Solar System

Sedna has an absolute magnitude (H) of 1.6,[3] and it is estimated to have an albedo of 0.16 to 0.30,[4] thus giving it a diameter between 1,200 and 1,600 km.[4] At the time of its discovery it was the largest object found in the Solar System since the discovery of Pluto in 1930. Mike Brown and colleagues now believe it to be the fifth largest known trans-Neptunian object after Eris, Pluto, Makemake, and Haumea.[4][20] In 2004, the discoverers placed an upper limit of 1,800 km on its diameter,[21] but by 2007 this was revised downward to less than 1,600 km after observation by the Spitzer Space Telescope.[5] As Sedna has no known moons, determining its mass is very difficult. However, if the above estimates for its diameter are coupled with Pluto's density of 2.0 g/cm3, the resultant estimated mass range is 1.8–4.3 x 1021 kg.[a]
Observations from the SMARTS telescope show that in visible light Sedna is one of the reddest objects in the Solar System, nearly as red as Mars.[12] Chad Trujillo and his colleagues suggest that Sedna's dark red colour is caused by a surface coating of hydrocarbon sludge, or tholin, formed from simpler organic compounds after long exposure to ultraviolet radiation.[22] Its surface is homogeneous in colour and spectrum; this may be because Sedna, unlike objects nearer the Sun, is rarely impacted by other bodies, which would expose bright patches of fresh icy material like that on 8405 Asbolus.[22] Sedna and two other very distant objects ((87269) 2000 OO67 and 2006 SQ372) share their colour with outer classical Kuiper belt objects and the centaur 5145 Pholus, suggesting a similar region of origin.[23]



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