Out of this world: Finding exoplanets

By Trish Koh, Class of 2016


First direct image of an exoplanet (lower left red spot) orbiting its star (centre blue spot). Credit: European Southern Observatory via their website

Movies have long explored the ideas of discovering exotic new landscapes that we could colonise and the potential to find extra-terrestrial lifeforms. Whenever we find a new planet, these reveries have the potential to become real and people understandably get very excited.

Recently, a new planet named Proxima B has been discovered. Proxima B is an exoplanet, a planet located outside our solar system. It’s orbiting the closest star to our Sun (only a stone’s throw at 40 trillion kilometres away) but that’s not the interesting thing about Proxima B. Astronomers have deemed Proxima B as suitable for life.

How do we know a planet is “suitable” for life?

To be deemed suitable, a planet has to meet a huge range of criteria (arguably more extensive than that of the US presidency candidates). To be habitable, or at least our idea of habitable, a planet needs to have a few characteristics that Earth has.

One important characteristic is the ability to support liquid water. A planet that is located too close to its star may be too hot and all potential water on the surface would be vaporised. On the other hand, a planet located too far away from its star may be too cold and all the water is only present as ice. There’s a range of distances where the conditions are just right, called the Goldilocks Zone. Planets within the Goldilocks zone are not uncommon and planets with the potential for liquid water aren’t as rare as originally thought.

There are many other characteristics such as the planet needing to be rocky and having both the right atmosphere and the essential chemicals. But even if we know what criteria makes a planet, there’s still the question of finding an actual planet.

The search for exoplanets

It’s actually very difficult trying to look for exoplanets. Planets are obviously large but the universe in which they are located is vast. Looking for an exoplanet is like looking for a needle in a barn full of haystacks. Sometimes, we can look and find an exoplanet by looking through a telescope but this is very rare and only possible for planets that both very large and very hot. So to find exoplanets, astronomers use other more indirect methods. Here are two techniques that have discovered the majority of our known exoplanets.

Radial velocity method

When a star is orbited by planets, it doesn’t remain stationary but actually “wobbles” due to the gravitational force exerted by those planets. We can detect this wobbling by looking at the type of light emitted by the star. When a star moves towards an observer, the light emitted shifts to blue and when it moves away, the light shifts to red. This is called the Doppler effect and it also explains why an ambulance siren is high-pitched as the ambulance approaches you, but as it moves away it becomes low pitched.

The radial velocity method has found most of our exoplanets but has some disadvantages. Firstly, it can only detect stars with orbiting exoplanets if the orbit plane is within a certain range of angles. Secondly, radial velocity cannot accurately determine the mass of a planet, something that needs to be known to determine suitability for life.

Transit photometry

This method relies on not detecting the type of light emitted by a star but the amount it emits. When a planet moves in between a star and an observer, the amount of light emitted drops a little bit. When the planet moves away, the light emitted is at full strength again. Transit photometry relies on detecting this dimming and brightening to determine if a star has orbiting planets. This is what the Kepler Spacecraft uses to find exoplanets and it’s detecting them at a higher rate than the radial velocity method.

Kepler does occasionally get it wrong though. So when Kepler detects a potential star, another method such as radial velocity is used for confirmation. Also, transit photometry is completely reliant on catching the alignment of a star, its orbiting planet and the observer at the exact right time and this event is rare.

I spy with my little telescope…

But when the star, exoplanet and Kepler align and a ground-based telescope confirms the existence of the exoplanet, we have our candidate exoplanet. Hubble Space Telescope is then used to determine the characteristics of that candidate exoplanet and if it meets the criteria, that’s when we get exciting news of exoplanets like Proxima B.

Find out more:

NASA on detection methods: https://exoplanets.nasa.gov/interactable/11/

Another great website on detection methods: http://www.planetary.org/explore/space-topics/exoplanets/how-to-search-for-exoplanets.html

Audio version:

Here’s me talking about exoplanets on 3RRR’s science show Einstein-a-go-go: http://www.rrr.org.au/program/einstein-a-go-go?an_page=2016-09-25

Exoplanets is on at 34.30 but I strongly encourage you to listen to our whole student show!