Pluto was found to have an atmosphere from an
occultation observation in 1985 (IAU Circ. 4097; MNRAS 276, 571); the
finding was confirmed and significantly strengthened by extensive observations
of another occultation in 1988. When an object with no atmosphere occults a
star, the star abruptly disappears; in the case of Pluto, the star dimmed out
gradually. From the rate of dimming, the atmosphere was determined to have a
pressure of 0.15
Pa, roughly 1/700,000 that of Earth.
In 2002, another occultation of a star by Pluto was observed and analyzed by
teams led by Bruno Sicardy of the
and by James Elliot of
Jay Pasachoff of
Surprisingly, the atmosphere was estimated to have a pressure of 0.3 Pa, even
though Pluto was further from the Sun than in 1988, and hence should be colder
and have a less dense atmosphere. The current best hypothesis is that the south
pole of Pluto came out of shadow for the first time in 120 years in 1987, and
extra nitrogen sublimated from a polar cap. It will take decades for the excess
nitrogen to condense out of the atmosphere.
spectroscopic discovery of ethane (C2H6) on Pluto's surface, presented by Dale
Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and
colleagues was announced. This ethane is produced from the photolysis or
radiolysis (i.e., the chemical conversion driven by sunlight and charged
particles) of frozen methane (CH4) on Pluto's surface and suspended in its
The MIT-Williams College team of James Elliot and
Pasachoff and a
Southwest Research Institute team led by Leslie Young observed a further
occultation of a star by Pluto on 12 June 2006 from sites in Australia. (Elliot,
J. L., Person, M. J., Gulbis, A. A. S., Adams, E. R., Kramer, E. A., Zuluaga, C.
A., Pike, R. E., Pasachoff, J. M., Souza, S. P., Babcock, B. A., Gangestad, J.
W., Jaskot, A. E., Francis, P. J., Lucas, R., Bosh, A. S. 2006, "The Size of
Pluto's Atmosphere As Revealed by the 2006 June 12 Occultation," Pasadena
Division of Planetary Sciences meeting, October 2006.)
The surface of Pluto is remarkably
heterogeneous, as evidenced by its lightcurve, maps of its surface
constructed from Hubble Space Telescope observations, and by periodic variations
in its infrared spectra. The face of Pluto oriented toward Charon has more
while the opposite face has more ices of
carbon monoxide. This makes Pluto the second most contrasted body in the
Solar System after
Pluto's orbit is very unusual in comparison to the planets of the solar
system. The planets orbit the Sun close to an imaginary flat
plane called the
plane of the ecliptic, and have nearly circular orbits. In contrast, Pluto's
orbit is highly
inclined above the ecliptic (up to 17° above it) and very
eccentric (non-circular). Owing to the orbit’s inclination, Pluto's
perihelion is well above (~8.0
AU) the ecliptic. The high eccentricity means that part of Pluto's orbit is
closer to the Sun than
perihelion, Pluto gets closer to the Sun than Neptune; the most recent
occurrence of this phenomenon lasted from
Mathematical calculations indicate that the previous occurrence lasted only
fourteen years from
September 15, 1749.
However, the same calculations indicate that Pluto was closer to the Sun than
April 30, 1483
and July 23,
1503, which is
almost exactly the same length as the 1979 to 1999 period. Recent studies
suggest each crossing of Pluto to inside Neptune's orbit lasts alternately for
approximately thirteen and twenty years with minor variations.
Pluto orbits in a 3:2
orbital resonance with Neptune. When Neptune approaches Pluto from behind
their gravity starts to pull on each other slightly, resulting in an interaction
between their positions in orbit of the same sort that produces
points. Since the orbits are eccentric, the 3:2 periodic ratio is favoured
because this means Neptune always passes Pluto when they are almost farthest
apart. Half a Pluto orbit later, when Pluto is nearing its closest approach, it
initially seems as if Neptune is about to catch up with Pluto. But Pluto speeds
up due to the gravitational acceleration from the Sun, stays ahead of Neptune,
and pulls ahead until they meet again on the other side of Pluto's orbit.
