Pluto Information 
Pluto
was considered the ninth planet in the solar system, until 2006. Now,
Pluto is regarded as a minor planet or a dwarf planet. Discovered in 1930 and immediately classified as a planet, its status is currently under dispute. Pluto has an eccentric orbit that is highly inclined in respect to the other planets and takes it inside the orbit of Neptune. Its largest moon is Charon, discovered in 1978; two smaller moons were discovered in 2005. Pluto's astronomical symbol is a P-L monogram, ♇. This represents both the first two letters of the name Pluto and the initials of Percival Lowell, the man who lent his name to the observatory that was used to find Pluto. An alternate symbol resembles that of Neptune, but has a circle in place of the middle spoke in the top center.
Due to its size and unusual orbit, there has been debate regarding Pluto's classification as a major or a minor planet, and there is increasing momentum for recognizing "dual status." Pluto is classified as a trans-Neptunian object. As of July 31, 2005, one other trans-Neptunian object, 2003 UB313, had been found that is larger than Pluto. Physical Characteristics More than 75 years after its discovery, many facts about Pluto remain unknown, mainly due to the fact that it is the only planet that has not been visited by human spacecraft and that it is too far away for in-depth investigations with telescopes from earth. What is known are the few physical properties listed below. Mass & Size Pluto is not only smaller and much less massive than every other planet, at less than 0.2 lunar masses it is also smaller and less massive than seven moons: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa and Triton. However, Pluto is more than twice the diameter, and a dozen times the mass, of Ceres, the largest minor planet in the asteroid belt, and it was larger than any other object known in the trans-Neptunian Kuiper belt until 2003 UB313 was announced in 2005.
Pluto's mass and diameter could only be estimated for many decades after its discovery. The discovery of its satellite Charon in 1978 enabled a determination of the mass of the Pluto-Charon system by simple application of Newton's formulation of Kepler's third law. Later Pluto's diameter was measured when it was occulted by Charon, and its disk can now be resolved by telescopes using adaptive optics. Atmosphere Pluto's thin atmosphere is most likely nitrogen and carbon monoxide, in equilibrium with solid nitrogen and carbon monoxide ices on the surface. As Pluto moves away from its perihelion and farther from the Sun, more of its atmosphere freezes. Pluto was found to have an atmosphere from an occultation observation 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 Paris Observatory and by Jim Elliot of MIT and Jay Pasachoff of Williams College. 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. Appearance Pluto's apparent magnitude is fainter than 14 m and therefore a telescope is required for observation. To be easily seen, a telescope of around 30 cm aperture is desirable. It looks star-like even in very big telescopes, because its angular diameter is only 0.15″. The colour of Pluto is light brown with a very slight tint of yellow. Orbit Pluto's orbit is unlike those of the other planets. It is highly inclined above the plane of the ecliptic, and highly eccentric (non-circular). The eccentricity of its orbit is such that it crosses the orbit of Neptune, and making Pluto only the eighth-most distant planet from the Sun for part of each orbit; the most recent occurrence of this phenomenon lasted from February 7, 1979 through February 11, 1999. Mathematical calculations indicate that the previous occurrence only lasted fourteen years from July 11, 1735 to September 15, 1749. However, the same calculations indicate that Pluto was the eighth-most distant planet between 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 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 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 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.
Beginning in the 1990s, other trans-Neptunian objects have been discovered, and a certain number of these also have a 3:2 orbital resonance with Neptune. TNOs with this orbital resonance are named "plutinos", after Pluto. Pluto's Satellite System The planet Pluto has three known moons. The largest, Charon, is proportionally larger than any other satellite in the solar system compared to its planet. The other two moons, as yet unnamed, are much smaller. Their discovery was announced on 2005 October 31. The innermost moon, Charon, was discovered by James Christy on 1978 June 22, nearly half a century after Pluto. Two outer moons were imaged by the Hubble Space Telescope Pluto Companion Search Team on 2005 May 15, and precovered from Hubble images taken in June 2002. The team has scheduled further Hubble observations for February 2006. Once the orbits are confirmed, the International Astronomical Union can consider names for the outer moons. Given the sensitivity of the Hubble images, and the fact that the entire region of space dominated by Pluto's gravitational field was imaged, Pluto is not expected to have any other moons larger than approximately 20 km in diameter.
Pluto and Charon have been called a double planet because Charon is larger compared to Pluto (half its diameter and an eighth its mass) than any other moon is to a planet; indeed Charon is massive enough that, despite their proximity, Pluto orbits the system's barycenter at a point outside its surface. Charon and Pluto are also tidally locked, so that they always present the same face toward each other.
The Plutonian system has not been visited by spacecraft, but a flyby by the New Horizons mission is planned for 2015. The Plutonian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in light purple. Pluto has been added for comparison, for it orbits a point outside itself. Name (spheroidal moons in bold)
(Pronunciation key) | Mean diameter (km) | Mass (kg) | Semi-major
axis (km) | Orbital period (days) | Discovery date |
|---|
| (Pluto) | (2284) | (13�1021) | (2450) | (6.3872) | (1930) | | Pluto I | Charon | shar'-ən** | 1192 | 1.6�1021 | 19,410 | 6.3872 | 1978 | | S/2005 P 2* | 32-145 | < 5�1018 | 49,400 � 600 | 25.5 � 0.5 | 2005 | | S/2005 P 1* | 52-160 | < 5�1018 | 64,700 � 850 | 38.2 � 0.8 | 2005 |
Notes:
* Awaiting orbital confirmation and naming.
