Globular clusters held an early fascination for me, and I guess anyone who encounters these rich cities of stars for the first time wonders what it would be like to be on a planet deep inside one of them. The clusters appear to be distributed in a spherical halo around the galactic center, ancient collections of stars much lower in heavy elements than stars in the galactic disk (although globular clusters in some other Local Group galaxies seem younger). The thought of the night sky on a planet embedded in such a place makes the mind reel, star upon star upon star filling the view.

Image: The globular cluster 47 Tucanae, the second brightest globular cluster orbiting the Milky Way (behind Omega Centauri). Imagine the night sky deep within such a cluster. Credit: South African Astronomical Observatory.

But a new paper suggests that at least one category of planets may be rare in such clusters. It follows up on an earlier survey of the cluster 47 Tucanae which examined some 34,000 stars and came up empty. The theory here is that the high density of stars in globular clusters disrupts planetary orbits. Even more significant, tidal effects upon planets much closer to their star than Mercury is to our Sun eventually cause ‘hot Jupiters’ to experience orbital decay and an early demise. Add this to the low metallicity (few elements heavier than hydrogen and helium) in globular clusters and you have an environment not conducive to planet building in the first place.

Brian Jackson (NASA GSFC) puts it this way: “Globular clusters turn out to be rough neighborhoods for planets, because there are lots of stars around to beat up on them and not much for them to eat.” Working with colleague John Debes, Jackson argues that any ‘hot Jupiters’ in globular clusters would be destroyed quickly by tidal effects if nothing else, their orbits gradually moving closer to their star until the planet is torn apart by the star’s gravity or crashes into it.

Thus we can explain the dearth of planets in 47 Tucanae. The researchers’ simulations using tidal effects and the proximity of nearby stars showed that hot Jupiters would be unlikely to survive even when metallicity is left out of the equation. In fact, Debes and Jackson have found that approximately one third of hot Jupiters won’t survive the first billion years of a cluster’s life, not to mention the eleven billion years 47 Tucanae has been in existence. The simulations suggest that at 47 Tuc’s age, at least 96 percent of the hot Jupiters would have perished.

Here Kepler becomes a relevant tool. The mission has four open clusters (much less dense than globular clusters) in its survey field, in a range of ages from less than half a billion to nearly 8 billion years old. All of these clusters appear to have the raw materials from which planets are formed, making them a good test for the tidal decay model. The new work suggests that Kepler should find up to three times more Jupiter-class planets in the youngest cluster than in the oldest one. Fewer expected hot Jupiters, in other words, as cluster age increases and, as a corollary, increasingly tight orbits for detected planets. We should have some answers soon.

And this is interesting, from the paper on this work, on the results of a planet being consumed by its star:

If planets are engulfed, one would expect a signature of pollution in the stellar atmosphere… or an increase in stellar rotation rates… If the process strips just the envelope but leaves a dense core, there might be an excess of ?5-10 ME planets with short periods above that expected through orbital migration alone. There might even be a mass-period relationship for the remnant cores, analogous to that observed for tidally-stripped white dwarfs… Although transit surveys may have difficulty detecting these small stranded remnant cores, their detection would provide an important clue to the fate of close-in gaseous planets.

As to other planets in globular clusters, we’re forced to look for smaller worlds in far more distant orbits. Says Debes: “The big, obvious planets may be gone, so we’ll have to look for smaller, more distant planets. That means we will have to look for a much longer time at large numbers of stars and use instruments that are sensitive enough to detect these fainter planets.” Personally, I hope we start finding them, if only to validate those spectacular sky scenes I’ve long imagined.

The paper is Debes and Jackson, “Too Little, Too Late: How the Tidal Evolution of Hot Jupiters affects Transit Surveys of Clusters,” accepted by The Astrophysical Journal (preprint).

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