Sednoid

• 1. Introduction

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  Sedna sits roughly 84 AU from the Sun today, drifting through a darkness so profound that it will not reach its closest approach to the Sun for several centuries. It is a dwarf planet with an orbit so stretched, so improbable, that astronomers coined an entirely new category to describe objects like it: the sednoids. These are trans-Neptunian objects defined by a distant perihelion, a large semi-major axis, and eccentricities greater than roughly 0.7. Only four objects are generally agreed to belong to this population. Four worlds at the absolute edge of our solar neighborhood, and none of them should be where they are. How did they get there? And what does their presence at the far reaches of the solar system tell us about the Sun's earliest years?

• 2. Orbits That Should Not Exist

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  If you formed a planet where Sedna now travels, in the cold far beyond Neptune's reach, the physics would not cooperate. Accretion, the gradual coalescence of smaller bodies into larger ones, requires that nearby objects move at similar velocities. At Sedna's distance, the relative velocities between planetesimals would be so large that collisions would shatter rather than build. For Sedna and its kin to have grown there, their orbits must originally have been nearly circular. The elliptical paths they follow today came later, shaped by forces that are still debated.

  Neither the gravitational tugging of the giant planets nor the slow sweep of galactic tides can account for these orbits. Four competing hypotheses remain on the table. A nearby star may have passed through the Sun's birth cluster and "lifted" the perihelia of these objects away from the planetary disk. These worlds may have been captured directly from around passing stars, most likely during the Sun's earliest cluster years. A planet-sized body beyond the Kuiper belt, the hypothetical Planet Nine, could be sculpting their paths. Or a rogue planet, temporarily present in the early solar system and long since ejected, may have disturbed them before departing.

• 3. The Four Known Members

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  Sedna was the first sednoid discovered, in 2003. Its perihelion sits at 76.29 AU, its semi-major axis at 506 AU, and its orbital period stretches to roughly 11,400 years. Its diameter is estimated at about 906 kilometers.

  The second confirmed sednoid carries the informal nickname Biden. Its perihelion is 44 AU with a semi-major axis of 440 AU, and it was discovered in 2012.

  Leleākūhonua, catalogued as 541132, was announced on the 1st of October 2018 under a different provisional designation. Its perihelion of 65 AU and semi-major axis of roughly 1,094 AU make its aphelion exceed 2,300 AU, pushing it further from the Sun at its most distant point than Sedna ever reaches. Its orbital period runs to about 41,200 years and its estimated diameter falls between 220 and 380 kilometers.

  The fourth member carries the informal name Ammonite and was announced in 2023 with a perihelion of 54.4 AU and a semi-major axis of roughly 832 AU. All four objects have perihelia greater than 60 AU by the consensus standard, placing them clearly beyond any meaningful gravitational reach of Neptune. Each also qualifies as a detached object, a class defined by perihelion distances large enough that Neptune's gravity does not strongly govern their motion.

• 4. A Signal in the Angles

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  The first three known sednoids share something that has no obvious explanation. Their arguments of perihelion, the angle describing the orientation of their orbits, all cluster near zero degrees. More broadly, 27 known objects with semi-major axes greater than 150 AU, perihelia beyond Neptune, and observation arcs longer than one year share an argument of perihelion near 340 degrees. Interaction with the giant planets should have randomized these angles over time, through precession periods ranging from 40 million years to 1.5 billion years for Sedna itself. The fact that they remain clustered is not an artifact of how astronomers look for these objects; observational bias cannot account for it.

  One explanation holds that a super-Earth at roughly 250 AU could cause sednoids to librate around this orientation for billions of years. Such a body would have an apparent magnitude below the detection limits of current all-sky surveys, particularly if it has a low albedo. Larger, more distant perturbers would be even fainter and equally undetectable. This hypothetical super-Earth has been given the name Planet Nine. A targeted shift-stacking search by Malena Rice and Gregory Laughlin, applied to data from TESS sectors 18 and 19, produced 17 new outer solar system body candidates at distances between 80 and 200 AU, though early follow-up with the William Herschel Telescope failed to confirm two of them.

• 5. Fossil Record of Formation

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  Mike Brown, speaking in 2006, described Sedna as a fossil record of the earliest solar system. He said that eventually, when other fossil records are found, Sedna will help tell us how the Sun formed and how many stars were close to the Sun when it formed. That framing drives the scientific urgency around finding more sednoids. A survey Brown conducted with Rabinowitz and Schwamb between 2007 and 2008 was sensitive enough to detect objects out to 1,000 AU. It found Gonggong, now regarded as a likely dwarf planet, but turned up no new sednoids. Simulations built from that survey's data suggest about 40 Sedna-sized objects probably exist in the same region, with the brightest expected to reach roughly the magnitude of Eris.

  Each proposed formation mechanism would leave a different fingerprint on the wider population. A trans-Neptunian planet sculpting these orbits would produce perihelia clustering near 80 AU. Capture from a star rotating in the same direction as the solar system would yield low-inclination orbits with semi-major axes between 100 and 500 AU. A counter-rotating capture scenario would split the population into two groups by inclination. Stellar flybys would scatter perihelia and inclinations widely, in proportion to the number and geometry of those encounters. Distinguishing between these scenarios requires a much larger sample than four objects.

• 6. An Inner Oort Cloud Population

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  Some astronomers place sednoids within a broader structure called the Inner Oort Cloud, also known as the Hills cloud. This region spans from about 1,000 to 10,000 AU from the Sun. Sednoids, at their extreme but still comparatively modest distances, may represent the innermost edge of a vast and largely unseen reservoir of small bodies.

  Following the discovery of Leleākūhonua, Scott Sheppard and colleagues concluded that its existence implies a population of roughly 2 million Inner Oort Cloud objects larger than 40 kilometers. Their combined mass estimate falls around the mass of Pluto and amounts to several times the mass of the asteroid belt. That projection rests on just four confirmed members and carries considerable uncertainty. The spectral slope of Biden differs sharply from that of Sedna, which suggests the sednoids may have formed through more than one process, even if they now occupy the same orbital class. What appears to be a unified population may prove, with more data, to be a mix of survivors from different chapters of the solar system's earliest history.