The logic is simple. Extrasolar planets orbit faraway stars. Star clusters are home to lots and lots of stars. So where better to look for new exoplanets? Unfortunately, astronomers think the extreme gravity of these clusters is ripping exoplanets apart.
We've found about 450 exoplanets around distant stars, but pretty much all of them were found around single stars, not in larger clusters. A recent survey of the globular cluster 47 Tucanae is representative - astronomers searched through some 34,500 stars that were good candidates to possess planets. With those sorts of numbers, a reasonable expectation would be to find at least a dozen or so stars. And yet 34,500 stars later, no new exoplanets were discovered.
According to NASA scientists John Debes and Brian Jackson, the gravitational tidal forces within the clusters are just too severe for many planets to form, and those that do don't last very long. Neighboring stars can kick a planet out of its natural orbit, sending it hurtling out of its solar system. Gravitational pressures can even create bulges on the surface of a star, which slowly saps energy from its exoplanet and pulls it into the star over billions of years.
Extreme gravity isn't the only problem for exoplanets in star clusters. Debes and Jackson point out that most star clusters have lots of hydrogen and helium but lack the heavier elements that planets need to exist. In such a starved environment, fewer exoplanets would exist to begin with. But even so, they believe this factor is less important than the issue of gravity.
The researchers ran a series of simulations in which they placed a number of so-called "Hot Jupiters" - exoplanets much bigger than Jupiter that are thought to be failed stars - in these star clusters. The simulation showed again and again that the vast majority of stars would be destroyed by tidal forces, making it unlikely a planetary survey conducted by Earth astronomers would find anything.
Their numbers suggest about a third of a star cluster's planets will be destroyed within the first billion years of the cluster's existence. 47 Tucanae is about 11 billion years old, and that stage the astronomers estimate 96% of the cluster's planets will have been destroyed. As such, they suggest the best use of our planet-hunting resources is to focus on younger star clusters, where there are still enough surviving planets that we're likely to find something.