Improving the search
In 1998, an even more sensitive spectrograph, named CORALIE, was installed at La Silla Observatory in Chile, which again was searching for planets using the radial velocity technique. In 2002, Michel Mayor began overseeing HARPS (High Accuracy Radial velocity Planet Searcher) at the same site, using a spectrograph capable of detecting exoplanets about the size of Earth. The wobble method of detection was very slow, so new techniques of spotting exoplanets were developed. The most successful method was the transit method, which looked for periodic changes in the brightness of a star. These changes were very small and happened when a planet transited the star, passing between the star and the observer, and causing it to dim very slightly. The best place to look for exoplanets by the transit method was out in space and so, in 2009, the Kepler observatory, named after the man who first described planetary orbits, was launched to do just that.
Staring at one place
Kepler was placed in a heliocentric orbit, trailing behind Earth as it circled the sun. The craft was designed to keep its aperture firmly fixed on a single patch of space, called the Kepler field. This made up only about 0.25 percent of the whole sky, but the spacecraft could see 150,000 stars in that area. To find exoplanets, Kelper would have to concentrate on this single field of view for years on end. It was unable to see individual exoplanets, but could identify stars that were likely to have them.
Kepler could only detect the transits of exoplanets with orbital paths that crossed the spacecraft’s line of sight. Many exoplanets would be orbiting at the wrong angle for that. Those that were correctly oriented would only transit their star once every orbital period (the planet’s year), so Kepler’s method was better at finding planets that orbited close to their star, taking a few years and months (or even weeks and days) to complete each revolution.
“We were not expecting to find a planet with a 4-day [orbital] period. No one was expecting this.” Michel Mayor
By the start of 2013, Kepler had identified about 4,300 candidate stars that might have extrasolar planetary systems. Unfortunately, the guidance system used to keep Kepler locked on target then failed, bringing its planet hunt to an end about three years sooner than expected. However, the data it had collected was enough to keep researchers busy for years to come. Kepler’s candidate stars could only be confirmed as planetary systems using radial velocity measurements from ground-based observatories, such as HARPS in Chile and the Keck Telescope in Hawaii. (Radial velocity is the velocity of the star in the direction of Earth.) So far, about a tenth of Kepler’s candidate stars have proved to be false positives but, after three years of analysis, the program had identified 1,284 exoplanets, with more than 3,000 stars left to examine. The statistics for the exoplanets in the Kepler field are striking— most stars are part of a planetary system. This means that the number of planets in the universe is likely to exceed the number of stars.
The amount of dimming during a transit gives an indication of how big an exoplanet might be, but the study of an exoplanet’s size and characteristics is still in its early stages. The light reflected from a planet is about 10 billion times fainter than the star it orbits. Astronomers are waiting for the James Webb Space Telescope in 2018 and the European Extremely Large Telescope in 2024 to image this light directly and analyze the chemistry of exoplanets. Until then, they have to speculate using very little data: an approximate mass of the planet, its radius, the orbital distance, and the temperature of the star. This tells them what the planet is probably made of and allows them to conjecture what the surface conditions are likely to be.
“Red dwarfs with rocky planets could be ubiquitous in the universe.” Phil Muirhead