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Finding planets with cosmic blips

starlight peeks round a planet

You only have to look up at night to discover planets close to us like Mars, Venus and Jupiter, but how do scientists find planets all the way out in the centre of our galaxy? Out there planets get hidden by the brightness of the stars, so astronomers cunningly hunt for the distant planets’ traces in the night sky. In order to do that they rely not just on their own observatories, but also lots of linked computers and telescopes around the world.

OK, start by bending space...

The trick to find faraway planets all began with Einstein. About a hundred years ago he predicted that mass would actually bend space. That means really huge objects like stars and planets bend the space around themselves a lot – and light passing next to them gets bent around too. It’s like a big celestial lens.

If one star drifts in front of another as our galaxy moves around, the more distant star will look extra bright for about a month or so. That’s because the mass of the closer star is magnifying the light from the one further away. Now imagine that the closer star also has a planet revolving round it. That planet will spend a bit of time lined up with the light from the distant star too, and that’s when it’ll show itself to scientists. Keith Horne and Martin Dominik, astronomers from the University of St Andrews, explain how. “The result is when the planet lines up, we’ll have a very brief flash or dip in light – a blip – that reveals the location of the planet and the size of it,” Martin says. "So typically that would last a few days if it’s a big planet like Jupiter and if it’s a planet like the Earth, that lasts only a few hours."

Knowing where to look

Since the blips are so short and the sky is, well – rather large, Keith explains that "the trick is to figure out which star to look at, at the right time." The way the astronomers accomplish this is with some serious computing power. After figuring out which stars undergo the lensing effect (about a thousand every year), they start each day by calculating which of them have the best chance of revealing a new planet. Then they train their telescopes on the field of stars around their target.

Keith tells what happens next. "Each time we take a picture of the star field, we use our computers to find the right star in that picture, measure the brightness of it, and determine if that target is being lensed by a planet or not. And when we find one that is, the computer will tell the telescope to go back and look at that one again, just to make sure. And when that is confirmed, then it tells all of our telescopes to go. So we have an alert system under way here that’s all coordinated through the internet."

All those connected telescopes and computers means that even though we may not actually see those distant planets, when they do peek round at the Earth there’s a better chance an astronomer will catch it.