What Pluto, Ceres, Haumea, Makemake and Eris, the dwarf planets lack in size they make up for in mystery. 

A new type of planet

Pluto was a planet for 76 years. Following the discovery of Eris (an even larger object on the edge of the Solar System), a new classification of ‘dwarf planet’ was introduced and its current membership includes Ceres, the largest asteroid, Pluto and Eris and the two that have been added since the classification was created: Haumea and Makemake.

Pluto's discovery

Pluto was discovered in 1930 by Clyde Tombaugh at the Lowell Observatory as a result of astronomers comparing the observed positions in the sky of the two 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.

Pluto's surface

Its surface temperature is about -230°C. 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.

New Horizons spacecraft

The New Horizons mission to Pluto launched on 19 January 2006 and arrived in July 2015, sending data back to Earth. New Horizons will then head into the Edgeworth-Kuiper belt and take a close look at some of its many Pluto-like minor planets.

Pluto had only been visible in fairly large telescopes where it appeared 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.

Ceres

While Pluto was downgraded to ‘dwarf planet’ status, Ceres received an upgrade from large asteroid in 2006 by the International Astronomical Union. Ceres is about 20 times less massive than Pluto and about a thousand kilometres in diameter. It was however discovered a long time before Pluto back in 1801. NASA’s spacecraft Dawn began its orbit of Ceres on 6 March 2015. Pictures revealed puzzling bright spots.

Eris

The ‘new kid on the block’, Eris was first discovered in 2003 and initially went under the name 2003 UB313 before taking the name of the Greek goddess of discord in 2006. Eris seems to be similar to Pluto in composition, with deposits of methane ice on the surface.

Eris is between 2330 and 3550 km across though, making it larger than Pluto. With such a large size, the discovery team were pushing for the object to be classed as a tenth planet. However, many astronomers took a different view and argued that Pluto should be downgraded to be 'just' another TNO (see below). The debate was finally concluded by the International Astronomical Union at their 2006 General Assembly, when a new definition for a planet was agreed.

Trans-Neptunian Objects or TNOs

Almost 800 small bodies (Trans-Neptunian Objects or TNOs) have been found in the region beyond Neptune, usually referred to as the Edgeworth-Kuiper belt. But until Eris, all the TNOs discovered were significantly smaller than Pluto, which is 2320 km across.

Many of these are possible candidates for dwarf planet status, but currently only four from this region have been accepted along with Ceres from the inner Solar System.