“We call it the ‘globular cluster opportunity,’” says Di Stefano. This would make interstellar communication and exploration significantly easier. In contrast, the nearest star within a globular cluster could be about 20 times closer - just one trillion miles away. The nearest star to our solar system is four light-years, or 24 trillion miles, away. Such a civilization would enjoy a very different environment than our own. So if habitable planets can form in globular clusters and survive for billions of years, what are the consequences for life should it evolve? Life would have ample time to become increasingly complex, and even potentially develop intelligence. “Once planets form, they can survive for long periods of time, even longer than the current age of the universe,” explains Di Stefano. Any potentially habitable planets they host would orbit nearby and be relatively safe from stellar interactions. The predominant stars in globular clusters are faint, long-lived red dwarfs. Brighter stars also live shorter lives, and since globular clusters are old, those stars have died out. While brighter stars have more distant habitable zones, planets orbiting dimmer stars would have to huddle much closer. However, a star’s habitable zone–the distance at which a planet would be warm enough for liquid water–varies depending on the star. A neighboring star could wander too close and gravitationally disrupt a planetary system, flinging worlds into icy interstellar space. “It’s premature to say there are no planets in globular clusters,” states Ray.Īnother concern is that a globular cluster’s crowded environment would threaten any planets that do form. And while Jupiter-sized planets are found preferentially around stars containing higher levels of heavy elements, research finds that smaller, Earth-sized planets show no such preference. Exoplanets have been found around stars only one-tenth as metal-rich as our Sun. However, Di Stefano and her colleague Alak Ray (Tata Institute of Fundamental Research, Mumbai) argue that this view is too pessimistic. In fact, only one planet has been found in a globular cluster to date. Some scientists have argued that this makes globular cluster stars less likely to host planets. As a result, their stars contain fewer of the heavy elements needed to construct planets, since those elements (like iron and silicon) must be created in earlier generations of stars. They formed about 10 billion years ago on average. Our Milky Way galaxy hosts about 150 globular clusters, most of them orbiting in the galactic outskirts. (Image courtesy NASA, ESA, and the Hubble Heritage Team) Globular star clusters like this one, 47 Tucanae, might be excellent places to search for interstellar civilizations.
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