Monday, January 18, 1999 Published at 12:23 GMT
Pluto may be demoted
Pluto and its companion Charon as seen by the Hubble Space Telescope
by BBC News Online's Science Editor Dr David Whitehouse
In just a few weeks time tiny Pluto will regain its crown as the most distant planet orbiting the Sun. Soon afterwards however it may lose its planetary status forever.
That will end at 11.22 GMT on February 11th when it will cross Neptune's path and once again become the solar system's most distant planet.
It will be 220 years before it again comes closer than Neptune. But long before that it may have been demoted from its status as a planet.
Depending upon an email vote among astronomers our solar system may soon have eight instead of nine planets as all the textbooks say.
Discovered in 1930 Pluto almost defies classification, there is no other body quite like it. It is only two-thirds the diameter of our Moon and it has a relatively large companion Charon, discovered in 1978.
Charon may have been born through a head-on collision between Pluto and another large ice body, in much the same way as the Earth-Moon system is believed to have formed.
According to computer models, some of the debris from this giant impact on Pluto went into orbit around Pluto and coalesced to form Charon.
Made of a mixture of rock and ice Pluto has always been an oddity. It neither qualifies as an Earth-like or a gas giant planet.
Experts disagree about what it is, but a growing number say that if it was discovered now, it would never even occur to them to call it a proper planet.
It may be the last survivor of a lost population of ice dwarfs that inhabited the primeval solar system. It may even be an escaped satellite of Neptune.
Last to be visited
Pluto's significance in the solar system has been a point of contention since soon after it was discovered in 1930 by Clyde Tombaugh, an astronomer at the Lowell Observatory in Arizona.
Since Tombaugh's death in 1997 pressure has grown for the International Astronomical Union, the international authority for naming celestial bodies, to take a tough line on Pluto.
The latest blow came in 1995 with the discovery of the first object in the so-called Kuiper Belt. Since then about 60 more objects, made of rock and ice and a few hundred km in size, have been found in the solar systems cold outer reaches.
Pluto remains the only major body in the solar system not to have been visited by a spacecraft.
One of the reasons scientists want to see it at close quarters is that despite its small size and remote location, Pluto undergoes dramatic seasonal changes.
As Pluto recedes from the Sun, much of its atmosphere is believed to freeze out onto the surface. This explains the observation of fresh white ice on its surface.