Exoplanets in nearby globular cluster not likely to be habitable

Stellar density at cluster's core would lead to gravitational interaction between stars, disrupting planetary systems.
By Laurel Kornfeld | Jan 07, 2019
Exoplanets orbiting stars in the Milky Way's largest globular cluster, Omega Centauri, likely orbit in densely packed systems too close to their parent stars to be habitable, according to a new study accepted for publication in the Astrophysical Journal.

Globular clusters are spherical groupings of stars that are gravitationally bound together, often found in galactic halos. They typically host older stars than the less dense open clusters often found in galactic disks.

Omega Centauri is located about 16,000 light years away and has approximately 10 million stars. It is visible from Earth with the naked eye and has been studied with the Hubble Space Telescope (HST).

Researchers led by Stephen Kane of the University of California at Riverside (UCR) looked at 350,000 stars in Omega Centauri, whose temperatures and ages make them most likely to host habitable planets. For each star, they calculated the habitable zone, where temperatures would allow liquid water to exist on a planet's surface.

The majority of the stars studied are red dwarfs, which are cooler and dimmer than our Sun and whose habitable zones are therefore much closer to the stars.

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

"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," he added. "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."

However, the researchers determined that planetary systems around Omega Centauri's stars are likely very compact, with habitable zones extremely close to the stars, making any planets in these locations vulnerable to radiation from the stars. At such close distances, orbiting planets would likely be tidally locked to the stars, with one side always facing its parent star and the other always facing away from it.

Furthermore, stars in the cluster's core are so close to one another that they would gravitationally interact, perturbing individual planetary systems. The average separation between core stars is 0.16 light years. In contrast, the Sun is 4.22 light years away from its closest neighboring star system.

With stellar distances of just 0.16 light years, neighboring stars would interact with one another approximately once every million years, making it impossible for those stars to maintain the stable planetary systems necessary for life.

Globular clusters that are less dense, with larger distances between their stars, could host habitable planets, the researchers noted.



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