Pluto Planet: Facts & Information

Pluto Planet Facts

Pluto was formerly regarded as the outermost and small- est planet. It also was considered the most recently  discovered planet, having been found in 1930. In August  2006, however, the International Astronomical Union, the  organization charged by the scientifi c community with clas- sifying astronomical objects, voted to remove Pluto from  the list of planets and give it the new classifi cation of dwarf  planet. The change refl ects astronomers’ realization that  Pluto is a large member of the Kuiper Belt, a collection of  debris of ice and rock left over from the formation of the  solar system and now revolving around the Sun beyond  Neptune’s orbit.  

Pluto is not visible in the night sky to the unaided eye. Its largest moon, Charon , is close enough in size to the dwarf  planet that it has become common to refer to the two bodies as a double system. Pluto is designated by the symbol ♇.  Named for the god of the underworld in Roman myth- ology (the Greek equivalent is Hades), Pluto is so distant  that the Sun’s light, which travels about 300,000 km  (186,000 miles) per second, takes more than fi ve hours to  reach it. An observer standing on Pluto’s surface would see  the Sun as an extremely bright star in the dark sky,providing Pluto on average 1/1,600 of the amount of sunlight that reaches  Earth. Pluto’s surface temperature there- fore is so cold that common gases such  as nitrogen and carbon monoxide exist  there as ices.  

 Because of Pluto’s remoteness and

small size, the best telescopes on Earth

and in Earth’s orbit have been able to

resolve little detail on its surface. Indeed,

such basic information as its radius and

mass have been diffi cult to determine;

most of what is known about Pluto has

been learned since the late 1970s as an

outcome of the discovery of Charon. Pluto

has yet to be visited by spacecraft, though

the U.S. spacecraft New Horizons

departed Earth for the Pluto-Charon sys-

tem in 2006 and will arrive there in July

2015; many key questions about it and its

environs can be answered only by close-

up robotic observations.

Basic astronomical data

Pluto’s mean distance from the Sun,

about 5.9 billion km (3.7 billion miles or

39.5 AU), gives it an orbit larger than that

of the outermost planet, Neptune. Its

orbit, compared with those of the planets,

is atypical in several ways. It is more

elongated, or eccentric, than any of the

planetary orbits and more inclined (at

17.1°) to the ecliptic, the plane of Earth’s

orbit, near which the orbits of most of the

planets lie. In traveling its eccentric path

around the Sun, Pluto varies in distance

from 29.7 AU, at its closest point to the

Sun (perihelion), to 49.5 AU, at its far-

thest point (aphelion). Because Neptune

orbits in a nearly circular path at 30.1 AU,

Pluto is for a small part of each revolu-

tion actually closer to the Sun than is

Neptune. Nevertheless, the two bodies

will never collide, because Pluto is locked

in a stabilizing 3:2 resonance with

Neptune—i.e., it completes two orbits

around the Sun in exactly the time it

takes Neptune to complete three. This

gravitational interaction affects their

orbits such that they can never pass

closer than about 17 AU. The last time

Pluto reached perihelion occurred in

1989; for about 10 years before that time

and again afterward, Neptune was more

distant than Pluto from the Sun.

Observations from Earth have

revealed that Pluto’s brightness varies

with a period of 6.3873 Earth days, which

is now well established as its rotation

period (or sidereal day). Of the planets,only Mercury, with a rotation period of

almost 59 days, and Venus, with 243 days,

turn more slowly. Pluto’s axis of rotation

is tilted at an angle of 120° from the per-

pendicular to the plane of its orbit, so that

its north pole actually points 30° below

the plane. (For comparison, Earth’s north

polar axis is tilted 23.5° away from the

perpendicular, above its orbital plane.)

Pluto thus rotates nearly on its side in a

retrograde direction; an observer on its

surface would see the Sun rise in the west

and set in the east.

Compared with the planets, Pluto is

also anomalous in its physical character-

istics. Pluto has a radius less than half

that of Mercury; it is only about two-thirds

the size of Earth’s Moon. Next to the outer

planets—the giants Jupiter, Saturn,

Uranus, and Neptune—it is strikingly tiny.

When these characteristics are combined

with what is known about its density and

composition, Pluto appears to have more

in common with the large icy moons of

the outer planets than with any of the

planets themselves. Its closest twin is

Neptune’s moon Triton, which suggests a

similar origin for these two bodies.

