit
Part 3 – The Ongoing Debate
And third, it must have cleared other objects out of the way in its orbital neighborhood.
To clear an orbit, a planet must be big enough to pull neighboring objects into the
planet itself or sling-shot them around the planet and shoot them off into outer space.
According to the IAU, Pluto does not meet this third requirement but is now in a new
class of objects called "dwarf planets." It is this third part of the definition that has
sparked debate.

6. To distinguish a planet from a round asteroid in the asteroid belt, a planet must be massive enough to
clear smaller objects – like asteroids – from their own orbit. The gravity of the planet would pull in
smaller objects which would become part of the new planet.
The problem for Pluto
The problem for Pluto is the fact that its orbit is in the Kuiper Belt along with 43 other
known Kuiper Belt Objects (KBOs). There are possibly billions of objects in the Kuiper
Belt that have not been cataloged yet. Scientists have even found 8 KBOs between
Neptune and Pluto. Some scientists view the new definition as unclear. Exactly how
much does Pluto have to "clear" from its neighborhood to be considered a planet? And
how much has Pluto already influenced its own neighborhood since the planet formed?
These and other questions have been raised in response to the IAU's definition of a
planet.
Consider this: Pluto crosses into Neptune's orbit, but Neptune is still classified a planet.
This is because of the orbits of Pluto and Neptune and that they never get closer to
each other than 17AU (AU=distance from Earth to the Sun). Pluto may cross orbits with
many other Kuiper Belt Objects, but how close do these objects get to Pluto? How close
to objects have to get to Pluto to be considered "in" Pluto's neighborhood?
Diagram of the planet orbits in our solar system, including Pluto, distinctly shows the cross over of
Neptune's and Pluto's orbits.
Journey to the edge
NASA's New Horizon spacecraft is speeding toward the edge of the solar system on its
mission to Pluto. Launched in January 2006, it will not be until July 2015 that we will
reach Pluto. It will swing past Jupiter for a gravity boost and scientific studies in
February 2007, and reach Pluto and its moon, Charon, in July 2015. Then, as part of an
extended mission, the spacecraft would head deeper into the Kuiper Belt to study one
or more of the icy mini-worlds in that vast region, at least a billion miles beyond
Neptune's orbit. Sending a spacecraft on this long journey will help us answer basic

7. questions about the surface properties, geology, interior makeup and atmospheres on
these bodies.
The Planets of the Solar System
Click here to go to The Nine Planets home page
Including...
The Inner Planets
Mercury
Venus
The Earth/Moon system (a separate file)
Mars
The Outer Planets
Jupiter
Saturn
Uranus
Neptune
Pluto
The Inner (Terrestrial) Planets
Mercury
The innermost planet

8. One of the brightest objects in the sky
But rarely seen - WHY? (Never more than 28 deg. from the sun)
Fastest moving of the planets (named for the Greek & Roman God of Speed)
Orbits sun in 88 earth days at 48 km/s
Orbit is most varied of all planets (except Pluto)
Highly eccentric (0.206)
Perihelion: 46 million kilometers
Aphelion: 70 million kilometers
Also, inclined 7 deg. to plane of ecliptic
Very small, but with a similar density to earth (5.5)
Seems to be similar in composition to earth, but different proportions
Core accounts for 60% of its total mass (fig 13.22, pg. 230)
Basically a small metal ball with a thin silicate crust
Surface features
Quite a range of temperatures
Up to 400 deg. C at 'noon'
Proximity to sun supplies the daytime heat
Drops to -175 deg. C just before dawn
Lack of atmosphere allows the heat to escape at night
This would give your heat pump a real workout
Heavily cratered like the moon
With areas that have been flooded by basalt

