A trans-Neptunian object (TNO) is any object in the Solar System that orbits the Sun at a distance greater on average than that of Neptune. Into this category fall the following:
Most of the 1,000 or so TNOs known are hundreds of kilometers in diameter. However, in 2006, astronomers announced the discovery of 58 TNOs measuring only 20 to 100 m across. The objects are much too small to show up in images, even with the most powerful telescopes. Instead, the researchers detected them when they passed in front of the distant X-ray source Scorpius X-1, causing sudden dips in its brightness. Based on this sample, they estimate there may be one million billion TNOs of 100-m size in the Solar System beyond Neptune.