The idea sounds perfectly logical: Life evolved on Earth partly because our entire solar system happened to coalesce within a relatively quiet corner of the Milky Way. This "habitable zone" is far from the lethal radiation, fast-flying supernova debris, and black holes that afflict other parts of the galaxy. The problem is that the theory may not be true, according to new research. The sun and all its planets may have been born in a far more dangerous corner of the galaxy and only later migrated to the suburbs.
The new findings suggest that many young stars, our sun included, can migrate from one part of the galaxy to another. If the finding is correct, it means that models of the Milky Way's evolution "are significantly incomplete," says astronomer Roelof de Jong of the Space Telescope Science Institute in Baltimore, Maryland.
The clues come from the chemical composition of our sun, which differs from that of nearby stars of about the same age. Astronomers believe that as stars form in particular parts of a galaxy, they should resemble their neighbors because they all congealed from the same clouds of gas and dust. But our sun is an oddball among its nearby companions, containing a higher proportion of heavy elements, such as iron. Why the discrepancy?
One possibility is a concept called radial migration, in which gravitational interactions among young stars can set them spiraling either far outward or inward from the galactic region in which they formed. Astronomers have tested the idea from time to time using computer simulations, and the results have suggested that radial migration occurs.
Now researchers in the United States, the United Kingdom, and Canada have run a 100,000-hour supercomputer simulation that strongly supports the concept of radial migration. They started with the birth of a hypothetical galaxy about 9 billion years ago--at the point at which the gigantic cloud of dust and gas had formed but had not yet coalesced into a disk--and ran the simulation to the present day. In the 10 September Astrophysical Journal Letters, the team reports that the simulation showed that the orbits of as many as half of the stars in the galaxy had moved thousands of light-years away from where they started.
"It was an exciting thing to stumble upon," says astronomer and lead author Rok Roškar of the University of Washington, Seattle, who notes that recent observations support the model's conclusion that radial migration of stars might be quite pervasive in the Milky Way. If so, he says, then astronomers will have to take the phenomenon into account when studying the histories of those stars. This is particularly true when attempting to define the conditions necessary for the onset of life, because radial migration suggests that the sun and Earth might have passed through such radiation and life somehow survived.
As for our own sun, Roškar says its cousins are probably scattered across the galaxy by now.