LECTURE 9: THE TRANS-NEPTUNIAN, KUPIER BELT AND OORT CLOUD REGIONS
Characteristically, big (bright) Trans Neptunian Objects are typically on inclined orbits, while the invariable plane re-groups mostly small and dim objects. With the exception of Sedna, all big TNOs: Eris, 2005 FY9, 2003 EL61, Charon, and Orcus display neutral colour (infrared index V-I < 0.2), while the relatively dimmer bodies (50000 Quaoar, Ixion, 2002 AW197, and Varuna), as well as the population as the whole, are reddish (V-I in 0.3 to 0.6 range). This distinction leads to suggestion that the surface of the largest bodies is covered with ices, hiding the redder, darker areas underneath.[6]
The diagram illustrates the relative sizes, albedos and colours of the biggest TNOs. Also shown, are the known satellites and the exceptional shape of 2003 EL61 resulting from its rapid rotation. The arc around 2005 FY9 represents uncertainty given its unknown albedo. The size of Eris follows Michael Brown’s measure (2400 km) based on HST point spread model.[11] The arc around it represents the thermal measure (3000 km) by Bertoldi (see the related section of the article for the references).
Eris (pronounced /ˈɪərɪs/, also /ˈɛrɪs/ as in Greek Έρις[7]), formally designated 136199 Eris and formerly 2003 UB313 (see minor planet names), is the largest known dwarf planet in the Solar System and the ninth largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto.[8][5]
Eris was first spotted in 2003 by a Palomar Observatory-based team led by Mike Brown but not identified until 2005. It is a trans-Neptunian object (TNO) native to a region of space beyond the Kuiper belt known as the scattered disc. Eris has one moon, Dysnomia; recent observations have found no evidence of further satellites. The current distance from the Sun is 96.7 AU, roughly three times that of Pluto. With the exception of some comets, the pair are the most distant known bodies in the Solar System.[9]
Because Eris is larger than Pluto, its discoverers and NASA called it the Solar system’s tenth planet. This, along with the prospect of other similarly sized objects being discovered in the future, motivated the International Astronomical Union (IAU) to define the term "planet" for the first time. Under a new definition approved on August 24, 2006, Eris was designated a "dwarf planet" along with Pluto and Ceres.[10]
Currently lying at 97 AU away, Eris is the farthest known object in the solar system, and the third brightest of the TNOs. Classified as a scattered disk object (SDO), Eris follows an orbit at 10 billion kilometres from the Sun, completing it in 560 years at an unusual 45-degree angle.
