Close encounters between stars in Omega Centauri, the only star cluster visible to the naked eye, leave little room for habitable planets, according to new research.
An artist’s conception of exoplanets in a dense star cluster. Image credit: Michael Bachofner.
Omega Centauri, also known as NGC 5139, is about 18,000 light-years away, 150 light-years in diameter and 12 billion years old.
Of the 200 or so globular clusters that orbit the Milky Way, this cluster is the largest, containing over 10 million stars.
It is also the brightest globular cluster in our Galaxy, at apparent visual magnitude 3.9 it is visible to southern observers with the unaided eye.
In the hunt for habitable exoplanets, Omega Centauri seemed like a good place to look.
“Despite the large number of stars concentrated in Omega Centauri’s core, the prevalence of exoplanets remains somewhat unknown,” said Dr. Stephen Kane, an astrophysicist in the Department of Earth Sciences at the University of California, Riverside.
“However, since this type of compact star cluster exists across the Universe, it is an intriguing place to look for habitability.”
Starting with a rainbow-colored assortment of 470,000 stars in Omega Centauri’s core, Dr. Kane and colleagues homed in on 350,000 stars whose color means they could potentially harbor life-hosting planets.
For each star, they then calculated the habitable zone, the orbital region around each star in which a rocky planet could have liquid water.
Since most of the stars in Omega Centauri’s core are red dwarfs, their habitable zones are much closer than the one surrounding our own larger Sun.
“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. An example of such a system is TRAPPIST-1,” Dr. Kane said.
Ultimately, though, the cozy nature of stars in Omega Centauri forced the team to conclude that such planetary systems, however compact, cannot exist in the cluster’s core.
While our own Sun is a comfortable 4.22 light years from its nearest neighbor, the average distance between stars in Omega Centauri’s core is 0.16 light years, meaning they would encounter neighboring stars about once every 1 million years.
“The rate at which stars gravitationally interact with each other would be too high to harbor stable habitable planets,” said Sarah Deveny, a graduate student at San Francisco State University.
“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 findings will be published in the Astrophysical Journal.
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Stephen R. Kane & Sarah J. Deveny. 2018. Habitability in the Omega Centauri Cluster. ApJ, in press; arXiv: 1808.00053
