600 extrasolar planets and counting....

A planet around the star Beta Pictoris: a beautiful example of how we can see actual images of planets around other stars. Whilst we may not be able to glean much information from images such as this, the technique is in its infancy and yet still provides some of the most poignant pictures of the cosmos. (Image credit: ESO/A.-M. Lagrange et al.)

Since the nineteen-nineties, astronomers have been scrutinizing the skies in search of planets that lie outside of our own solar system, orbiting other stars, and as of today, there are 603 of these extrasolar planets which have been confirmed. Descriptively named, they range from the gargantuan CD-35 2722 b, a whopping 31 times as massive as Jupiter, to the celestial weight-watchers weighing in at a mere 2-3 times the mass of the Earth. They have been spotted as nearby as a mere ten years or as distant as twenty five thousand, and we may even have found a planet composed of diamond, but how is it we know these planets are there?

Only a handful of planets outside our solar system have been large enough and close enough to be directly imaged, and astronomers often have to be more subtle to tease out information from these incredibly remote stellar wanderers.

Most extrasolar planets have been discovered by the examining their 'radial velocity'. As a planet orbits a star, it causes the star to wobble. As the star moves away from us, its light is moved towards the red end of the spectrum (red shifted). As it moves back towards us, the light is blue shifted, and this change in the light allows astronomers to determine the mass of the orbiting planet.

If we're lucky, we can spot planets around other suns just by looking at how bright the star is over time. If the path of a planet's orbit brings it between its host star and ourselves, we will observe it periodically dimming and brightening again as the planet first passes in front of the star and then passes away from from our line of sight. Known as the transiting method, this sounds like a synch, but it is far from easy. Often much brighter than the sun, and impossibly distant, these stars are thousands of times wider than their companions. Like looking for a tiny dark needle in a blazing haystack!

More complicated than the radial velocity and transit methods, 'gravitational microlensing' tracks the brightness of an object as its light is interfered with by the gravitational influence of a star or planet that passes in front of it. Although the light from our target planet may be much too dim to see, the way it bends the light from the background star allows us to find and identify it. Not dependent only on visible light, the gravitational microlensing effect allows us to identify objects that emit all kinds of electromagnetic radiation. Verification of gravitational lensing in general was found in 1979 after having been a prediction of Einstein's General Theory of Relativity since 1936.

In 1992, the first planet outside of our solar system was discovered orbiting a pulsar a mere eighteen million billion kilometers away in the constellation Virgo. A pulsar is a small but massively dense object, the leftover remains of a supernova, which rotates sometimes thousands of times per second. These rotations can be timed with incredible precision, meaning that pulsars can be as accurate as some atomic clocks. As the pulsar rotates, it emits beams of electromagnetic radiation which sometimes sweep across the distant Earth. As we see with the radial velocity method, as a planet orbits a star, the star shows tell-tale wobbles. This wobble affects the timing of the beams of radiation from the pulsar, giving away the otherwise impossible hiding place of the orbiting planet.

There have also been twenty four extrasolar planets discovered by direct imaging alone. Whilst not nearly as successful as the radial velocity method, direct imaging may be the most inspiring way of hunting for our planetary neighbours. When I was a child, not that long ago, the question 'are there any planets outside the solar system?' was yet to be answered, but now we can literally see them for ourselves, and detect hundreds more with ingenious technology and cunning know-how. Some planets are baked in ferocious heat, hurtling around dangerously close to their parent stars while others linger out in the cold and dark, taking hundreds and even thousands of years to circle their distant host. The number of extrasolar planets has just passed 600, and it is likely to reach a thousand within the next few years; a number which will grow exponentially in the coming decades. With such a high abundance and a plethora of sizes, types and compositions, it surely only a matter of time before we find a planet significantly like our own. Of course, once we do begin to find earth-like worlds scattered across the galaxy, we will come tantalizing close to finding whether life is extraordinarily rare, or staggeringly common.

GAS