Though hot Jupiters prefer circles

The high frequency of elliptical orbits is a striking feature of the population of exoplanets. We cannot invoke some defect of the observing process to explain this: distinguishing an elliptical orbit from a circular one poses few problems, whether it is the radial velocity or the transit method which is in use. For any

Radial velocities and orbital ellipticities. The radial-velocity method can easily distinguish circular from elliptical orbits. Planets in circular orbits cause regular, symmetrical variations in velocity, expressed by sinusoidal curves. Such is the case with the planet HD 330075 b, discovered by HARPS (lower diagram). High eccentricity produces sharp dips in the velocity curve, as seen in the upper diagram for the star HD 37605. Its companion planet, with orbital eccentricity of 0.74, was discovered in 2004 by the Hobby-Eberly telescope.

48 Twelve years of discovery

Mercury - an eccentric of the solar system. Among the planets of the solar system, only one - Mercury - has an eccentricity greater than 0.2. Mercury is the least well investigated of the terrestrial planets, having been visited by only one spaceprobe, Mariner 10 (in 1974-75), which charted 45% of the planet's surface. A few years from now we will have a much better picture of this planet when NASA's Messenger mission, launched in 2004, enters into orbit around it in 2011. Later, ESA's BepiColombo spacecraft - due for launch in 201 3 - will study Mercury. Due to tidal effects caused by the Sun, Mercury completes three rotations on its axis in two Mercurian years (similarly, the Moon completes one rotation during one revolution about the Earth, which explains why it always presents the same face towards us). Such tidal effects are probably significant in the case of all exoplanets orbiting near their stars, and resonances involving rotation and revolution must be common. On a 'Mercury' with a 3:2 resonance and an eccentric orbit, the apparent path of the Sun would seem a little bizarre. As the Sun climbs into the sky it appears to grow bigger, and when it reaches the zenith it stops, turns back on itself, and stops again before descending towards the horizon, 'shrinking' as it goes.

given orbital period, as the orbit becomes more elliptical, the velocity of the planet at periastron (the nearest point to the star) increases, and so detection becomes easier. At the same time, however, the planet spends less time at periastron, and the chance of missing this episode increases. Simulations suggest that the two effects compensate for each other. In summary: of the 185 objects of which the orbital eccentricity is known, at least half have orbits more eccentric than any found in the solar system. Only a few comets have orbits of similar eccentricity.

Why should this be problematic? Again, it seems that the solar system is a special case, and that our theories of its formation do not apply in the case of other planetary systems. In the protoplanetary disk that was the cradle of the solar system, planetesimals grew by accretion from smaller objects, dust and ice (see p. 43). The orbits of these particles were undoubtedly highly elliptical; and the more elliptical their orbits, the more probable that they would intersect the


Eccentricities of exoplanets: from circle to flattened ellipse. The orbits of more than half the known exoplanets are more elliptical than that of Pluto. Ellipticity is not the rule, however, and more than twenty exoplanets have practically circular orbits. All the hot Jupiters fall into this category; and are the closest to their stars. The arrows indicate the eccentricities of Mercury (0.206), Pluto (0.246) and Mars (0.093).

orbits of other particles. This natural selection rapidly eliminated the planete-simals in elliptical orbits, eventually leaving only bodies in near-circular orbits, arranged tidily around the Sun. However, this is not at all what is observed when we look at stars other than the Sun.

There are, of course, mechanisms that can be introduced to explain the elliptical orbits: for example, interactions between numbers of planets. But we then have to understand why such mechanisms have not come into play within our planetary system. The eccentricity of an orbit is an important factor in deciding the habitability of a planet. On Earth, long-term climatic variations and glaciations are linked not only with the changing inclination of our planet's axis, but also with variations in the shape of its orbit, its eccentricity and Earth's distance from the Sun at any given time in the cycle of the seasons. Eccentric orbits are common throughout the family of exoplanets, whatever their size, and whether their orbits are close to or far from their stars. Only in the case of planets very close in are all orbits near-circular, and all objects with periods less than 6 days have eccentricities below 0.1. This is easily understood. At such short distances, stars produce enormous tidal effects on these planets. The resulting deformations are considerable, but are stable if the planet's orbit is circular. Planets with initially elliptical orbits will be distorted as the tidal effects dictate, and the dissipation of energy involved will cause the planet to be locked into a synchronous rotation as it revolves in a more circular orbit, as is the case with the Earth's Moon.

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