Asked by Griff Smith
Any object in our Solar System that orbits the Sun at a greater average distance than Neptune is known as a trans-Neptunian object (TNO). The most famous TNO is the dwarf planet Pluto which was discovered in 1930, over 60 years before any other TNOs were found. Despite this large gap in discoveries, over 1200 TNOs have now been found, including a small number that have been designated as Plutoids.
Plutoids are defined to be any TNOs that are also dwarf planets, when a dwarf planet is an object orbiting the Sun with enough mass to become spherical under its own gravity but which has many other objects of similar sizes around the same orbital path. Not a huge amount is known about TNOs as they are very difficult to observe, but there is a spacecraft on its way. New Horizons launched in 2006 and is on a nine year journey to explore Pluto before, subject to a mission extension from NASA, examining another TNO and sending back insights on the types of worlds closer to the edge of our Solar System.
Answered by Megan Whewell from the National Space Centre
Image courtesy of NASA