In our solar system Jupiter is the king of planets. It is 2.5 times more massive than the other planets combined. But it isn’t the most massive planet we know of. In the search for planets around other stars, we’ve found planets with masses up to 20 times that of Jupiter. All things being equal, we can imagine Jupiter-like planets having a fairly even distribution of sizes, but it turns out that’s not the case.
A team of astronomers looked at the mass of hundreds of Jupiter-type planets, and found they tend to fall in two groups. One with masses of 1 – 4 Jupiters, and another with much larger masses. Large-mass Jupiters were found around all types of stars, but small-mass Jupiters were only seen around stars with a higher metallicity.
Planet mass vs metallicity for the analyzed stars. In the plot one can see the position of the two populations of giant planets. Credit: Santos et al. 2017
Stars are mostly hydrogen and helium. In astronomy other elements are referred to as “metals.” The metallicity of a star is a measure of how much of these metals a star has. The higher the metallicity, the more metals. Our Sun, for example, has a relatively high metallicity. The team found that metal rich stars like our Sun tend to have planets in the 1 – 4 Jupiter-mass range, while metal poor planets tend to have planets with 4 – 20 Jupiter masses.
The key to this mass difference could be in the way they form. There are two major ideas about how large planets can form. One model is the core accretion model, where a dense metal core forms first, and its gravity then attracts surrounding gas and dust to form a large planet. The other is the gravitational instability model, where gas and dust over a large area becomes gravitationally unstable and collapses under its own weight. It seems that the smaller Jupiters form via core accretion, which limits their mass, while larger Jupiters form by gravitational instability, which allows them to grow quickly.
It’s a bit early to conclude that formation mechanism is the cause of the two mass types. We’ll need to look at a larger sample of planetary systems to be sure. But this work does demonstrate the role of metallicity in planetary formation. Our solar system is just one example of a diverse range of planetary systems, and the formation of these systems was deeply dependent on the characteristics of their home stars.
Paper: N. C. Santos, et al. Observational evidence for two distinct giant planet populations. Astronomy & Astrophysics Vol. 603, A30, DOI: 10.1051/0004-6361/201730761 (2017)