Science Puzzle
How Can We Spot a Planet Round Another Star?
Astronomers now know of thousands of planets going round other stars. Almost none of them has ever been seen directly: they're far too small, too dim and too close to the glare of their stars. NASA's Kepler space telescope alone found more than 2,600 of them, all without a single picture of a planet.
How did Kepler find them?
The Answer
By watching stars dim. When a planet passes in front of its star, it blocks a tiny part of the starlight. Kepler measured the brightness of about 150,000 stars, over and over, and looked for small dips that repeated regularly.
The dips are minute. A planet the size of Jupiter crossing a star like the Sun blocks about 1 per cent of its light. An Earth-sized planet blocks about 0.008 per cent, roughly one part in 12,000, like a moth crossing a streetlight seen from many kilometres away. Kepler, launched in 2009, stared at the same patch of sky for years and could measure changes that small.
One dip could be anything, but dips that repeat with clockwork regularity give the planet away. The time between dips is the length of its year, and how much light it blocks tells you how big it is compared with its star.
The method only works if the planet's orbit is lined up edge-on as seen from Earth, so for every planet Kepler caught, many more must have been passing above or below their stars, unseen. Telescopes such as the James Webb Space Telescope now study the starlight that shines through a transiting planet's air, to find out what that air is made of.
The principle: The transit method. A planet passing in front of its star blocks a tiny, regular fraction of its light, so precise brightness measurements reveal planets far too faint to see.