At the other end of the scale, brown dwarfs and gas giant planets up to tens of times the mass of Jupiter are all approximately the same size: as large as or a little bit larger than Jupiter.

But very late-M dwarfs can be tiny, down to about 0.1 the radius of the Sun. These are just hot enough to sustain the hydrogen burning that distinguishes them from brown dwarfs.

But at the extreme ends of the scale, planets can be almost as big as their stars! There's a lot of current interest in detecting planets around the smaller, cooler, late-spectral type stars such as M-dwarfs.

There is also a bias towards finding big planets around small stars. Credit: LCOįor most sun-like stars, an orbiting planet even as large as a brown dwarf will only cause an observed reduction in brightness of the star of a few percent or less during a transit. Like the radial velocity method, this method has a bias towards discovering large planets orbiting close to their stars, because larger planets block more light and transit more frequently so they are easier to detect. For planets that do transit, astronomers can get valuable information about the planet's atmosphere, surface temperatures and size.Įxamples of different exoplanet orbit orientations, showing exoplanets that do transit and ones that don't. Smaller planets in larger orbits are even less likely to be aligned in such a way that we can observe transits. This method will not work for all systems, however, because only about 10% of hot Jupiters are aligned in such a way that we see them transit. Credit: LCOĪ planet does not usually block much light from a star, (only 1% or less) but this can be detected. This method only works for star-planet systems that have orbits aligned in such a way that, as seen from Earth, the planet travels between us and the star and temporarily blocks some of the light from the star once every orbit.Įxample of an exoplanet transit.

Transit