Pluto is also affected by the
Kozai mechanism, which causes its closest approach to the Sun to occur when
it is farthest out of the plane of the Solar System, again keeping Pluto from
getting too close to Neptune.
Beginning in the 1990s, other trans-Neptunian objects (TNOs) were discovered,
and a certain number of these also have a 3:2 orbital resonance with Neptune.
TNOs with this orbital resonance are named "plutinos",
Pluto's orbit is often described as 'crossing' that of Neptune. In fact,
nodes (the points at which the orbit crosses the ecliptic) are both situated
outside Neptune’s orbit and are separated by a distance of 6.4 AU (that is, over
six times the distance of the Earth from the Sun). Furthermore, due to the
orbital resonance between them, Pluto executes 2 full cycles while Neptune
makes 3; this means that when Neptune reaches the 'closest' point on the orbit,
Pluto remains far behind and when Pluto in turn reaches that point, Neptune is
far (over 50°) ahead. During the following orbit of Pluto, Neptune is half an
orbit away. Consequently, Pluto never gets closer than 30 AU to Neptune at this
point in its orbit.
The actual closest approach between Neptune and Pluto occurs at the opposite
part of the orbit, some 30 years after Pluto's
(its last aphelion was in 1866) when Neptune catches up with Pluto (i.e.
Neptune and Pluto have similar
longitudes). The minimum distance was 18.9 AU in June 1896. In other words,
Pluto never approaches Neptune much closer than it approaches
In the 1950s it was suggested that Pluto was an escaped moon of Neptune,
knocked out of orbit by
Triton, Neptune's largest moon. This notion has since been discredited.
Triton shares many atmospherical and geological composition similarities with
Pluto and is believed to be a captured Kuiper belt object.
Kuiper belt is believed to be the source for all
short-period comets, and Pluto, like other Kuiper Belt objects, shares
features in common with
is gradually blowing Pluto's surface into space, in the manner of a comet.
If Pluto were placed near the Sun, it would develop a tail, like comets do.
Pluto has three known
Charon, first identified in 1978 by astronomer James Christy; and two
Hydra, both discovered in
The Pluto-Charon system is noteworthy for being the largest of the solar
system's few binary systems, i.e. whose
barycenter lies above the primary's surface (617
Patroclus is a smaller example). This and the large size of Charon relative
to Pluto lead some astronomers to call it a dwarf
double planet. The system is also unusual among planetary systems in that
they are both
tidally locked to each other: Charon always presents the same face to Pluto,
and Pluto also always presents the same face to Charon.
|Pluto and Charon, compared to Earth's
||Orbital radius (km)
|Orbital period (d)
Nix and Hydra
Two additional moons of Pluto were imaged by astronomers working with the
Hubble Space Telescope on
2005, and received
provisional designations of S/2005 P 1 and S/2005 P 2. The International
Astronomical Union officially christened Pluto's newest moons
(or Pluto II, the inner of the two moons, formerly P 2) and
Hydra (Pluto III, the outer moon, formerly P 1), on
These small moons orbit Pluto at approximately two and three times the
distance of Charon: Nix at 48,700 kilometres and Hydra at 64,800 kilometers from
the barycenter of the system. They have nearly circular
prograde orbits in the same orbital plane as Charon, and are very close to
(but not in) 4:1 and 6:1 mean motion
orbital resonances with Charon.
Observations of Nix and Hydra are ongoing to determine individual
characteristics. Hydra is sometimes brighter than Nix, speculating that it
either is larger in dimension or different parts of its surface may vary in
brightness. Sizes are estimated from albedos. The moons' spectral similarity
with Charon suggests a 35% albedo similar to Charon's; this results in diameter
estimates of 46 kilometers for Nix and 61 kilometers for brighter Hydra. Upper
limits on their diameters can be estimated by assuming the 4% albedo of the
darkest Kuiper Belt objects; these bounds are 137 ± 11 km and 167 ± 10 km
respectively. At the larger end of this range, the inferred masses are less than
0.3% of Charon's mass, or 0.03% of Pluto's.
