This article was written in May 2011 and was shortlisted for the Wellcome Trust Science Writing Prize, in association with the 'Guardian' and the 'Observer'. It has since been published on the Wellcome Trust Blog.
The desire to explore our surroundings is part of human nature. It's that which drove our ancestors to leave Africa tens of thousands of years ago, walking across continents, and traversing unmapped oceans in simple rafts[1]. It's that which led explorers like Christopher Columbus and Sir Walter Raleigh to rediscover these 'new worlds' during the European Renaissance, and it's that which eventually put twelve people on the Moon.
With the Earth mapped and human exploration of the rest of the Solar System still many years away, we may feel that there is nothing left to discover. We will never get to experience what it felt like for those early explores upon encountering an unimagined animal, or new mountain range, or waterfall, for the first time.
But all that is changing. In 2011 we discovered that there are many more worlds, with new lands, resources, and possibly even wildlife, left to explore. In February, NASA Space Scientist William Borucki stated that there should be at least 50 billion planets in our galaxy alone, and at least 500 million may be capable of hosting life[2]. We have only located a small proportion so far, but these include a planet that may have mountains made of diamond[3], a planet that may be one giant ocean[4], and a planet covered in 'hot' ice[5].
Scientists have been discovering exoplanets for almost two decades but, thanks to the launch of NASA's Kepler spacecraft, members of the public can now contribute to the exploration of the universe from their own home. In February 2011, NASA announced the discovery of over 1000 new planetary candidates[6a], and over 60 of these were found by amateurs[7].
Scientists working on the Kepler spacecraft. Image credit: NASA/Troy Cryder/Public domain.
Kepler detects planets by looking for the dip in brightness that occurs when a planet passes in front of its star. By studying patterns in the starlight, scientists can work out how massive a planet is, how hot it is, and what it is made of. If the planet has an atmosphere, then it is often possible to determine its composition, and with the launch of Hubble's replacement, the James Webb Space Telescope, we will also have the ability to directly detect oceans[8].
Kepler finds planets by measuring the dip in brightness as they pass in front of their host star. Image credit: Helen Klus/CC-NC-SA.
In order to contain an ocean of liquid water, a planet must be within the 'habitable zone' of its star. This means it must not be so close that all the water boils away, but it must not be so far away that it freezes. Over 50 of Kepler's planetary candidates are located in this zone, and five are close to the size of the Earth[6b]. Scientists hope that planets so similar to Earth may also contain life.
Once these planets have been confirmed, scientists will want to look at the composition of their atmospheres. If large amounts of methane and oxygen were found, then this would suggest that something is continually producing these gases, like plants do on Earth[9]. In the next few decades, more precise observations could reveal the presence of chlorophyll, the chemical that gives plants their green colour, on the surface of a planet[10]. Intelligent life could be spotted by the presence of artificial compounds like CFCs[11].
If we do find signs of life on another planet, then the next steps will be to explore using robotic probes, and to try to communicate by sending them a message, which would travel at the speed of light. Two messages have already been sent to the star system thought most likely to contain life. The first was composed of over 500 pictures and messages submitted to social networking site Bebo[12]. The second contained over 25,000 messages written by members of the public[13]. Both are due to arrive in the next 20 years.
More opportunities to send messages are bound to occur as new planets are confirmed to be habitable. In the meantime, you have the chance to discover a world of your own by logging onto 'planethunters.org' and following a simple tutorial.
UPDATE: As of 2017, Kepler has found over 4000 new planetary candidates, over 2000 of which have already been confirmed. Assuming that the small area of the Galaxy surveyed so far is representative of the rest of the disc of the Galaxy, then there should be about 100-400 billion exoplanets in the Milky Way, which contains about 300 billion stars, and at least 10% should be Earth-sized.