Beyond the familiar orbits of the planets, in the cold and silent immensity, the solar system hides an innumerable population. Billions of relics, witnesses to planetary formation, compose a true "fossil archipelago" of which we are only beginning to discover the first objects.
In the icy darkness where the Sun is just a particularly bright star, enigmatic worlds evolve: the extreme trans-Neptunian objects (ETNO). Unlike classical Kuiper Belt objects, these icy bodies follow very particular orbits with a perihelion (closest point to the Sun) always very far from Neptune (> 40 AU) and a semi-major axis extremely long (> 150 AU, sometimes reaching over 1000 AU). Their trajectories are often very inclined and eccentric, radically distinguishing them from the planets and most small bodies of the solar system.
This unique configuration protects them from strong gravitational perturbations from the known giant planets. For astronomers, they are not just distant pebbles, but true relics of primordial chaos that reigned during the formation of the solar system. Their orbits are fossil archives, preserved by cold and distance, which could contain clues to ancient cataclysmic events, such as the passage of nearby stars or the migration of giant planets, or reveal the presence of an unidentified perturber.
The real enigma around ETNOs appeared at the beginning of the 21st century, when studies highlighted that their orbits were not randomly distributed. Against all expectations for objects supposed to be isolated, certain orbital parameters, such as the argument of perihelion, seemed to cluster. The probability that this arrangement is the result of chance is extremely low, on the order of a few tenths of a percent.
N.B.:
The argument of perihelion \((\omega)\) is an angle that describes the orientation of an object's orbit in its own plane. More precisely, it measures the position of the closest point to the Sun (the perihelion) relative to the point where the orbit crosses the reference plane of the solar system (the ascending node). If the orbits of a group of objects are randomly oriented, their arguments of perihelion should be uniformly distributed between \(0^\circ\) and \(360^\circ\). The fact that those of certain ETNOs cluster around a similar value is a strong statistical signal that an external force, such as the gravity of an undetected massive body, has "locked" them into this configuration over time.
It is this statistical anomaly that led astronomers Konstantin Batygin and Mike Brown (both born in 1975) to formulate in 2016 a bold hypothesis: the existence of an undetected giant planet, located on a very distant and eccentric orbit. This hypothetical Planet Nine, with an estimated mass between 5 and 10 times that of Earth, would be the invisible "puppeteer" whose gravity, over billions of years, would have sculpted and clustered the orbits of the ETNOs we observe.
It is not a "rogue" planet from outside, but was probably ejected to the outer reaches of the solar system during its turbulent youth, more than 4 billion years ago. The hunt for this gravitational ghost is now open. Telescopes like Subaru in Hawaii are methodically scanning the regions of the sky where it might be hiding, but the task is immense: its apparent motion is very slow and its brightness extremely faint.
If it exists, Planet Nine does not evolve on a circular orbit, but on a very elliptical trajectory, a signature of a chaotic past. Current models estimate that its semi-major axis (average distance) would be between 400 and 800 AU (Neptune → 30 AU). Concretely, its distance from the Sun would vary enormously: it could approach to about 200-300 AU at its perihelion, before moving away to more than 1200 AU at its aphelion.
For Planet Nine, models published by Konstantin Batygin and Mike Brown suggest an eccentricity between \( e \approx 0.2 \) and \( e \approx 0.6 \). The most commonly cited value in recent studies is around \( e \approx 0.5 \) (Neptune → 0.008).
Unlike the eight main planets whose orbits are close to the plane of the ecliptic (inclination < 3° for Neptune), Planet Nine would be on a highly inclined orbit, between 15° and 25°. This major difference is a "dynamic signature" that suggests it could not have formed in its current location. Astronomers believe it was catapulted onto this inclined orbit by gravitational interactions with the gas giants, more than 4 billion years ago. This extreme orbit, coupled with its presumed low brightness, explains the difficulty of its direct detection.
The discovery of emblematic objects like Sedna (discovered in 2003) paved the way for the revolutionary hypothesis of a massive ninth planet at the edge of the solar system. With a perihelion at 76 AU and an aphelion (farthest point) at about 900 AU, its orbit is completely decoupled from Neptune's influence. More recently, 2012 VP113 ("Biden") and especially 2015 TG387 ("The Goblin"), have confirmed the existence of this distinct population. Each new member discovered, like the very distant 2018 AG37 ("FarFarOut"), refines the model and constrains the possible parameters of Planet Nine.
N.B.:
For ETNOs, a perihelion greater than 40 Astronomical Units (AU) ensures that they never approach Neptune, thus avoiding any major gravitational perturbation that would unpredictably modify their orbit.
| Object name | Year of discovery | Semi-major axis (AU) | Perihelion (AU) | Comment |
|---|---|---|---|---|
| 90377 Sedna | 2003 | ~ 506 AU | 76 AU | Prototype object; orbit completely isolated from Neptune. |
| 2012 VP113 ("Biden") | 2012 | ~ 261 AU | 80 AU | Its orbit contributed to highlighting the clustering of the arguments of perihelion. |
| 2014 FE72 | 2014 | ~ 1550 AU | 36 AU | Immense orbit (aphelion > 3000 AU), potentially influenced by nearby stars. |
| 2015 TG387 ("The Goblin") | 2015 | ~ 1050 AU | 65 AU | Discovered near Halloween; its orbit strongly supports the Planet Nine hypothesis. |
| 2018 VG18 ("Farout") | 2018 | ~ 90 AU (est.) | ~ 42 AU | Former holder of the title of most distant object; characteristic pinkish color. |
| 2018 AG37 ("FarFarOut") | 2018 | ~ 101 AU (est.) | ~ 27 UA | Temporary holder of the title of the most distant object observed. |
| 2021 DR15 | 2021 | ~ 700 AU (est.) | ~ 39 AU | Recent discovery confirming the persistence of the ETNO population. |
Source: Data compiled from the archives of the Minor Planet Center (MPC) and publications from The Astrophysical Journal.
While the Planet Nine hypothesis is the most publicized, it is not the only one on the table. The scientific community is exploring other avenues to explain the enigmatic arrangement of ETNO orbits:
The pursuit of this gravitational ghost, whether planetary or the result of another phenomenon, is an observational and modeling challenge that mobilizes astronomers. Large sky surveys like that of the future Vera C. Rubin Observatory (planned for the mid-2020s) promise to revolutionize the field of ETNOs.
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