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Science and Engineering Research Council

Royal Greenwich Observatory

Information Leaflet No. 45: 'Pluto'.


Pluto is, on average, the most distant planet from the Sun. For the next few years, however, it is in fact closer to the Sun than is its closest rival, Neptune. The reason for this is that the orbit of Pluto around the Sun is an ellipse with quite a large eccentricity. This means that it is more 'oval-shaped' than circular. At present Pluto is near perihelion, its closest distance to the Sun, and is about 4,440 million kilometres away. At its furthest from the Sun, aphelion, which will be reached in about 124 years time (half its orbital period), it will be 7,395 million kilometres away.

Pluto was discovered in 1930 by Clyde Tombaugh at the Lowell Observatory. It was discovered as a result of astronomers comparing the observed positions in the sky of the planets Uranus and Neptune, with positions predicted from their orbits about the Sun. Small departures from the predicted positions indicated that the paths of these two planets were being disturbed by the gravitational pull of another body.

In 1978 it was discovered that Pluto has a very close satellite, now called Charon.
Charon orbits Pluto at a distance of 20,000 kilometres, in 6.4 days. From these facts we can determine that Pluto has a mass only 0.2% of the Earth. Its diameter is about 2,500 kilometres, and so Pluto has a density much less than the Earth. It is also very black and it has been supposed by some astronomers that it is more like a giant comet nucleus than a planet. Its surface temperature is about -230°C; too cold for there to be much of an atmosphere.

From recent observations in the infrared, Pluto is known to have on its surface, solid ices of nitrogen, methane and carbon monoxide. This implies that there will be a thin atmosphere of these gases around the planet. The only information about surface detail comes from an analysis of the variation in the observed brightness over 5 years, during which Pluto's satellite, Charon, occulted differing parts of its surface. From these measures it has been deduced that Pluto's south pole has recently received a new layer of methane ice, giving it a high reflectivity of about 90 percent, whereas other parts of the surface only reflect less than 30 percent of the sunlight.

Pluto is only visible in fairly large telescopes, where it appears as a star-like object of 14th magnitude. Because of its great distance from the Sun, Pluto only moves very slowly across the sky. At present it lies close to the borders of the constellations Libra and Serpens Caput.

Pluto's orbit has the highest eccentricity and largest inclination to the ecliptic of all the planets.

Due to its great distance, very little, apart from the facts given above, is known about Pluto.

ARVAL's Note:

 [Pluto and Charon]

This is the clearest view yet of the distant dwarf planet Pluto and its moon, Charon, as revealed by NASA's Hubble Space Telescope (HST). The image was taken by the European Space Agency's Faint Object Camera on February 21, 1994 when the planet was 2.6 billion miles (4.4 billion kilometers) from Earth; or nearly 30 times the separation between Earth and the Sun.

Hubble's corrected optics show the two objects as clearly separate and sharp disks. This now allows astronomers to measure directly (to within about 1 percent) Pluto's diameter of 1,440 miles (2,320 kilometers) and Charon's diameter of 790 miles (1,270 kilometers).

The Hubble observations show that Charon is bluer than Pluto. This means that both worlds have different surface composition and structure. A bright highlight on Pluto suggests it has a smoothly reflecting surface layer.

Pluto typically is called the double planet because Charon is half the diameter of Pluto (our Moon is one-quarter the diameter of Earth).

Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA.

For more details (and more resolution) on HST's image of Pluto and Charon, link to Pluto and Charon at HubbleSite, May 16, 1994.

Produced by the Information Services Department of the Royal Greenwich Observatory.

PJA Thu Nov 25 10:45:33 GMT 1993

Pluto is now a "dwarf planet" by the IAU definition of "planet" and "dwarf planets" (IAU 2006 General Assembly, August 24 '06).
See Observatorio ARVAL: Solar System Data.

On 13 September '06 the IAU Minor Planet Center assigned to Pluto the asteroid number 134340.
See IAU Minor Planet Center Circular 8747 (.pdf).

New Horizons Mission to Pluto and Charon - Kuiper Belt Objects. The first reconnaissance of Pluto and Charon - a "double planet" and the last planet in our solar system to be visited by spacecraft. Later, as part of an extended mission to 2020, New Horizons will visit one or more objects in the Kuiper Belt region beyond Neptune. Launched on January 19, 2006.
Jupiter Encounter: Closest approach occurred on February 28, 2007. New Horizons flew about 3 to 4 times closer to Jupiter than the Cassini spacecraft. New Horizons has also crossed the orbits of Saturn, on June 8, 2008, Uranus on March 18, 2011, and Neptune on August 2014. It transitioned from hibernation to active mode on Dec. 6, 2014, in preparation for the Pluto-System Encounter close approach on July 14, 2015 (11:49:59 UTC), about 12,500 kilometers (7,750 miles) from Pluto.
See New Horizons - The Flyby.

See Observatorio ARVAL: New Horizons Photograph of Pluto Shows Surface Features.

Updated: September 15 '06, June 26 '14, July 13 '15

Best seen with Font Verdana.
See About the Web Pages of Observatorio ARVAL.

For some illustrative images and excellent texts, link to:
Pluto in Calvin J. Hamilton's Views of the Solar System

For more details on Clyde Tombaugh and his discovery of Pluto, link to Clyde Tombaugh, 1906 - 1997 in ARVAL

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