The first world beyond our solar system was discovered in 1992, and since then astronomers have been finding such ‘extrasolar planets’ (also abbreviated to ‘exoplanets’) at an ever quickening rate. As techniques and telescopes increase in sensitivity, the race is on to be the first to announce the discovery of a truly terrestrial exoplanet – a Second Earth.
Astronomers use various different methods to spot distant worlds. Some planet-hunters look for variations in the wavelength of distant stars’ light, caused by changes in their speed as an orbiting planet makes them ‘wobble’. Another very successful technique scrutinises nearby stars for a recurring dip in their brightness as a planet passes in between the Earth and this star, a so-called transit.
Telescopes using these two methods are becoming more sensitive all the time, and some are being sent into space to spy on the far reaches of our galaxy. One such space telescope, Kepler, will be launched by NASA in March 2009 and is expected to have spotted 50 transiting Earth-like planets by 2013. But what actually do we mean by an Earth-like planet?
Planetary scientists have drawn up a shopping list of the attributes we’d be looking for in a true sister-world: one with the potential to support life like us. Firstly, for an exoplanet to be Earth-like it must be a terrestrial world – small and rocky – rather than a gas giant like Jupiter or Saturn. Even the smallest exoplanet with the potential for life discovered so far is thought to be at least five times larger than our homeworld. This ‘super-Earth’ is probably much more like Neptune, shrouded in a crushingly-thick atmosphere.
It’s not just the size of the planet that’s important, the kind of star it orbits is crucial as well. Stars much larger than the sun burn themselves out too quickly for life to develop, whereas stars smaller than the sun bring their own share of problems. The smaller a star, the cooler it is and so the more tightly a planet must orbit it to receive enough warmth for lakes and oceans of liquid water – placing it in the so-called habitable zone around a star. But orbiting closely to a star, even a cool red one, is fraught with dangers such as floods of stellar radiation, and so even though planets orbiting within the habitable zone of cool stars are relatively easy to spot, they’re unlikely to have astrobiologists holding their breath.
So in summary, we’re most interested in finding Earth-sized planets orbiting sun-like stars at just the right distance for oceans of water. Or to put it another way, medium-sized lumps of wet rock with an orbit of almost exactly one year.
Only a matter of time
Pinpointing planets as small as the Earth is pretty tricky, especially with the added constraint that they must be orbiting large, bright stars like the Sun. However, astronomers are pretty confident we’ll have bagged at least a few by 2013, and if we get lucky with a random search strategy called gravitational microlensing, we might even spot our first sister planet this year! The trouble with microlensing is that any discovered planets are very far away – halfway to the galactic centre – and we’ll probably never be able to follow-up with any of the other techniques. Think how frustrating that would be – knowing a Second Earth is out there, but that we’ll never see it again!
Exoplanet researchers are optimistic that finding the first truly Earth-like world is now just around the corner. And the next generation of space telescopes should be able to read the chemical make-up of these distant lands – water has already been found in the atmosphere of an exoplanet. The tell-tale signatures of biology, a mix of methane and oxygen for example, could even be discovered in an exoplanet’s atmosphere within the next decade. Just imagine walking out into your back-garden in 2019 to point up to an otherwise unremarkable star in the night sky, and knowing that there is alien life thriving on this new world.
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