MOST stands for Microvariability and Oscillations of STars or Microvariabilité et Oscillation STellaire.
It is Canada's first space telescope.
It is also the only telescope and instrument in the world – in space or on Earth – that is capable of measuring reflected light from planets orbiting around other stars.
MOST was actually designed to study vibrations of stars for stellar seismology, but more recently we pointed it at stars that have mysterious exo-planets (extra-solar planets) around them, and it is giving us clues about the atmospheres of these planets that no one else can obtain.
The way it does this is to look for subtle variations in the light coming from these star systems, either from the star itself or from the planet that is orbiting around it.
How subtle are these variations?
MOST can see changes in the brightness of the star, or the planet associated with it, down to levels of one part in a million: that's one ten thousandth of a percent.