| Subject: KBO 2001 QG298 |
Author:
Blobrana
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Date Posted: 15:48:10 11/29/04 Mon
Scott S.Sheppard (Carnegie Institution of Washington) and David C. Jewitt (University of Hawaii) may have found a strange Kuiper-Belt object orbiting beyond Pluto.
The object, 2001 QG298, orbits far from the Sun, and seem to have a peanut shape.
After measuring it’s changing brightness in 2002 and 2003 with the University of Hawaii's 2.2-meter telescope and the 10-meter Keck I telescope, they noticed that its brightness varies by a 1.14 magnitude every 6.89 hours.
Moreover, the object's colours do not change, which suggests that dark spots rotating in and out of view are not causing the brightness changes.
Only three other solar-system objects larger than 50 kilometres across range in brightness by more than 1 magnitude. Two of them are
contact binaries, main-belt asteroid 216 Kleopatra and Trojan asteroid 624 Hektor; the third is Saturn's peculiar moon Iapetus, which displays a very dark leading hemisphere and a very bright trailing hemisphere.
However, Iapetus represents an unusual case because it is locked in
synchronous rotation with Saturn, so its leading edge may be sweeping
up dark material kicked off from Saturn's outer satellites.
With an average diameter of about 180 kilometres, 2001 QG298 is large
enough that it should be nearly spherical. Nevertheless, the object is not
spinning fast enough for rotation to whirl it into an elongated shape.
The simplest explanation for the brightness variations is that two
roughly spherical and equal-sized bodies eclipse each other
periodically every 6.89 hours, which means they must be very close together. We view them along their equators, which maximizes the eclipsing effect.
"We believe 2001 QG298 is a contact binary,"
Sheppard and Jewitt have found other possible contact-binary Kuiper
Belt objects (KBOs). Given the number of KBOs they have observed, and the fact that other KBO contact binaries might be viewed pole-on (which makes them harder to categorize), Sheppard and Jewitt estimate that at least 10 to 20 percent of all large KBOs might be contact binaries with similarly-sized components.
"The number of contact binaries with one component much larger than the other is probably much higher, but these don't make as large brightness variations and thus are not as easy to distinguish."
These close pairs probably formed early in the solar system's history
when two bodies approached each other and went into mutual orbit after exchanging orbital energy with other bodies nearby.
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