With the discovery of the two small moons, Pluto may possess a variable
system. Small body impacts can create debris that can form into a ring
system. Data from a deep optical survey by the
Advanced Camera for Surveys on the
Hubble Space Telescope suggests that no ring system is present. If such a
system exists, it is either tenuous like the
Rings of Jupiter, or it is tightly confined to less than 1000km in width.
The distribution of Plutonian moons is highly unusual compared to other
observed systems. Moons could potentially orbit Pluto up to 53% (or 69%, if
retrograde) of the
sphere radius (stable gravitational zone of influence) of 6.0 million
kilometers. In simple terms, an imaginary sphere is drawn around an object to
represent the potential of an object to have other objects orbit it stably. For
Psamathe orbits Neptune at 40% of the Hill radius. In the case of Pluto,
only the inner 3% of the zone is known to be occupied by satellites. In the
discoverers’ terms, the Plutonian system appears to be "highly compact and
In imaging the Plutonian system, observations from Hubble placed limits on
any additional moons. With 90% confidence, no additional moons larger than 12 km
(or a maximum of 37 km with an albedo of 0.041) exist beyond the glare of Pluto
5 arcseconds from the dwarf planet. This assumes a Charon-like albedo of 0.38;
at a 50% confidence level the limit is 8 kilometers.
Exploration of Pluto
Pluto presents significant challenges for space craft because of its small
mass and great distance from Earth. Voyager 1
could have visited Pluto, but controllers opted instead for a close flyby of
Saturn's moon Titan, which resulted in a trajectory incompatible with a
Voyager 2 never had a plausible trajectory for reaching Pluto.
In 2000, NASA cancelled the Pluto Kuiper Express mission, citing increasing costs and launch vehicle
The first spacecraft to visit Pluto will be NASA's New
Horizons, launched on
2006. The craft
will benefit from a
gravity assist from
the closest approach to Pluto will be on
observations of Pluto will begin 5 months prior to closest approach and will
continue for at least a month after the encounter. New Horizons captured
its first images of Pluto in late September 2006, during a test of the Long
Range Reconnaissance Imager (LORRI).
The images, taken from a distance of approximately 4.2 billion kilometres (2.6
billion miles), confirm the spacecraft's ability to track distant targets,
critical for manoeuvring toward Pluto and other Kuiper Belt objects.
New Horizons will use a remote sensing package that includes imaging
instruments and a radio science investigation tool, as well as spectroscopic and
other experiments, to characterize the global geology and morphology of Pluto
and its moon Charon, map their surface composition and characterize Pluto's
neutral atmosphere and its escape rate. New Horizons will also photograph
the surfaces of Pluto and Charon. Some of the ashes of Pluto's discoverer, Clyde
W. Tombaugh, are aboard the spacecraft.
Discovery of moons Nix and Hydra may present unforeseen challenges for the
probe. With the relatively low escape velocity of Nix and Hydra, collisions with
Kuiper belt debris may produce a tenuous dusty ring. Were New Horizons to fly
through such a ring system, there would be an increased potential for
micrometeorite damage that could damage or disable the probe.
Planetary status controversy
Pluto's official status as a planet has been a constant subject of
controversy, fueled by the past lack of a clear
definition of planet, since at least as early as 1992, when the first
Kuiper Belt Object,
(15760) 1992 QB1, was discovered. Since then, further discoveries
intensified the debate in the 21st century.
Omission from museum models
Museum and planetarium directors occasionally created controversy by omitting
Pluto from planetary models of the solar system. Some omissions were
Hayden Planetarium reopened after renovation in 2000 with a model of 8
planets without Pluto. The controversy made headlines in the media at the time.
Commemoration as a planet
Pluto is shown as a planet on the
Pioneer plaque, an inscription on the space probes Pioneer 10
and Pioneer 11, launched in the early 1970s. The plaque, intended to give
information about the origin of the probes to any alien civilization that might
in the future encounter the vehicles, includes a diagram of our solar system,
showing nine planets. Similarly, an analog image contained within the
Voyager Golden Record included on the probes Voyager 1
and Voyager 2 (also launched in the 1970s) includes data regarding Pluto and
again shows it as the ninth planet.
