The star-packed core of Omega Centauri, one of the largest globular clusters in the Milky Way and a familiar targer for amateur astronomers. This stunning view was captured by the Hubble Space Telescope. Image: NASA, ESA, and the Hubble SM4 ERO Team
Omega Centauri, the largest globular star cluster in the Milky Way and a favourite target for amateur astronomers in southern latitudes, is packed with some 10 million stars. One can only imagine the glory of the night sky an alien might enjoy from a planet orbiting a star in the cluster’s core. But based on a new study, aliens – and stable solar systems – are unlikely to be found.
Writing in The Astrophysical Journal, Stephen Kane, a planetary astrophysicist at the University of California-Riverside, and Sarah Deveny, a graduate student at San Francisco State University, conclude stars are packed so closely together in the core of Omega Centauri that disruptive gravitational interactions would occur every million years or so.
That almost certainly would prevent the formation of solar systems with planets in stable orbits over the timescales necessary for life to evolve.
For comparison, the nearest star to Earth’s Sun is in the Alpha Centauri system some 4.2 light years – 40 trillion kilometres (25 trillion miles) – away. In the core of Omega Centauri, the average distance between stars is just 0.16 light years, or 1.5 trillion kilometres (940 billion miles).
Omega Centauri, as seen by the European Southern Observatory’s VLT Survey Telescope. Image: ESO/INAF-VST/OmegaCAM. Acknowledgement: A. Grado, L. Limatola/INAF-Capodimonte Observatory
“The rate at which stars gravitationally interact with each other would be too high to harbour stable habitable planets,” Deveny said. “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.”
The conclusions were based on an analysis of 350,000 stars in Omega Centauri’s core with the proper temperature and age, based on their colour, to host life-bearing exoplanets. Because most of the stars in the core are red dwarfs, their habitable zones, where liquid water could exist, would be much closer to their parent stars than the habitable region Earth enjoys orbiting the Sun.
“The core of Omega Centauri could potentially be populated with a plethora of compact planetary systems that harbour 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.”
But the numbers don’t appear to work out for stars in the cores of large globular clusters. When astronomers and amateurs train their telescopes on Omega Centauri, it’s unlikely anyone is looking back.