Space exploration does not stop at the edge of the solar system. Despite the fact that technology has not yet advanced enough for us to send a probe outside the solar system (although Pioneers 10 and 11 are heading into interstellar space after completing their missions, they are no longer being tracked), we can still 'see' planets orbiting other stars by measuring the 'wobble' of the star. This 'wobble' is due to the gravitational interaction between the star and the planet, and can be detected by space telescopes such as Hubble, or ground telescopes like Keck on Mauna Kea in Hawaii. Due to recent technological advances, some planets have now been detected by their light alone.
Measuring the 'Wobble'
When the spectra of distant stars are recorded by a telescope, the 'signatures' of specific elements are easily observable. This is how we know about the composition of stars. If a star is moving relative to the Earth in a longitudinal direction, these 'signatures' are shifted slightly in the spectrum, according to the direction and magnitude of the movement. This is called the Doppler Effect or Doppler Shift. For things moving away from us, waves are effectively stretched so the wavelength of the light is apparently longer, and the signatures are shifted towards the red end of the spectrum. We say that the light from objects moving away from us is red-shifted. The opposite is true for bodies moving towards us, and we say that these are blue-shifted.
If a planet is orbiting a star, the gravitational interaction makes the star move, and some of its movement will be in a longitudinal direction, so the resultant periodic shift in the spectrum can be observed and from that scientists can infer not only whether or not a planet is present, but also the size and orbital radius of the planet. The diagram above shows the changing relative velocity of a star with respect to time (courtesy of NASA).
Why Search for Extrasolar Planets?
The search for extrasolar planets is important because discoveries in this field impact on our understanding of our own solar system. Although we have a good idea of how planets form, in more detailed areas of the subject there are still aspects of planet formation that we would like to know more about.
A current example of this is the discovery by Keck of a Jupiter-like planet orbiting one star in a triplet system. This is astonishing for two reasons: firstly, it was believed until now that no planet could possibly survive the intense gravitational forces present within such a system; secondly, in our most recent model for planetary formation gas giants are expected to be formed in a 'doughnut' of dust, called a proto-planetary disc, at the very edges of the solar system and later migrate inwards. However, in this system two other stars circle the central star beyond the planet's orbit, so the hypothesis does not really provide a satisfactory explanation of how this planet came to exist.
A major incentive to search for other planets is the hope that we might discover an Earth-like planet that is also within the 'habitable zone' around its star. This zone is defined by the temperature change with increasing distance from the star. No planet except Earth is currently in this zone in our solar system, although it is possible that Mars was at some point in time. (Diagram courtesy of NASA).
The part of this research with the most public appeal is undoubtedly the fact that if we do find an Earth-like planet within a solar system's habitable zone, then we might be one step closer to discovering extraterrestrial life. There is also speculation that primitive life forms may live in the subsurface ocean of Jupiter's moon, Europa, in our own solar system.
Whatever we find is unlikely to resemble E.T. of Hollywood fame, but even finding the most primitive bacteria on another world will show us that we are not alone. If life can evolve on other worlds, it is statistically probable that somewhere in the vastness of the universe there are beings at least as intelligent as us, if not more so. Meanwhile, the search goes on. NASA's PlanetQuest mission to find 'another Earth' and the SETI (Search for ExtraTerrestrial Intelligence) Institute aim to make significant contributions, as does ESA's Aurora Exploration Programme. Whether they find what they are seeking or not, the results will have great implications for our understanding of our own planet and of humanity.