In order to investigate the possibility of observing a planet orbiting a nearby star, let us construct an example. Imagine a 'pseudo-solar-system' in which a Jupiter-sized planet orbits a Sun-like star at a distance of 5 AU. The star is 5 parsecs (16 light-years) away from us. In the sky, the angle separating the planet from the
The first image of an extrasolar planet, taken in 2004 with one of the 8.2-metre VLT telescopes (ESO) using the NACO adaptive optics system, which partially corrects for atmospheric distortion. The planet here appears in red, and at centre Is the brown dwarf 2M 1207 (see opposite).
2.2 Problems with direct imaging 17
giant candidate planet fl
Jupiter Saturn « f solar system
Jupiter Saturn « f
The system of the brown dwarf star 2M 1207 and its planetary companion. A team of French and American observers, studying the region around the very low-mass star 2M 1207, discovered a very faint object, apparently a companion to the star. This turned out to be a planet of about 5 Jupiter masses, orbiting at 55 AU from it (twice the distance of Neptune from the Sun). Later observations confirmed that the 'companion' was not a more distant star in the same direction, as the two objects moved together against the sky background. This, then, was the first time that a planet had been observed directly. Several favourable elements came into play in this case: the brown dwarf star is not very bright, and the planet is very big, with a contrast in brightness of only 100:1. The planet is a very long way from its star. Certainly, this is no exoEarth - not an environment where life is likely to flourish!
star would be 1 arcsecond (second of arc): 1/3,600 of a degree, equivalent to 1/ 1,800 the diameter of the full Moon. In theory, a sufficiently large telescope will separate and reveal two stars 1 arcsecond apart; but in the case of a star and its planet the contrast in brightness is far too great. Because the star is larger and hotter than the planet, it appears much brighter. The star's temperature would be of the order of 5,600 K, while that of our imaginary Jupiter would be 130 K. In visible light the star would seem about a billion times brighter than the planet! With a normal CCD camera it would not be possible to observe the planet, drowned in the light of its star. In the infrared, at a wavelength of 5-10 pi, the contrast is less unfavourable, but it would still be of the order of one million.
Direct imaging of planets with Earth-based instruments is therefore very difficult. However, in the case of giant exoplanets it will become possible during the years to come. There are two techniques which offer some hope. First, there is coronography, whereby the light from the central star is masked, just as the Sun can be masked during studies of its corona. Then there is nulling ('black fringe') interferometry in the infrared. This technique combines the signal from the starplanet system using a number of different telescopes working as an interferometer, in such a way that the light from the centre of the image (the star) is cancelled out. It seems likely, though, that the imaging of exoEarths will remain beyond the capacity of telescopes, and space missions are therefore being planned in order to achieve this ambitious goal. The American TPF (Terrestrial Planet Finder) and European Darwin projects are in process of development.
Other techniques have been studied for the indirect detection of planets orbiting nearby stars. The astrometric method is based on the principle of measuring the displacement of the central star in relation to the centre of gravity of the star-planet system. In the case of our imaginary solar system, this displacement would be only 0.001 arcsecond. The Doppler-velocimetric method involves measuring variations in the velocity of the star induced by the 'swaying' of the star around the centre of gravity. It is this second method which has led to the discovery of the majority of known exoplanets. Yet another technique is beginning to bear fruit: the observation of transits of exoplanets across the face of their stars. Pulsar timing and gravitational microlensing have also registered remarkable successes. In the following pages we shall examine some of the many detection methods in detail.
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