Globular clusters may be the perfect location to find extraterrestrial civilisations, according to new research announced today at the 227th American Astronomical Society meeting in Florida.
Globular clusters are ancient balls of stars that exist in the halo of galaxies. Our own Milky Way Galaxy sports around 150 of them, each one containing anywhere from a hundred-thousand to several million stars crammed into volumes about 100 light years across. They are believed to have formed around 12 billion years ago, during intense bursts of star formation as the galaxies around them were assembled.
Rosanne Di Stefano of the Harvard–Smithsonian Center for Astrophysics and Alak Ray of the Tata Institute of Fundamental Research in Mumbai, India, believe that conditions within globular clusters may be just right to facilitate long-lived, space-faring civilisations.
Traditionally, the outlook for life evolving in globular clusters has been poor. Elements heavier than helium – so called ‘metals’ – are produced inside stars and released into the cosmos when they die. These elements are vital for the formation of planets and for providing the raw materials for life. Because the cluster stars are so old, the amount of heavy elements that they contain – their ‘metallicity’ – had been assumed to be too low to form planets.
Our understanding is changing, however, thanks in part to NASA’s Kepler Space Telescope, which has shown that smaller, rocky planets can form around stars with metallicities as low as those found in globular clusters. Large gas giant planets, on the other hand, require more heavy elements to build up their massive cores. As such, we are unlikely to find Jupiter-like planets in globular clusters, but worlds like Earth and Mars might be common, say Di Stefano and Ray.
So far, only one exoplanet has been discovered in a globular cluster, found in the Messier 4 globular cluster in the constellation Scorpius. The planet orbits a pulsar in a binary system with a white dwarf. If this wasn’t bizarre enough, the planet’s orbit is strange too. Planets form in roughly circular orbits level with the plane of the system, but the pulsar planet’s orbit is highly inclined. This, coupled with the fact that it is unlikely a planet could survive the supernova explosion that created the pulsar, indicates that it has been captured from another star system within the cluster.
Such capture events are thought to be common in globular clusters given the proximity of their stars to each other – one can expect to find stellar neighbours just 1,000 astronomical units (one astronomical unit is the average distance between Earth and the Sun, which is 149.6 million kilometres) away. Compare that to the Sun’s nearest neighbour, Proxima Centauri, which is 4.2 light years away. If the pulsar planet has been captured, then it had to form somewhere else, which implies that planets should be common in Messier 4 and other globular clusters. So why have we not found them?
“The standard methods for finding exoplanets are very difficult to apply in globular clusters because these are very dense fields and it is hard to resolve the stars,” says Di Stefano.
Planets that are regularly swapped between their stars may be unlikely candidates for habitability, but the more compact a planetary system, the less likely a planet will be stolen by a passing star. Because they are cooler than the Sun, the habitable zones around red dwarf stars are much closer than Earth is to the Sun, and so we end up with scaled-down planetary systems. Di Stefano and Ray calculated that a red dwarf with a tenth of the mass of the Sun could hold onto its planets in the dense environment at the centre of a globular cluster for tens of billions of years. Meanwhile, in the suburbs of the cluster where the distances between stars are a little larger, a star with four-fifths the mass of the Sun could hold onto its planets indefinitely. Because lower mass stars live much longer than the Sun – red dwarfs could survive for hundreds of billions of years, perhaps even trillions – they would provide long term stability. That said, one caveat is that some red dwarfs are known to be violently active and to unleash powerful flares of radiation that could destroy any semblance of habitability on a planet.
Nevertheless, if intelligent, technological life can develop on a planet around a red dwarf inside a globular cluster, then it would find interstellar travel far more feasible than we do. Travelling at just one percent of the speed of light it would take them just four years to reach their nearest stars. Given that civilisations in globular clusters could be far older than the Earth, they could very well have colonised their entire cluster. A star-faring civilisation building colonies around many different stars has enhanced longevity because it becomes immune to many existential dangers. If war or disease breaks out on one planet, or another colony is hit by an asteroid, the civilisation would still survive on the other worlds.
Globular clusters have been considered before by scientiststaking part in SETI, the Search for Extraterrestrial Intelligence. In 2015, astronomers collaborating on the Ĝ (Glimpsing Heat for Alien Technologies, or ‘G-HAT’) search released the first results of their analysis of data from NASA’s Wide Field Infrared Explorer, or WISE. The idea is that a technological civilisation capable of building megastructures that collect all the energy radiated by a star would produce thermal leakage at infrared wavelengths. While Ĝ has not targeted any specific globular clusters yet, it is incorporating them into its search, according to one of the project’s leaders, Professor Jason Wright of Penn State University, USA.
“Globular clusters were included in the Ĝ search that we did for galaxies, as long as they were resolved (angularly) by WISE,” Wright tells Astronomy Now.
Detecting thermal leakage from a civilisation living in a globular cluster might prove quite difficult, though.
“Globular clusters have very low infrared emission naturally, so [a civilisation] would need really whopping excesses for us to have noticed them,” says Wright. “On the other hand, their low natural emission means that very low excesses should be easy to spot with more sensitivity. We haven’t done this yet for globulars or elliptical galaxies.”
In 1974 Carl Sagan and Frank Drake beamed a message from the Arecibo Radio Telescope in Puerto Rico towards the globular cluster Messier 13 in the constellation Hercules. They chose a globular cluster based on the logic that it has a high concentration of stars, increasing the chances that someone might actually be present to detect the message in 22,000 years when it reaches them. If Di Stefano and Ray are correct, then M13 may have been a good choice.
Says Di Stefano, “Globular clusters may indeed contain very old, advanced civilisations.”