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We use telescopes to
collect lots of light and
focus it onto detectors
using mirrors or lenses.
Large modern telescopes,
like the Keck Telescope
shown below, can collect
6,000,000 times as much
light as the human eye.
Keck Telescope
Eris is a dwarf planet in the outer reaches of our Solar system.
It was first sighted in 2005 and is currently 97 astronomical units away, putting it at more than twice the distance of Pluto. Following its discovery there was debate amongst astronomers as to whether the new object would be a planet, but the discussion eventually led to Pluto being de-classified as a proper planet in 2006.
Although we cannot yet be sure, we think that Eris is an icy, rocky object that is about three-quarters the size of our Moon. Like other dwarf planets, Eris's orbit is tilted by 44° to the orbits of the main planets.
Having an inclined orbit suggests that it did not form in the same way as the planets, and is simply a large lump of rock that was somehow captured into its highly eccentric 557 year orbit around the Sun.
Eris has one known moon, called Dysnomia, which is around 150km across and orbits the dwarf planet every 15.8 days.
| Facts and Figures | Ceres | Pluto | Haumea | Makemake | Eris |
| Orbital distance (AU)1 | 2.77 | 39.5 | 43.1 | 45.8 | 67.7 |
| Orbital Inclination | 10.6° | 17.1° | 28.2° | 29° | 44° |
| Radius (KM) | 487 | 1,137 | 1,436 | 750 | 1,300 |
| Mass (Earth Masses)2 | 0.00016 | 0.00213 | 0.0007 | 0.0007 | 0.003 |
| Year Length (Earth Years) | 4.6 | 248.5 | 283.3 | 309.9 | 557 |
| Day Length (Earth Days) | 0.38 | 6.39 | 0.16 | Unknown | Unknown |
| Surface Gravity (g)3 | 0.003 | 0.04 | 0.044 | 0.05 | 0.08 |
| Surface Temperature | about -105°C | about -230°C | about -240°C | about -243°C | about -240°C |
| Number of moons | 0 | 3 | 2 | 0 | 1 |