At least one in three exoplanets orbits closer in than Mercury

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The first exoplanet discovered, orbiting the star 51 Pegasi, caused a sensation when it was revealed that its orbital period was incredibly brief: 4 days! However, 51 Pegasi b is an extraordinary planet only when compared with those in our

3.3 Hot Jupiters and Pegasids 45

3.3 Hot Jupiters and Pegasids 45

The orbital periods of exoplanets are often short! The periods of the 161 exoplanets known by July 2005 are mostly short: half of them have periods of less than 267 days, and therefore orbit their stars at mean distances less than 0.8 times that of the Earth from the Sun. Theories of the formation of planetary systems suggest that these planets are too massive to have formed in situ. Once a large planetesimal has formed, it grows by 'sweeping up' all the matter located in its path. The final mass is grosso modo that of all the matter in a ring centred on the star and passing through the planet. However, at 1 AU from the star this ring does not contain sufficient material to form a large, Jupiter-like planet. So, the planet must have formed further out and thrn migrated inwards.

The orbital periods of exoplanets are often short! The periods of the 161 exoplanets known by July 2005 are mostly short: half of them have periods of less than 267 days, and therefore orbit their stars at mean distances less than 0.8 times that of the Earth from the Sun. Theories of the formation of planetary systems suggest that these planets are too massive to have formed in situ. Once a large planetesimal has formed, it grows by 'sweeping up' all the matter located in its path. The final mass is grosso modo that of all the matter in a ring centred on the star and passing through the planet. However, at 1 AU from the star this ring does not contain sufficient material to form a large, Jupiter-like planet. So, the planet must have formed further out and thrn migrated inwards.

own solar system. Within the family of exoplanets, at least a third have periods shorter than that of Mercury, which orbits in 88 days. Admittedly, these 'Pegasids', as they are known, are still relatively rare; but even so, around twenty known exoplanets have orbits of fewer than 4.2 days. The record-holder is OGLE TR 56 b - a planet of 1.45 Mj, discovered by the transit method, which takes only 1.2 days to revolve around its star. Kepler's third law enables us to calculate the distance between this planet and its star: just 0.02 AU. Astronomers reckon the surface temperature of OGLE-TR-56 b to be as much as 1,600° C - a truly lethal world. 51 Pegasi b may be a 'hot Jupiter', but it seems that there are also 'scorched Jupiters'.

Might we find planets even closer in? Researchers are divided on this question. At a distance of only hundredths of an AU from a star, not only is such a planet hot, but is subject to the irresistible pull of gravity of the star. It is possible that planets so close to their stars have not been found because there are none, as any finding itself in this position will have already been devoured by the star. But the question remains: why do we observe so many planets at around 0.05 AU from their stars? Are they in stable orbits, or merely on their way to being swallowed up by those stars? Astronomers cannot surmise how massive planets can form at distances within a few AU from stars; they must be forming further out, and then

(Left) The field of the star OGLE-TR-11 B - a star a little more massive and hotter than the Sun. It is 6,000 light-years away. The OGLE programme, using the transit method, has revealed three planets with orbital periods of less than 2 days. The method measures the size and period of the planet, but not its mass, and complementary radialvelocity measurements are necessary. As the stars studied are very distant, these observations are possible only through major instruments such as the ESO's Very Large Telescope (VLT). (Right). Light-curves from observations of the passage of a planet across the stars OGLE-TR-11 3 and the star OGLE-TR-1 32. The radial-velocity curves of these stars were based on observations with the Kueyen telescope of the VLT, which allowed the determination of the masses of the planets detected by the transit method.

(Left) The field of the star OGLE-TR-11 B - a star a little more massive and hotter than the Sun. It is 6,000 light-years away. The OGLE programme, using the transit method, has revealed three planets with orbital periods of less than 2 days. The method measures the size and period of the planet, but not its mass, and complementary radialvelocity measurements are necessary. As the stars studied are very distant, these observations are possible only through major instruments such as the ESO's Very Large Telescope (VLT). (Right). Light-curves from observations of the passage of a planet across the stars OGLE-TR-11 3 and the star OGLE-TR-1 32. The radial-velocity curves of these stars were based on observations with the Kueyen telescope of the VLT, which allowed the determination of the masses of the planets detected by the transit method.

migrating inwards. To cause such migrations, various processes may be at work, such as interaction between the planet and the protoplanetary disk in which it formed. The next (difficult) question which arises, though, is this. Once a planet has begun its migration, what is there to prevent it from falling into its star? Are all the close-orbiting planets so far discovered on their way to inevitable destruction? We must be thankful, it seems, that our own solar system is more stable! Computer models suggest that very massive planets are less likely to migrate than their smaller siblings. There are some indications favouring this idea, if we compare the masses of exoplanets to the semimajor axes of their orbits. Hot Jupiters with short periods are not very massive. Also, at distances greater than 1 AU, only planets more massive than Jupiter are found. So, not all exoplanets are scorched. Periods observed range from 1.2 days to 4,500 days, with half the population orbiting their stars in fewer than 300 days. It is difficult to estimate what bias might be introduced by this method of observation; for example, by the timing of the acquisition of data. It is clear that with time the number of known planets in longer orbits will increase. The detection, in 2002, of a planet with an orbital period of 14 years might seem difficult to understand, as before 1995 there were no observing programmes. However, in this case the planet in question is the third to be detected in the vicinity of the star 55 Cancri, and the adjustment of the radial velocity curve was more effective given a three-planet system, with the third in a long orbit.

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