In order to give birth to life, at least as we know, a planet must orbit a relatively calm and stable star. The planet's orbit must also be almost circular, so the planet experiences similar warmth throughout the year. And it must not be too hot so that any surface water does not boil; Don't get too cold so that the water doesn't get locked in the ice; But just right, keeping rivers and oceans liquid.
These features define the "habitable zone" around the star – places that are tempting targets when looking for life-friendly exoplanets. But scientists are increasingly conducting similar scrutiny of the entire galaxy. Just as continents with different biospheres have different flora and fauna, different regions of the Milky Way galaxy may have different populations of stars and planets. The Milky Way's turbulent history means that not all corners of the galaxy are created equal, and only a few regions of the galaxy may be just right to make planets that we think are habitable.
Jesper Nielsen, an astronomer at the University of Copenhagen, said that as exoplanet scientists fine-tune their ideas in the search for extraterrestrial life, they are now considering the origin of stars and their neighbors. New simulations, as well as observations from satellites searching for planets and monitoring millions of stars, are mapping out how different galaxy neighborhoods – and perhaps even different galaxies – form planets in different ways.
This, in turn, could help us better understand where the telescope is pointing**, Nelson said.
Today, the structure of the Milky Way is complex. Its central supermassive black hole is surrounded by a "bulge," a thick mass of stars that contains some of the oldest citizens of the Milky Way. The bulges are wrapped in "thin discs", and on clear, pitch-black nights, you can see this structure meandering overhead. Most stars, including the Sun, are located in the spiral arms of thin disks surrounded by a wider "thick disk" that contains older stars. A diffuse, mostly spherical halo of dark matter, hot gas, and a few stars envelops the entire building.
Merrill Sherman and Samuel Velasco Quanta.
For at least two decades, scientists have wondered if habitable conditions differ between these structures. The first study of the Milky Way's habitability dates back to 2004, when Australian scientists Charles Linewe**er, Yeshe Fenner and Brad Gibson modeled the history of the Milky Way and used it to study places where habitable zones might be found. They want to know which host stars have enough heavy elements (like carbon and iron) to form rocky planets, which stars have been around long enough to evolve complex life, and which stars (and any orbital planets) are safe for neighboring supernovae. They eventually defined a "galactic habitable zone," a doughnut-shaped region with a hole at the center of the Milky Way. The internal boundary of the region begins at about 22,000 light-years from the center of the Milky Way and its outer boundary ends at about 29,000 light-years.
In the two decades since, astronomers have tried to define more precisely the variables that control the evolution of stars and planets within the Milky Way, said Kevin Schlaufman, an astronomer at Johns Hopkins University. For example, he said, planets are born in a disk of dust around a newborn star, and simply put, 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 these planetary manufacturing components than others, and scientists are now working to understand the extent to which galactic communities influence the planets they own.
Of the approximately 4,000 known exoplanets, so far there are few rules governing which type of planet lives in **; None of the star systems look much like our own, and most of them don't even look quite like them.
Nelson and his colleagues wondered if planets could have formed differently in the thick disks, thin disks and halos of the Milky Way. In general, thin-disk stars contain more heavy elements than thick-disk stars, which means they grow out of clouds, which may also contain more planet-making components. Using data from the European Space Agency's stellar tracking Gaia satellite, Nelson and his colleagues first separated stars based on their abundance of certain elements. They then simulated planet formation in these populations.
Simulations they published in October suggest that gas giants and superEarths – the most common type of exoplanet – grow more in thin disks, probably because (as expected) these stars have more building materials to work with. They also found that younger stars with more heavy elements usually have more planets, and that giant planets are more common than smaller ones. In contrast, gas giants are virtually absent in thick disks and halos.
Schlaufmann, who was not involved in the work, said the results made sense. The composition of the dust and gas from which stars are born is crucial in determining whether stars will form planets or not. Although this composition may vary from location to location, he argues that while location may have laid the groundwork for the world-building of stars, it may not determine the final outcome.
Nielsen's simulations are theoretical, but some recent observations support his findings.
In June, a study using data from NASA's Kepler Space Telescope found that stars in the thin disk of the Milky Way have more planets than those in the thick disk, especially worlds the size of SuperEarth and sub-Neptune. Jessie Christiansen, an exoplanetary scientist at the California Institute of Technology and co-author of the study, said one explanation is that ancient thick-disk stars may have been born at a time when planets were scarcely manufactured, before generations of dying stars seeded the world's building blocks for the universe. Alternatively, a thick-disk star may have been born in a dense, high-radiation environment, where turbulence simply cannot prevent the baby planets from merging.
Christiansen says planets may perform better in open areas such as suburbs than densely populated "urban" areas. Our sun is located in such a sparsely populated suburb.
Christiansen's survey and Nielsen's simulations were among the first to study planetary occurrence as a function of the Milky Way's neighborhoods. Vedant Chandra, an astronomer at the Harvard-Smithsonian Center for Astrophysics, is preparing to go one step further and study whether planet formation might have been different in some galaxies consumed by the Milky Way as it grows. In the future, Nielsen hopes fine-tuned surveys and instruments, such as NASA's upcoming Nancy Grace-Rome Space Telescope, will help us understand planetary formation in the same way that demographers understand populations. Can we ** which types of stars will have which types of planets? Is the Earth more likely to form in certain communities? If we knew to go ** look, would we find something that looked back at us?
We know that we live in a habitable zone, in a world orbiting a quiet star. But how, and when and why, life on Earth began, is the biggest question in any field of science. Perhaps scientists should also consider the origin stories of our stars, and even the origin stories of the stellar ancestors who shaped the corners of our galaxy billions of years ago.
Is life on Earth inevitable? Isn't it special? Chandra asked. "It's only when you start to have this global picture. Can you start answering questions like this?