Methods of detection
There are several different ways in which we can determine whether a star has planet orbiting it.
- Direct imaging
- Pulsar timing
- Stellar wobble
This is sort of what it sounds like. Point a telescope and fortuitously see the planet. The problem is, most planets are quite small and not very bright, so this isn't the most reliable of methods. That's not to say it hasn't had some results. For example, Formalhaut B was discovered this way when the debris cloud surrounding the star was imaged.
This method requires a little bit more background physics. The Doppler effect is what happens when something which is emitting waves (for example, sound waves) is moving with regards to the observer. For example, if you are on a train, going past a level crossing with the ding-ding-ding-ing, it sounds higher-pitched when you're moving towards it because the waves seem more "bunched up", and then, as you go past, it suddenly sounds lower-pitched because they seem more "spread out". That's the Doppler effect.
Light is also a wave (or at least, often behaves as one). When a light source is moving towards you, the light waves will appear more bunched up and hence bluer (because blue light is a higher frequency than other visible colours). And if the light source, a star, for example, is moving away from you, the light will seem more spread out and hence redder.
Planets have a non-zero mass which means that while their star gravitationally pulls on them and keeps them in orbit, the planet also pulls on the star a bit. But because the planet is going around the star, it pulls in different directions at different times, making the star wobble. When the star wobbles towards Earth, it's light will look slightly bluer, and when it wobbles away, it will look slightly redder. Fancy spectrographs on telescopes can detect these slight variations in light output and hence, we can use this method to detect plants.
This only works on sufficiently heavy planets, sufficiently close to their stars. This has been the most popular method (up until Kepler, maybe, which I'll talk about below) for discovering exoplanets. It was the reason we suddenly discovered a whole lot of "hot Jupiters"—Jupiter sized, or bigger, planets close in to their stars—and had our pre-existing theories of planetary formation turned on their heads*.
*We were basing our theories on our own solar system which has large planets far out and small rocky planets close in. Suddenly, because of the detection methods we were capable of, we were seeing a lot of large planets close to their suns which we could not easily explain. However, it's quite likely that the plentitude of these hot Jupiters is actually a selection bias (as they are the easiest to find) rather than an indication that they are actually proportionally that common in the galaxy.
This method uses the fact that when a planet passes in front of its sun, it blocks out some (very small amount) of its light. Sensitive telescopes can pick up this dip in light output. The size of the dip gives an indication of the size of the planet and monitoring the star for long enough allows us to work out how quickly it goes around the star.
This is the method the Kepler telescope, currently in orbit, is using the discover a pile of exoplanets. And I do mean pile. So far more than 1200 planet candidates have been detected by Kepler. There are some really interesting stranger-than-fiction ones that I'll talk about in a later blog post.
Gravitational microlensing is a smaller-scale version of gravitational lensing, which I have briefly mentioned in the past. Remember (or follow the previous links to find out), if a massive object passes in front of a more distant light-source, then the massive object's gravity, which distorts spacetime a little bit, will cause the light from the background source to bend through the distortion. On a large scale, this can give us several images of the background source (example: Einstein's Cross). On a smaller scale (one might say a micro scale, heh), what happens when a moderately massive object like a star or a planet (or a star with a planet) passes in front of a background star, is the light from the background star is temporarily magnified.
If the foreground object is a star with a (sufficiently massive) planet around it, then the presence of the planet will make an extra peak in the magnification light curve. The height of this peak tells us about the mass and distance from the star of the planet.
I've put this in for completion, but there is no way for a pulsar to be human-habitable, even though some of them have planets. Pulsars are neutron stars—very, very dense stars made entirely of neutrons; sort of giant atoms. they emit radiation, mostly radio waves, from their magnetic poles which, because they spin very quickly, flashes past us in pulses, hence the name.
We can measure and time these pulses very precisely and usually vary in a very predictable way. Slight timing variations due to the gravitational tug of planets are very noticeable and some of the earliest exoplanets in the 90s were detected in this way.
The problem with most of these methods is that they require a fortuitous positioning of planets with respect to their suns and the Earth. We cannot yet look at a star and definitely say that there are no planets orbiting it. If we're lucky we can say there there definitely are planets there, but if we don't see any it could be because they're not lined up with us nicely, rather than because they're not there.