One of the most exciting areas of research today is the search for planets outside our own solar system. The number of confirmed extra-solar planets currently stands at around 60, but this is increasing all the time. This has important implications for the likelihood of finding alien life. The most likely place to find it seems to be on a planet like our own, and the more Earth-like planets that exist, the more chance there is that one of them supports life.
There are four main ways of detecting extra-solar planets:
At the time of writing, only the dynamical perturbations method has succeeded in discovering extra-solar planets, but other methods have been used for follow-up observations.
When a planet orbits a star, it pulls on the star just as the star pulls on the planet. As the star is much more massive, it will seem to stay stationary while the planet orbits, but will in fact wobble slightly as the planet moves round it. This wobble can be used by astronomers to detect the presence of a planet around a distant star.
The most important quantity that is measured is called the radial velocity of the star. This is measured using the Doppler shift of the light coming from the star. As the star moves slightly towards us in its wobble, the light is blue-shifted. As the star moves away from us it is red-shifted. This slight shift in wavelength can be used to measure the amount of wobble of the star, and therefore to calculate the mass of the planet orbiting it. So far this method has discovered all of the planets found around Main Sequence stars. The problem with this method is that planets the size of Earth do not make the star wobble very much, and so only the bigger planets (Jupiter-sized or above) tend to be found.
|Red-shifted spectrum, showing that the source is moving away from the Earth in its orbit|
|Spectrum as it would appear in the laboratory, in other words, if the source was not moving relative to the observer|
|Blue-shifted spectrum, showing that the source is moving towards the Earth in its orbit|
A better way of detecting Earth-like planets is to look at stars called pulsars. Pulsars are rapidly rotating neutron stars with high magnetic fields that give out a radio pulse at very exact intervals. They are so regular that they can be used to keep time more accurately than almost any clock on Earth. A planet orbiting a pulsar will cause it to wobble, just like with other stars, and this will cause a very slight difference in the period of the pulses. If this difference is timed, the mass of the planet can be calculated. The advantage of this method is that it can be used to find much lower mass planets, possibly even as low as the mass of the Moon.
This aims to detect planets by seeing the light they reflect from the star they are orbiting. This is very difficult because the planet is very faint in comparison to the star. Ground-based observations of these systems are also very difficult because atmospheric turbulence causes the star to shimmer, making accurate enough imaging almost impossible. One way of correcting this is to use adaptive optics. This uses a nearby reference star to look at how the atmosphere is moving. A signal is then sent to a series of small motors on the telescope mirror, which continuously deform the mirror to correct for variations in the atmosphere. Scientists are now also using a laser in place of the reference star, which means that this technique can be used in areas of the sky where there are no bright stars for reference.
When a planet passes in between its star and the Earth, the light we see from the star drops slightly. This happens periodically as the planet orbits. The length and level of this drop gives us an indication of the planet’s mass and distance from the star. This only works for planets that pass between the star and the observer. If the planet is orbiting in any other direction, it will not be detected. Planets generally orbit about the equator of a star, so the chances of detection can be improved by concentrating on stars that rotate in the correct orientation.
When a star passes in front of a distant galaxy, gravitational lensing can cause the light from the distant galaxy to be focussed and amplified. If the star is orbited by a planet, the light from the distant galaxy is distorted. By looking at these “light curves” it is possible to detect the presence of planets around these stars. The problem with this method is that each lensing event happens only once, so it is necessary to look at millions of stars in order to effectively find planets.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001