A Mission to Uranus Could Help Find Planet 9

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Based on orbits of know objects in the outer solar system, it is possible there is a very large "planet 9" out there.

A Mission to Uranus Could Help Find Planet 9

Uranus has a sideways orbit, unique weather patterns, and unusual rings, and its moons might have subsurface oceans. A mission to the ice giant has a great deal of scientific potential, and now there’s another compelling reason to visit: Data gathered on the way to the distant world could aid the search for an elusive ninth planet suspected to orbit beyond Neptune.


With current technologies, positional data gathered during the mission’s Jupiter-to-Uranus cruise stage could shrink the search window a thousandfold and make hunting for the planet with high-powered telescopes much more feasible, according to a team of doctoral students at the University of Zürich in Switzerland. Their results demonstrate that NASA’s proposed flagship mission to Uranus could yield scientific discoveries beyond the Uranian system.


Narrowing the Search Grid

In 2016, two astronomers noticed that the orbits of several small icy objects in the outer reaches of the solar system track too well with each other to be random. Computer modeling and subsequent observations suggested that an unseen body far beyond Neptune’s orbit might be gravitationally shepherding those objects into alignment. The astronomers dubbed the hypothesized body Planet 9 and have been trying to pinpoint its location ever since. (Not all astronomers are convinced.)



Zooming out from Neptune’s orbit, the orbits of six small icy bodies are traced out in pink. The inclinations of these orbits relative to the plane of the solar system are too similar to be random and hint at the presence of an as yet undiscovered ninth planet, whose possible orbit is traced in orange. If Planet 9 exists and has the predicted orbit, it would shepherd several other objects in the outer solar system into perpendicular orbits, which are traced in cyan. Credit: Caltech/R. Hurt (IPAC)
The planet, if it’s out there, would be very faint. However, the high-powered telescopes that could find it have narrow fields of view, suited for pinpoint targeting rather than sweeping searches. Astronomers would need to know exactly where to look, explained coauthor Jozef Bucko, and as of now the search grid covers too large a swath of sky to earn highly coveted telescope time. “To persuade the observational astronomers to focus a telescope and try to search for it—this is very expensive, and we need to have strong arguments.”


That’s where the proposed Uranus Orbiter and Probe mission comes in. During its travels to the outer solar system, it would occasionally ping a receiving station on Earth to let technicians know where it is, how fast it’s going, and the status of onboard systems. This is standard procedure. Mission teams use these kinds of ranging data to keep a spacecraft on course; anything with enough gravitational influence—such as planets, asteroids, and comets—could nudge it off its path.


“If there is a gravitational anomaly in the solar system, in this case, Planet 9, the trajectory of the spacecraft would be affected,” said coauthor Deniz Soyuer. The planet’s gravity would subtly tug the spacecraft toward it, registering as a small change in speed or direction during the craft’s 10- to 15-year cruise toward Uranus. Given the theorized mass and distance of Planet 9—6.3 Earth masses and 460 times the Earth–Sun distance—the planet “will definitely have a nonnegligible effect on the trajectory of a spacecraft,” they said.