Recently,
astronomers reported the surprising discovery of a very large diameter
Kuiper Belt planetoid — (90377) Sedna — on a distant,
12,500-year-long, eccentric orbit centered approximately 500
astronomical units from the Sun. Sedna’s estimated diameter is about
1,600 km, two-thirds that of Pluto. Initial studies of Sedna’s origin
have speculated that it might have been ejected from the giant planets
region of our solar system far inside the orbit of Pluto, or perhaps
was captured from a passing star’s Kuiper Belt.

In a report published in the January 2005 issue of The
Astronomical Journal, planetary scientist Dr. Alan Stern of the Space
Science and Engineering Division at Southwest Research Institute®
(SwRI®) shows Sedna could have formed far beyond the distance of
Pluto.

“If this is actually what happened,” Stern points out, “it would
indicate that our solar system’s planet factory operated across a much
larger region than previously thought.” It would also indicate that
the mysterious Kuiper Belt “edge” near 50 AU (one AU is the distance
from the Earth to the Sun) is not an outer edge, but simply the inner
edge of an annular trough, or gap, that is carved out of a much
broader structure that has been called the “Kuiper disk.”

The new Sedna formation study used a planetary accretion code
developed by Stern with funding from NASA’s Origins of Solar System’s
Program in the late 1990s for studies of the formation of Kuiper Belt
Objects. This software was used to explore the feasibility of building
Sedna from boulder-sized and other small bodies at distances between
75 AU (Sedna’s closest solar approach distance) and 500 AU (Sedna’s
average distance from the Sun). Stern’s Sedna formation simulations
assumed that Sedna’s original orbit, while distant from the Sun, was
circular. Astronomers agree that Sedna could not have formed in its
present, eccentric orbit because such an orbit allows only violent
collisions that prevent the growth of small bodies. Stern’s
simulations further assumed that the solar nebula — the disk of
material out of which the planets formed — was much more extended
than most previous simulations had assumed.

“The Sedna formation simulations assumed that the primordial solar
nebula was a disk about the size of those observed around many nearby
middle-aged stars — like the well-known example of the 1,500-AU-wide
disk around the star Beta Pictoris,” Stern says.

“The model calculations found that objects as large, or even
larger, than Sedna could easily form in circular orbits at distances
of 75 to 500 AU, and that their formation time could have been fairly
short — just a few percent the age of the solar system,” Stern
continues. “If Sedna did form this far out, it is likely to be
accompanied by a cohort of other large planetoids in this very distant
region of the solar system. One telltale sign that these objects were
formed where they are, rather than in another location, would be if a
good fraction of them are on near circular orbits.”

Editors: An image to accompany this story is available at
http://www.swri.org/press/sedna.htm

SwRI is an independent, nonprofit, applied research and
development organization based in San Antonio, with more than 2,800
employees and an annual research volume of more than $355 million.