 25 years ago, not a single planet outside the solar system had been detected. But remarkably, we now know of thousands and have studied many in surprising detail. ESO's observatories in Chile have been at the forefront of this enormous expansion in knowledge and their state-of-the-art instruments are continuing to discover and study the extraordinary diversity of exoplanets. This is the ESOcast, cutting-edge science and life behind the scenes at ESO, the European Southern Observatory. Looking up at the night sky, people throughout history have wondered if there are planets and especially planets bearing life beyond the solar system. Astronomers have also asked themselves these questions and many more. Are planets common or very rare? Do they resemble planets in the solar system or are they totally different? Frustratingly, until very recently, observational techniques were not advanced enough to be able to answer any of these questions. But in 1995, this changed overnight. The first exoplanet orbiting a sun-like star was detected. The monumental discovery was made by Geneva-based astronomers Michel Mayor and Didier Kélo around the star 51 Pegasi. The exoplanet named 51 Pegasi B has around half the mass of Jupiter and travels around its parent star in just over four Earth days. But this was only the beginning. The initial trickle of discoveries became a flood. Thousands of exoplanets have since been detected in a huge variety of sizes and orbits. Many of these discoveries have been made by ESO's observatories in Chile. But the hunt for exoplanets is a challenging one. These alien worlds hide in the shadows, giving off little or no light of their own. Any light that they do emit is swamped by the overwhelming brilliance of their parent star. However, advanced observational methods can be used to spot these elusive exoplanets. The weak gravitational pull of an exoplanet in orbit causes its parent star to wobble back and forth. This tiny motion causes a small shift in the star's spectrum, which extremely sensitive spectrographs, such as ESO's harps, can detect through radial velocity tracking. Harps, installed on the ESO 3.6-metre telescope at the La Silla Observatory, is the world's foremost exoplanet hunter. It's the most successful finder of low mass exoplanets to date. In 2010, the instrument discovered the richest planetary system yet. The system, located over 120 light-years away around the sun-like star HD-10180, contains at least five exoplanets. There is also tantalizing evidence that two more planets may be present in this system, one of which would have the lowest mass ever found. Planetary transits can also be utilized by astronomers to indirectly detect distant worlds. When an exoplanet passes in front of its parent star, as seen from the Earth, it blocks a small fraction of the star's light from our view. This creates a dip in the brightness of the star, which can be measured. In addition to determining the size of an exoplanet, planetary transits can reveal the composition of an exoplanet's atmosphere. The atmosphere around a super-Earth exoplanet was analyzed for the first time by astronomers using the very large telescope. The planet, which is known as GJ-1214b, was studied as it passed in front of its parent star, and starlight passed through the planet's atmosphere. This starlight revealed that the planet's atmosphere is either mostly water in the form of steam, or is dominated by thick clouds or hazes. Directly observing an exoplanet is a monumental feat, but one that was first achieved by ESO. The very large telescope obtained the first-ever image of a planet outside the Solar System. 2M-1207b is five times more massive than Jupiter. It orbits a failed star, a brown dwarf, at a distance 55 times larger than the Earth to the Sun. ESO's telescopes are equipped with state-of-the-art instruments, but to remain at the forefront of exoplanet research, ESO has recently commissioned two new instruments for the VLT. Sphere is able to find and study faint planets masked by the glare of their host stars. And in the near future, the espresso spectrograph will arrive at the VLT, where it will surpass harps. The European Extremely Large Telescope, which is currently under construction in Chile, will take the hunt for exoplanets even further. Once operational, this 39-meter telescope could detect Earth-like planets and possibly evidence of alien biospheres. The search for planets outside the Solar System constitutes a key element of what is possibly the greatest question of all. Is there life elsewhere in the universe? Over the past 20 years, our knowledge of exoplanets has advanced dramatically. But the quest for Earth-like planets and those that harbour life remains one of the great frontiers of astronomy. Are we alone? We do not know, but the answer is almost within reach. Transcribed by ESO, translated by —