It was the renaissance philosopher Giordano Bruno who first suggested there might be other worlds orbiting the stars of the night sky. Bruno's heretical philosophising came to a fiery end, when in 1600 he is believed to have been burned at the stake. However, his musings set the stage for one of astronomy's 'Holy Grails' - the search for planets around other stars. Bruno's death at the hands of the Holy Inquistion was followed by fruitless searches over the following 395 years. In 1991 the first extrasolar planet (exoplanet) based on its low-mass was discovered in close-orbit around a pulsar using timing measurements (Wolzcan & Frail 1992). A further three planets have now been discovered around PSR 1257+12 and even a possible comet, as well as a planet around PSR B1620-26 (Backer, Foster & Sallmen 1993; Sigurdsson et al. 2003). While these are landmark discoveries, the planets' location, next to a stellar remnant and perhaps forming after its collapse, probably helps little in understanding our own Solar System. Nonetheless it gives the impression that planet formation is robust and fuels our idea that planets are common throughout the universe.

In 1995 Mayor & Queloz (1995) announced the detection of the first exoplanet around a Sun-like star. The radial velocity of the G2V star 51 Pegasi was used to infer the presence of a Jupiter mass planet in a 4.2 day orbit. The discovery was quickly confirmed independently (Marcy & Butler 1996) and also corroborated by Doppler evidence for Jupiter mass planets in close-orbit around a number of other nearby stars. Our knowledge of exoplanets has been fuelled by the growth in the sheer number and also by the broad range of parameter space now populated. However, close-orbiting planets characterised with a combination of precise radial velocity measurements and transit photometry have played a key role. In these systems we can determine the mass and radius of the planet, which in turn yields constraints on its physical structure and bulk composition. The transiting geometry also permits the study of the planetary atmosphere without the need to spatially isolate the light from the planet from that of the star. This technique is known as transit spectroscopy or sometimes occultation spectroscopy and has allowed for photometric and spectroscopic measurements of exoplanets to be made.

Fig. 6.1. In an artist's impression, a "hot Jupiter" in tight orbit about its parent star, is seen to puff up under the intense heat and its outer gases boil off into space. Image courtesy NASA, ESA, and G. Bacon (STScI).



time from center of transit (days)

Fig. 6.2. Light curve from Brown et al. (2001) obtained by observing four transits of the planet of HD209458 using the STIS spectrograph on the Hubble Space Telescope. The folded light curve can be fitted within observational errors using a model consisting of an opaque circular planet transiting a limb-darkened stellar disk. In this way the planetary radius is estimated as 1.347 ± 0.060 Rjup, the orbital inclination 86.6 ± 0.14, the stellar radius 1.146 ± 0.050 R0. Satellites or rings orbiting the planet would, if large enough, be apparent from distortions of the light curve or from irregularities in the transit timings. No evidence is found for either satellites or rings, with upper limits on satellite radius and mass of 1.2 RJUP and 3 MJUP, respectively. Opaque rings, if present, must be smaller than 1.8 planetary radii in radial extent.

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