The astronomers sought but did not find

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In order to find exoplanets it is useful to know at least something of their characteristics. Because direct imaging of them is impossible with current instruments, the method employed relies on precise observations of the motion of a star and the detection of the regular perturbations induced in its motion caused by an orbiting planet. The nature of these perturbations depends on the mass of the planet compared with that of the star, on the planet's orbital distance, and on the shape of its orbit. These criteria are estimated in order to determine observational strategies. For example: if we are searching for a planet at the same distance from the star as Jupiter is from the Sun, we know that the period of revolution will be about 12 years. Daily observations will therefore be of little use, but a schedule will need to last for several years.

Naturally enough, all the researchers took the solar system as their model: larger planets on the outside, and smaller planets nearer to the star. The Sun's family of planets can be divided into two classes. The first group comprises rocky planets with an obvious surface, such as Mercury, Venus, the Earth and Mars: the 'terrestrial' planets. The second group comprises the four giants, Jupiter, Saturn, Uranus and Neptune, which are much more massive than the terrestrials, and consist mostly of hydrogen and helium. Until August 2006 Pluto was the ninth planet of the solar system, but with the decision of the General Assembly of the International Astronomical Union it is now classed as a 'dwarf planet'. It is one of the 'trans-Neptunian objects' - small bodies of rock and ice moving far from the Sun beyond the orbit of Neptune.

1.3 The solar system: an atypical planetary system? 7

1.3 The solar system: an atypical planetary system? 7

A terrestrial planet: Mars. The fourth planet from the Sun is half the size of the Earth. It has a rocky surface and a thin atmosphere, 95% of which is carbon dioxide. Mars is accompanied by two tiny satellites: Phobos and Deimos.

A gas giant: Jupiter-the king of the eight planets of the solar system. The diameter of Jupiter is only ten times less than that of the Sun, and its thick hydrogen-helium atmosphere surrounds an icy core about ten times the mass of the Earth. Jupiter now has more than sixty recorded satellites - the most famous being the four Galileans - lo, Europa, Ganymede and Callisto - named after the Italian astronomer Galileo Galilei, who was the first to observe them with a telescope.

A terrestrial planet: Mars. The fourth planet from the Sun is half the size of the Earth. It has a rocky surface and a thin atmosphere, 95% of which is carbon dioxide. Mars is accompanied by two tiny satellites: Phobos and Deimos.

A gas giant: Jupiter-the king of the eight planets of the solar system. The diameter of Jupiter is only ten times less than that of the Sun, and its thick hydrogen-helium atmosphere surrounds an icy core about ten times the mass of the Earth. Jupiter now has more than sixty recorded satellites - the most famous being the four Galileans - lo, Europa, Ganymede and Callisto - named after the Italian astronomer Galileo Galilei, who was the first to observe them with a telescope.

The two classes of planets differ not only in their natures, but also in their distances from the Sun. Terrestrials circle near the Sun, and giants further away. The more distant they are, the longer it takes them to revolve around it. Of the inner, terrestrial planets, the orbit of Mercury, nearest the Sun, lasts 88 days, while that of Mars lasts 687 days. The orbital periods of the giant, outer planets range from 12 years (Jupiter) to 165 years (Neptune). All these orbits are more or less circular, except that of Mercury, which is a little more elliptical.

Armed with this knowledge to guide them into the unknown, the planet hunters of the 1990s thought they knew what they were looking for: exojupiters and exoSaturns with circular orbits and periods of more than 10 years. Their instruments were incapable of detecting less massive planets. The observational strategies they evolved reflected these expectations. However, although the first exoplanet to be detected near a Sun-like star had a mass 0.47 times that of Jupiter, it orbited its star in only 4.2 days! This was such an unexpected discovery that one of the two teams then investigating exoplanets, led by American astronomer Geoff Marcy, missed the object. They thought they would have to wait for several years for a positive detection, and had not begun to examine their data just weeks into the project.

Orbits in the solar system

The eight planets move in ellipses around the Sun, which lies not at the centre C, but at one of the foci F. Most of the planets of the solar system have only slightly elliptical orbits, and are near-circular (a circle being a particular case where C and F are the same).

The two characteristics of an ellipse are its eccentricity — with values between 1 and 0, where 0 denotes a circle and values approaching 1 denote a very flattened ellipse - and the semimajor axis (cj), which is the longest distance between the centre and a point on the ellipse.

The further a planet from the Sun, the longer it takes to perform a complete revolution. The period of revolution 7" of a planet depends on the semimajor axis a, according to Kepler's third law. If T is expressed in years and a in

Astronomical Units (AU, the mean Sun-Earth distance — 1 49,600,000 km), then a1 IT2 = 1. In any planetary system, o3/r2 is proportional to the mass of the central star.

Definition of the semimajor axis of an ellipse with centre C and focus F.

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