Densely packed stellar balls, known as globular clusters, could be the best places to search for alien life, according to research presented today at the 227th annual meeting of the American Astronomical Society.
Globular clusters pack nearly one million stars into an area only 100 light-years across, and are nearly as old as the Milky Way itself. Approximately 150 of these ancient stellar relics lurk on the outskirts of our galaxy.
"Globular clusters could be the first place in which intelligent life is found in our galaxy. This research shows they could be great places to look for other intelligent civilizations," Rosanne DiStefano, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), said during the meeting.
Globular clusters are estimated to have formed over 10 billion years ago, and as a result the stars within the cluster contain fewer of the ingredients needed to form planets. However if habitable planets do form within the cluster, they will likely be around for a lot longer than planets that form around massive stars, giving life more time to evolve.
Habitable zones—the area around a star that can support liquid water—vary greatly depending on the size of the star. The ones surrounding more massive stars will be further away, while these zones around smaller, dimmer stars will be much closer. This is good news in the crowded, stellar neighborhood of a globular cluster. It means any potential habitable planets would huddle close to their host star and be safe from the gravitational tug of other stars.
The majority of stars residing within these clusters are called "red dwarfs." These types of stars are usually smaller than our Sun, and so they have longer lives and can support planets for billions of years, providing any potential life forms ample time to evolve beyond microbes into intelligent beings.
"Once a planet has formed, it can survive for long periods of time, even billions of years," DiStefano explained.
But don't get too excited yet, so far only one planet has been detected in a globular cluster. Most astronomers think these stellar populations are too crowded for planets to form, and are concerned about the lack of planet-forming ingredients like silicon and iron.
DiStefano and her colleagues remain optimistic and think we shouldn't rule out clusters completely. DiStefano explained that exoplanets have been detected around metal-poor stars, and while massive Jupiter-sized planets seems to prefer metal-rich stars, smaller Earth-sized planets seem to have no preference and can form around a variety of stars. Good news for alien hunters.
Life on a planet within one of these clusters would be vastly different than what we experience here on Earth. The closest star to our Solar System is four light-years or 24 trillion miles away. However, within a cluster stellar neighbors are only about one trillion miles away, helping facilitate interstellar communication and exploration.
DiStefano refers to this as "the globular cluster opportunity" and believes the close proximity would help support any potential life, making interstellar outposts easier to establish. Travel time within the cluster would take less time, and if there was a civilization at our technological level, sending probes between outposts would definitely be possible, according to DiStefano.
So, how do we make contact? The first step is identifying more planets within clusters. Since clusters are densely packed and contain dim stars, detecting planets within the crowded cores of clusters are very difficult to identify, and DiStefano believes that it could be easier to detect planets on the outskirts.
Another, more intriguing method might be to partner up with SETI to look for signs of radio communications from the cluster, something DiStefano would like to get more involved with. DiStefano is confident many more planets within these clusters will be discovered; it's just a matter of time. "In my mind there is no doubt," she said. "More planets within these clusters are waiting to be found."