Sunday Science: Exoplanets

This week, NASA got everyone jolly excited with an announcement about "exoplanets". Simply put, an exoplanet is a planet that does not orbit our Sun - but orbits around another star.

End of post? Not quite.

While the definition of an exoplanet is very simple, finding these far-flung worlds is not. Here's how you can find an exoplanet:

1. Watching wobbles

When a star has planets orbiting around it, then the planets cause the star to "wobble". Scientists pick up these wobbles as changes in the colour of the light emitted by the star.

2. Searching for shadows

When a planet passes in front of a star, then a tiny amount of the star's light will be blocked. This is the most popular way to find planets - NASA's Kepler spacecraft has found thousands of candidate planets using this method since 2009.

It's called the transit method. Here's a hulk (planet) and giant Lego head (star) to demo how this is done:

3. Say cheese

A tiny amount (around 1%) of exoplanets have been found by directly imaging them in orbit around a star. It's incredibly difficult to do (imagine trying to photograph a speck of dust in front of a lightbulb) but here's a short movie of four planets more massive than Jupiter orbiting the young star HR 8799. These images were taken over seven years by the W.M. Keck observatory in Hawaii.

4. Bending beams

Another tricky method is to detect how a star's light is bent around a planet due to gravity. Only a handful of planets have been found this way.

How many exoplanets are there?

We will never know! But we have (at the time of writing) discovered just under 3,500 extra-terrestrial worlds.

Scientists are also busy trying to work out if these planets could hold life, which is why the recent NASA announcement is particularly exciting - the space agency has found SEVEN Earth-sized planets all within the "habitable zone" of a star ( a place in the orbit where life could survive as it's not too hot or too cold - which is why this is sometimes called the "Goldilocks zone").

It's a fascinating concept and one that will keep scientists and science fiction writers busy for some time. Imagine planet hopping between worlds with different civilisations.

You may want to download the "eyes on exoplanets" app to fly through the planets and see an artist's concept of their surfaces.

NASA announced the discovery of the most Earth-sized planets found in the habitable zone of a single star, called TRAPPIST-1. Image courtesy of NASA.

To find out more about exoplanets and where to find them, check out NASA's exoplanet exploration site.