Main Page | See live article | Alphabetical index

Pluto (planet)



Hubble Space Telescope photo of Pluto.
Currently our best view of this distant object.
Discovered byClyde Tombaugh
Discovered in1930
Orbital characteristics
Mean radius5.91352×109 km
Revolution period248y 197d 5.5h
Synodic period366.7 days
Avg. Orbital Speed4.7490 km/s
Number of satellitess1
Physical characteristics
Equatorial diameter2320 km
Surface area17 million km2
Mass1.290×101022 kg
Mean density2.05 g/cm3
Surface gravity0.6 m/s2
Rotation period6d 9h 17.6m
Axial tilt119.61°
Escape Speed1.2 km/s
Surface temp
Atmospheric characteristics
Atmospheric pressure0 - 0.01 kPa

Pluto is the ninth and smallest planet of our solar system (although there is some debate regarding whether Pluto should be classified as a planet - see below for details). It was discovered by the astronomer Clyde Tombaugh at the Lowell Observatory in Arizona on February 18, 1930 (although the body was first photographed on March 19, 1915). Tombaugh was searching for a "Planet X" to explain the orbit of Neptune; further analysis, with seven decades more data about Neptune's position, has resolved the perceived anomaly without need for an additional gravitational pull on Neptune.

Table of contents
1 Physical characteristics
2 The Pluto debate
3 Exploration of Pluto
4 Atmosphere

Physical characteristics

Its highly eccentric orbit makes Pluto the eighth-most distant planet from the Sun for part of each orbit; this most recently occurred from February 7, 1979 through February 11, 1999. Pluto orbits in a 3/2 orbital resonance with Neptune. When Neptune approaches Pluto from behind their gravity start to pull on each other slightly, resulting in an interaction between their positions in orbit of the same sort that produces Trojan points. Since the orbits are eccentric, the 3/2 periodic ratio is favoured because this means Neptune always passes Pluto when they're almost furthest apart. Half a Pluto orbit later, when Pluto is nearing its closest approach, it initially seems like Neptune's about to catch up to 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.

Because of its small size and eccentric orbit, there has been some debate over whether it truly should be classified as a planet. There is mounting evidence that Pluto may in fact be a member of the Kuiper belt, only one of a large number of distant icy bodies. A subclass of such objects have been dubbed plutinos, after Pluto.

Pluto has an atmosphere when it is close to perihelion; the atmosphere freezes out as Pluto moves further from the Sun.

Pluto has one natural satellite, Charon. Little is known about Pluto because of its great distance from Earth and because no exploratory spacecraft have visited Pluto yet. Pluto and Charon are noteworthy for being the only planet/moon pair in the solar system whose barycenter lies above the planet's surface. Pluto and Charon are also unique among planets in both being tidally locked to each other. This means that Charon always presents the same face to Pluto but Pluto also always presents the same face to Charon. Note that some binary asteroids may also possess both of these traits, and that the Jupiter/Sun barycenter is above the Sun's surface, so neither is completely unique.

Some researchers have suggested that Pluto and its moon Charon were moons of Neptune that were knocked out of Neptune's orbit. It is now thought that not only was Pluto never Neptune's moon, but that Triton was originally an independent body much like Pluto which was captured by Neptune.

The Pluto debate

The planet Pluto was originally discovered in 1930 in the course of a search for a body sufficiently massive to account for supposed anomalies in the orbits of Uranus and Neptune. Once it was found, its faintness and failure to show a visible disc cast doubt on the idea that it could be Lowell's Planet X.

In the following decades estimates of Pluto's mass and diameter were the subject of debate as telescopes and imaging systems improved. The consensus steadily favored smaller masses and diameters as time passed. Indeed, one observer waggishly pointed out that if the trend were extrapolated the planet seemed to be in danger of vanishing altogether.

In an attempt to reconcile Pluto's small apparent size with its identification as Planet X, the theory of specular reflection was proposed. This held that observers were measuring only the diameter of a bright spot on the highly reflective surface of a much larger planet which could thereby be massive without having an exceptionally high density.

The uncertainty was conclusively resolved by the discovery of Pluto's satellite Charon in 1978. This made it possible to determine the combined mass of the Pluto-Charon system which turned out to be lower even than that anticipated by skeptics of the specular reflection theory, which was then rendered completely untenable. The accepted figure for Pluto's diameter today makes it comparable in size with the Moon and less massive on account of its being largely composed of ice.

At the time of Pluto's discovery it was the farthest object known in the solar system and we can now recognize that its discovery was as much due to luck as to the diligence of Tombaugh's search. While Pluto's identification as Planet X was then doubted, it was nevertheless identified as the solar system's ninth planet.

In September of 1992 scientists began discovering hundreds of other, smaller, icy bodies in the area of the solar system beyond the orbit of Neptune. These objects are now deemed members of the Edgeworth-Kuiper belt. The continued discovery of these objects began a debate that goes on to this day: is Pluto a planet or simply the largest (known) example of an Edgeworth-Kuiper Belt object?

This planetary sciences debate landed in newspaper headlines, editorials, and on the Internet in early 1999. Thoughts that Pluto might be "demoted" as a planet left certain sectors of the public angry. Such news outlets as the BBC News Online, the Boston Globe, and USA Today all printed stories noting that the International Astronomical Union was considering dropping Pluto's planetary status. "Save Pluto" websites sprang up, and school children sent letters to astronomers and the IAU.

On February 3, 1999, Brian Marsden of the Minor Planets Center inadvertently fueled the debate when he issued an editorial in the Minor Planets Electronic Circular 1999-C03 noting that the 10,000th minor planet was about to be numbered and this called for a large celebration (the IAU celebrates every thousands numbered minor planet in some way). He suggested that Pluto be honored with the number 10,000, giving it "dual citizenship" of sorts as both a major and a minor planet.

Between the media reports and the Minor Planets Electronic Circular, IAU General Secretary Joannes Anderson issued a press release that same day stating there were no plans to change Pluto's planetary status.

The debate continues, and recent discoveries have made the position of Pluto as a major planet perhaps even harder to sustain. On October 7, 2002, Mike Brown and Chad Trajillo announced at a meeting of the American Astronomical Society their discovery of Quaoar. This new object in the Edgeworth-Kuiper Belt is 1280 km in diameter, making it a bit more than half the size of Pluto. Quaoar is the largest object discovered in the solar system since Pluto itself in 1930. Some astronomers think it is only a matter of time before a Edgeworth-Kuiper Belt object larger than Pluto is discovered.

Exploration of Pluto

NASA has approved a mission to Pluto, to be conducted by The Southwest Research Institute. (Referred to as "New Horizons")

The planet is named both for the Roman god Pluto, and for the astronomer Percival Lowell, who predicted that a planet would be found beyond Neptune.


Pluto was determined to have an atmosphere from an occultation observation in 1988. When a planet or asteroid occults a star, if it has no atmosphere, 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 1.5 microbars. This thin atmosphere is most likely nitrogen, in equilibrium with solid nitrogen on the surface.

In 2003, another occultation of Pluto was observed and analyzed by Bruno Sicardy [1]. Surprisingly, the atmosphere was estimated to have a pressure of 3 microbars, even though Pluto is farther away 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 in 1987 (for the first time in 120 years), and extra nitrogen sublimated from a polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere.

The Solar System

Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets - Kuiper belt - Oort cloud