9. Volcanics very early in planets history (4 b.y.)
No evidence of plate tectonics
Isolated 'scarps' indicate shrinkage during cooling (fig. 13.25, pg. 232)
General tectonic history indicates:
Early expansion while hot
Releasing basaltic flows
Later shrinkage during cooling
Causing scarps due to compression/contraction
Venus
Very similar to earth on overall features
Physical features nearly identical (Table 15.1; pg. 258)
Surface is only lightly cratered
Dominated by volcanic activity
Definite "continents" as on earth (2 of them)
Indirect evidence for surface water in the past
One difference is its retrograde rotation
"One of the most beautiful objects in the night sky"
Named for the Goddess of Love and Beauty
Beauty is clearly "only skin deep" because its a rather ugly place at the surface
Extremely harsh surface conditions
Temperature well above 400 deg. C

10. Atmospheric pressure 90X that of earth (we would implode!)
Both the result of the extremely dense atmosphere (96% CO2)
Thick cloud cover is the result of H2SO4 droplets in the atmosphere
Probably derived from extensive volcanic activity
Greenhouse effect
DIGRESS TO: runaway greenhouse effect
Venus used to be more like Earth
Almost certainly had large amounts of surface water
Initial slow surface heating due to small increase in atmospheric CO2
Leads to increased evaporation and H2O content in air
Leads to more heat retention, and the "Runaway Greenhouse Effect" cycle
Carried to its logical conclusion...
Leads to evaporation of any surface waters and a "hot water" atmosphere
Water vapor is not stable in UV light and breaks down into atomic form
Hydrogen escapes into space
Oxygen combines with iron, etc. at the surface
Therefore, the loss of surface water is permanent
Could this happen on the earth?
Mars
The "red planet" named for the God of War
Much smaller than the earth

11. Approx. 11% earth's mass
Atmosphere similar to Venus in composition (95% CO2)
But not in density - .006 bar (Mars) to 1 bar (Earth) to 90 bar (Venus)
Surface similar to earth 200 mya when Pangea was complete
Southern highland (continent) which is heavily cratered (probably older)
Surrounded by younger volcanic plains (not covered by water)
Several kilometers lower in elevation than the "continent"
Extensive tectonic and volcanic activity
No direct evidence of plate tectonic activity
Several features indicating tectonic/volcanic activity
Most 1-3 billion years old
Tharsis Bulge - active region the size of North America
Concentration of "recent" volcanic activity
Olympus Mons (fig. 14.8, pg. 241)
Probably largest volcano in solar system (fig. 15.14, pg. 268)
Possibly still intermittently active!
Valles Marineris (fig. 14.10, pg. 243)
A tectonic feature so not really a "valley"
Basically tension cracks on the edge of the Tharsis Bulge
Similar to tensional features in Africa
Big! 5000 km X 100 km X 7 km deep
Possibly plate-style activity may have started long ago

12. Did not develop like on earth due to smaller mass, quicker overall cooling
Much evidence for surface water (See: photo pg. 234; fig. 14.8b, pg. 241)
Most drainage features limited to older cratered highland areas
Two kinds of drainage patterns
Normal dendritic patterns (fig. 14.12, pg. 244)
Developed on the older cratered upland areas
Evidence for catastrophic floods (fig. 14.21, pg. 250)
From the upland onto the lava plains
Like the Channeled Scablands of Eastern Washington
Evidence for glacial ice ages in the Martian past?
Supports theory that surface water was present during 2 periods in the past
The first 4 billion years ago related to "normal" rainfall/runoff
Then later a sudden release of frozen water by volcanic heating, or?
All surface water now frozen into polar ice caps (fig. 14.2, pg. 236)
Possibility for life in the past (maybe now?)
Mars has been the origin of some of earth's most fearsome alien creatures
Invaders From Mars, War of the Worlds, My Favorite Martian
None identified by any of the Martian probes
Intense UV from sun would make "life as we know it" unlikely
However, "water is life" and Mars has water
Stay tuned for an update

