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Pluto - the dwarf planet?

Pluto and Charon

It is hard to believe that in the early years of the third millennium still relatively little is known about one of the planets in the Solar System, but Pluto is the least explored planet of all and remains something of a mystery. Its sheer distance from Earth has prohibited space missions from sending probes and Pluto has yet to be visited up close by any spacecraft. It is just a pinprick of light when viewed with the naked eye. Earth- based telescopes can not pick it out and even the Hubble Space telescope can only see the large features of Pluto so great is its distance from Earth. However as telescopes become more powerful astronomers and scientists are beginning to learn more about the planet which lies in the furthest reaches of the Solar System .

Pluto was discovered in 1930 by an American astronomer, Clyde W Tombaugh whilst working in the Lowell Observatory in Arizona. He was continuing the work of Percival Lowell who had been searching for a ninth planet- Planet X. At the time this was assumed to be influencing the orbit of Neptune and Uranus. Tombaugh work was to systematically take pairs of photographs of the celestial sky one or two weeks apart and then compare the photo plates to see if any change had occurred in the star field. In Feb of 1930 Tombaugh discovered a possible moving object on the photographic plates that had been taken on Jan 23rd and Jan 29th of that year. Further confirmatory photos were taken and Harvard Observatory was informed of the news of the discovery of a new planet, although not Lowell Planet X as it turned out, in March 1930.

The right to name the planet belonged to the Lowell Observatory and its director. Suggestions of names for this new planet poured in from all over the world, some being greeted with more enthusiasm than others, but eventually it was the name Pluto, the Roman equivalent of Hades, suggested by Venetia Burney, an 11 year old girl from Oxford, that was chosen.

What is known about Pluto?

  • Pluto has an eccentric orbit that sets it apart from other planets. Once every 248 Earth years Pluto swings inside the orbit of Neptune and remains there for between 13 and 20 years.
  • Pluto does not have a significant atmosphere. As Pluto move closer to the sun in its elliptical orbit, frozen methane and nitrogen at the poles of Pluto thaw, rise and temporarily form an atmosphere. As Pluto moves further away from the sun this atmosphere collapses onto the surface of the planet. Astronomers expect this to happen next in 2020 but cannot be absolutely sure, as it has never actually been observed.
  • Pluto composition is not known for sure but it is assumed to be a mixture of rock and water.
  • Pluto diameter is estimated to be 2320 kilometres
  • Charon, Pluto largest moon, discovered in 1978 by Jim Christy and Rob Harrington has a diameter estimated to be more than half the diameter of Pluto itself. The planet and its moon are locked together in synchronisation as Charon orbits the planet in the same time that it takes Pluto takes to rotate. Charon keeps the same face towards Pluto and Pluto always keeps the same face towards Charon. Some astronomers refer to the Pluto-Charon system as a double planet.
  • Nix and Hydra, two additional moons, were only discovered in 2005 by astronomers working with the Hubble Space Telescope and observations of these two natural satellites are on -going.

Planet or Comet?

In August 2006 424 astronomers at the International Astronomical Union Conference held in Prague controversially voted to strip Pluto of its status as a planet and reclassify it as a dwarf planet. Pluto's status as a planet has been questioned several times before owing to its position in the Solar System, its size and its physical make up. When Pluto was discovered it was assumed that it had a mass comparable to that of Earth's but this is far from the truth as are several other assumptions made at the time it was discovered. Over the years astronomers have constantly had to revise their knowledge about Pluto as advances in telescope technology have enabled more accurate observations to be made and the growth of stellar sciences has forced a new look in the twenty first century at the meaning of the term 'planet' Pluto lies furthest away from the sun in an area of space called the Kuiper Belt. This is a disc-shaped region of icy objects outside the orbit of Neptune and now considered to be the source of the short -period comets many of which impact on Earth. Below is the complete text of Definition of a Planet in the Solar System adopted at the IAU general Assembly in Prague on August 24th 2006.

Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation planets. The word planet originally described wanderers that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

The IAU therefore resolves that planets and other bodies in our Solar system can be defined into three distinct categories in the following ways.

1. A 'planet' is a celestial body that a) is in orbit around the Sun, b) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and c) has cleared the neighbourhood around its orbit.

2. A 'dwarf planet' is a celestial body that a) is in orbit around the Sun, b) has sufficient gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, c) has not cleared the neighbourhood around its orbit, and d) is not a satellite.

3. All other objects except satellites orbiting the Sun shall be referred to collectively as 'small solar-system bodies'.

The IAU further resolves that Pluto is a 'dwarf planet' by the above definition and is recognised as the prototype of a new category of trans-Neptunian objects.

This announcement by the IAU has provoked much discussion and argument amongst astronomers both professional and amateur. Many people have criticised the Prague Conference for adopting the resolution when the voting figures represented less than four percent of the worlds professional astronomers. Some feel that this definition brings into question the status of Earth, Mars, Jupiter and Neptune who also share their orbits with asteroids while others welcome Plutos de-classification explaining that the planet never exhibited the characteristics of the other eight planets. More than one amateur astronomer has voiced their disquiet that Clyde Tombaughs contribution to astronomy has been diminished with Plutos relegation to the status of "dwarf planet" and there has even been criticism of the term "dwarf planet which many feel implies a little planet` which is not what the IAU intended. Rick Feinberg writing in Sky & Telescope Nov 2006 believes that at the next General Assembly due to held in August 2009 in Rio de Janeiro the IAUs definition might well be revised.

What of the Future?

On January 19th 2006 the New Horizons space probe was launched from Cape Canaveral. It is designed to help scientists understand worlds at the edge of our solar system by being the first probe to attempt a reconnaissance of the Pluto- Charon system and the first mission to a binary object of any kind. The USA has already made history by being the first nation to reach every planet from Mercury to Neptune with a space probe and this latest space mission allows it to complete the reconnaissance of the Solar System.

The aims of the New Horizons Mission are to determine the atmosphere and composition of Pluto and Charon, map its surface temperature, search for rings and additional satellites around Pluto and then as part of an extended mission visit one or more objects in the Kuiper Belt region beyond Neptune so giving scientists and astronomers their first glimpse of an unexplored region of the universe. The space probe will not reach its destination until summer 2015 even though it is travelling at 45,000 mph so great is the distance of Pluto from Earth. Meanwhile as it continues on its nine-year journey, New Horizons is sending back photographs of space and scientists await its findings with eager anticipation. It really is a case of watch this space.


Please see our catalogue for these.

Useful websites

BBC Science & Nature webpage concerning outer space.

NASA's website dedicated to the New Horizons mission.

The Solar Space Station.

New Scientist website

NASA website

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