How to find a planet

Its been announced that the Kepler telescope has found a ‘scorched super-Earth’ exoplanet (see my previous post). Ever wondered how astronomers actually find planets orbiting distant stars?

By the way, the Kepler telescope is in space, it orbits the Earth, which is pretty cool.

Here are the main ways of finding planets, some others exist, but these are the most popular:

1. The most obvious way is just to look for the planets with a telescope, similar to looking at Jupiter with a hand telescope; this is usually called direct imaging. Unfortunately stars are big and bright, and planets are actually pretty small and not very bright; planets don’t make their own visible light like stars do, they reflect light emitted from their star and emit some infrared light as they are heated by their stars. So trying to directly image a distant planet is a bit like trying to observe a tiny-little insect buzzing around a huge halogen light, at night. With current technology it’s extremely difficult, but NASA did directly observe some exoplanets in 2010, and future innovations may make this an increasingly feasible technique, although to-date no confirmed exoplanets have actually been discovered via direct imaging.

2. Another method; is called the transit method or sometimes the occultation method. The idea is that as a planet orbits its star it will periodically cross in front of its star and will cause a temporary, and very small, dip in the star’s brightness. This dip in brightness will tell us the volume of the planet, as the larger a planet’s volume, the greater the drop in brightness. The main problem with this technique is that we need to be in a position in which we can actually see the planet cross its star; ideally we need to be side-on from the system, if we saw the star from the top, we would never see the planet cross in front of it. However, the technique has proved successful, with around 20% of exoplanets discovered using this technique, as telescopes can be trained on huge numbers of stars at the same time, waiting to capture and temporary drops in brightness. Two of the most famous spacecraft dedicated to exoplanet detection use this method, COROT and Kepler.

3. Another technique also looks at changes in a star’s brightness, but this time scientists’ look for a periodic increase in brightness, it’s usually called gravitational microlensing. Einstein found that space is a bit like rubber, and that heavy objects, like stars, placed on space cause it to stretch and deform, a bit like putting a bowling ball on a rubber sheet, the ball will sink and stretch the rubber around it. This stretching of space affects the passage of light rays and even changes the speed at which time passes. Weird huh? This is one of the reasons Einstein was so magnificent, he was able to understand that space itself could behave in such a strange way, with only his tiny human brain. Now, if a star passes in front of a more distant star, the distortion of space caused by the nearer star will make the light from the distant star become momentarily brighter, its called lensing as the gravitational field of the nearer star acts as a lens for the light of the further star, a bit like a glass lens on spectacles can make distant objects more easy to see. If the nearer, lensing star has planets, these planets will add to the gravitational field of the lens, and thus their mass can be inferred based on the increase in lensing above that expected by the star alone. To date, this technique has only found 4 confirmed exoplanets and 7 candidate exoplanets, as although it is sensitive enough to find Earth-sized planets it depends upon a chance alignment of the Earth and two stars, so observers must train their telescopes on many stars and for a long time in order to see enough of these chance events.

4. The most successful method to date (roughly 70% of exoplanets discovered) is called the Doppler spectroscopy or radial velocity method. Ever heard of red-shift and blue-shift? Well, stars emit rays of light, and these light rays have a specific wavelength. Think of it like waves in the sea, the wavelength is the distance between two wave crests, so a short wavelength means the crests of the waves are close together, and a large wavelength means the crests are further apart. Different wavelengths in rays of light is what gives light different colours, red for longer-wavelengths, blue for shorter. Now imagine if an object emitting waves is moved closer towards you, it will cause the wavelengths of its waves to be compressed as the distance between the object and you is shortened. If the object is moved away from you, the wavelengths of its waves become longer, as the waves are stretched over a greater amount of space; this is called the Doppler effect. This happens with stars too, if a star moves away from the Earth the wavelength of the light it emits becomes stretched and its light gets redder (red-shift), and if the star moves towards the Earth the wavelength of the light it emits becomes compressed and its light gets bluer (blue-shift). When planets orbit their star, they actually cause the star to wobble back-and-forth, which alternatively moves the star towards and away from the Earth. Astronomers can look at the wavelengths of light emitted from stars to try to find such alternate blue and red-shifts. The bigger the planet, the greater the star’s wobble and the greater the Doppler effect, so this technique can be used to work out the mass of an exoplanet. Often this data is combined with the volume measurements from the transit method so the density of exoplanets can be estimated (mass / volume = density), which gives an indication of what the planet is made out of. As with the transit method, Doppler spectroscopy works best when the star and orbiting planet are seen side-on from Earth, rather than top-down. The technique is also most sensitive to large planets orbiting close to their stars, hence the bias for finding Hot Jupiters. The most famous telescope that uses Doppler spectroscopy is the Keck telescope in Hawaii.

5. The last method is called astrometry, or sometimes the wobble method. Remember I said that planets orbiting a star cause the star to wobble back-and-forth, this is the idea behind the Doppler spectroscopy technique. Well rather than looking for changes in the wavelength of light, astronomers can actually look for the wobble of the star itself by precisely measuring a star’s position in the sky compared to its neighbours. It’s a relatively simple technique but unfortunately the wobble caused by orbiting planets is so small that current technology is not sensitive to detect it, and to-date no confirmed exoplanets have been found with this method. As technology becomes more refined though, it is likely to become a viable method, and NASA planned to launch a space detector using this method (Space Interferometry Mission) but unfortunately it looks as though this project will be shelved for budgetary reasons, damn those global financiers and their demolition of the world economy!

There, the biggest bog post I’ll ever write. There are a few other methods, like the pulsar timing technique, but these are the only ones likely to ever come up in a pub quiz. Now if only we could find that pesky Nibiru (spoiler alert, if doesn’t actually exist).

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