Searching for Earth: The Hunt for Habitable Exoplanets
An exoplanet is any planet that is in another solar system, orbiting around a star other than our Sun. In this piece, I’ll be looking at some of the methods we have for detecting exoplanets and some of the interesting things we find when we take the time to observe them. In my opinion, the study of exoplanets is one of the most interesting in astrophysics due to our current environmental crisis here on earth. Considering who we are relying on to fix our planet (a pair of dumb blondes), it may become necessary to look to the skies for our new home. Or, if you want to take a more positive perspective;
ALIENS! COOL! YAY!
So how is it that we can see planets that are so far away? The short answer is that we can’t. Though astrophysicists can’t always see the thing we are studying, whether it is with the naked eye or even a regular old telescope, there are ways of inferring information about the celestial bodies. If an exoplanet is particularly large, physicists can detect it using a technique with a very funky name; the ‘stellar wobble method’.
You may have seen those ads for science week on the DART which said something along the lines of ‘everyone is attractive to everyone around them’. The point of those highly cheesy ads was that everything, no matter how small or large, has a gravitational pull. We may not always notice these forces at work, but that is only because we are so infinitesimally tiny on a universal scale. Every planet exerts some sort of gravitational force, but the larger the planet, the greater the pull.
When a distant star wobbles from side to side, physicists can detect this movement by observing colour changes in a spectroscope reading. If something is moving away from you, the colours will shift towards the red end of the spectrum. As the object moves away, the distance between you and it gets larger and the wavelength of the light is stretched out. This is called the doppler effect and is the same reason an ambulance siren will appear to become higher-pitched as it moves towards you, then lower-pitched as it moves away. If this colour-shift method reveals that a distant star is wobbling from side to side, the best explanation is that there is something orbiting that star; an exoplanet! The seemingly invisible planet pulls the star towards it as it rotates around, creating a wobble that can be measured and analysed. When I was researching an exoplanet, I used a technique with a less funky name; ‘transit photometry’. This method is perfect for smaller planets with weaker gravitational fields. In transit photometry, the first step is to observe the light coming from a distant star using a telescope. When an exoplanet passes in front of the star, it casts a shadow. The characteristics of that shadow can then tell us everything from the size of the planet to the distance from its star. With the right algorithm, we can infer quite a lot about distant planets just by observing these tiny dips in the light coming from distant stars.
The habitable zone (HZ) is traditionally defined as the area around a star where an earth-sized planet with an atmosphere similar to ours can sustain liquid water on its surface. If the planet is too close to its star it is too hot and there will be no water, as is the case with Venus. If it is too far from the star, however, it gets too cold and filled with CO2 as a result of volcanic activity, as is the case with Mars.
But what if the exoplanet is not like earth? The majority of the exoplanets we have discovered are not earth-like. So is studying them completely useless if we’re looking for somewhere that could sustain life? Not exactly. One of the most interesting things I found out when studying a large exoplanet was that if the planet is in the habitable zone and has a moon, then that moon could be habitable. The large number of moons in the solar system indicates a high probability of moons orbiting giant exoplanets. Jupiter alone has 79 moons that we know of!
Exomoons have the potential to be ‘super habitable’ because their host planets offer them a diverse range of energy sources. The biosphere of a super-habitable exomoon could receive energy from the reflected light and emitted heat of its host planet and from the planet’s gravitational field. In a sense, the giant exoplanet would act as a miniature host star for its exomoon. It is expected that an exomoon would have a more stable, longer period in which the conditions would be suitable for life to form.
It is likely that each of these giant exoplanets has more than one exomoon, just like Jupiter or Saturn in our own solar system. If this is the case, then the number of potentially habitable exomoons is far greater than the number of potentially habitable exoplanets. I don’t know about you, but I find that pretty damn exciting!
Georgia Stynes – Science Writer