May Have Been Captured In "Close" Encounter
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."