Sedna (2003 VB12)
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On May 25, 2011, at the 218th American Astronomical Society Meeting, NASA space scientist Jack J. Lissauer led a talk on the possibility of a "A Jovian Mass Object in the Oort Cloud?" that perturbed the orbit of Sedna from the Edgeworth-Kuiper Belt into its current, highly elliptical orbit in the inner Oort Cloud. Lissauer presented updated dynamical and statistical analysis of cometary orbits (with AAS presentation abstract co-authors John J. Matese and Daniel P. Whitmire) as potential evidence that the Sun may have a Jovian mass companion orbiting in the outer regions of the Oort Cloud. Astronomer Ned (Edward L.) Wright, principal investigator of NASA's Wide-field Infrared Survey Explorer (WISE) mission, noted that such a large object might be detectable as a "possible low-mass brown [dwarf]" in observational data already collected by WISE (now being processed) if it has at least two Jupiter-masses (AAS presentation abstract by Lissauer et al, 2011; and John Matson, blog at Scientific American, May 27, 2011).
Cassini-Huygens Mission to
Saturn and Titan
Larger image (Jupiter with Europa).
A planet at least as massive as Jupiter
may have perturbed Sedna's original
orbit in the Edgeworth-Kuiper Belt
to its current, highly elliptical orbit
within the inner Oort Cloud.
On March 15, 2004, a team of astronomers (including Mike Brown, Chad Trujillo, and David Rabinowitz) announced the discovery of a very large planetary body in one of the most distant planetary orbits yet discovered within the Solar System. Confirmed by the Spitzer (infrared) Space Telescope, it was given a provisional designation as 2003 VB12. Subsequently, however, the IAU approved the discovery team's proposed permanent name of Sedna (after the Inuit Goddess of the Sea from which all creatures of the very cold Arctic sea were created) for the object. On August 24, 2006, the International Astronomical Union (IAU) voted to establish a new category of Solar System objects called "dwarf planets." Inner Oort Cloud member Sedna (2003 VB12) is large enough to have a spherical shape and so is a candidate for IAU designation as a dwarf planet.
Sedna is currently located around 90 AUs from our Sun, Sol -- an orbital distance that is roughly three times farther out than that of Pluto or Neptune -- but will eventually move as much as 10 times farther away (around 990 AUs) in a 12,260-year orbit around Sol; it's orbital semi-major axis is around 532 AUs with an extremely high eccentricity of 0.857. Based on its current distance, brightness, and presumed albedo or ability to reflect light, Sedna has been estimated to have more than half the diameter of Pluto and may be larger than any other planetary body found since Pluto -- initially estimated at 730 to 1,470 miles or 1,180 to 2,360 kilometers (km) across (compared to Pluto's diameter of around 1,440 miles or 2,320 km) but an August 2006 table posted by Mike Brown (a member of the discovery team) now lists a diameter of around 1,100 miles (1,800 kilometers), or more than three-fourth's Pluto's diameter.
Sedna is the second most reddish planetary body in the Solar System, after Mars. Although originally believed to have a slow rotation of 40 days which suggested that Sedna might have its own moon, astronomers eventually determined in 2005 that its rotation speed is actually around 10 hours (more). The icy object will move closer to the Sun over the next seven decades years -- to 76 AUs of Sol -- before receding back towards the inner Oort Cloud. Because Sedna so located so far from the Sun, its surface temperature never rise above minus 240 degrees Celsius (minus 400 degrees Fahrenheit) and is usually even colder as it approaches the Sun only briefly during its 10,500-year solar orbit. (More information and images can be found from the NASA press release, Spitzer press release, astronomer Mike Brown's Sedna Page, Astronomy Picture of the Day, and the Kuiper Belt Page).
R. Hart, Spitzer, CalTech, JPL, NASA -- larger illustration
Although Sedna has an extremely large orbit around the Sun, it appears to
move well within the hypothesized location of the inner Oort Cloud, (more).
Sedna appears to lie in a transition region between the Edgeworth-Kuiper Belt and the Oort Cloud. As of late 2005, only a few Edgeworth-Kuiper objects (EKOs) such as 2004 XR190 were known to have orbits that stay mostly beyond 50 AUs. Unlike Scattered Disk objects, these EKOs do not move inward within Neptune's gravitational reach of Neptune but have named members of the "Extended Scattered Disk," as defined by some astronomers. Two of these objects (1995 TL8 and 2000 YW134) move inward to within to 40 AUs of the Sun but have fairly elliptical orbits that take them back out beyond 60 AUs.
Extended Scattered Disk objects, however, also include 2000 CR105, which moves inward to within 44 AUs of the Sun but then outward beyond 500 AUs, and Sedna, which never comes closer to the Sun than 76 AUs before moving to around 900 AUs away (into the realm of the inner Oort Cloud). Given the large orbital eccentricities of these two objects (which move beyond 500 AUs of the Sun), some astronomers have argued that they were likely to have been strongly perturbed by a massive celestial object (which is unlikely to have been Neptune as they do not come close enough to its gravitational influence) such as a rogue planet or passing star, which could have dragged the two objects farther out after initial orbital perturbation by Neptune. (More discussion on such scenarios with illustrations from computer models are available from a 2005 Powerpoint presentation by Brett Gladman and Collin Chan.)
Although inclined by only around 11.9 degrees from the ecliptic where the eight major planets orbit, Sedna's distant orbit is extremely elliptical indicating that its formation and orbit may have been influenced by a passing nearby star during the early years of the Solar System, when Sol formed out of a molecular cloud with many other closeby stars around 4.6 billion years ago. Like 2000 CR105, Sedna may have been perturbed by a Solar-mass star at around 800 AUs from Sol more than 100 million years after its birth, given today's observed numbers of Oort Cloud comets (Morbidelli and Levison, submitted 2004).
David Seal (a mission planner and engineer at NASA's Jet Propulsion Laboratory at CalTech) has a web site that generates simulated images of the Sun, planets, and major moons from different perspectives and at different times of the year. Try his Solar System Simulator.
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