A globular cluster is a glorious thing. Consider Omega Centauri, a vast city of stars about 15,000 light years from Earth. Clusters like this one are composed of millions of Population II stars, meaning they’re among the oldest observed stars and may date back as far as twelve billion years. A result of their early formation is that they remain deficient in metals (in astronomical terms, the elements above hydrogen and helium), making them ideal laboratories for a particular branch of exoplanet studies.

Omega Centauri cluster

Image: This image of Omega Centauri, the brightest and largest globular cluster in the sky, was obtained with the Danish 1.5 m telescope at the ESO La Silla observatory. It shows the central part only; the cluster is actually much larger than the field reproduced here. Credit: European Southern Observatory.

A growing assumption about the massive ‘hot Jupiters’ we’ve found in our early planet hunting is that their existence depends upon a relatively high metallicity in their host star. The planets we’ve found certainly fit that bill, but is this just an observational bias, given that we’re looking at nearby stars that are all metal-rich to begin with? To test the idea, a team led by David T.F. Weldrake (Max-Planck Institut für Astronomie, Heidelberg) has been looking at Omega Centauri and 47 Tucanae, the two brightest clusters in the sky, to see if metal-poor stars produce such planets.

Working with Australian National University’s 1-meter telescope and Wide Field Imager, the team compiled 56 nights of data and gathered photometry on 53,000 main sequence stars, sampling the outer halos of each cluster. Analyzing the occurrence frequencies expected for gas giants in close orbits and factoring in the transit probabilities, the team expected to find 7 planets in 47 Tucanae and 5.3 in Omega Centauri. The actual result: not a single transit around any of the sampled cluster stars.

Note the method here. The work proceeded under the assumption that stellar metallicity does not affect the occurrence of these planets in globular clusters. If that were the case, the numbers expected should have been found. The result is statistically significant. From the paper:

…our null result suggests that stellar metallicity, not dynamics, is the dominant effect limiting the frequency of short period massive planets in globular clusters, and places an observational constraint on planetary frequency at such a low metallicity. Perhaps the low metallicity does not affect planet formation, but does affect planetary migration. If true, then long period planets should still exist in these clusters, undetectable in our work.

Centauri Dreams‘ take: The upshot is that we have to be careful in stretching our conclusions too far. What we can say is that the existence of metals in stellar atmospheres is implicated as the leading factor in preventing hot Jupiters from forming in these clusters. But the other great imponderable, how such planets migrate to the inner system, is still in play. Its possible role in planetary formation needs further study.

And because we’ve speculated in these pages about the view from a terrestrial world in a globular cluster, let’s add one more thing. While the planet/metallicity connection seems to be firming up rather well for gas giants, we know little about its effect on smaller, rocky worlds. It seems a natural assumption that these glorious cities of stars would be bereft of such planets, but the present work does not address that question.

The paper is Weldrake et al., “Searching for Planetary Transits in Globular Clusters – 47 Tucanae and ω Centauri,” slated to appear in the PASP proceedings of “Transiting Extrasolar Planets Workshop” (Heidelberg), and available as a preprint online.