We examined the past history of the three most detached TransNeptunian
Objects (TNOs) -- Sedna, 2012 VP113, and Leleakuhonua (2015 TG387) -- the three
clearest members of the dynamical class known as sednoids, with high perihelia
distances q. By integrating backward their nominal (and a set of cloned)
orbits for the Solar System's age, we surprisingly find that the only time all
their apsidal lines tightly cluster was 4.5 Gyr ago, at perihelion longitude
Ï– of 200{\deg}. This "primordial alignment" is independent of the
observational biases that contribute to the current on-sky clustering in the
large-semimajor axis Kuiper Belt. If future sednoid discoveries confirm these
findings, this strongly argues for an initial event during the planet formation
epoch which imprinted this particular apsidal orientation on the early detached
TNO population and then subsequently modified only by the simple precession
from the 4 giant planets. If other sednoids also cluster around the same
primordial value, various models suggesting a still present planet in the outer
Solar System would be incompatible with this alignment. We inspected two
scenarios that could potentially explain the primordial alignment. First, a
rogue planet model (where another massive planet raises perihelia near its own
longitude until ejection) naturally produces this signature. Alternatively, a
close stellar passage early in Solar System history raises perihelia, but it is
poor at creating strong apsidal clustering. We show that all other known
35<q<55 au TNOs are either too perturbed or orbits are still too uncertain to
provide evidence for or against this paradigm.Comment: 9 pages, 4 figures, submitted to ApJ