Discovered in 1930, Pluto is the second closest dwarf planet to the Sun and was at one point classified as the ninth planet. Pluto is also the second most massive dwarf planet with Eris being the most massive.
Pluto Dwarf Planet Profile
Mass: 13,050,000,000,000 billion kg (0.00218 x Earth)
Diameter: 2,368 km (+- 20km)
Known Moons: 5
Notable Moons: Charon, Nix, Hydra, Kerberos and Styx
Orbit Distance: 5,874,000,000 km (39.26 AU)
Orbit Period: 246.04 Earth years
Surface Temperature: -229°C
Discovery Date: 18th February 1930
Discovered By: Clyde W. Tombaugh
Pluto (minor-planet designation: 134340 Pluto) is the largest object in the Kuiper belt,the tenth-most-massive known body directly orbiting the Sun, and the second-most-massive known dwarf planet, after Eris. Like other Kuiper belt objects, Pluto is primarily made of rock and ice,and is relatively small, about 1⁄6 the mass of the Moon and 1⁄3 its volume. It has an eccentric and highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. Hence Pluto periodically comes closer to the Sun than Neptune, but an orbital resonance with Neptune prevents the bodies from colliding. In 2014 it was 32.6 AU from the Sun. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance (39.4 AU).
Discovered in 1930, Pluto was originally considered the ninth planet from the Sun. Its status as a major planet fell into question following further study of it and the outer Solar System over the next 75 years. Starting in 1977 with the discovery of the minor planet Chiron, numerous icy objects similar to Pluto with eccentric orbits were found.The scattered disc object Eris, discovered in 2005, is 27% more massive than Pluto.The understanding that Pluto is only one of several large icy bodies in the outer Solar System prompted the International Astronomical Union (IAU) to formally define "planet" in 2006. This definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category (and specifically as a plutoid). Astronomers who oppose this decision hold that Pluto should have remained classified as a planet, and that other dwarf planets and even moons should be added to the list of planets along with Pluto.
Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Nix, Hydra, Kerberos, and Styx.Pluto and Charon are sometimes described as a binary system because the barycenter of their orbits does not lie within either body. The IAU has yet to formalise a definition for binary dwarf planets, and Charon is officially classified as a moon of Pluto.On July 14, 2015, the Pluto system is due to be visited by spacecraft for the first time.The New Horizons probe will perform a flyby during which it will attempt to take detailed measurements and images of Pluto and its moons.Afterwards, the probe may visit several other objects in the Kuiper belt.
In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus.Subsequent observations of Neptune in the late 19th century caused astronomers to speculate that Uranus's orbit was being disturbed by another planet besides Neptune.
In 1906, Percival Lowell—a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona, in 1894—started an extensive project in search of a possible ninth planet, which he termed "Planet X".By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death in 1916, but to no avail. Unknown to Lowell, on March 19, 1915, surveys had captured two faint images of Pluto, but they were not recognized for what they were.There are fifteen other known prediscoveries, with the oldest made by the Yerkes Observatory on August 20, 1909.
Because of a ten-year legal battle with Constance Lowell, Percival's widow, who attempted to wrest the observatory's million-dollar portion of his legacy for herself, the search for Planet X did not resume until 1929,when its director, Vesto Melvin Slipher, summarily handed the job of locating Planet X to Clyde Tombaugh, a 23-year-old Kansan who had just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.Tombaugh's task was to systematically image the night sky in pairs of photographs, then examine each pair and determine whether any objects had shifted position. Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. A lesser-quality photograph taken on January 21 helped confirm the movement After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930
Pluto's orbital period is 248 Earth years. Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits around the Sun close to a flat reference plane called the ecliptic. In contrast, Pluto's orbit is highly inclined relative to the ecliptic (over 17°) and highly eccentric (elliptical). This high eccentricity means a small region of Pluto's orbit lies nearer the Sun than Neptune's. The Pluto–Charon barycenter came to perihelion on September 5, 1989 and was last closer to the Sun than Neptune between February 7, 1979, and February 11, 1999.In the long term, Pluto's orbit is in fact chaotic. Although computer simulations can be used to predict its position for several million years (both forward and backward in time), after intervals longer than the Lyapunov time of 10–20 million years, calculations become speculative: Pluto is sensitive to unmeasurably small details of the Solar System, hard-to-predict factors that will gradually disrupt its orbit.Observations by the Hubble Space Telescope place Pluto's density at between 1.8 and 2.1 g/cm3, suggesting its internal composition consists of roughly 50–70 percent rock and 30–50 percent ice by mass.Because the decay of radioactive elements would eventually heat the ices enough for the rock to separate from them, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of ice. The diameter of the core is hypothesized to be approximately 1700 km, 70% of Pluto's diameter. It is possible that such heating continues today, creating a subsurface ocean layer of liquid water some 100 to 180 km thick at the core–mantle boundary.The DLR Institute of Planetary Research calculated that Pluto's density-to-radius ratio lies in a transition zone, along with Neptune's moon Triton, between icy satellites like the mid-sized moons of Uranus and Saturn, and rocky satellites such as Jupiter's Io.
Pluto's atmosphere consists of a thin envelope of nitrogen (N2), methane (CH4), and carbon monoxide (CO) gases, which are derived from the ices of these substances on its surface Its surface pressure ranges from 6.5 to 24 μbar (0.65 to 2.4 Pa). Pluto's elongated orbit is predicted to have a major effect on its atmosphere: as Pluto moves away from the Sun, its atmosphere should gradually freeze out, and fall to the ground. When Pluto is closer to the Sun, the temperature of Pluto's solid surface increases, causing the ices to sublimate into gas. This creates an anti-greenhouse effect; much as sweat cools the body as it evaporates from the surface of the skin, this sublimation cools the surface of Pluto. In 2006, scientists using the Submillimeter Array discovered that Pluto's temperature is about 43 K (−230 °C), 10 K colder than would otherwise be expected.
The presence of methane (CH4), a powerful greenhouse gas, in Pluto's atmosphere creates a temperature inversion, with average temperatures 36 K warmer 10 km above the surface.The lower atmosphere contains a higher concentration of methane than its upper atmosphere.
Evidence of Pluto's atmosphere was first suggested by Noah Brosch and Haim Mendelson of the Wise Observatory in Israel in 1985,and then definitively detected by the Kuiper Airborne Observatory in 1988, from observations of occultations of stars by Pluto. When an object with no atmosphere moves in front of a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually.From the rate of dimming, the atmospheric pressure was determined to be 0.15 Pa, roughly 1/700,000 that of Earth.
In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory, James L. Elliot of MIT,and Jay Pasachoff of Williams College. Surprisingly, the atmospheric pressure was estimated to be 0.3 pascal, even though Pluto was farther from the Sun than in 1988 and thus should have been colder and had a more rarefied atmosphere. One explanation for the discrepancy is that in 1987 the north (or positive) pole of Pluto came out of shadow for the first time in 120 years, causing extra nitrogen to sublimate from the polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere as it freezes onto the south (or negative) pole's now continuously dark ice cap.Spikes in the data from the same study revealed what may be the first evidence of wind in Pluto's atmosphere.Another stellar occultation was observed by the MIT-Williams College team of James L. Elliot, Jay Pasachoff, and a Southwest Research Institute team led by Leslie A. Young on June 12, 2006, from sites in Australia.
In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his colleagues announced the spectroscopic discovery of ethane (C2H6) on Pluto's surface. This ethane is produced from the photolysis or radiolysis (i.e. the chemical conversion driven by sunlight and charged particles) of frozen methane on Pluto's surface and suspended in its atmosphere