Beyond the solar system exoplanets

Our Sun and its system of planetary bodies are not alone in the universe. Since 1995, astronomers have discovered more than 270 planets in orbit

Figure 14.3 The Pleiades (or Seven Sisters) is a cluster of young stars formed at the same time from a gas cloud of uniform composition. It lies about 400 light years from Earth in the constellation Taurus. The blue haze around the stars, called a reflection nebula, is caused by interstellar dust that scatters and reflects blue light. There may be planetary systems forming around some stars in this cluster. (Photo: J. Wilkinson)

around other stars. Most of the stars with planets are similar in size to our Sun. Several of these stars have more than one or two planets orbiting them. Since our Sun is a fairly typical star in the Milky Way galaxy, one would expect that other planetary systems would be common. Planets that exist around other stars are called extra-solar planets or exoplanets. Part of the enormous attraction of searching for exoplanets is the possibility that some of these new-found worlds will be able to support life.

Detecting exoplanets

Astronomers detect planets around other stars by a number of methods. One method involves accurately measuring changes in the intensity of light coming from a star as a planet passes in front of the star (the transit technique). If such changes are regular, then it is likely a planet is orbiting the star. Another method involves an accurate analysis of the motion of the star - a wobble in a star's motion could be caused by the gravitational pull of its planets. Astronomers also use spectral analysis to detect exoplanets. An exoplanet's properties are determined by combining information about a planet's brightness, colour, spectral properties and the variability of these over time.

Detection of planets around a distant star is extremely difficult from Earth because the intense brightness of a star tends to block out any planets. Very high resolution instrumentation is needed to separate the planet from its parent. Most of the known exoplanets have been discovered using Earth-based telescopes, but astronomers also use space telescopes (that is, those in orbit above Earth's atmosphere).

Most of the exoplanets astronomers have already discovered have been around stars similar to our Sun. It is estimated that at least 12 per cent of sun-like stars have planets orbiting them. Most known exoplanets are also 'giant' planets, typically as large or larger than Jupiter. The difficulty of detecting exoplanets means that current techniques are suited to discovering large exoplanets very close to their parent stars.

Figure 14.4 The star HD189733, an orange dwarf 63 light years away in Vulpecula, has a Jupiter-mass planet very closely circling it every 2.2 days and transiting the star's face once per orbit (artist's interpretation). (Photo: J. Wilkinson)

Probing for exoplanets

Space telescopes orbiting Earth are used to search for exoplanets. Examples include the Hubble Space Telescope (launched in 1990), Spitzer Space Telescope (2003), MOST (Microvariability and Oscillation of STars, 2003), and the French and European Space Agency's COROT (COnvection, ROtation and planetary Transits, 2006).

NASA's Kepler Mission and Space Interferometry Mission (SIM) and the ESA's Eddington Mission are designed to search for terrestrial planets in the Sun's neighbourhood of the Milky Way galaxy. These probes are scheduled to be launched before 2010 and are the first missions capable of finding Earth-size and smaller planets around other stars via space telescopes. The James Webb Space Telescope is scheduled for launch by NASA and the ESA in 2013.

NASA's Terrestrial Planet Finder and the European Space Agency's Darwin are two other missions designed to search for exoplanets. These

Table 14.1 Significant missions to search for exoplanets


Country of origin

Date of origin


Hubble Space Telescope

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