Trapping the stars

Most of these planets have been found using a method called radial velocity, also known as Doppler spectroscopy. This method is independent of distance and is good at finding massive planets close to stars that are relatively nearby, about 160 light-years from Earth. However, it can't determine the planet's physical aspects such as its actual size, orbit and whether it has an atmosphere, so it is often used in combination with what's called the transit method. Combining this with radial velocity enables astronomers to determine the density of an exoplanet, and hence learn something about its physical structure.The European Southern Observatory (ESO) uses this combination of techniques at its various telescopes around the Atacama Desert in Chile. Its latest addition called Trappist (TRAnsiting Planets and Planeteslmals Small Telescope) relies on the transit method but whereas the ESO's other telescopes are manned, Trappist is a robotic instrument, controlled remotely. It's also designed to study comets orbiting around the Sun but, whichever the task, it receives its instructions from the University of Liege inBelgium, nearly 7,500 miles (12,000km) away.