RETURN TO THE WORLD SCIENCE HOME PAGE

Alien Worlds May Be in Our Backyard

Two researchers reached this conclusion while working to explain the mystery object Sedna, a world almost as large as the planet Pluto but located much farther from the Sun.

Sedna’s discovery in 2003 puzzled astronomers because of its unusual orbit-a 10,000-year-long oval whose closest approach to the Sun, 70 astronomical units, is well beyond the orbit of Neptune. (One astronomical unit, abbreviated A.U., is the average distance between the Earth and the Sun, or about 93 million miles.)

Understanding Sedna is a challenge because its orbit is far away from the gravitational influence of other planets in our solar system. However, the gravity of a passing star can pull objects into orbits like Sedna’s if they start out beyond the orbit of Neptune, in the Kuiper Belt. The Kuiper belt is a region beyond Neptune in which at least 70,000 small objects, including tiny planets, orbit.

Astronomer Scott Kenyon of the Smithsonian Astrophysical Observatory in Cambridge, Mass., and physicist Benjamin Bromley of the University of Utah performed computer simulations to show how this stellar fly-by could have occurred. Their results are to be published in the Dec. 2 issue of the research journal Nature.

The fly-by must have met two key requirements, say Bromley and Kenyon. First, the star must have stayed far enough away that it did not disrupt Neptune’s nearly circular orbit. Second, the encounter must have happened late enough in our solar system’s history that Sedna-like objects had time to form.

Kenyon and Bromley suggest that the near-collision occurred when our Sun was between about 30 million years and 200 million years old. The fly-by distance might have been between 150 and 200 A.U., they say.

The passing star’s gravity would sweep clear the outer solar system beyond about 50 A.U., even as our Sun’s gravity pulled some of the alien planetoids into its grasp. The model explains both the orbit of Sedna and the observed sharp outer edge of our Kuiper Belt, where few objects reside beyond 50 A.U., the researchers say.

“A close fly-by from another star solves two mysteries at once. It explains both the orbit of Sedna and the outer edge of the Kuiper Belt,” says Bromley.

A Crowded Birthplace

But where did such a star come from, and where did it go? Since the fly-by happened more than 4 billion years ago, any suspects have long since escaped the Sun’s neighborhood, say the researchers. Thus there is no practical way to find the culprit today.

The visitor’s origin may seem equally mystifying because the Sun currently lives in a sparse region of the Milky Way. Our closest neighbor is a distant 4 light-years away, and stellar close encounters are correspondingly rare.  However, a near-collision would be much more likely for a young Sun if it were born in a dense star cluster, as recent evidence suggests.

“We believe that 90 percent of all stars form in clusters with a few hundred to a few thousand members,” says Charles Lada of the Harvard-Smithsonian Center for Astrophysics. “The denser the cluster, the more likely the chance for an encounter between member stars.” The new research is “an important piece of evidence that the Sun formed in near proximity to other stars,” he adds.

Searching for Adopted Worlds

Kenyon and Bromley’s simulations indicate that thousands of alien Kuiper Belt Objects were stripped from the passing star. However, none have been positively identified. Sedna is probably homegrown, not captured. Among the known Kuiper Belt Objects, an icy rock dubbed 2000 CR105 is the best candidate for capture given its unusually elliptical and tilted orbit.

* * *