To harbor life, at least as we know it, a planet must orbit a star that is relatively calm and stable. The planet’s orbit must also be nearly circular so the planet experiences similar warmth throughout its year. And it must be not too hot, lest any surface water boil off; not too cold, lest that water remain locked in ice; but just right, so that rivers and seas remain liquid.
These characteristics define a “habitable zone” around stars — tantalizing places to target in the search for life-friendly exoplanets. But scientists are increasingly subjecting the entire galaxy to similar scrutiny. In the same way that continents with distinct biospheres host distinct flora and fauna, different regions of the galaxy could harbor different populations of stars and planets. The Milky Way’s turbulent history means that not all corners of the galaxy are the same, and that only some galactic regions might be just right for making planets we think could be inhabited.
As exoplanet scientists fine-tune their ideas about where to look for alien life, they are now considering the origin of a star and its neighborhood, said Jesper Nielsen, an astronomer at the University of Copenhagen. New simulations, along with observations from satellites that hunt for planets and monitor millions of stars, are painting a picture of how different galactic neighborhoods — and maybe even different galaxies — form planets differently.
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