Two gas giants discovered around the star HD 155358 raise again the question of planetary formation and the mechanisms behind it. Most planets detected through radial-velocity methods, which measure the effects unseen companions have on a star’s motion, have been found to orbit stars that are high in metal content. ‘Metals’ in this context means elements higher than hydrogen and helium, and of the two primary models for planetary formation, high metal content seems to favor the one known as the core accretion model, about which more in a moment.
What to do about a low metal star whose system is dominated by two massive planets? HD 155358 contains only 20 percent of the metal content of our Sun. Such a finding may favor the rival disk instability model. Here the notion is that the rotating disk of gas and dust in a protoplanetary system becomes unstable not long after it forms, causing it to fragment. As clumps begin to appear, they become large enough to cause their gases to collapse under gravitational forces. And you wind up with a planet forming quite quickly, perhaps in as little as a few centuries.
But the University of Texas team that discovered the two Jupiter-like planets around HD 155358 doesn’t rule out core accretion. That model, which works on much longer timescales, says that a Jupiter-like planet gradually forms by accumulation of solid materials into a core that develops over about a million years, finally reaching the point where its gravity is large enough to pull large amounts of gas along with it. A gas giant forms, though in a few million years rather than a few hundred.
Nonetheless, if the two new planets did form through core accretion, the results are unusual, since the star has little of the material needed for that kind of planet-building. But maybe what’s significant here isn’t just the metallicity of the star in question but the size of the protoplanetary disk, says Michael Endl (University of Texas at Austin):
“The major result of our discovery is that these planets required a very massive disk to form, several times more massive than we think our solar system disk was. This demonstrates that disk masses can vary significantly and might even be the most crucial factor in planet formation.”
The core accretion/disk instability argument has a long run ahead of it, though core accretion seems to have the upper hand in most exoplanetary systems we’ve examined thus far. As for the new planets, one is at least 90 percent the mass of Jupiter and orbits at about 0.6 AU, while the other appears to have half of Jupiter’s mass, orbiting at 1.2 AU. HD 155358 is somewhat hotter than the Sun but also less massive. The Texas team estimates its age at some 10 billion years, though we’ve seen recently how much play there can be in stellar age calculations.
The paper on this discovery is Cochran et al., “A Planetary System Around HD 155358: The Lowest Metallicity Planet Host Star,” in press at The Astrophysical Journal and available online. The intricate orbital resonances of these two worlds — when one planet’s orbit becomes more circular, the other becomes more eccentric — make for interesting reading here.
