ESO Astronomical Glossary - T
T Tauri star
Named after the first star of its type to be discovered, T Tauri stars are 'protostars' in the final stages of formation to become a stable main sequence star. The nuclear reactions in their core have not yet stabilised and the stars are known for the variability of their brightness.
In astronomy, the term 'transit' denotes an event where one astronomical object appears to move across the face of another. As seen from Earth, the planets Venus and Mercury are seen to transit the Sun. We can also observe satellites transit the face of their host planet. In recent years, exoplanets have been discovered by studying periodic 'dips' in brightness of stars, suggesting that a planet is moving in front of the star along our line of sight.
Trans-Neptunian Object (TNO)
Trans-Neptunian Objects are objects in orbit around the Sun at a larger distance than the distance between Neptune and the Sun. This includes several dwarf planets, such as Pluto, and many small solar system bodies.