Globular
star clusters like this one, 47 Tucanae, might be excellent places to
search for interstellar civilizations. Their crowded nature means
intelligent life at our stage of technological advancement could send
probes to the nearest stars. Credit: NASA, ESA, and the Hubble Heritage Team. High Resolution (jpg) - Low Resolution (jpg)
Kissimmee, FL - Globular star clusters are extraordinary in almost every way. They're densely packed, holding a million stars in a ball only about 100 light-years across on average. They're old, dating back almost to the birth of the Milky Way. And according to new research, they also could be extraordinarily good places to look for space-faring civilizations.
"A globular cluster might be the first place in which intelligent life is identified in our galaxy," says lead author Rosanne DiStefano of the Harvard-Smithsonian Center for Astrophysics (CfA).
DiStefano presented this research today in a press conference at a meeting of the American Astronomical Society.
Our Milky Way galaxy hosts about 150 globular clusters, most of them
orbiting in the galactic outskirts. They formed about 10 billion years
ago on average. 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. Some
scientists have argued that this makes globular cluster stars less
likely to host planets. In fact, only one planet has been found in a
globular cluster to date.
However, DiStefano and her colleague Alak Ray (Tata Institute of
Fundamental Research, Mumbai) argue that this view is too pessimistic.
Exoplanets have been found around stars only one-tenth as metal-rich as
our Sun. 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.
"It's premature to say there are no planets in globular clusters," states Ray.
Another concern is that a globular cluster's crowded environment
would threaten any planets that do form. A neighboring star could wander
too close and gravitationally disrupt a planetary system, flinging
worlds into icy interstellar space.
However, a star's habitable zone - the distance at which a planet
would be warm enough for liquid water - varies depending on the star.
While brighter stars have more distant habitable zones, planets orbiting
dimmer stars would have to huddle much closer. Brighter stars also live
shorter lives, and since globular clusters are old, those stars have
died out. The predominant stars in globular clusters are faint,
long-lived red dwarfs.
Any potentially habitable planets they host would
orbit nearby and be relatively safe from stellar interactions.
"Once planets form, they can survive for long periods of time, even
longer than the current age of the universe," explains DiStefano.
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.
Such a civilization would enjoy a very different environment than our
own. The nearest star to our solar system is four light-years, or 24
trillion miles, away. In contrast, the nearest star within a globular
cluster could be about 20 times closer - just one trillion miles away.
This would make interstellar communication and exploration significantly
easier.
"We call it the 'globular cluster opportunity,'" says DiStefano.
"Sending a broadcast between the stars wouldn't take any longer than a
letter from the U.S. to Europe in the 18th century."
"Interstellar travel would take less time too. The Voyager probes are
100 billion miles from Earth, or one-tenth as far as it would take to
reach the closest star if we lived in a globular cluster. That means
sending an interstellar probe is something a civilization at our
technological level could do in a globular cluster," she adds.
The closest globular cluster to Earth is still several thousand
light-years away, making it difficult to find planets, particularly in a
cluster’s crowded core. But it could be possible to detect transiting
planets on the outskirts of globular clusters. Astronomers might even
spot free-floating planets through gravitational lensing, in which the
planet’s gravity magnifies light from a background star.
A more intriguing idea might be to target globular clusters with SETI
search methods, looking for radio or laser broadcasts. The concept has a
long history: In 1974 astronomer Frank Drake used the Arecibo radio
telescope to broadcast the first deliberate message from Earth to outer
space. It was directed at the globular cluster Messier 13 (M13).
Headquartered
in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics
(CfA) is a joint collaboration between the Smithsonian Astrophysical
Observatory and the Harvard College Observatory. CfA scientists,
organized into six research divisions, study the origin, evolution and
ultimate fate of the universe.
For more information, contact:
Christine Pulliam
Media Relations Manager
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu
