Yesterday, the Texas group announced their discovery of a new two-planet system orbiting HD 155358. Assuming that they’ve drawn a more-or-less edge-on configuration, the inner planet has a bit less than a Jupiter mass and orbits the solar-type parent star in 195 days. The outer planet has about half a Jupiter mass and orbits in 530 days. Dynamically, the system is reminiscent of an overclocked Jupiter and Saturn (although the planets lie far enough away from the 5:2 commensurability so as to avoid the indignities associated with the great inequality).
The main angle on HD 155358 is the low metallicity. The star has [Fe/H]=-0.68, which means that its iron abundance is only 21% that of the Sun. It’s rare to find giant planets around a star that’s so anemic. What exactly happened that allowed HD 155358b and c to beat the odds by assembling cores and accreting enough gas to become full-fledged giant planets?
There were probably a number of contributing factors. HD 155358 may have had a relatively long-lived protostellar disk. In all likelihood, that disk was probably considerably more massive than average. Although HD 155358 is iron-poor, I bet it’ll turn out to be relatively overabundant in oxygen and silicon (that is, a core-accretion formation scenario would prefer supersolar [Si/Fe] and [O/Fe] for HD 155358, see here for more details). Giant planet cores are made from volatiles, and so it’s the oxygen, not the iron, that’s the critical element.
HD 155358, with its ~10 billion year age, and (possibly) enhanced [O/Fe] would be very much at home in a giant elliptical galaxy like M87.
At times, oklo.org likely seems rather provincial. The scope of discussion here rarely ranges beyond the distances of a few hundred light years that mark our local stellar neighborhood. It’s easy to forget that there are a hundred billion galaxies within our cosmological horizon. Each galaxy contains billions of planets.
A bruiser like M87 packs trillions of stars, many of which formed during the ferocious galactic mergers that occurred roughly 10 billion years ago at redshift z~2. (I like this Java applet for computing ages, redshifts and lookback times for the Universe as a function of fundamental cosmological parameters). Many of the stars in giant ellipticals have metallicities that are similar to or even greater than solar, and because older stellar populations tend to have higher [O/Fe], it’s nearly certain that collossal numbers of planets were forming during the epoch when the giant ellipticals were being assembled.
To the best of our knowledge, it takes 4.5 billion years from the epoch of planetary formation to the point where technology and directed information processing emerge. This means that when we look back at elliptical galaxies at redshift z~0.65, we’re seeing what may have been the Universe’s golden age — the time and the environment when the density of civilizations was the highest that it will ever be. What happened to them? Where are they now?