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.
“That, in turn, can help us better understand where to point our telescopes,” Nielsen said.
Galactic Geography
Today, the Milky Way has a complicated structure. Its central supermassive black hole is surrounded by the “bulge,” a thick mass of stars that contains some of the galaxy’s most senior citizens. The bulge is encased by the “thin disk,” the structure you can see winding overhead on a clear, dark night. Most of the stars, including the sun, are found in the thin disk’s spiraling arms, which are embraced by a wider “thick disk” containing older stars. And a diffuse, mostly spherical halo of dark matter, hot gas and some stars envelops the entire architecture.
For at least two decades, scientists have wondered whether habitable conditions vary among those structures. The first study of galactic habitability dates to 2004, when the Australian scientists Charles Lineweaver, Yeshe Fenner and Brad Gibson modeled the history of the Milky Way and used it to study where habitable zones might be found. They wanted to know which host stars had enough heavy elements (like carbon and iron) to form rocky planets, which stars had been around long enough for complex life to evolve, and which stars (and any orbiting planets) were safe from neighboring supernovas. They ended up defining a “galactic habitable zone,” a doughnut-shaped region with the hole centered at the center of the galaxy. The region’s inner boundary starts about 22,000 light-years from the galactic center, and its outer boundary ends about 29,000 light-years out.
In the two decades since, astronomers have tried to more precisely define the variables that control both stellar and planetary evolution within the galaxy, said Kevin Schlaufman, an astronomer at Johns Hopkins University. For example, he said, planets are born in dusty disks that surround newborn stars, and, put simply, if “a protoplanetary disk has a lot of material that can make rocks, then it will make more planets.”
Some regions of the galaxy may be more densely seeded with those planet-making ingredients than others, and scientists are now working to understand how much galactic neighborhoods influence the planets they harbor.
Here Be Exoplanets
Among the roughly 4,000 known exoplanets, so far there are few rules governing which types of planets live where; no star systems look quite like our own, and most of them don’t even look much like each other.
Nielsen and his colleagues wanted to know whether planets might form differently in the Milky Way’s…
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