The universe is huge. It is so big in fact, that scientists can't find an end to it. The earth orbits the sun in a solar system. While it seems like this solar system could be unique it is highly unlikely because there are just so many other stars in the universe. Once scientists were able to determine the stars of the Milky Way galaxy and where they are, it was time to look even closer. Perhaps some of these stars had planets orbiting them, just like our solar system. The first, confirmed, planet orbiting another star was confirmed in the year 1993. Since this planet was not within our own solar system, the phrase extrasolar was coined by scientists searching for these alien worlds. These new discoveries brought the question of whether these planets could sustain life and furthermore, already have life living on them. Considering how large the universe is, it seems very unlikely that our own planet, earth, would be the only one that could sustain life. Recently, though, there has been many strides in discovering these extrasolar planets and as of the beginning of this century, approximately two dozen are being discovered each year. It certainly seems that finding a planet with life is inevitable. Unfortunately, it could be a very long time before scientists develop the technology for finding planets that are similar to earth on a regular basis.
There is an interesting history to just how the first exoplanets were discovered. The first ever extrasolar planets were discovered around a millisecond pulsar. Pulsar planets are discovered through pulsar timing measurements, to detect oddities in the pulsation period. Anything orbiting the pulsar will cause regular changes in its pulsation. Since pulsars normally rotate at near-constant speed, any changes can easily be detected with the help of precise timing measurements. Unfortunately, pulsar planets would be impossible to support life due to the extreme radiation given off from the quickly spinning pulsars. The planets however, are believed to have been formed from the remnants of the supernova that preceded the pulsar. Since then however, scientists have focused more on discovering extrasolar planets around main-sequence stars like our own sun. This seems like the most likely place life will be found since it would be just like our own solar system which has developed life. The first exoplanet around a main-sequence star was discovered October 6, 1995. Most of these planets discovered so far have been very large - most larger then Jupiter - and gas giants. However, on January 25, 2006 astronomers announced a rocky or icy planet of about 5 times the mass of Earth. “As of May 7, 2006, there were 152 known planetary systems around main sequence stars, containing at least 184 known planets. In addition, there were 4 known pulsar planets orbiting two pulsars,” (Wikipedia).
There are currently seven methods of detecting extrasolar planets. Detecting the planets optically is currently impossible since they are so much fainter compared to their parent star. The methods currently used are pulsar timing, astrometry, radial velocity, gravitational microlensing, the transit method, circumstellar disks, and direct observation. Each method has its own advantages and disadvantages but currently the most used method is radial velocity.
The first method mentioned, pulsar timing, is how the first extrasolar planets were discovered. It's a fairly simple method that isn't too hard for scientists to perform and is probably why it was the first method used. Basically, scientist monitor the pulses given off by pulsars - extremely quick spinning neutron stars - and look for any anomalies caused by an outside force. When these anomalies are found, then the scientists search for a source. In most cases it turns out to be a planet orbiting the pulsar. Since pulsars give off radiation at such a regular pace in an easily detected pattern, it is very easy to spot these anomalies and locate the planet within the system. This method is often used to detect pulsar companions but is not always used to specifically find planets.
The second the method mentioned, astrometry, is very simple but at the same time very complicated. The problem with astrometry is that scientists simply don't have equipment precise enough to accurately discover planets. The concept of astrometry however, is quite simple. A number of exoplanets have been found this way but none of them are definitely confirmed and most astronomers have given up on this method because of the difficulty to confirm the planets existence. This method involves measuring the proper motion of a star in the search for a disturbance caused by its planets but unfortunately, changes in proper motion are so small that scientists simply don't have the technology to detect these changes.
The next method of detection, and most used and successful method, is radial velocity. Radial velocity is used by measuring changes in the speed in which the star moves away from or towards earth. That means that the way scientists see it changes with relative velocity to the earth. “The radial velocity can be deduced from the displacement in the parent star's spectral lines due to the Doppler effect,” (Wikipedia). This means, in essence, that the radial velocity is seen from the lines of light through a spectrum and creates a Doppler effect on the earth. The changes are caused by the planet orbiting the star and so the scientists can measure the speed at which the planets and suns go around their center of masses. The velocity of the star around the center of mass is much smaller then the planet's since the planet is actually moving. Velocities as low as 1 m/s can be detected with modern spectrometers like the HARPS (High Accuracy Radial Velocity Planet Searcher) spectrometer at the ESO 3.6 meter telescope in La Silla Observatory, Chile. The radial velocity method is also known as the “wobble method” or “Doppler method”. This method only works for nearby stars, about 160 light years out. Also, it only works for planets that are relatively close to their stars and have a small revolution period. It is very hard for this method to find planets orbiting at great distances. It cannot be used to detect planets in solar systems where orbits exist on a plane perpendicular to the line between earth and that system's star. This method can also help to confirm the transit method discoveries.
