Earth seems like a troubled place right now, but at least our cosmic environment seems peaceful. But this is not universally true, as explosions, collisions and other dangers lurk in other places and times. In our new research, published in Monthly Notices of the Royal Astronomical Society, we explore simulated universes to find the friendliest places for life. We discovered something reassuring: the Sun, sitting here on the outskirts of the Milky Way, is as good as it gets. The Goldilocks Zones The regions of the universe that would be conducive to life are known as habitable or Goldilocks zones: they are perfect. Whether life really exists is another question. While astronomers and biologists explore how life could form on exoplanets (planets outside the Solar System), astrophysicists follow the structure and evolution of the universe to identify the most beautiful places, as well as those where life could become extinct in an instant. So where should we look for life? For example, there is a Goldilocks zone in the vicinity of the stars. Too hot or too cold will not do. This “circumstellar habitable zone” is ideal for liquid water rather than vapor or ice. The circumstellar habitable zone near a star is “perfect”: neither too hot nor too cold. NASA By zooming out, we can look for the best places and times to live in our own galaxy, the Milky Way. A young star that is too close to the galactic center will suffer the hustle and bustle of all nearby stars, while stars that are too far away will be in a sterile environment, devoid of the chemicals essential for life. Just as the Earth is in the Sun’s circumstellar habitable zone, the Sun is in the “galactic habitable zone” of the Milky Way. Let’s get even further away. What about other galaxies? Simulating the universe We follow the synthetic evolution of galaxies using the EAGLE (Evolution and Assembly of GaLaxies and Their Environments) project, one of the most realistic simulations of the observed universe. They track the expansion of space, the pull of gravity, and the physics of gas, stars, and black holes, giving us a detailed map of what the universe looks like across hundreds of millions of light years. Faced with a map like this, we ask ourselves: where could life be most welcome? Finding life, avoiding catastrophes Without observations of extraterrestrial life, we must make assumptions. The hydrogen and helium produced in the Big Bang are too simple to form organisms, so we focus on generations of stars that have seeded the cosmos with heavier elements essential for life. Observations of exoplanets reveal that rocky Earth-like planets require a certain concentration of those heavier elements. However, too much could produce a hot Jupiter, a gas giant that sweeps through the planetary system, devouring rocky planets in its path. It is better to avoid this. Our advancement involves detailed modeling of those things that are bad news for life. Big stars explode, sterilizing their local environment. Larger bursts, such as gamma ray bursts, can burn a much larger volume. Paleontologists believe that the Ordovician mass extinction 450 million years ago could have been the result of a nearby explosion. There are other dangers to avoid. We wouldn’t want another star wandering into our Solar System and disrupting Earth’s pleasant circular orbit. Therefore, stay away from crowded galactic neighborhoods. Then there are black holes. Most of them are small and pose no danger, but monsters hide in the hearts of galaxies. Weighing more than a million suns, these can come to life with a roar as they swallow matter that gets too close. So stay away as this can cause intense radiation that can strip away your planet’s atmosphere. A home among the galaxies Although we do not know if life would form in these beautiful places, they are the ones that offer the best conditions. And the results tell us something else: the Milky Way is quite beautiful. This is not just a fondness for our own neighborhood. We could have discovered that life should exist in earlier galaxies, smaller or farther from galactic centers. We found that our universe has created many opportunities for habitation, although it appears to be slowing as the pace of catastrophes catches up to the pace of new planet creation. Our Milky Way seems typical of galaxies that have managed to convert the raw materials of the cosmos into environments conducive to life. Similarly, the Milky Way and its kin have left their energetic and dangerous days behind and have settled into relaxed retirement. Today, even in a billion years of wandering our Milky Way, fewer than one star in a thousand would experience an extreme extinction event that would wipe out life. The story is different for smaller galaxies, which are much more numerous than the Milky Way, as they are too crowded and have too much energy. So, rest easy. And if you have the opportunity, buying property in the Milky Way is probably a safe investment, astronomically speaking.