The atmosphere

Although the detection of methane ice

on Pluto’s surface in the 1970s gave scien-

tists confidence that the body had an

atmosphere, direct observation of it had

to wait until the next decade. Discovery

of its atmosphere was made in 1988 when

Pluto passed in front of (or occulted) a star as observed from Earth. The star’s

light gradually dimmed just before it dis-

appeared behind Pluto, demonstrating

the presence of a thin, greatly distended

atmosphere. Because Pluto’s atmosphere

must consist of vapours in equilibrium

with their ices, small changes in tempera-

ture should have a large effect on the

amount of gas in the atmosphere. During

the years surrounding Pluto’s perihelion

in 1989, when Pluto was slightly less cold

than average, more of its frozen gases

vaporized; the atmosphere was then at or

near its thickest, making it a favourable

time to study the body. Astronomers esti-

mate a surface pressure in the range of a

few to several tens of microbars. At aph-

elion, when Pluto is receiving the least

sunlight, its atmosphere may not be

detectable at all.

Observations made during occulta-

tions cannot provide direct information

about atmospheric composition, but they

can allow determination of the ratio of

mean molecular weight to temperature.

Using reasonable assumptions about the

atmospheric temperature, scientists have

calculated that each particle—i.e., each

atom or molecule—of Pluto’s atmosphere

has a mean molecular weight of approxi-

mately 25 atomic mass units. This implies

that significant amounts of gases heavier

than methane, which has a molecular

weight of 16, must also be present.

Molecular nitrogen, with a molecular

weight of 28, must in fact be the domi-

nant constituent, because nitrogen ice known to be more volatile than methane

ice. Nitrogen is also the main constituent

of the atmospheres of both Triton and

Saturn’s largest satellite, Titan, as well

as of Earth.

Although ongoing Earth-based obser-

vations will add to knowledge about the

atmosphere and other aspects of Pluto,

major new insights will likely require a

close-up visit from a spacecraft. Scientists

looked to the U.S. New Horizons space-

craft mission, launched in 2006, to Pluto,

Charon, and the outer solar system

beyond to provide much of the needed

data. The mission plan called for a nine-

year flight to the Pluto-Charon system

followed by a 150-day flyby for investiga-

tion of the surfaces, atmospheres,

interiors, and space environment of the

two bodies.

The surface and interior

Observations of Pluto show that its colour

is slightly reddish, although much less

red than Mars or Jupiter’s moon Io. Thus,

the surface of Pluto cannot be composed

simply of pure ices, a conclusion sup-

ported by the observed variation in

brightness caused by its rotation. Its aver-

age reflectivity, or albedo, is 0.55 (i.e., it

returns 55 percent of the light that strikes

it), compared with 0.1 for the Moon and

0.8 for Triton.

The first crude infrared spectroscopic

measurements reveal that Pluto’s south

polar region is unusually bright. Scientists

find such variation in Pluto’s surface striking because, with the exception of

Saturn’s mysterious moon Iapetus , all the

other icy bodies in the outer solar system

exhibit much more uniform surfaces.

Brightness maps based on observations

with the Earth-orbiting Hubble Space

Telescope reveal some of this heteroge-

neity, but only visiting spacecraft can

provide the spatial resolution needed to

make associations between brightness

and surface composition or topography.

Pluto’s moons

Pluto possesses three known moons.

Charon, by far the largest, is fully half the

size of Pluto. It revolves around the dwarf

planet—more accurately, the two bodies

revolve around a common centre of

mass—at a distance of about 19,640 km.

(12,200 miles), equal to about eight Pluto

diameters. (By contrast, Earth’s Moon is a

little more than one-fourth the size of

Earth and is separated from the latter by

about 30 Earth diameters.)

Charon’s period of revolution is

exactly equal to the rotation period of

Pluto itself; in other words, Charon is in

synchronous orbit around Pluto. As a

result, Charon is visible from only one

hemisphere of Pluto. It remains above the

same location on Pluto’s surface, never

rising or setting (just as do communica-

tion satellites in geostationary orbits over

Earth). In addition, as with most moons

in the solar system, Charon is in a state of

synchronous rotation—i.e., it always pres-

ents the same face to Pluto.

Charon is somewhat less reflective

(has a lower albedo—about 0.35) than

Pluto and is more neutral in colour. Its

spectrum reveals the presence of water

ice, which appears to be the dominant

surface constituent. There is no hint of

the solid methane that is so obvious on

its larger neighbour. The observations to

date were not capable of detecting ices of

nitrogen or carbon monoxide, but, given

the absence of methane, which is less vol-

atile, they seem unlikely to be present.

