Heavy metal planets in a globular cluster

Question

Globular clusters are old, dense groups of stars that tend to have relatively few metals (in this case meaning any elements heavier than helium). Clusters near the galactic core tend to have more metals, but still have notably less metallicity than our solar system, which almost entirely precludes any notable quantities of metals heavier than iron.

Is it feasible for there to be a group of planetary systems within a globular cluster that have higher metallicity than our solar system and notable quantities of metals heavier than iron?

A few stipulations:

• The more planetary systems the better, but at minimum I'd like at least a couple of systems, preferably within a few thousand AU of each other.
• Since the point of this is to have planets rich in heavy metals, I'd like each system to be able to support at least a couple terrestrial planets.
• The type of star doesn't really matter. G-type stars would be nice, but red dwarfs are just fine.
• Habitability isn't a concern, but I'd prefer the systems not be unreasonably dangerous to mine (e.g. no nearby x-ray bursters or instantly lethal levels of radiation).
• The size of the globular cluster is entirely up to you.
• I'd really like this to work for a globular cluster, but if that's just not possible, I'm open to other locations for a group of systems like this. Perhaps the center of a large open cluster?

What I have so far:

Supernovas are one of the primary sources of elements heavier than oxygen, and quark novas (massive explosions resulting from the hypothetical conversion of a neutron star to a quark star) are theorized to be a good source of heavier metals such as platinum.

Since globular clusters are so old, any massive stars they once possessed would have long since gone supernova and drifted to the center of the cluster. Evidence suggests that higher-metal stars can form in globular clusters, which seems to indicate that a second round of stellar formation can occur in the wake of supernovas.

These two factors would seem to make the center of a globular cluster a good place to get high-metallicity planetary systems, except that stars tend to be so close together in the center of globular clusters that any planetary systems would almost certainly be disrupted. Higher-metallicity planetary systems might form outside the core of a cluster, but still might not have much in the way of heavy metals because of the distance between the novas.

On a smaller scale, a few supernovas in a relatively small area might cause the formation of planetary systems, but it's entirely possible that the number of supernovas necessary to create enough metals to form planetary systems with higher metallicity than our own would push away too much material to allow said systems to form at all, especially if the supernovas occur over a long period.

More massive stars create heavier elements and bigger supernovas, but also push away more stellar material during their lifetime, which may or may not prevent the creation of relatively nearby systems. It seems possible that a number of massive stars close enough to each other (in any combination of multiple-star systems and neighboring systems) might trigger a "chain reaction supernova", destabilizing each other enough that they all supernova within a relatively short period. This Astronomy SE answer suggests that this is unlikely under normal stellar densities, but globular clusters might be a different story.

Enriching an existing planetary system with heavy metals seems pretty unlikely to occur naturally. Supernovas are obscenely powerful, so any major effect they have on a nearby system would be more along the lines of stripping away material rather than depositing it. The one possibility I can see here would be for a supernova to scour a system of most of its light elements, increasing its proportion of heavier elements.

Stars can survive the nova of a companion, but I can't find any evidence on whether or not they would be able to keep any of their existing planets, or if they would be able to have enough material to form new planets.

Answer 1

I think what you are talking about is not really the stellar metallicity but the heavy metal abundance relative to rock (e.g., measured as X/Si). To have a planet with a high abundance of heavy metals, you need the ratio of metals to rock to be high. I think that is what you are after: rocky planets with lots of heavy metals, right? The stellar metallicity is the grouped abundance of everything heavier than He relative to Hydrogen. So, a higher metallicity gives you more planetary building blocks but not more metals relative to rock.

One important fact is that the occurrence of low-mass (presumably terrestrial) planets does not correlate with metallicity. This is an observed fact (for example, see this paper: http://adsabs.harvard.edu/abs/2012Natur.486..375B). Higher metallicity stars form more gas giant planets, which tend to become unstable and destroy terrestrial planets.

Back to your setup. First, it's definitely plausible to imagine lots of planetary system in relatively dense stellar clusters. There are plenty of planets in open clusters. Globular clusters are an extreme example but why not. As long as there are no encounters closer than, say, 5 times the planets' orbital distance.

Now, how to get your planets lots of heavy metals? Well, to enrich them in, anything heavier than iron requires R-process nucleosynthesis and therefore a supernova. Different types of supernovae produce different elemental patterns but I don't remember enough from grad school about the differences (https://en.wikipedia.org/wiki/Supernova#Type_II_2). I know that supernova ejecta is clumpy, so it's easy to imagine some stars being polluted by heavy metal-rich clumps while others are not.

Of course, there are other ways of packing lots of rocky planets within 1000 AU (shameless plug: https://planetplanet.net/2016/04/13/building-the-ultimate-solar-system-part-6-multiple-star-systems/)

Answer 2

What you want to look at is metallicity of globular clusters. Global clusters have metallicities distributed in a bimodal distribution, for reasons that are not entirely understood. If I'm reading this right, a bit of ad-hoc statistics from Caltech's data here suggests that about 3.6% of globular clusters have at least the same metallicity as our sun, meaning that about 46% of all galaxies similar to ours possess at least one such cluster (we happen to have one ourselves: BH 176).

Not being an astronomer, I don't precisely know how that translates to rocky planetary systems, but it's a little encouraging. As far as I can tell, global clusters are planet-poor in general, with terrestrial planets even rarer, if they exist at all.

However, there might be other ways to guarantee planets exist. Start with some metallic star systems. Then some catastrophe occurs (hypergiant star passes very close, astroengineering, etc.) and the stars get a small kick upward, passing into the gravitational influence of an out-of-plane globular cluster. Over billions of years, chance perturbations lower the systems into its heart. Thus, you end up with an anomalous globular cluster with at least several metal-rich stars.

In sum, I estimate that a highly metallic globular cluster is rare, but not impossible, and that this corresponds to at least a few systems with metal-rich planets. However, making a cluster substantially more metallic (say, factor of 2) is unlikely to be realistic, unless you posit some highly unusual astrodynamics.