It has been the darling of science fiction; inhabited planets in other solar systems. I started reading SF in my teens and fondly remember the hard-sf of Arthur C Clarke and Isaac Asimov in particular. Has science fact finally caught up with SF?
All stars were formed out of huge swirling clouds of gas and it is surmised that most have planets orbiting them. Because of the glare of the host star it is impossible to see any planets directly. Our sun, for example, is 100x the diameter of the Earth so comparative size compounds the problem
Since 1995 astronomers have found firm evidence of exoplanets using indirect methods.
Two of the indirect methods for finding exoplanets are The Transit Method and the Doppler/Radial Velocity Method.
In the Transit Method the faint dimming of a star is recorded as the planet orbits across the star relative to the observer. The occurrence of transits may be very infrequent and only last a short time so multiple data taken over months and years is required. The Hubble Space Telescope has been one of the telescopes used for transit recording. It is possible to determine the size of the planet with this method. The Kepler space telescope is another instrument which has found Earth-like planets and even planets orbiting binary star systems.
Stars called M-Dwarfs have been found too and they have a lower mass than our sun and a longer life so that there is potentially more time for biology to have developed. It has been estimated that these M-Dwarf stars could have five billion Earth-like planets.
The Transit Method relies on sensitive instrumentation so that a star dimming by as little as one percentage of the star’s light can be recorded.
The Doppler Method is possible because in any star-planetary system there is a common centre of gravity which causes the host star to ‘wobble’ around the centre of gravity. This wobble is too small to be seen directly so that is where the Doppler Shift comes in, recorded by special equipment. If the star is moving towards the observer ( that is; the telescope) the wave-lengths of light will be ‘bunched up’ or shifted towards the blue end of the spectrum. And if moving away, towards the red end of the spectrum.
If you imagine looking down on our solar system from ‘above’ there would be little movement of the planets towards or away from you. So, for this method to work the star system has to be observed in the equatorial plane which obviously will mean only a small percentage of all systems will be viable.
This ‘radial velocity’ method has been used by the Keck Telescope in Hawaii amongst other instruments looking for exoplanets. The method can be combined with the transit method and with the use of spectroscopy the composition of the planet can be roughly determined.
What about life on these exoplanets? It would seem incredible if ours was the only planet in the universe which has life! Through spectroscopy ‘biomarkers’ can be detected – that is, the gases which may be signs of life such as oxygen, carbon dioxide and water. The James Webb Telescope is the next space-telescope to search for exoplanets in the infra-red spectrum so watch out for news in the coming years.