Planet Nine

Planet Nine is a proposed, undiscovered planet far beyond Neptune. Its hypothesized gravity could explain the clustered orbits of some distant trans-Neptunian objects.

Orbital resonance: A relationship between orbital periods in which orbiting bodies exert repeated, coordinated gravitational influences on one another. Resonant interactions are among the mechanisms proposed to shape distant objects’ orbits.
Trans-Neptunian object: A small Solar System body that orbits the Sun beyond Neptune. Planet Nine’s proposed evidence comes from the orbits of selected distant members of this population.
Konstantin Batygin: A planetary scientist whose research includes the dynamics and formation of planetary systems. Batygin co-developed the influential 2016 Planet Nine hypothesis.
Observational selection bias: A distortion in observed data caused by the methods or conditions under which observations are collected. Uneven sky coverage can create apparent orbital clustering without a planet.
Subaru Telescope: An 8.2-meter optical-infrared telescope atop Mauna Kea in Hawaii. Its wide-field observations are used to search the predicted distant sky for Planet Nine.
Kozai–Lidov mechanism: A gravitational effect that exchanges orbital inclination and eccentricity in hierarchical systems of orbiting bodies. A distant perturber could drive long-term changes in trans-Neptunian objects’ eccentricities and inclinations.
Extreme trans-Neptunian object: A trans-Neptunian object with a very distant orbit, typically marked by a large semimajor axis and perihelion. The initial Planet Nine argument focused on the clustered orbits of extreme examples.
Michael E. Brown: An astronomer known for discoveries of distant Solar System bodies and research on planetary populations. Brown co-developed the 2016 hypothesis and helped identify the orbital pattern it sought to explain.
Kuiper belt: A region beyond Neptune containing many small icy bodies, including dwarf planets and comets’ source populations. The known Kuiper belt is distinct from the much more distant region where Planet Nine is proposed to orbit.
Dark Energy Survey: An astronomical survey that mapped a large portion of the southern sky in optical and near-infrared light. Survey data can reveal distant Solar System objects and test how strongly discovery bias shaped the orbital sample.

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