- Roughly 10 billion icy objects outside the orbit of Neptune.
- Although believed to exist since 1930's, only discovered in 1992 because objects are far away and thus faint.
- Most of the bright KBOs are the same as a 100 watt light bulb on the Moon.
- Exception is Pluto because it is covered by bright frost and so reflects 90% of sunlight.
- Objects like the Moon reflect only a few percent.
- That is why scientists originally believed that Pluto was much bigger than it is.