An illustration Imagines what a hidden ninth planet in the outer solar system might look like. Photo by NASA

July 13 (UPI) -- Astronomers have amassed more evidence of a ninth planet hiding in the outer solar system.

Last month, researchers published a study suggesting the unique tilt of the orbital planes of several rings of rocks in the Kuiper Belt could be explained by the presence of a ninth planet.


However, subsequent research showed the skewed distribution of the orbital paths of trans-Neptunian objects, or TNOs, could also be explained by observation bias -- no ninth planet necessary.

Scientists at the Complutense University of Madrid in Spain decided to analyze the orbits of a group of Kuiper Belt objects less vulnerable to observation bias. Extreme trans-Neptunian objects, or ETNOs, are Kuiper Belt objects that don't cross the path of Neptune's orbit. They're positioned more than 150 astronomical units from Earth.

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Astronomers analyzed the nodes of dozens of ETNOs, the two points where the objects' orbit intersects the solar system's orbital plane. ETNOs are most likely to be influenced by other objects near these two points.

"If there is nothing to perturb them, the nodes of these extreme trans-Neptunian objects should be uniformly distributed, as there is nothing for them to avoid, but if there are one or more perturbers, two situations may arise," astronomer Carlos de la Fuente Marcos said in a news release. "One possibility is that the ETNOs are stable, and in this case, they would tend to have their nodes away from the path of possible perturbers. But if they are unstable, they would behave as the comets that interact with Jupiter do, tending to have one of the nodes close to the orbit of the hypothetical perturber."

In analyzing the objects' nodes, researchers found a correlation between the nodal positions and the inclination, or orientation, of the objects' orbital paths. Scientists say the presence of a ninth planet explains the unexpected correlation.

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"Assuming that the ETNOs are dynamically similar to the comets that interact with Jupiter, we interpret these results as signs of the presence of a planet that is actively interacting with them in a range of distances from 300 to 400 AU," said de la Fuente Marcos. "We believe that what we are seeing here cannot be attributed to the presence of observational bias."

The new findings, detailed this week in the Monthly Notices of the Royal Astronomical Society, lend credence to the previous hypotheses of researchers at Caltech and the University of Arizona.