What are Extrasolar Planets?:
To define an Extrasolar  planet we must first define a planet. There are two types: the Earth type which is rocky and contains lots of iron but its not very big and the Jupiter or 'Jovian' type planet which is gaseous and mainly consists of Hydrogen and Helium. These planets are usually very massive and can be up to 13 times the mass of Jupiter. At 13x the mass of Jupiter the object would be a brown dwarf. A Brown dwarf can 'burn' deuterium, an isotope of hydrogen which is more easily fused together, but it is not have enough gravity and density to 'burn' hydrogen . Once the mass is in excess of 80x of the mass of Jupiter,  the object is classed as a star (in the case of 80x Jupiter, an M class star) and 'burns' hydrogen fuel which gives out light and heat before it dies after its life cycle.
So, ExtraSolar Planets are planets outside of our solar system that orbit stars other than our own. Mankind has been trying to prove their existence since the telescope was invented in 1610.

Why?
The ulimate goal of this search is to find a planet that would be capable of supporting life i.e. an Earth-like planet that would be a suitable distance from the sun for it to support life. Yet along the way we have also made new discoveries that have added new ideas and views to our research. One such example of this is the fact that many Jupiter like planets, in their size and composition have been found to be orbiting very close to their parent star when previously it was thought that only rocky, Earth-like planets did so and that the gaseous giants were further away. In fact most of the gaseous giants discovered are in fact closer to their sun than even Mercury is in relative distance to our own sun.

When was the first Extrasolar planet discovered?:
The first Extrasolar planet was discovered in 1995 orbiting 51 Pegasi. It was therefore named 51 Pegasi B. This planet is 0.6x the size of Jupiter with a tug on its parent star of 57m/s. It has been speculated that 51 Pegasi B (an artist's impression: Fig 1.2)  is craterless, with the surface being constantly refreshed by volcanic eruptions of molten rock, as on Earth.

AAn Artist's impression of 51 Pegasi B (Fig 1.2)
 

This would be because of the planet's distance from its star (as shown in the third diagram in Fig 1.3). The closeness of the planet to the star would have the same effect that Jupiter has on its moon, Io, which erupts sulphur and ice.  N.B. The first diagram shows the inner solar system which is in comparison to the other 5 Extrasolar planets.

Fig 1.3
the orbital period of 51 Pegasi B, the planet shown in this figure closest to its sun, is 4 .23 days whereas the orbital period of  47 UMa b, shown in this figure as the planet furthest away from its star, is 1089 days (2.98 years)

Who discovered it?:
M. Mayor and D. Queloz.

How was it discovered?:  The first Extrasolar planet was discovered very much by chance. The silhouette of the planet was cast upon its parent star as it passed in front (as shown by the first picture). A more  refined method which relies less on chance and has greater success is by comparing redshift and blueshift. This all ties in with the idea of the Doppler effect and the effect of the gravity of two cellestial bodies acting upon each other. This is further explained later on.
 

Detection

There are four methods for detecting planets around other stars, one of them has already been mentioned: The Doppler effect. The planet's gravity, although much less than the star's, does have an effect on the star itself and vica versa. So the two celestial bodies orbit each other. The central mass point depends on the masses of the bodies and since there is such a difference in the two masses, the  central mass point is usualy somewhere inside the star. Both the planet and the star revolve around this point and, providing we see the star at the correct orientation, the star will move towards us and then away from us. This is the Doppler effect and can be detected by red shift and blue shift in the spectrum. Blue shift  is created when the star is moving towards Earth on its orbit and redhift when the star is moving away (Fig 1.4)

   Fig 1.4

The doppler effect has been the most successful method of detecting Extrasolar planets so far in our search.
Planets can also be detected by Astrometry. If a particular star is closer to Earth than a set of stars behind it then the stars can be used as background stars and constants from which to observe the movement of the closer star. If the star moves and has a sort of 'wobble' relative to the background stars then it must be orbiting something which must be a planet and this uses the same theory as the Doppler effect.
The third method is the 'Transit' method. The light from a star is fairly constant unless something blocks the light's path. When the planet passes in front of the star there is a 'dip' in the amount of light given off by the star (Fig 1.5).


Fig 1.5

Because of this dip, we know that something must be orbiting the star and for it to be big enough to obsure this amount of light, it could be a planet. This is the way in which the first Extrasolar planet was discovered.

Finally, the last detection method is Gravitational lensing. This involves the passage of a star infront of another star. The effect that this has is that the star infront magnifies the light of the star behind it.  And this is shown as a smooth curve. But if the star that is doing the magnifying has a planet orbiting it and the planet passes in front of the magnified light it will magnify it further and cause a glich in the smooth curve.

   Fig 1.6

A graph with a smooth curve indicates that the star doing the magnifying does not have a planet orbiting it but if the  graph has a glitch  (as shown in Fig 1.6) this indicates that there is something else there which has done an extra bit of magnifying and this must be large enough to cause such a glitch. This glitch is caused by a planet which is large enough in mass to magnify the star's light quite a lot. The problem is with the last two methods is that they rely on events happening tht are not predictable and therefore they are not the best methods of detection.
All of these methods generally apply to large planets that are nearer to Jupiter's mass rather than Earth-like, rocky planets which are more difficult to detect.

Examples of Planets discovered
Other extrasolar planets disovered are numerous. I have mentioned just a few of these.
The star 47 Ursae Majoris has been found to have two planets orbiting it:  47 Ursae Majoris b (47 UMb) and 47 Ursae Majoris c (47 UMc). 47 UMb was discovered in 1996 by G. Marcy and P. Butler using the Doppler method of detection. It is a Jovian (Jupiter-like) planet. Its mass is 2.54 times that of Jupiter and its average distance from its parent star is 2.09 AUs, which is twice the distance of Earth from the Sun making the planet possibly colder than Mars.
An artist's impression: Fig 1.7. The planet orbiting 47 Ursae Majoris is close enough to its sun for liquid water to exist on its moons. Here we see a moon slightly smaller than Mars covered in glaciars. Under the frozen surface, a deep ocean of liquid water, and perhaps life, may reside.

