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
Pluto’s
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.