** Astronomers use this idiosyncratic pronunciation, not the classical kair'-ən. The diameters of objects can be estimated from their assumed surface reflectivity (albedo). The low estimates correspond to a 35% albedo like Charon, while the larger ones correspond to the 4% albedo of the darkest KBOs.
Further observations will determine if S/2005 P 2 and S/2005 P 1 are in orbital resonance with Charon and Pluto. Their currently estimated periods are four and six times that of the Charon-Pluto orbital period. Charon The Pluto-Charon system is noteworthy for being the only planet/moon system in the solar system whose barycenter lies above the planet's surface, thus prompting some astronomers to label it a double planet (a term complicated by the discovery of two more Plutonian moons).
The Pluto-Charon system is also unusual among planetary systems in that they are tidally locked to each other: Charon always presents the same face to Pluto, and Pluto also always presents the same face to Charon.
The discovery of Charon allowed astronomers to determine the mass of the Pluto-Charon pair from their observed orbital period and separation by a straightforward application of Kepler's third law of planetary motion. The mass was found to be lower than even the lowest earlier estimates.
The discovery also led astronomers to alter their estimate of Pluto's size. Originally, it was believed that Pluto was larger than Mercury but smaller than Mars, but that calculation was based on the premise that a single object was being observed. Once it was realized that there were in fact two objects instead of one, the estimated size of Pluto was revised downward. Today, with modern adaptive optics, Pluto's disc can be resolved and thus its size can be directly determined.
Charon's discovery also resulted in the calculation of Pluto's albedo being revised upward; since the planet was now seen as being far smaller than originally estimated, by necessity its capacity to reflect light must be greater than what had been formerly believed. Current estimates place Pluto's albedo as marginally less than that of Venus, which is fairly high. Previously, some researchers had theorized that Pluto and its moon Charon were moons of Neptune that were knocked out of Neptunian orbit when Triton was captured. Triton, the largest moon of Neptune, which shares many atmospherical and geological composition similarities with Pluto, may once have been a Kuiper belt object in a solar orbit, and today it is widely accepted that Pluto never orbited Neptune.
An occultation of a star by Charon in 2005, observed in South America by teams from MIT-Williams College, the Paris Observatory, and the Southwest Research Institute has led to improved knowledge of Charon's parameters. The Outer Moons Two additional moons were imaged by astronomers working with the Hubble Space Telescope on May 15, 2005, and have received provisional designations of S/2005 P 1 and S/2005 P 2. They were confirmed with "precovery" Hubble images from June 14, 2002. Observations suggest they orbit Pluto at at least twice the distance Charon does. P2 stays about 49,000km from the planet, P1 lies even further away at 65,000km. The two candidate moons seem to orbit Pluto in an anti-clockwise direction. Preliminary observations are also consistent with the outer moons lying in the same orbital plane as Charon, and orbiting at distances two and three times farther away, with orbital resonances of 4:1 and 6:1 with Charon.
Both objects appear to be on the order of 50-150 km in diameter, compared to Charon's 1,200 km, and are thought to have masses less than 0.3% of Charon's (or 0.03% of Pluto's mass). The discovery team plans follow-up observations with Hubble in February 2006 to work out the precise orbits, but ground-based observatories will attempt to image the moons as well. Once the orbits are confirmed, the moons can be given permanent names. Specifications Orbital Characteristics Semi-M ajor Axis 5,906,376,272 km (39.481 686 77 AU)
Orbital Circumference 36.530 Tm (244.186 AU)
Eccentricity 0.248 807 66
Perihelion 4,436,824,613 km (29.658 340 67 AU)
Aphelion 7,375,927,931 km (49.305 032 87 AU)
Orbital Period 90,613.3055 days (248.09 a)
Synodic Period 366.73 days
Avg. Orbital Speed 4.666 km/s
Max. Orbital Speed 6.112 km/s
Min. Orbital Speed 3.676 km/s
Inclination 17.141 75� (11.88� to Sun's equator)
Longitude of the Ascending Node 110.303 47�
Argument of the Perihelion 113.763 29�
Number of Satellites 3 Physical Characteristics Diameter 2390 km (0.188 Earths or 1,485 miles)
Surface Area 1.795�107 km2 (0.033 Earths)
Volume 7.15�109 km3 (0.0066 Earths)
Mass 1.25�1022 kg (0.0021 Earths)
Mean Density 1.750 g/cm3
Equatorial Gravity 0.58 m/s2 (0.059 gee)
Escape Velocity 1.2 km/s
Rotation Period 6.387 days (6 d 9 h 17.6 min)
Rotation Velocity 47.18 km/h (at the equator)
Axial Tilt 122.54� (to orbit) 115.60� (to the ecliptic)
Right Ascension of North Pole 133.02� (8 h 52 min 5 s)
Declination -9.09�
Albedo 0.30
Min. Surface Temp. 33 K
Mean Surface Temp. 44 K
Max. Surface Temp. 55 K
Adjective Plutonian Atmospheric Characteristics Atmospheric Pressure = 0.30 Pascals (summer maximum)
Composition Nitrogen & Methane ** Information provided by Wikipedia, the free online encyclopedia. **
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