Elements 92, 93, and 94 are named
Neptune, and Pluto. The Disney character
Pluto, introduced in 1930, was also named in honour of the planet.
New discoveries ignite debate
Continuing advances in telescope technology allowed for further discoveries
of Trans-Neptunian objects in the 21st century, some of comparable size to that
of Pluto. In 2002, 50000 Quaoar was discovered, with a diameter of 1,280
kilometers, about half that of Pluto. In 2004, the discoverers of 90377 Sedna
placed an upper limit of 1,800 kilometers on its diameter, near Pluto's diameter
of 2,320 kilometers.
On July 29,
Trans-Neptunian object later named
Eris was announced, which on the basis of its
magnitude and simple
considerations is assumed to be slightly larger than Pluto. This was the largest
object discovered in the solar system since
Neptune in 1846. Discoverers and media initially called it the "tenth
planet", although there was no official consensus at the time on whether to call
it a planet. Others in the astronomical community considered the discovery to be
the strongest argument for reclassifying Pluto as a minor planet.
The last remaining distinguishing feature of Pluto was now its large moon,
Charon, and its atmosphere; these characteristics are probably not unique to
Pluto: several other Trans-Neptunian objects have satellites; and
Eris' spectrum suggests that it has a similar surface composition to Pluto,
as well as a moon,
Dysnomia, discovered in September 2005. Trans-Neptunian object
2003 EL61 (nicknamed "Santa") has two moons (one of which is
nicknamed "Rudolph") and is the fourth largest TNO behind
Eris, Pluto, and
2005 FY9 (nicknamed "Easterbunny").
The debate came to a head in 2006 with an
IAU resolution that created an official definition for the term "planet".
According to this resolution, there are three main conditions for an object to
be considered a 'planet':
- The object must be in orbit around the
- The object must be massive enough to be a sphere by its own gravitational
force. More specifically, its own gravity should pull it into a shape of
- It must have
cleared the neighborhood around its orbit.
Pluto fails to meet the third condition.
The IAU further resolved that Pluto be classified in the simultaneously created
planet category, and that it act as prototype for a yet-to-be-named category
trans-Neptunian objects, in which it would be separately, but concurrently,
Impact of the IAU decision
There has been resistance amongst the astronomical community towards the
dubbed the "Great Pluto War" by some astronomers.
principal investigator with
Horizons" mission to Pluto, has publicly derided the IAU resolution, stating
that "the definition stinks" albeit "for technical reasons."
Stern's current contention is that by the terms of the new definition Earth,
Mars, Jupiter and Neptune, all of which share their orbits with asteroids would
However, his own published writing has
supported the new list of planets, as "our solar system clearly contains"
eight planets that have cleared their neighborhoods, however, he does not deny
planetary classification to objects like Pluto & Ceres that have not "cleared
Marc W. Buie of the Lowell observatory has voiced his opinion on the new definition
on his website and is one of the petitoners against the definition. Others
have supported the IAU. Mike Brown, the astronomer who discovered
Eris, said "through this whole crazy circus-like procedure, somehow the
right answer was stumbled on. It’s been a long time coming. Science is
self-correcting eventually, even when strong emotions are involved."
Among the general public, reception is mixed amidst widespread media
coverage. Some have accepted the reclassification, while some are seeking to
overturn the decision, with online petitions urging the IAU to consider
reinstatement. A resolution introduced by some members of the California state
assembly light-heartedly denounces the IAU for "scientific heresy," among other
Others reject the change for sentimental reasons, citing that they have always
known Pluto as a planet and will continue to do so regardless of the IAU
The words "plutoed" and its variant "to pluto" were coined in the aftermath
of the decision. In January 2007, the
American Dialect Society chose "plutoed" as its 2006 Word of the Year,
defining "to pluto" as "to demote or devalue someone or something",
an example being "as happened to the former planet Pluto when the General
Assembly of the International Astronomical Union decided Pluto no longer met its
definition of a planet."