ONE of astronomy’s biggest changes of perspective in recent years has been the realisation that planets are abundant in the cosmos. But not everywhere. Collections of stars called globular clusters seem bereft of them.
Globular clusters are roughly spherical collections of hundreds of thousands of stars. These, in turn, are among the oldest stellar inhabitants of galaxies. But though the Milky Way, the Earth’s home galaxy, has more than 150 globular clusters, so far only a single planet has been spotted in one: Messier 4 (see picture).
Nothing daunted, Rosanne Di Stefano of the Harvard-Smithsonian Centre for Astrophysics and Alak Ray of the Tata Institute of Fundamental Research, in India, told the 2016 meeting of the American Astronomical Society why they think globular clusters are a good place to go hunting for advanced civilisations.
First, the clusters’ very age means that life will have had the best chance of coming into existence and then climbing the ladder of complexity to the point where it can travel from star to star. Second, that age also means clusters have stopped being disrupted by life-destroying stellar explosions like supernovae and gamma-ray bursts. Third, the proximity of a cluster’s stars to one another means interstellar travel is not nearly as onerous as it would be for humanity. A spacefaring cluster-inhabitant would have to travel, on average, only about 1,000 times the distance from Earth to the sun to get to its nearest stellar neighbour. For humans, that distance is 275,000 times the Earth-sun distance. Not only does this make travel easier, it also makes communication practical. Messages between a home planet and its outposts could be sent and received with the same sort of delay as those between European countries and their colonies before the invention of the electric telegraph.
This speculation does, of course, require the existence of many more globular-cluster planets than the lone example so far discovered. But Dr Di Stefano and Dr Ray are optimistic about that. Their models suggest that such planets will often be in stable orbits in the “Goldilocks” zones (not too hot and not too cold) of their host stars. It may be, then, that the galactic empires dreamed of by science-fiction writers were wrong only in scale, rather than concept. Colonising an entire galaxy was always going to be a big ask. Annexing a cluster, though, looks eminently doable.