Our sun was born 4.6 billion years ago near the crowded center of the Milky Way and then migrated roughly 10,000 light-years outward to the peaceful galactic suburbs it currently occupies. Now a pair of recent studies in Astronomy and Astrophysics argues that the sun did not make this trip alone.
Details of the sun’s journey can be found in its chemical composition, says Tokyo Metropolitan University astronomer Daisuke Taniguchi, a co-author on both studies. “Astronomers know that the sun’s birthplace lies closer to the galactic core than its current position,” Taniguchi explains. The Milky Way’s dense inner regions formed stars faster and accumulated heavy metals far more quickly than the outer edges—and a star with the sun’s age and chemical components would not have been able to form at its present location. But getting there required crossing a dramatic border.
Milky Way observations have revealed an enormous rotating barlike structure made of gas, dust and millions of stars that slices through our galaxy’s center. This bar creates a distinct gravitational phenomenon known as the corotation barrier, which prevents inner stars from traveling to the galactic outskirts. Computer simulations suggest that only about 1 percent of stars born at the sun’s presumed original location could successfully breach this barrier to reach our present neighborhood within a 4.6-billion-year time frame. Yet Taniguchi and his colleagues discovered that thousands of “solar twin” stars, each with a mass and a metal makeup similar to those of the sun, managed to do so.
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To catalog these stellar migrants, the researchers turned to the European Space Agency’s Gaia satellite, an observatory tracking the positions, movements and light wavelengths of more than two billion stars. They identified 6,594 solar twins within about 1,000 light-years of Earth.
When the scientists looked at the age distribution in their catalog, they saw two distinct peaks: one narrow spike of stars around two billion years old that probably formed locally and a broad, massive grouping of stars between six billion and four billion years old that included our sun—“a large population of stars that migrated from their birthplace to their current position,” Taniguchi proposes.
Alice C. Quillen, a physicist and astronomer at the University of Rochester, who was not involved in Taniguchi’s studies, warns that there’s a chance that the broad peak of solar twins might be an artifact resulting from the sample-selection method—a mere statistical illusion. “The sample is distance-limited, and most of it would be stars that make it into the solar neighborhood,” Quillen says. This factor could favor stars with more oblong orbits, which tend to be older, because younger stars with circular orbits wouldn’t have made it to our vicinity yet. But Taniguchi says his team addressed this bias and found no strong effect of age on the distribution of orbital shape among the solar twins.
His team proposes that the corotation barrier did not stop the migration of the sun and its cohort, because the barrier was not fully formed when it happened. In fact, Taniguchi suggests, the growing galactic bar could have pushed the migration forward instead of restricting it. The sun and its thousands of solar twins could have been propelled by the combined gravitational forces of the forming bar, the Milky Way’s spiral arm structure and, most likely, close passages of the nearby Sagittarius dwarf galaxy.
Rosemary Wyse, an astrophysicist at Johns Hopkins University, who was not involved in the studies, says that the researchers’ argument is persuasive but adds that (as the study authors note) the exact timescales remain uncertain. “The field of galaxy dynamics is itself dynamic,” she says.