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EAGLE universe simulations rank the Sun's spot on the Milky Way's outskirts among the best places for life
Researchers using the EAGLE universe simulations report in MNRAS that the Sun's place in the Milky Way's outskirts is about as good for life as locations get. The ranking rests on modeled hazards and assumed chemistry, so it says where conditions favor life and leaves open whether any life is there.
The Scientist · Science desk
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What happened
- The team used EAGLE, a simulation that tracks cosmic expansion, gravity, gas, stars and black holes to map the universe across hundreds of millions of light-years.
- Their hazard model penalizes supernovae, gamma-ray bursts, close stellar passes in crowded regions and radiation from black holes of over a million suns at galaxy centers.
- Across the simulated universe, new opportunities for habitation appear to be slowing as the rate of catastrophes catches up with the formation of new planets.
- The Milky Way comes out as typical of the galaxies that have been efficient at turning raw cosmic material into life-friendly environments.
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Why it matters
- constraint Changing the assumed heavy-element limits or the hazard list could reorder the ranking, so the result is only as firm as inputs chosen without any observed alien life to check them.
- decision Search planners gain a galaxy-scale argument for environments like the Sun's own, while choosing individual target stars remains a separate problem.
- constraint If habitable environments are forming more slowly as catastrophes catch up, estimates of how common life-friendly worlds are must account for when planets formed as well as where.
"With no observations of alien life, we must make assumptions," the researchers wrote in their account of the work on phys.org [14]. Their assumptions start with chemistry. Hydrogen and helium from the big bang are too simple to build organisms, so the team followed the generations of stars that seeded space with heavier elements [4]. That input has a ceiling as well as a floor. Exoplanet observations show that Earth-like rocky planets need a certain concentration of heavy elements, but too much can produce a hot Jupiter, a gas giant that sweeps through its planetary system and eats rocky planets along the way [5].
The authors put the novelty in the next step, describing their "breakthrough" as "detailed modeling of those things that are bad news for life" [13]. Paleontologists think the Ordovician mass extinction 450 million years ago might have resulted from a nearby stellar explosion [7]. The approach scales up an older idea. Inside the Milky Way, astronomers already describe a galactic habitable zone: young stars too close to the center suffer from crowding, while stars too far out lack the chemicals life needs [8].
The authors anticipate the charge that a search for good places will find that home is best. "We could have found that life should exist in earlier galaxies, smaller ones or farther from galaxy centers," they wrote [10]. The point is fair as far as what the simulation was free to return. It protects less well against the inputs. Every criterion still had to be assumed, because there is no observed alien life to calibrate against [14]. I'd expect a location like the Sun's to score well on criteria built this way.
The ranking puts the Sun's location at "about as good as it gets" [2]. The published summary does not say how many simulated galaxies were compared, or by what margin the Sun's kind of location beats the alternatives.
The study does not show whether anything lives in the good spots, and the authors say as much. "While we don't know if life would form in these nice spots, they are the ones that offer the best conditions," they wrote [9].
What to watch
- The full MNRAS paper's quantitative ranking: how many simulated galaxies were scored and by what margin Sun-like locations lead.
- Whether other cosmological simulations, or altered hazard and heavy-element assumptions, reproduce the Sun's near-top placement.