Observing ex-solar planets

One thing that emerges from the variety of theories about the relationships between stars and planets is that the existence of ex-solar planets cannot be deduced simply from theories. Confirmation must be derived from observation. Observations of ex-solar planets can be categorized in terms of direct and indirect strategies. Direct strategies involve trying to extract a faint planetary  image from the bright image of the star. The advantage of this method is fast verification with no requirement to observe a complete orbit, which could take hundreds of years. However, direct observation is extremely difficult. The Earth would not be discernible with a telescope even one-hundredth of a light year away. Planets do not emit light and their size in relation to their parent stars will be very small. Jupiter, the largest planet in the solar system, is 300 times the mass of the Earth but 1,000 times less than the Sun. Light from the Sun is about a billion times brighter than the light reflected from Jupiter. Using the best observational equipment available, an astronomer on one of  the nearest stars – for example, Proxima Centauri which is 4.2 light years away – would not be able to distinguish Jupiter from the Sun. It would be lost in a glare of sunlight.

There are several promising techniques underway. For example, techniques known as ‘optical’ and ‘nulling interferometry’ aim at suppressing the glare of starlight that would otherwise conceal planets. This involves the use of two or more telescopes located at some distance from each other thereby gaining the resolution of a single telescope. Optical interferometry, although  very complex, will detect a star’s wobble caused by the surrounding planets and even directly image these planets and possibly analyse their atmospheres. Nulling interfer-ometry will employ two mirrors to collect light from a star and combine it so that  the light from one mirror is slightly out of step with the light from the other. The idea is that the light waves from the two mirrors slightly suppress each other, while the light coming from objects near the star is not delayed. Plans are underway for the launch of the Space Interferometer Mission in 2005, which will employ six telescopes aligned with mirrors that ought to detect or rule out the existence of Earth-type planets around the nearest twenty or so stars.

Can planets be observed indirectly? Indirect strategies involve methods of sensing the effect the planet may have on the star. Two kinds of indirect approaches were outlined by J.D. Scargle (1988). First, there was the Photometric Planetary Detection Method. If the planet transits in front of the star as seen from Earth, the light from the star will be diminished. This would create problems for detection as one would have to view the orbital plane edge on. Consequently this technique is only applicable to planets whose orbital plane is near our line of sight at the time of transit. One major problem is that this technique is non-repeatable, as the star, the intervening object and the Earth, are unlikely to be aligned again. Furthermore, there are problems involved with attempts to separate luminosity fluctuations from the star itself from those caused by the planet. This would be particularly difficult in the case of small planets like Earth, although large planets like Jupiter can be detected with this method.

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Since 1994 a binary star system, CM Draconis, which is about 60 light years away, has been observed by a research team who have employed ten telescopes distributed around the Earth. Several astronomers recorded a dimming of the star’s light during what might be inferred as transits as a small planet passed in front. It now seems that the scientific community are prepared to accept that the  observational technique actually works. On these terms proof of the existence of ex-solar planets is not via direct observation, but follows predictable observations, based on calculations of planetary orbits determined by regular dimming of the light from the parent star (Seife, 1998a: 23). A most promising proposal is the photometric space-based telescope known as  KEPLER, which is planned for 2003 and will orbit the Sun seeking potential alignments between ex-solar planets and their stars. It is hoped that KEPLER will find 500 rocky Earth-type planets, 160 close-orbiting giants and 24 outer-orbiting gas giants, during its proposed four-year programme.

The second indirect method is based on measuring gravitational perturbation. This is known as astrometry – the detection of the sources of gravitational pull on stars. Planets can influence their parent stars by means of their mass. Jupiter certainly affects the Sun’s motion. Some nearby stars appear to wobble as they move through space which suggests that they could be gravitationally affected by unseen planets. Barnard’s star, which is the second nearest star to the Earth, was widely proclaimed to have one or more planets when Peter van de Kamp, an astronomer at Sproul Observatory, announced the discovery of a wobble in 1963. It was then inferred that it had a ‘companion’ which was about 50 per cent heavier than Jupiter. Unfortunately, a fresh look at the original measurements suggested that the ‘wobble’ was due to a series of mechanical adjustments which had been made to the telescope.

Many searches are now underway, and over thirty ex-solar planets have been detected since 1995. There are also estimated to be numerous ‘dark companions’ among the nearest forty or so stars. However, there are difficulties in measuring the wobble. Although measuring of gravitational effects is one of the better indirect methods, there is a major problem in separating the effects caused by the planet from other motions in the stellar atmosphere. Moreover, this might only be possible in the case of very large planets. For example, the star’s position would have to be measured relative to a set of different stars seen in the same direction. But these measurements are unlikely to be accurate enough to detect the influence of a planet. It would be impossible to measure the influence of Jupiter on the Sun from an observatory in Proxima Centauri. It should also be noted that full confirmation of a planetary orbit can take a long time. For example, the planet, Jupiter, takes twelve years to orbit the Sun, and astronomers beyond our solar system would have to spend decades confirming its existence.

At the University of British Columbia Bruce Campbell, Gordon Walker and Stephenson Yang conducted a twelve-year-long search using the 3.6 metre Canada–France–Hawaii telescope on Mount Kea, Hawaii. They employed a ‘Doppler technique’ which involves observing the motion of a star and

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