Is there an unseen planet — a “planet X” or “Planet Nine” — in the outer solar system? That possibility is supported by calculations in a new study that shows the hypothetical planet could have a mass 4.4 that of Earth and orbit almost 10 times farther out than the most distant planet, Neptune.
Accepted for publication in The Astrophysical Journal, the paper looks at the position of trans-Neptunian objects in the solar system that have an orbit beyond Neptune. The largest TNOs known are Pluto, Eris, by Haumea, Makemake, Gonggong, Quaoar, Sedna and Orcus, but this study is of TNOs much further out.
Beyond Neptune
It reveals a significant clustering in the orbital properties of TNOs about six times farther than Neptune. The research then uses several statistical tests to refine the predicted location and characteristics of the unseen planet.
“Using the largest sample of distant and stable TNOs to date across a battery of different statistical tests, we find significant clustering in a particular angle that describes the orientation of orbits,” said Amir Siraj, lead author and an astrophysicist at Princeton University, in an email.
Closer Orbit
The resulting model of Planet X’s characteristics differs significantly from previous studies, specifically that of Caltech’s Mike Brown and Konstantin Batygin, who in 2016 published evidence for a theoretical planet about 1.5 times the size of Earth in the outer solar system.
“Compared to the Batygin & Brown “Planet Nine,” our work finds that Planet X has a closer-in and more elliptical (more eccentric) orbit that is better aligned with the orbits of the planets in the solar system (lower inclination) — as well as having a lower mass,” said Siraj. “Overall, our work predicts different planet characteristics than Batygin & Brown, with only 0.06% agreement between our models.”
Imminent Search
Does Planet X exist? If the new paper is correct, it may be easier than previously thought to find it. “As a result of the different characteristics, our model for Planet X predicts a higher likelihood of the planet being discovered by the Vera C. Rubin Observatory,” said Siraj.
The $1.9 billion Vera C. Rubin Observatory’s revolutionary 3,200-megapixel camera, designed for the Legacy Survey of Space and Time, will map billions of celestial objects, uncover cosmic mysteries like dark energy and dark matter, and redefine time-domain astronomy with real-time imaging of the dynamic universe. Crucially, the paper also gives astronomers a “concentrated high-likelihood area on the sky,” according to Siraj.
Unseen Objects
The Vera C. Rubin Observatory on Cerro Pachón, Chile, is poised — from 2026 — to transform astronomy with a 3,200-megapixel resolution sensor — the world’s largest astronomical camera. Attached to the Simonyi Survey Telescope, it will undertake the Legacy Survey of Space and Time, a project to map 40 billion celestial objects, including galaxies, supernovas, potentially hazardous asteroids — and unseen planets.
Rubin’s wide field of view will enable it to produce alerts for transient events like supernovas or asteroids within 120 seconds, generating 20 terabytes of data each night, ultimately creating the largest astronomical movie ever. “It’s beautifully designed and has an excellent chance of spotting Planet X — if it exists — as it blinks across the night sky,” said Siraj.
Wishing you clear skies and wide eyes.
