Behind every man now alive stand thirty ghosts, for that is the ratio by which the dead outnumber the living. Since the dawn of time, roughly a hundred billion human beings have walked the planet Earth.
Now this is an interesting number, for by a curious coincidence there are approximately a hundred billion stars in our local universe, the Milky Way. So for every man who has ever lived, in this Universe there shines a star.
But every one of those stars is a sun, often far more brilliant and glorious than the small, nearby star we call the Sun. And many - perhaps most - of those alien suns have planets circling them. So almost certainly there is enough land in the sky to give every member of the human species, back to the first ape-man, his own private, world-sized heaven - or hell.
Arthur C. Clarke, 1968
To date, over 6,000 exoplanets have been discovered. There are billions more waiting in the Milky Way. In 1968, that was speculation, hardly more than science fiction. Today astronomers are discovering new planets as fast as they get a coveted slice of telescope time.
The first method used to discover exoplanets was the radial velocity method. This involves measuring a star's slight wobble caused by the gravitational pull of a nearby planet. In 1995, Swiss astronomers Michel Mayor and Didier Queloz used this method to discover the first exoplanet, 51 Pegasi b. This hot Jupiter, which orbits close to its star, challenged existing theories about the formation and behavior of planets.
In 1999, the transit method was used to discover an exoplanet for the first time. This method involves measuring the small dips in a star's brightness caused by a planet passing in front of it. The Hubble Space Telescope and ground-based observatories have made many discoveries using this method. The Kepler space telescope launched in 2009 also relied heavily on the transit method and revealed the existence of thousands of exoplanets.
There are several categories exoplanets, including gas giants, super-Earths (or sub-neptunes), and rocky planets. Gas giants are similar in size to Jupiter and Saturn and are predominantly made up of hydrogen and helium gas. Super-Earths are larger than Earth but smaller than gas giants, and may have rocky surfaces. Rocky planets are similar to Earth, with solid surfaces and atmospheres. Some exoplanets are found within their star's habitable zone, or the distance where conditions may allow for the existence of liquid water and potentially, life.
The study of exoplanets has led to numerous exciting discoveries, including the first exoplanet with an atmosphere, HAT-P-11b, and the closest potentially habitable exoplanet known as Proxima Centauri b. Scientists continue to study these distant worlds in the hopes of unlocking the secrets of planet formation, the potential for habitable worlds, and the possibilities for life beyond our solar system.