Extract from ABC News
They sound like distant destinations ripped from the pages of science fiction: a deep blue gas giant where it rains molten glass, or a little iron "cannonball" world which cuts a lap around its star every eight hours.
In recent years, astronomers have not only discovered these extra-solar planets, or exoplanets — and more — they can also paint often incredibly vivid planetary descriptions.
Yet with very few exceptions, exoplanets can't be seen directly.
They're too small or far away to see with a telescope, or the brilliance of their sun overwhelms what little light they emit.
So with this in mind, how do astronomers know what an exoplanet is made of — and if it might be a contender for harbouring life?
How to spot an exoplanet
For a start, astronomers need to detect these far-away solar systems. There are two main ways they do this.
The first relies on the fact that stars with planets don't sit motionless in the centre of their solar system. They're pulled into their own small circular or elliptical path by the gravitational tug of their planets.
It's generally too hard for telescopes to see a star's subtle side-to-side shimmy.
But as a star moves away from and towards Earth, its light stretches and becomes redder, then compresses and becomes slightly bluer — a change that telescopes can pick up.
This way of detecting exoplanets is known as the radial-velocity method, or, as University of Southern Queensland astronomer Jonti Horner calls it, the "wobble method".
And it was the dominant way of finding exoplanets for a decade or so after the first confirmed detections in the 1990s.
But spotting these stellar wobbles is labour-intensive, sometimes requiring years of staring at a single star to see if its light appears to get redder and bluer over time.
Enter: the transit — or "wink" — method, the second primary way to detect exoplanets.
If an exoplanet's journey around its star is in Earth's line of sight, we'll see that starlight momentarily, but periodically, dip in brightness.
"The beauty of that is that you can use a wide-field camera and simultaneously measure the brightness of a large number of stars at a time," Professor Horner said.
"Some of them will have planets that are lined up just right that they pass between us and their star every time they go around, and you'll see that star periodically winking."
The winking method's clocked up more than 3,700 exoplanets; the wobble method's found around 900.
Add to these another 120 exoplanets that were spotted using a technique called gravitational lensing, where light from a star far, far off in the distance bends around an exoplanet.
Only around 50 exoplanets have been directly imaged.
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