Frozen water could be found on the moons of this planet as the rules of distance tends to be that the further the non-jovian world is from its sun, the more ice and less rock there is in its mass. This is proven in the case of Jupiter's and Saturn's moons which contain lots of ice and are further away from the Sun that other cellestial bodies that tend to contain more rock than ice such as The Moon.
The core and mantle of the moons could be rocky but the surface could be covered with ice. The most interesting possibilty about the planet's moons is that if it is tectonically active then an ocean could be formed underneath the ice crust and here could reside a world teaming with life that has never seen the light of its sun. (Fig 1.7). The second planet, 47 UMc, is much more mysterious. It was only discovered in 2001 by D. Fischer and has a mass of 0.76 Jupiters. However, as of yet no speculations as to its composition and such properties have been made.
70 Virginis has one planet orbiting it: 70 Virginis b which is 6.6 Jupiter masses. It orbits at a distance of 0.43 AUs (shown in Fig 1.3, the fifth diagram down) which is at a distance slightly further than Mercury. It was discovered by G. Marcy and P. Butler in 1996.
70 Virginis b is one of the more massive planets known. It is a promising planet. Although the planet itself is almost certainly a lifeless gaseous giant, like Jupiter, it orbits close enough for liquid water to exist and where there is liquid water life is possible.  Recent information gathered by the Galileo probe told us that Jupiter's atmosphere, rather than consisting of a high quantity of water, was mainly composed of hydrogen and helium. It is therefore reasonable to assume that 70 Virginis b is likely to be the same. Though life is unlikely on the planet itself, a system of moons, possibly the size of Mars or Earth, could orbit this planet and hold some promise of life.  The inner moons, cooked and damaged by the tidal forces and intense radiation belts of 70 Virginis b, would be inhospitable. However the outer moons could resemble Earth or Mars. Some of the larger moons could have a Nitrogen atmosphere, which although unsuitable to humans, could allow organisms that live in the possible small lakes and warmed by the sun to evolve, crawling along biological evolution (Fig 1.8).
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An artist's impression:

Fig 1.8. A moon half the size of Earth orbits at a comfortable distance from 70 Virginis b's magnetic field. Under its thin atmosphere we see lakes of liquid water in which simple forms of life may exist.

The Upsilon Andromedae system is the first multiple planet star system to be found with a sun-like star. The three planets vary in size from the b planet which is 0.72 masses of Jupiter (an epistellar giant), the c planet which is 1.98 Jupiter masses (a near epistellar giant) to the d planet which is 4.11 Jupiter masses (an eccentric giant). Each of the three planets are Jovian. The b planet is nearest to the sun and smallest. It was discovered in 1996 by Marcey and Butler. Because of the planets distance from its sun it is likely that it is 'tidally locked' meaning that the same side of the planet is always facing its sun, just as the moon does with the Earth. If this is true then the spot on the planet would always be facing the sun and would therfore be the hottest part of the planet and so here the gases would be superheated and eventually the would explode. When they got to the far side of the planet they would cool. The gases would then sink to the lower atmosphere
and be circulated back to the starside of the planet. This world is constantly turning itself inside out.
Because of the tidal lock of this planet, its moons would have probably  lost their orbital momentum long ago and crashed into their parent planet.
 

Future

So far there have been about 200 extrasolar planet's existance confirmed using the methods aforementioned; but what have we actually learnt from all this? Well for a start (as mentioned in the introduction) we have learned that gaseous giants do exist at a very close proimity to their star. We have also learnt that our Sun is not as unique as we once thought and that maybe neither are we. If other Jupiters and Satrurns can exist around distant stars then why not other Earths? A scary thought that perhaps we are not as unique and important as we have always assumed that we are. And an even more daunting thought is that we might not be alone in the universe and that maybe on one of the yet to be discovered planets Earth-like beings are at this very moment trying to detect us and see if it is possible that we exist.
As for the future of detection, one moment that human beings have longed is the moment that we could see an actual image of a planet that is not of our own solar system. Yet all of the detection methods that we ue do not allow this to happen and the only images we have are images impressions.
However, there are plans for two projects over the next decade that would allow imaging to take place: the Stellar Interferometry Mission (SIM) and the Terrestrial Planet Finder (TPF).  The missions propose to use interferometry, the use of of the interferance of waves for the precise determination of such things as distance or wavelength, to accuratly measure the positions of stars and find planets around them.  Multiple telescopes will fly at different distances and act like a pair of binoculars.  An extension of this idea would be for 25 40-meter telescopes to be separated by distances of hundreds of meters. The difficulty would be that they would have to 'know' each others positions extremely accuratly in order for an image to be created of even a giant planet/gaseous giant. Such a system should be able to create images of Earth-like planets around stars within about 50 light-years (see Fig 1.9).

The 'Darwin' project has also been set up to try and discover if extrasolar planets are capable of supporting life. The satellites (shown in Fig 1.9) target a star where they know a planet resides. The project would concentrate on earth-like planets, in terns of the composition of rock and iron and an abundance of water, orbiting sun-like stars. A comparison would be made between our atmoshere, particularily the contents of oxygen and ozone and the atmosphere of the targeted planet. If the atmosphere was capable of supporting a carbon based life form then we would then know that it is possible for life to exist on planets other than our own.

   Fig 1.9