Charon’s density implies that the moon

contains materials such as silicates and

organic compounds that are denser than

water ice. The disposition of these mate-

rials inside Charon is even more

speculative than it is for Pluto.

Discovery of Pluto and its moons

When Pluto was found, it was considered

the third planet to be discovered, after

Uranus and Neptune, as opposed to the

six planets that have been visible in the

sky to the naked eye since ancient times.

The existence of a ninth planet had been

postulated beginning in the late 19th cen-

tury on the basis of apparent perturbations

of the orbital motion of Uranus, which

suggested that a more-distant body was

gravitationally disturbing it. Astronomers

later realized that these perturbations

were spurious—the gravitational force

from Pluto’s small mass is not strong

enough to have been the source of the

suspected disturbances. Thus, Pluto’s dis-

covery was a remarkable coincidence

attributable to careful observations rather than to accurate prediction of the exis-

tence of a hypothetical planet.

 The search for the expected planet

was supported most actively at the Lowell

Observatory in Flagstaff , Ariz., U.S., in

the early 20th century. It was initiated by

the founder of the observatory, Percival

Lowell , an American astronomer who

had achieved notoriety through his

highly publicized claims of canal sight-

ings on Mars. After two unsuccessful

attempts to fi nd the planet prior to

Lowell’s death in 1916, an astronomical

camera built specifi cally for this purpose

and capable of collecting light from a

wide fi eld of sky was put into service in

1929, and a young amateur astronomer,

 Clyde Tombaugh , was hired to carry out

the search. On Feb. 18, 1930, less than one

year after he began his work, Tombaugh

found Pluto in the constellation Gemini.

The object appeared as a dim “star” of

the 15th magnitude that slowly changed

its position against the fi xed background

stars as it pursued its 248-year orbit

around the Sun. Although Lowell and

other astronomers had predicted that the

unknown planet would be much larger

and brighter than the object Tombaugh

found, Pluto was quickly accepted as the

expected ninth planet. The symbol

invented for it, ♇, stands both for the fi rst

two letters of Pluto and for the initials of

Percival Lowell.A team of nine astronomers

working  in the United States discovered 


two small moons, Hydra and Nix, in 2005

via images made with the Hubble Space

Telescope during a concerted search for

objects traveling around Pluto as small as

25 km (16 miles) in diameter. To confirm

the orbits, the astronomers checked

Hubble images of Pluto and Charon

made in 2002 for surface-mapping stud-

ies and found faint but definite indications

of two objects moving along the orbital

paths calculated from the 2005 images.

Origin of

Pluto and its moons

Before the discovery of Charon, it was

popular to assume that Pluto was a for-

mer moon of Neptune that had somehow

escaped its orbit. This idea gained sup-

port from the apparent similarity of the

dimensions of Pluto and Triton and the

near coincidence in Triton’s orbital

period (5.9 days) and Pluto’s rotation

period (6.4 days). It was suggested that

a close encounter between these two

bodies when they were both moons led

to the ejection of Pluto from the

Neptunian system and caused Triton to

assume the retrograde orbit that is pres-

ently observed.

Astronomers found it difficult to

establish the likelihood that all these

events would have occurred, and the dis-

covery of Charon provided information

that further refuted the theory. Because

the revised mass of Pluto is only half

that of Triton, Pluto clearly could not

have caused the reversal of Triton’s orbit.

Also, the fact that Pluto has a propor-

tionally large moon of its own makes the

escape idea implausible. Current think-

ing favours the idea that Pluto and

Charon instead formed as two indepen-

dent bodies in the solar nebula, the

gaseous cloud from which the solar sys-

tem condensed. Just as the Moon

appears to be deficient in volatile ele-

ments relative to Earth as a consequence

of its high-temperature origin, so also

can the absence of methane on Charon,

along with the relatively high densities

of both Pluto and Charon, be explained

by a similar process.

Astronomers have argued that

Pluto’s two small moons also are prod-

ucts of the same collision that resulted

in the present Charon. The alternative

scenario—that they formed indepen-

dently elsewhere in the outer solar

system and were later gravitationally

captured by the Pluto-Charon system—

does not appear likely given the

combination of circular coplanar orbits

and multiple dynamic resonances that

currently exist for the two small bodies

and Charon. Rather, these conditions

suggest that material in the ring of

debris that was ejected from the colli-

sion accreted into all three moons—and

possibly into others yet to be found.

This collision scenario implies that

at the time the Pluto-Charon system

formed, about 4.6 billion years ago,

the outer solar nebula contained many

icy bodies with the same approxi-

mate dimensions as these two. The

bodies themselves are thought to have

been built up from smaller entities that

today would be recognized as the nuclei

of comets.

