Planetary Systems

Exoplanets en famille

Twenty-one of the 200 planetary systems so far discovered are multiple systems in which two or three planets have been detected, and even four in the case of 55 Cancri. This is, perhaps, not many when compared with the number of members of the Sun's planetary family, but we must be mindful of the considerable limitations of current detection methods. If we were situated a few light-years from the solar system and had begun to observe it fifteen years ago with instruments such as ELODIE, we would just about have been able to detect Jupiter. It is very probable that systems of planets are the rule rather than the exception. For once, we can say that our solar system is not a special case, for it seems that planets occur more often in families than alone.

In a few known planetary systems, the orbits of the planets are in a particular configuration known as resonance. The ratio between the orbital periods of their two planets is equal to the ratio of two small whole numbers; for example, 3:2. In the cases of HD 82943 and GJ 876, the outer planet orbits the star in a period almost twice as long as that of the inner planet. With 55 Cancri, the ratio is almost 3:1. Such resonances are well known within the solar system. Pluto and Neptune orbit in a 3:2 resonance. The phenomenon is almost universal in the saturnian system with its many rings and satellites: for example, the satellites

In the system of HD 82943 the orbital period of the outer planet (c) is 444 days - double that of the inner planet (b). The two planets are in a 2:1 resonance. The radial-velocity curves assume the shape characteristic of the presence of two planets. (After data from the Geneva Observatory.)

Saturn s satellite Titan, imaged by the Cassini spacecraft. Titan is in a 4:3 resonance with another of Saturn's satellites, Hyperion, which is a rare example of a chaotically rotating body in the solar system.

In the system of HD 82943 the orbital period of the outer planet (c) is 444 days - double that of the inner planet (b). The two planets are in a 2:1 resonance. The radial-velocity curves assume the shape characteristic of the presence of two planets. (After data from the Geneva Observatory.)

Saturn s satellite Titan, imaged by the Cassini spacecraft. Titan is in a 4:3 resonance with another of Saturn's satellites, Hyperion, which is a rare example of a chaotically rotating body in the solar system.

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Planetary systems: single, double, triple... Among the exoplanetary systems known there are 21 which have more than one planet; and to these must be added the system of pulsar PSR 1257+12, with its three planets (and possibly a fourth, unconfirmed). Planets in these multiple systems greatly resemble 'solitary' planets in characteristics such as mass, period and eccentricity; but this is hardly surprising, as the latter are doubtless 'solitary' only because current instruments are not sufficiently powerful to detect their neighbours. On the graph, the figures give the masses of the first exoplanets to be discovered as a function of distances from their stars. (After www.exoplanets.org.)

Titan and Hyperion are in a 4:3 resonance. To this we can add the multiple planetary system of the pulsar PSR B1257+12, with at least three planets; of which two, planet b (4.3 Earth masses) and planet c (3.9 Earth masses), are in orbits with a resonance of almost 3:2. It is also interesting that these planets have been shown to be orbiting in practically the same plane. Even if the neutron star associated with the phenomenon of a pulsar is nothing like the Sun, this suggests that the process of planetary formation is the same with both types of object, with an initial disk of gas and dust rotating around the star.

The observed resonances show that dynamical effects may be important in these multiple systems. If two planets have achieved resonant orbits, the gravitational interactions between them repeat at the same stages of their orbits and may be amplified. It is not impossible, therefore, that one of the planets might eject the other from its orbit. Those who specialise in celestial mechanics are very interested in this question of the stability of multiple systems, and are delighted to have something outside the solar system to study! The results from the planets in a 2:1 resonance are, however, slightly disturbing. Over a million years it is quite possible that the larger of the two might find itself ejected by the smaller.

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