[2][5][6] Nonetheless, some astronomers question this conclusion and instead assert that the clustering of the ETNOs' orbits is due to observational biases, resulting from the difficulty of discovering and tracking these objects during much of the year.

[7] Based on earlier considerations, this hypothetical super-Earth-sized planet would have had a predicted mass of five to ten times that of the Earth, and an elongated orbit 400–800 AU.

[20][21] The announcement in March 2014 of the discovery of a second sednoid with a perihelion distance of 80 AU, 2012 VP113, in a similar orbit led to renewed speculation that an unknown super-Earth remained in the distant Solar System.

[27] Using a larger sample of 39 ETNOs, they estimated that the nearer planet had a semi-major axis in the range of 300–400 AU, a relatively low eccentricity, and an inclination of nearly 14°.

[40][41] Given a hypothesized ~10 Earth masses and using a theory of exoplanet sizes in the Kepler-454 system, Esther Linder and Christoph Mordasini assumed that Planet Nine's radius would be 3.66 times that of Earth's (23,300 km versus 6,378 km), and that its internal composition would be similar to Uranus and Neptune's: Planet Nine would likely have a hydrogen-helium atmosphere averaging 47° Kelvin, with a core composed of iron and a mantle filled with magnesium silicate and water ice.

In their initial article, Batygin and Brown proposed that Planet Nine formed closer to the Sun and was ejected into a distant eccentric orbit following a close encounter with Jupiter or Saturn during the nebular epoch.

[53] However, while the Sun remained in the open cluster where it formed, any extended disk would have been subject to gravitational disruption by passing stars and by mass loss due to photoevaporation.

[68] A 2017 article by Carlos and Raúl de la Fuente Marcos noted that distribution of the distances to the ascending nodes of the ETNOs, and those of centaurs and comets with large semi-major axes, may be bimodal.

[111] Numerical simulations of the Nice model show that the ejection of the fifth giant often leaves a gravitational signature in the form of altered orbits for the remaining planets and small bodies.

[112] The observed clustering of certain trans-Neptunian objects (TNOs) has been cited as indirect evidence of Planet Nine's gravitational influence, possibly originating from its early interactions with the outer Solar System.

OSSOS, a well-characterized survey of the outer Solar System with known biases, observed eight objects with semi-major axis a > 150 AU with orbits oriented in a wide range of directions.

After accounting for the observational biases of the survey, no evidence for the arguments of perihelion (ω) clustering identified by Trujillo and Sheppard was seen,[J] and the orientation of the orbits of the objects with the largest semi-major axis was statistically consistent with being random.

[119] A later analysis of the discovery biases of fourteen ETNOs used by Brown and Batygin determined the probability of the observed clustering of the longitudes of perihelion and the orbital pole locations to be 0.2% .

Their simulations also showed that the perihelia of the ETNOs rose and fell smoothly, leaving many with perihelion distances between 50 and 70 AU where none had been observed, and predicted that there would be many other unobserved objects.

The self-gravity of this disk would cause its spontaneous organization, increasing the inclinations of the objects and aligning the arguments of perihelion, forming it into a cone above or below the original plane.

[124] Mike Brown considers Planet Nine a more probable explanation, noting that current surveys have not revealed a large enough scattered-disk to produce an "inclination instability".

[130][131] Simulations show that the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) project should be able to supply strong evidence for or against the ZM belt.

[40] Antranik Sefilian and Jihad Touma propose that a massive disk of moderately eccentric TNOs is responsible for the clustering of the longitudes of perihelion of the ETNOs.

[2][5] After numerical simulations showed that the arguments of perihelion should circulate at varying rates, leaving them randomized after billions of years, they suggested that a massive planet in a circular orbit at a few hundred astronomical units was responsible for this clustering.

Their work is very similar to how Alexis Bouvard noticed Uranus' motion was peculiar and suggested that it was likely gravitational forces from an unknown 8th planet, which led to the discovery of Neptune.

[147] Edward Witten proposed a fleet of probes accelerated by radiation pressure that could discover a Planet Nine primordial black hole's location, however Thiem Hoang and Avi Loeb showed that any signal would be dominated by noise from the interstellar medium.

[150][151] In 2023, it was shown that a gravity theory known as modified Newtonian dynamics (MOND), which can explain galactic rotation without invoking dark matter, can provide an alternative explanation using secular approximations.

[161][162] Michael Medford and Danny Goldstein, graduate students at the University of California, Berkeley, are also examining archived data using a technique that combines images taken at different times.

[12][163] Malena Rice and Gregory Laughlin applied a targeted shift-stacking search algorithm to analyze data from TESS sectors 18 and 19 looking for Planet Nine and candidate outer Solar System objects.

[164] Their search generated no serious evidence for the presence of a distant planet, but it produced 17 new outer Solar System body candidates located at geocentric distances in the range 80–200 AU, that need follow-up observations with ground-based telescope resources for confirmation.

In late April and early May 2020, Scott Lawrence and Zeeve Rogoszinski proposed the latter method for finding it as multiple spacecraft would have advantages that land-based telescopes do not have.

[42] This radiation signature could be detected by Earth-based submillimeter telescopes, such as ALMA,[173] and a search could be conducted by cosmic microwave background experiments operating at mm wavelengths.

[162][188] William Folkner, a planetary scientist at the Jet Propulsion Laboratory (JPL), has stated that the Cassini spacecraft was not experiencing unexplained deviations in its orbit around Saturn.

[198] A 2025 study by Amir Siraj, Christopher F. Chyba, and Scott Tremaine using an expanded sample of 51 ETNOs to inform 300 simulations in the Rebound program, proposed new orbital characteristics for Planet Nine: that its semi-major axis is 290 ± 30 AU, its eccentricity is 0.29 ± 0.13, and its inclination is roughly 6°.

[207] Persephone, the wife of the deity Pluto, had been a popular name commonly used in science fiction for a planet beyond Neptune, most notably in the works of Arthur C. Clarke and Larry Niven.