Thursday, February 4, 2016

Globular Clusters are Perfect for Advanced Civilisations

Globular clusters are dense, spherical clusters of stars that are typically found in the halo of galaxies. The Milky Way galaxy is estimated to have over 150 globular clusters. A globular cluster can contain anywhere from a hundred thousand to several million stars packed in a relatively small region of space measuring just ~100 light years across. Other galaxies, such as the Andromeda galaxy, have as many as 500 globular clusters, while giant elliptical galaxies can have thousands of globular clusters.

Globular clusters are ancient objects that formed ~10 billion years ago. The oldest globular clusters can be as old as ~13 billion years. As a result, globular clusters have a low abundance of heavy elements such as iron and silicon that are necessary for the formation of rocky worlds like the Earth. Nevertheless, data from NASA’s Kepler space telescope has shown that rocky planets can readily form around stars with abundances of heavy elements as low as those found in globular clusters.

In contrast, Jupiter-like planets require a higher abundance of heavy elements to form their massive cores. For this reason, Jupiter-like planets are expected to be very rare in globular clusters, while rocky Earth-like worlds might be quite common. The compact nature of globular clusters means that the stars in globular clusters are much closer to each other than the stars in the Sun’s stellar neighbourhood. Distances between stars in a globular cluster can be as small as hundreds to thousands of AU, whereby one AU is the average Earth-Sun distance.

It turns out that globular clusters might be the perfect place for advanced civilisations to inhabit. The small distances between stars means that interstellar communication between stars is expected to take only days, weeks, or months. For comparison, a signal sent from Earth to Alpha Centauri, the nearest star system, would take over 4 years. An advanced civilisation in a globular cluster that has the same level of technology as the one currently on Earth would have already known a great deal about the nearest ~100,000 stars. Most of the planets around these stars would have already been detected and even characterised. Sending exploratory probes to these planets would be much easier since signals sent from the home planet takes take only days, weeks, or months to reach them.

However, planets in globular clusters can have their orbits disrupted by close encounters with passing stars. This is due to the dense stellar environments of globular clusters. Fortunately, globular clusters are ancient objects and so they only contain low-mass stars as more massive stars have shorter lifespans. The majority of stars in globular clusters are expected to be low-mass red dwarf stars.

Because the luminosities of stars drop steeply with decreasing mass, the habitable zone around low-mass stars is expected to be much closer-in than Earth is from the Sun. This means that planets orbiting within the habitable zone of stars in globular clusters are sufficiently close-in that their orbits are unlikely to be disrupted due to interactions with passing stars. Basically, the more compact a planetary system is, the less likely it will be disrupted by passing stars.

Red dwarf stars can live for hundreds of billions to trillions of years. As a result, planets in the habitable zone of red dwarf stars can support the development and evolution of life for extremely long periods of time. If an advanced civilisation developed on a planet around a red dwarf star in a globular cluster, it would find interstellar space travel far more feasible. Travelling at just one percent the speed of light, it could reach the nearest stars in only a few years.

Given that the stars in a globular cluster are billions of years older than the Sun, an advanced civilisation residing in a globular cluster could be far older than the one on Earth and could have already colonised the entire cluster. An advanced civilisation with colonies around many different stars would be immune to many existential threats. If an apocalyptic event occurs on one planet, the civilisation would still continue on other worlds. Finally, the view from the surface of a rocky planet in a globular cluster would be breathtaking. The entire night sky would be densely packed with stars.

Stefano & Ray (2016), “Globular Clusters as Cradles of Life and Advanced Civilizations”, arXiv:1601.03455 [astro-ph.EP]