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 meters 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-meter size in the Solar System beyond Neptune.