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A sednoid is a trans-Neptunian object with a perihelion greater than and a semi-major axis greater than . Only two objects are known from this population, 90377 Sedna and , both of which have perihelia greater than ,〔(【引用サイトリンク】publisher=JPL Solar System Dynamics )〕 but it is suspected that there are many more. These objects lie outside an apparently nearly empty gap in the Solar System starting at about 50 AU, and have no significant interaction with the planets. They are usually grouped with the detached objects. Some astronomers, such as Scott Sheppard, consider the sednoids to be inner Oort cloud objects (OCOs), though the inner Oort cloud, or Hills cloud, was originally predicted to lie beyond 2,000 AU, several times as far as the aphelia of the two known sednoids. ==Unexplained orbits== The sednoids' orbits cannot be explained by perturbations from the giant planets, nor by interaction with the galactic tides.〔 If they formed in their current locations, their orbits must originally have been circular; otherwise accretion (the coalescence of smaller bodies into larger ones) would not have been possible because the large relative velocities between planetesimals would have been too disruptive. Their present elliptical orbits can be explained by several hypotheses: # These objects could have had their orbits and perihelion distances "lifted" by the passage of a nearby star when the Sun was still embedded in its birth star cluster. # Their orbits could have been disrupted by an as-yet-unknown planet-sized body beyond the Kuiper belt. # They could have been captured from around passing stars, most likely in the Sun's birth cluster.〔〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「sednoid」の詳細全文を読む スポンサード リンク
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