The Star Garden
Exoplanets
The first planets formed along with the second generation of stars. Matter forms a flat disc around the core of a protostar and denser regions collapse into planets, moons, asteroids and comets over a period of about ten million years.
The first extra-Solar planets (exoplanets) were confirmed by Polish astronomer Aleksander Wolszczan and Canadian astronomer Dale Frail in 1992 and over five hundred have been discovered since. These include a planet which may have mountains made of diamond (Wasp 12b), a possible ocean planet (GJ 1214 b), a planet which migrated from another galaxy (HIP 13044 b), a planet where a year lasts ten hours (SWEEPS-10), a planet made of hot ice (Gliese 436 b), and a planet where it rains rocks (Corot-7b). Given the amount of planets that have been discovered so far, it is estimated that there should be at least fifty billion planets in the Milky Way, and at least five hundred million may be capable of containing life.
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Artist's conception of the Gliese 581 System Image Credit: Lynette Cook
The most popular methods for finding exoplanets are the radial velocity and the transit methods. The radial velocity method detects changes in the rotational speed, or 'wobble' of a star, with respect to the Earth, due to the mass of orbiting planets. The radial velocity is calculated using spectroscopy and can be used to determine the minimum mass of a planet. The true mass of a planet can be found by combining this method with the transit method.
The transit method detects planets by looking for the small drop in brightness that occurs when a planet passes in front of its star, with respect to the Earth. Discoveries made using the transit method are particularly exciting because they provide more information about the planet than any other method. The size of a planet can be determined from the star's light curve and, once the mass is known, its composition can be determined from its density. The composition of a planet's atmosphere can be discovered using spectroscopy and the temperature can be found by studying its intensity as the planet orbits.
The easiest planets to find using the transit method are those that obscure the most light. These are known as hot-Jupiters, they are at least as large as Jupiter and mostly orbit closer to their host star than Mercury. Hot-Jupiters migrated from further out in the stellar system. Most do not orbit in the same plane as their host star and some even orbit in the opposite direction.
NASA's Kepler spacecraft, which was launched in 2009, has surveyed over a hundred and fifty thousand stars using the transit method. In 2011, it was announced that they had found over a thousand new planetary candidates more than ninety percent of these are expected to be confirmed.
Water is thought to be vital for life and Kepler has found over fifty candidates which are within the 'habitable zone', an area where a planet it at the right temperate for surface water to remain liquid. Five of these candidates are similar in size to the Earth. The next step is to determine if these planets have atmospheres. If large amounts of methane and oxygen are found then this would suggest that something is continually producing these gases, like plants do on Earth. More precise observations could reveal the presence of chlorophyll or artificial compounds like CFCs.
If we do find signs of life on another planet then we could explore using robotic probes or try to communicate by sending them a message at the speed of light. Two messages have already been sent to Gliese 581, the star system thought most likely to contain life. Both contained text composed by members of the public and translated into binary and both due to arrive in the next twenty years.
References
See NASA's profile of exoplanets and the Kepler spacecraft.