The simulation used for the study maps out the distribution of gas, which provides fuel for stars and material for planets. It includes the Milky Way (MW) and Andromeda (M31) galaxies.

The simulation used for the study maps out the distribution of gas, which provides fuel for stars and material for planets. It includes the Milky Way (MW) and Andromeda (M31) galaxies.

K. Riebe/CLUES project (www.clues-project.org)

The most likely spots for life in the Milky Way

Our home galaxy isn’t as hospitable to life as you might think. Cosmic radiation, supernova explosions, and collisions with small galaxies make much of the Milky Way too hellish for biology. But a detailed new simulation locates quiet and fertile neighborhoods, including a surprising locale: wispy streams of stars flung far beyond the main body of the Milky Way.

To support life as we know it, planets must have liquid water and orbit in the right place in their solar systems, not too close and not too far from their star. Similarly, life will not emerge or survive for long near the centers of galaxies. Here, the high density of stars means that at any given time several could be exploding, frying off a planet’s ozone layer and exposing any surface life to deadly ultraviolet rays.

So in the new study, researchers led by physicist Duncan Forgan of the University of St. Andrews in Fife, U.K., focused on the regions far from a galaxy’s center. They used computer simulations to model an entire Milky Way–like galaxy and its neighbors, the Andromeda and Triangulum galaxies. They then simulated the distribution of gas, stars, and planetary systems within those whorls of stars. Finally, they allowed these galaxies to evolve over billions of years, while mapping out their evolving habitable zones. “We’re the first to look at how the history of galaxies affects their habitability,” Forgan says.

For every type of star in the simulation, Forgan and his colleagues estimated the probability that terrestrial planets would form, some of which might be Earth–like or might be as inhospitable as Mercury. They also estimated the chance that a giant planet as large as Neptune would form near the star, as it would disrupt potential earths that could have assembled there. Then they analyzed the likelihood of short-lived life-friendly worlds that happened to be in stellar systems too close to dying, exploding stars.

The team’s simulations show, perhaps not surprisingly, that potentially habitable planets are more likely to remain so if they form in areas far from dense conglomerations of stars, where more supernova explosions occur. The results indicate that for the Milky Way and other spiral galaxies, the most dangerous regions are in the galactic centers, whereas the more diffuse spiral arms pose fewer hazards and are therefore more hospitable to life. Earth lies near the inner edge of this habitable zone.

Observations from NASA’s Kepler space telescope strongly suggest that, “basically every star has a planet, on average, which is pretty mind-boggling,” Forgan says. Because the team’s simulation has many stars in the inner regions of galaxies, many planets form there, and some will be habitable but with a low chance of escaping irradiation from supernovae. The odds of a planetary system containing habitable worlds far enough away from these stellar explosions increases far from the galactic center, peaking in the outer edges of the spiral arms, the team will report in an upcoming issue of the International Journal of Astrobiology.

The scientists modeled not just the stars, but also the evolution of the galaxies themselves. Over billions of years, gravity pulls neighboring galaxies together, just as the Milky Way and Andromeda now experience that inexorable force. Forgan and his co-authors found that when galaxies collide, the habitable zone is transformed and then gradually settles back to its general trend: Stars at larger distances from the galactic center have higher chances of hosting planets hospitable to life.

Forgan and his team also widened the scope of their model to account for objects beyond the Milky Way’s spiral arms. These include filamentlike streams of stars—remnants of galactic collisions—as well as small “satellite” galaxies orbiting the Milky Way. These objects turn out to have “pockets of habitability,” with many life-friendly stars beyond where astronomers have traditionally pointed their telescopes.

These simulations “guide us about where to look” for habitable worlds, says physicist Anupam Mazumdar of Lancaster University in the United Kingdom, who was not involved in the study. Current telescopes and surveys are scanning wide areas in the Milky Way, but future telescopes could engage in more focused searches, he suggests.

Mazumdar notes that the simulation lacks fine resolution and cannot probe scales smaller than a hundred light years. Because the study cannot yet capture the details of individual stars and their planetary systems, the scientists must make approximations about how supernova explosions affect planets within those systems.

Habitable planets face additional threats missing from this research, including radiation spewing from gamma ray bursts, which are rarer and more intense than supernovae, says astronomer Neil Gehrels of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

There is also “some uncertainty in the distance range and rate of supernovae,” which determine how hazardous a galactic region might be and how far a planet must be from a supernova to remain safe, he says. “But their basic idea has got to be right that there are regions in our galaxy where there’s too much star formation going on.”

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