A new kind of planet
On October 6, 1995, astronomers Michel Mayor and Didier Queloz announced the discovery of 51 Pegasi b, the first exoplanet found orbiting a star similar to our own Sun. The planet was detected using the ESO 3.6-metre telescope at the La Silla Observatory, located at an altitude of 2,400 meters on the edge of the Chilean Atacama Desert. The discovery was made with the ELODIE spectrograph, an instrument designed to detect the subtle "wobble" of a star caused by the gravitational pull of an orbiting planet. This detection method is known as the radial velocity technique.
The planet itself was unexpected. Named 51 Pegasi b, it has about half the mass of Jupiter but orbits its star in just 4.2 days. Its proximity to its star means its surface temperature is estimated to be around 1,000 degrees Celsius. This type of large, close-orbiting gas giant became known as a "hot Jupiter" and was the first of many new classes of exoplanets to be identified. The discovery of 51 Pegasi b opened up a new field of astronomy, and Mayor and Queloz were awarded the Nobel Prize in Physics in 2019 for their work.
A factory for finding worlds
The ESO 3.6-metre telescope, which began operations in 1977, is a leading facility for exoplanet research. Its original instrument has been replaced by the High Accuracy Radial velocity Planet Searcher (HARPS), which was installed in 2003. HARPS is one of the world's most precise spectrographs, capable of detecting stellar wobbles as slow as 3.5 km/h, or a walking pace. This incredible precision allows it to find low-mass, Earth-like planets. HARPS has been responsible for the discovery of more than 130 exoplanets.
La Silla has other significant telescopes. The 3.58-metre New Technology Telescope (NTT), inaugurated in 1989, introduced the use of active optics. This technology uses computer-controlled actuators to constantly adjust the shape of the main mirror, correcting for distortions and producing sharper images. Active optics is now standard on all large modern telescopes. The observatory also has the TRAPPIST–South telescope, a smaller 60-centimeter robotic telescope. In 2016, this telescope discovered the famous TRAPPIST-1 system, which contains seven Earth-sized rocky planets orbiting an ultracool dwarf star just 40 light-years away.