13. The Outer (Jovian) Planets
General features of Jupiter and Saturn
Largest planets in our solar system
Jupiter is almost a binary partner to the sun
Each has a system of orbiting satellites (moons) and rings
Can be quite extensive
Saturn has an impressive ring structure and 19 moons
Basically mini-suns - Composed of hydrogen and helium
Theoretical internal structure - fig. 16.3, pg. 277
Pressure at depth compresses the hydrogen into liquid, then "metallic" form
With a small "ice and rock" core
Central cores may represent the original rock/ice bodies which accumulated from the
nebula
With the hydrogen being "captured" at a later time
Not "rock" as we recognize it due to extreme pressure and temperature
Some form of iron, silica, and oxygen
The "ice" is also probably different from what goes into a glass of Pepsi
Any combination of hydrogen with carbon, nitrogen, or oxygen
Both radiate impressive amounts of heat from 2 sources
Residual heat from the initial condensation of the nebula
Newly generated heat from continued contraction of the gas
Well developed "atmospheres"

14. Hydrogen and helium, with methane (CH4) and ammonia (NH3)
Jupiter
In orbit: 16 moons, faint ring
Has an extensive cloud cover
Vivid colors (white, orange, red, brown)
Essentially condensed ammonia (anyone for a walk in the rain?)
Great Red Spot (fig. 16.2, pg. 276) and (fig. 16.12, pg. 284)
A large "storm" in the atmosphere - almost 30,000 km across!
Has been "stable" for at least 300 years
How can it last for so long?
Nothing solid to interfere with the circulation of the gas
Saturn
In orbit: 19 moons. extensive rings
Uranus
In orbit: 15 moons, intricate system of dark rings
Neptune
In orbit: 8 moons, faint rings
Great Dark Spot: similar to GRS on Jupiter

15. Atmospheric storm 10,000 km across
Pluto
Discovered after a systematic search
Its presence was indicated by "wobbles" in Neptune's orbit
In orbit: a single large moon (Charon)
Essentially a binary system
Let’s find out why Pluto is no longer considered a planet.
Pluto was first discovered in 1930 by Clyde W. Tombaugh at the Lowell Observatory in Flagstaff Arizona. Astronomers had long predicted
that there would be a ninth planet in the Solar System, which they called Planet X. Only 22 at the time, Tombaugh was given the laborious
task of comparing photographic plates. These were two images of a region of the sky, taken two weeks apart. Any moving object, like an
asteroid, comet or planet, would appear to jump from one photograph to the next.
After a year of observations, Tombaugh finally discovered an object in the right orbit, and declared that he had discovered Planet X. Because
they had discovered it, the Lowell team were allowed to name it. They settled on Pluto, a name suggested by an 11-year old school girl in
Oxford, England (no, it wasn’t named after the Disney character, but the Roman god of the underworld).
The Solar System now had 9 planets.
Astronomers weren’t sure about Pluto’s mass until the discovery of its largest Moon, Charon, in 1978. And by knowing its mass (0.0021
Earths), they could more accurately gauge its size. The most accurate measurement currently gives the size of Pluto at 2,400 km (1,500
miles) across. Although this is small, Mercury is only 4,880 km (3,032 miles) across. Pluto is tiny, but it was considered larger than anything
else past the orbit of Neptune.
Over the last few decades, powerful new ground and space-based observatories have completely changed previous understanding of the
outer Solar System. Instead of being the only planet in its region, like the rest of the Solar System, Pluto and its moons are now known to be
just a large example of a collection of objects called the Kuiper Belt. This region extends from the orbit of Neptune out to 55 astronomical
units (55 times the distance of the Earth to the Sun).
Astronomers estimate that there are at least 70,000 icy objects, with the same composition as Pluto, that measure 100 km across or more in
the Kuiper Belt. And according to the new rules, Pluto is not a planet. It’s just another Kuiper Belt object.