Most of these icy planetesimals were

incorporated into the cores of the giant

planets during their formation. Many

others, however, are thought to have

remained as the unconsolidated debris

that makes up the Kuiper Belt, which

includes the outer part of Pluto’s orbit.

After more than 1,000 Kuiper Belt objects 

(KBOs) were directly observed starting

in the early 1990s, astronomers came to

the conclusion that Pluto and Charon

likely are large members of the Kuiper

Belt and that bodies such as Chiron,

Neptune’s moon Triton, and a number of

other icy moons of the outer planets

originated as KBOs.(solar system before). 

This flux comes  from the fringe of the Oort 

cloud. He  identified it by looking at the distribution  

of the original values of the total energies

of cometary orbits. These energies are in

proportion to a−1, with a being the semim-

ajor axis of the cometary orbit. The

original value of a refers to the orbit when

the comet was still outside of the solar

system, as opposed to the osculating

orbit, which refers to the arc observed

from Earth after it has been modified by

the perturbations of the giant planets.

Passages through the solar system pro-

duce a rather wide diffusion in orbital

energies (in a −1). In 1950 Oort accounted

for only 19 accurate original orbits of

long-period comets. The fact that 10 of

the 19 orbits were concentrated in a very

narrow range of a−1 established that most

of them had never been through this dif-

fusion process due to the planets. The

mean value of a for these new comets

suggested the distance they were coming

This distance is also  the place where perturbations resulting  

from the passage of nearby stars begin to

be felt. The distance coincidence sug-

gested to Oort that stellar perturbations

were the mechanism by which comets

were sent into the planetary system.

Subsequently, using a much larger

number of observed orbits, the American

astronomer Brian Marsden calculated

that the part of the Oort cloud where new

comets originate—the more distant part

of the cloud—is between 40,000 and

50,000 AU from the Sun. At such dis-

tances, the orbits of the tiny icy bodies

The Oort Cloud

Beyond the Kuiper Belt lies the Oort

cloud, an immense, roughly spherical

cloud of icy small bodies that are inferred

to revolve around the Sun at distances

typically more than 1,000 times that of

the orbit of Neptune, the outermost

known major planet. Named for Jan Oort,

who demonstrated its existence, the Oort

cloud comprises objects that are less than

100 km (60 miles) in diameter and that

number perhaps in the trillions, with an

estimated total mass 10–100 times that of

Earth. Although too distant to be seen

directly, it is believed to be the source of

most of the historically observed long-

period comets—those that take more than

200 years (and usually much longer) to

orbit the Sun.

The Estonian astronomer Ernest J.

Öpik in 1932 suggested the possible pres-

ence of a distant reservoir of comets,

arguing that, because comets burn out

relatively quickly from their passages

through the inner solar system, there

must exist a source of “fresh” comets,

which steadily replenishes the comet

supply. Although these comets have

never been in the inner solar system

before, they are difficult to distinguish

from older long-period comets because,

by the time they are first observed, their

orbits already have been gravitationally

perturbed by the outer planets.

In 1950, Oort showed by statistical

arguments that a steady flux of a few

“new” comets are observed per year.

The heliopause

Between the Kuiper Belt and the Oort

cloud lies the heliopause, the teardrop-

shaped region around the Sun that is

filled with solar magnetic fields and the

outward-moving solar wind consisting of protons and electrons. Nearer the

Sun than the heliopause lies the helio-

sheath, a region of transition where the

solar wind slows to subsonic speeds,

that is, slower than the speed with which

disturbances travel through the inter-

stellar medium.

The tail of the heliopause is estimated

to be between 110 and 170 AU (17 and 26

billion km [10 and 16 billion miles]) from

the Sun. Its shape fluctuates and is influ-

enced by a wind of interstellar gas caused

by the Sun’s motion through space. The

orbits of all the major planets, including

Earth’s, lie well within the heliopause.

No satellite has yet reached the helio-

pause, although the Voyager 1 and 2

probes launched in 1977 are the closest to

it at distances from the Sun of 105 and 85

AU (16 and 13 billion km [10 and 8 billion

miles]), respectively. (Voyager 1 and 2

crossed into the heliosheath at distances

from the Sun of 94 and 84 AU in 2004 and

2007, respectively.) What is known about

the heliopause is deduced by its effects

on cosmic-ray particles coming into the

solar system after passing through it and

by the radio emission generated when

material thrown off by the Sun in coronal.

mass ejections crosses it.

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