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Astronomical Observatory: Cool Images


Sedna, the limit of the observable solar system



What is Sedna? The movie loop shows a small patch of sky (6.2 by 2.4 arcminutes) on a sequence of four nights. The object moving down and to the right across the center of the field is called Sedna (90377). Discovered with the 48 inch Palomar Schmidt Camera just over one year ago by M. Brown, C. Trujillo, and D. Rabinowitz, Sedna is the furthest solar system object known. At a current distance of 88 AU it is about three times the distance of Pluto. As with asteroids, the apparent motion from night to night is largely a response (called parallax) to the earth's own orbital motion. However, the angle nearby asteroids move in an hour takes the more distant Sedna an entire day. It is estimated to be between 1300 and 1800 km across, compared to about 2 km for asteroids of similar apparent brightness (300,000,000 times more volume!).

Why is Sedna important? Sedna is in an elliptical orbit, bringing it as close as 76 AU to the sun and as far as 930 AU. Hence, at its closest approach it is still outside the Kuiper belt, a band of minor planets extending from Pluto's orbit out to 50 AU with orbits little changed since the planet forming era of the early solar system. Likewise, at its furthest distance it is not far enough to be influenced by the tidal force of the Galactic plane or passing stars, as are the long period comets that originate in the Oort cloud. In summary, Sedna's unique orbit rules out membership in either the Kuiper belt or the Oort cloud! Its existence requires the introduction of a new chapter in the dynamic history of the solar system. Possible new elements include a new planet just beyond the Kuiper belt, a very close encounter with a passing star, or the formation of the solar system within a cluster of stars. Each of these ideas has different implications for how to put objects into Sedna-like orbits, and can be tested in coming years as more Sedna-class objects are discovered, and their collective orbital properties can be determined.

How faint is Sedna? In these images, Sedna is 21st magnitude. That is some 1400 times fainter than the "bright" star at the right side of the image, which is 13.1 magnitude. It in turn is 1700 times fainter than the faintest star that can be seen by the unaided eye on a dark night. Altogether that makes Sedna 2.5 million times too faint to see by eye. It is a demonstration of the quality of our Rehoboth, New Mexico, observing site that Sedna can just be seen in individual 5 minute exposures. For the movie loop, an hour's worth of exposures were averaged together for each night, making Sedna easy to see.

Where is Sedna? Sedna is currently in the constellation Cetus, just 12 degrees south of the ecliptic, next to the zodiacal constellation of Aries. We measured precise positions (see the table below) using the PinPoint Astrometric software package, which compared star positions in our images to positions in the US Naval Observatory A2.0 catalog. This procedure yields positions repeatable within a few tenths of an arcsecond. The positions agree with JPL predictions to within 0.6 arcseconds.

Brown, Mike "Sedna" (2003 VB12).

Observation Details
RA (J2000)
Dec (J2000)


Exposure time per filter
12 x 300s
10 x 300s
10 x 300s
11 x 300s
2004 Dec. 10.23 UT
2004 Dec. 11.25 UT
2004 Dec. 12.26 UT
2004 Dec. 13.19 UT
Observer Larry Molnar


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