Plenty of Jupiters but few superJupiters

Bearing in mind all the caveats mentioned, what can we now say about the large family of exoplanets? Let us first look at their minimum masses, which range from 0.02 Mj to 17.5 Mj. The lower figure represents merely the limit of detection in the most favourable case. The histogram of masses shows a marked preponderance of masses below 2 Mj, and half the planets detected are of mass less than 1.6 M(. Many low-mass planets must have 'fallen through the net', as

3.2 Exoplanets: a heavyweight family? 43

3.2 Exoplanets: a heavyweight family? 43

The birth of the solar system. According to the standard theory of the formation of the solar system, the Sun formed at the heart of an immense disk of gas (hydrogen and helium), dust and ice. The disk collapsed under its own gravity, but as it was rotating it settled into a disk with the protoSun at its centre. Within the disk, embryonic planets (planetesimals) grew as they gathered up dust particles. In the central regions, ice particles were melted by solar radiation, and dust agglomerated to form rocky planets. In the outer regions there was sufficient material (dust, and especially ice) to form very large planetesimals, which became massive enough to draw the surrounding gas in upon themselves. The giant planets were born. This scenario explains the spatial separation between the terrestrial planets and the giants. It also explains why the orbits of the planets in the solar system are more or less in the same plane - that of the initial disk - and why these orbits are almost circular. Any planetesimals with orbits too elliptical would be destroyed in collisions with others. Unfortunately, the properties of most exoplanetary systems do not fit this scenario.

The birth of the solar system. According to the standard theory of the formation of the solar system, the Sun formed at the heart of an immense disk of gas (hydrogen and helium), dust and ice. The disk collapsed under its own gravity, but as it was rotating it settled into a disk with the protoSun at its centre. Within the disk, embryonic planets (planetesimals) grew as they gathered up dust particles. In the central regions, ice particles were melted by solar radiation, and dust agglomerated to form rocky planets. In the outer regions there was sufficient material (dust, and especially ice) to form very large planetesimals, which became massive enough to draw the surrounding gas in upon themselves. The giant planets were born. This scenario explains the spatial separation between the terrestrial planets and the giants. It also explains why the orbits of the planets in the solar system are more or less in the same plane - that of the initial disk - and why these orbits are almost circular. Any planetesimals with orbits too elliptical would be destroyed in collisions with others. Unfortunately, the properties of most exoplanetary systems do not fit this scenario.

they are much more difficult to detect than their larger neighbours, and there must be an even greater preponderance of small fry. This distribution - with its obvious preference for 'low' masses - is therefore quite a firm result, independent of random selection effects. Here we see a reflection of our own solar system: of its eight planets, the small ones are as numerous as the large.

The form of the distribution is not in itself remarkable, and the 'standard' theory of the formation of the solar system explains it well enough. If planets form by accretion, from protoplanets or planetesimals within a protoplanetary disk, logic suggests that more small objects than large ones will result, since more massive planets require more material. However, a nagging doubt remains. How can we explain the presence of really large planets of about 8-10 Mj? Such bodies would need tens of millions or even hundreds of millions of years to form; but all observational evidence indicates that after a few million years (10 million at most), young stars have lost their protoplanetary disks. How does this happen? All stars lose matter in the form of a continuous 'wind' of particles and eruptions. The Sun is no exception, and the solar wind is responsible for magnetic storms and polar aurorae on Earth. Young stars, however, emit fierce stellar winds,

The masses of exoplanets. The observed distribution of the 161 exoplanets known by July 2005 (pulsar planets not included). The diagram at right is a detail of the diagram at left, between 0 and 2 Mj. The masses of Jupiter, Saturn and Neptune are shown by arrows (mass of Saturn = 0.3 Mj; mass of Neptune = 0.05 Mj).

The masses of exoplanets. The observed distribution of the 161 exoplanets known by July 2005 (pulsar planets not included). The diagram at right is a detail of the diagram at left, between 0 and 2 Mj. The masses of Jupiter, Saturn and Neptune are shown by arrows (mass of Saturn = 0.3 Mj; mass of Neptune = 0.05 Mj).

which soon blow away the disks of gas and dust surrounding them at birth. Without a disk, gas and dust no longer remains to provide material for planets to grow to more than a few times larger than Jupiter. So, here is another mystery for us to solve. How do the heavyweight members of the family of exoplanets come to be?

When we are dealing with even greater masses, the word 'planet' does not really apply. There are a few interlopers within the exoplanet family: the failed stars known as 'brown dwarfs'. Ordinary stars like the Sun create their energy by the fusion of hydrogen into helium. Fusion in brown dwarfs involves deuterium - a heavy isotope of hydrogen with a nucleus comprising one neutron and one proton. Theory predicts that this process will be triggered in bodies of mass 13 Mj. If the effect of inclination is taken into account, any exoplanet with a minimum mass greater than 8 Mj is a suspect. About ten candidates present themselves, so 'brown dwarf contamination' is likely to be limited.

Can we extrapolate our observations to include planets of very low mass? We do not know if it is possible, but let us try anyway. We ought to be finding about 100 times as many planets of 0.1-30 Earth masses than in the range 1-2 Jupiter masses. So, a rich harvest awaits us.

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