A new interpretation of the giant planet-metallicity correlation


Since the discovery, at the Observatoire de Haute-Provence, of a planet orbiting 51 Peg in 1995, almost 300 planetary systems have been discovered. The only characteristic that distinguishes the host stars is their metallicity (the abundance of elements heavier than helium in the star atmosphere). These stars, which are mainly dwarfs (stars in the main sequence phase), are, in the mean, more metal-rich than most field objects. Up to now, this peculiarity has been interpreted as due to the enhanced rate of giant planets, or "Jupiters", formation in metal-rich circumstellar disks. A new interpretation suggests that the rate of detection of Jupiters around stars could depends on the density of H2 in the galactic disk, decreasing from the inner disk outwards. The observed correlation would then result from the presence in the solar neighbourhood of stars originating from the inner galactic disk.

Two recent observational facts have mitigated the evidence of the correlation between metallicity and the probability of detection of exo-planets:

We show in a recent article (Haywood 2009) that these two pecularities can be explained if the planet-metallicity correlation is given a radically different interpretation.

In the last years, radial mixing has proved to be a phenomenon of importance in the galactic disk. Radial mixing is a consequence of badly identified dynamical processes, which are responsible for the 'migration' of stars in the disk. Stars born in the inner parts of the disk may drift to the outer parts within a few billion years, and vice-versa. The inner disk evolving at a more rapid pace, stars born in these regions are, in the mean, more metal-rich than those born in the outer disk.

Radial mixing then provides a natural explanation to point (a) and (b). Indeed, giants and massive stars are in the mean younger than dwarfs. Radial mixing being a secular process, its effects are proportional to time. Hence, samples containing older objects are more polluted by wanderers from the inner disk.

It is also possible to explain point (b) above, because if thick disk stars are deficient in metals, their origin is also thought to be local or in the inner disk, i.e very different from the thin disk stars of the same metallicity.

All this suggest that, rather than metallicity, it is the distance to the galactic center that governs the rate of Jupiters. The new question is then: what is the cause of this dependence ? A interesting candidate is the density of H2 gas. Molecular hydrogen is indeed the main ingredient of circumstellar disks and Jupiters. 70% of galactic H2 resides within the solar galactocentric radius. Its density rises towards the inner disk to reach a maximum at 3-5 kpc from the Sun (the molecular ring), at 4 to 5 times the estimated local density, in proportion to the rate of exoplanets on local stars (4%) and metal-rich ones (25%).

Figure 1: The observed planet-metallicity correlation (in red) can be easily reproduced by a model (thin line) where the percentage of planets is a function of galactocentric radius, with metal-rich stars originating from the inner galactic disk having a high percentage (around 25%) of giant planets. No specific dependence between of the rate of giant planets and metallicity is necessary in this case, while the density of H2 in the galactic disk could be responsible for the dependence with galactocentric distance.

Reference

 On the correlation between metallicity and the presence of giant planets
Haywood M.
ApJ Letter, 698, L1

Contact

Misha Haywood (Observatoire de Paris, GEPI, et CNRS)