16. Here’s the problem. Astronomers had been turning up larger and larger objects in the Kuiper Belt. 2005 FY9, discovered by Caltech
astronomer Mike Brown and his team is only a little smaller than Pluto. And there are several other Kuiper Belt objects in that same
classification.
Astronomers realized that it was only a matter of time before an object larger than Pluto was discovered in the Kuiper Belt.
And in 2005, Mike Brown and his team dropped the bombshell. They had
discovered an object, further out than the orbit of Pluto that was probably the same size, or even larger. Officially named 2003 UB313, the
object was later designated as Eris. Since its discovery, astronomers have determined that Eris’ size is approximately 2,600 km (1,600 miles)
across. It also has approximately 25% more mass than Pluto.
With Eris being larger, made of the same ice/rock mixture, and more massive than Pluto, the concept that we have nine planets in the Solar
System began to fall apart. What is Eris, planet or Kuiper Belt Object; what is Pluto, for that matter? Astronomers decided they would make a
final decision about the definition of a planet at the XXVIth General Assembly of the International Astronomical Union, which was held from
August 14 to August 25, 2006 in Prague, Czech Republic.
Astronomers from the association were given the opportunity to vote on the definition of planets. One version of the definition would have
actually boosted the number of planets to 12; Pluto was still a planet, and so were Eris and even Ceres, which had been thought of as the
largest asteroid. A different proposal kept the total at 9, defining the planets as just the familiar ones we know without any scientific rationale,
and a third would drop the number of planets down to 8, and Pluto would be out of the planet club. But, then… what is Pluto?
In the end, astronomers voted for the controversial decision of demoting Pluto (and Eris) down to the newly created classification of “dwarf
planet”.
Is Pluto a planet? Does it qualify? For an object to be a planet, it needs to meet these three requirements defined by the IAU:
It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet.
It needs to have enough gravity to pull itself into a spherical shape – Pluto…check
It needs to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule breaker. According to this, Pluto is not a planet.

17. What does “cleared its neighborhood” mean? As planets form, they become the dominant gravitational body in their orbit in the Solar
System. As they interact with other, smaller objects, they either consume them, or sling them away with their gravity. Pluto is only 0.07 times
the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.
Any object that doesn’t meet this 3rd criteria is considered a dwarf planet. And so, Pluto is a dwarf planet. There are still many objects with
similar size and mass to Pluto jostling around in its orbit. And until Pluto crashes into many of them and gains mass, it will remain a dwarf
planet. Eris suffers from the same problem.
It’s not impossible to imagine a future, though, where astronomers discover a large enough object in the distant Solar System that could
qualify for planethood status. Then our Solar System would have 9 planets again.
Even though Pluto is a dwarf planet, and no longer officially a planet, it’ll still be a fascinating target for study. And that’s why NASA has sent
their New Horizons spacecraft off to visit it. New Horizons will reach Pluto in July 2015, and capture the first close-up images of the (dwarf)
planet’s surface.
Space enthusiasts will marvel at the beauty and remoteness of Pluto, and the painful deplaneting memories will fade. We’ll just be able to
appreciate it as Pluto, and not worry how to categorize it. At least now you know why Pluto was demoted.
Read more: http://www.universetoday.com/13573/why-pluto-is-no-longer-a-planet/#ixzz2WHovp9Oy
The definition of planet set in 2006 by the International Astronomical Union (IAU) states that, in
the Solar System, a planet is a celestial body which:
1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has "cleared the neighbourhood" around its orbit.
A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet". According
to the IAU, "planets and dwarf planets are two distinct classes of objects". A non-satellite body fulfilling
only the first criterion is termed a "small Solar System body" (SSSB). Initial drafts planned to include
dwarf planets as a subcategory of planets, but because this could potentially have led to the addition of
several dozens of planets into the Solar System, this draft was eventually dropped. The definition was a
controversial one and has drawn both support and criticism from different astronomers, but has remained
in use.
According to the definition, there are currently eight planets and five dwarf planets known in the Solar
System. The definition distinguishes planets from smaller bodies and is not useful outside the Solar
System, where smaller bodies cannot be found yet. Extrasolar planets, or exoplanets, are covered
separately under a complementary 2003 draft guideline for the definition of planets, which distinguishes
them from dwarf stars, which are larger.