Since the time of Galileo, astronomers have speculated that the stars we see at night are distant suns, some of which may be circled by planets. The idea has long been a science-fiction staple. What might such distant worlds be like? Could some of them harbour life? But despite burning interest, astronomers have faced a huge technical challenge. Even the nearest star is 40 trillion kilometres away. In general, the faint, reflected light of an orbiting planet is too difficult to separate from the light of the star it orbits.
Starting in 1978, Canadian astronomers Bruce Campbell and Gordon Walker developed a technique that would allow them to detect the back-and-forth wobble of a star if that star was being pulled by the gravitational tug of a large orbiting planet. Knowing that such a detection could take years to verify, they continued their observations through the 1980s. They were ultimately unsuccessful. In hindsight, they could have discovered the first planet outside our solar system had they been luckier in the stars they chose to observe or been able to observe more of them.
In 1995, competing Swiss and U.S. teams of astronomers using a method similar to that pioneered by Dr. Campbell and Dr. Walker reported the detection of the first planet around a star similar to our sun. The discovery came as a surprise because the planet was nothing like anything seen before. It was massive: similar to Jupiter in our solar system, but extremely close to its star, making a complete orbit in about four days. Over the next few years many similar "hot Jupiters" turned up, spawning a new field of research to explain how they form and migrate. The bigger message is that solar systems are incredibly diverse and ours may not be a "normal" one.
IN THE BLINK OF AN EYE
In 1999, astronomers including David Charbonneau, then a Canadian graduate student at Harvard University, made the first detection of a planet by spotting a slight but recurring dip in the brightness of a nearby star. The change is due to the planet crossing in front of its star, like a miniature eclipse. This technique, known as the "transit" method, provided additional information about the size of the planet and in some cases allowed for the detection of an atmosphere. It also opened the door to the first searches for planets that are similar in size to Earth.
TAKING IT TO SPACE
Drawn by the promise of clear, crisp views of the stars unaffected by the atmosphere, scientists designed and launched spacecraft to exploit the transit method and find many more planets. A French satellite known as Corot paved the way in 2006, followed by NASA's more powerful Kepler mission in 2009. Kepler was designed to stare continuously at one part in the sky containing approximately 100,000 stars. It detected thousands of possible planets before the spacecraft broke down in 2013; more than 100 of those planets have been confirmed with follow-up observations. The large sample size allowed astronomers to estimate that as many as one in five stars harbour Earth-size planets where temperatures could allow for the presence of liquid water.
DAWN OF DIRECT IMAGING
By 2009, astronomers were able to image a handful of planets directly. The method required a large telescope and sophisticated computer-controlled optics that can compensate for atmospheric distortion. The star's light is first steadied, then carefully subtracted from the centre of the image, leaving the planets behind. The Gemini Planet Imager, partly built in Canada, is the first camera dedicated to imaging planets. Starting in 2014, it will conduct a survey of hundreds of nearby stars, looking for giant planets in orbits similar to Jupiter or Saturn. While it's still not possible to directly image a planet the size of Earth, the new camera is expected to find solar systems that are more like our own, filling in a crucial gap in the data and helping to clarify how solar systems like ours form.
WHAT WE KNOW SO FAR
Planets are abundant. Many are gas-giant planets like Jupiter or even larger, and many of those are in orbits that would prevent a planet like Earth from existing. Surprisingly, the most common kind of planets appears to be a type that doesn't exist in our solar system. Bigger than Earth but smaller than Neptune, such planets could be made of rock or gas, or they could be vast ocean worlds primarily made of water.