A section of the globular star cluster Omega Centauri.
Image: NASA, ESA, and the Hubble SM4 ERO Team

Globular clusters are among the most fascinating celestial phenomena in the galaxy, packing a hideous amount of stars into a relatively tiny region of space. Given the sheer number and variety of stars within these clusters, it seems reasonable to think they’d also be packed with life. But as new research suggests, globular clusters are likely cosmic-scale wastelands.

Stars within the Omega Centauri globular cluster are located too close together to provide the necessary long-term conditions required to sustain life, according to new research set to be published in the Astrophysical Journal (a pre-print is available at the arXiv). So what appears to be an excellent candidate in the search for extraterrestrial life is instead a vast expanse of sterile space, if this conclusion is correct. The finding could very well apply to other globular clusters, too.

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The Omega Centauri star cluster.
Image: ESO

The two authors of the new study, Stephen Kane from the University of California, Riverside, and Sarah Deveny from San Francisco State University, set about the task of estimating the number of potentially habitable exoplanets within the Omega Centauri globular cluster. This cluster, the largest in the Milky Way, is packed with some 10 million stars. It’s located about 16,000 light-years from Earth, making it a good observational target for the Hubble Space Telescope.

“Despite the large number of stars concentrated in Omega Centauri’s core, the prevalence of exoplanets remains somewhat unknown,” said Kane in a statement. “However, since this type of compact star cluster exists across the universe, it is an intriguing place to look for habitability.”

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Out of a selection of 470,000 stars of various types, Kane and Deveny whittled down their sample pool to about 350,000, all of which, due to their temperature and age, could allow for the presence of habitable zones, and by consequence, habitable exoplanets. The area of each star’s habitable zone—that sweet-spot orbital range within which liquid water could exist on a planet’s surface—was calculated by the researchers. Most stars in the study were small red dwarfs, resulting in habitable zones at close distances owing to the stars’ low temperatures.

“The core of Omega Centauri could potentially be populated with a plethora of compact planetary systems that harbor habitable-zone planets close to a host star,” Kane said. “An example of such a system is TRAPPIST-1, a miniature version of our own Solar System that is 40 light-years away and is currently viewed as one of the most promising places to look for alien life.”

Globular cluster NGC 2108
Image: ESA/Hubble & NASA

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But when the researchers looked at the resulting data, they came to a rather grim realization: These stars are located too close together for stable planetary systems to exist. Take Earth, for example, which is located about 4.22 light-years from our nearest neighboring star, Alpha Centauri; it’s too far away for its gravity to influence the orientation of our planets.

Such is not the case in the Omega Centauri globular cluster, where the average distance between stars is about 0.16 light-years. At this distance, each star endures a close encounter with a neighboring star about once every million years. These encounters fundamentally alter the planetary architecture of each star system. An exoplanet once parked within the cozy confines of a habitable zone would suddenly find itself flung into the frigid outer realms of its star system, or tossed into a toasty closer orbit.

As the example on Earth shows, life requires thousands of millions of years to evolve complexity, so with this kind of disruption, it’s highly unlikely that Omega Centauri, or any globular cluster for that matter (there are about 200 globular clusters in the Milky Way, most of them located in the galactic halo beyond the galaxy’s bright center), contains the long-term conditions required to sustain life. If life did manage to emerge, say some kind of microbe, it would likely be snuffed out within a million years or so, unable to acquire complexity and evolve into things like fish, terrestrial vertebrates, or animals with human-like intelligence.

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“The rate at which stars gravitationally interact with each other would be too high to harbor stable habitable planets,” explained Deveny. “Looking at clusters with similar or higher encounter rates to Omega Centauri’s could lead to the same conclusion. So, studying globular clusters with lower encounter rates might lead to a higher probability of finding stable habitable planets.”

This isn’t the first study to question the habitability of globular clusters, but it does provide the first quantitative analysis of Omega Centauri and its potential for habitability. Still, other scientists have previously argued that star clusters could in fact harbor life.

The central portion of star cluster RCW 38.
Image: ESO/K. Muzic

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Needless to say, this study carries implications for both astrobiologists and SETI (the search for extraterrestrial intelligence). Life may be rarer in the galaxy than we thought—but that doesn’t mean globular clusters aren’t attractive to star-hopping alien intelligences. For advanced space-faring civilizations, a globular cluster, with its stars in close proximity, could be an ideal place to build an array of superstructures, such as Dyson spheres. Should they venture into these star-packed regions of space, they’d better bring their sunglasses.

[The Astrophysical Journal | arXiv]