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We're Closing in on Discovering Planet 9 From Outer Space

Astronomers have narrowed down the area in the sky where a hypothetical, Neptune-sized Planet 9 might be found, and in doing so may have solved a mystery about the Sun that has vexed scientists since the 19th century. Their latest findings were announced this week in Pasadena, California, at the annual meeting of the Division for Planetary Science of the American Astronomical Society.

“Right now the search for Planet 9 is as much about understanding the effects of Planet 9 on the solar system—understanding the physics of Planet 9 to help us understand where it is—as it is going to the telescopes and staring at the sky,” Mike Brown, a professor of planetary astronomy at the California Institute of Technology, said at the conference.

The planets of the solar system orbit in a single plane, with a variation of about one degree. And yet inexplicably, the Sun is tilted by 6 degrees compared to the planets. If, in the beginning of the solar system, the planets and the Sun formed in a swirling disc of gas, there’s no reason to think that would be the case. “This is such a central mystery in the solar system that nobody even talks about it anymore,” said Brown.

It turns out that if there is some massive object in the outer solar system on a highly inclined orbit, it could act as a kind of huge celestial lever on the entire solar system, slowly tilting the orbital plane of the planets in its direction.

Enter Planet 9.

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Brown and theoretical astrophysicist Konstantin Batygin first revealed evidence for the existence of a hypothetical new planet earlier this year and have worked since then to find it. When Brown—a.k.a. @plutokiller—suggests a possible new celestial body, people take notice. He’s discovered many trans-Neptunian objects, mostly famously Eris, a dwarf planet larger than Pluto; its discovery caused Pluto’s reclassification (thus Brown’s Twitter handle).

Based on simulations by his team using an understood set of standard parameters for the new world, if Planet 9 precessed (i.e., behaved like a spinning top) toward us over billions of years, it would cause the “north pole” of the solar system to peel away from the north pole of the Sun. In other words, according to this idea, the solar system itself would try to follow Planet 9 in its enormous orbit, which may take tens of thousands of years.

When the team starting performing the calculations, they had no idea what they would find. The results for their favorite Planet 9 configuration might have been that it tilted the Sun by 20 degrees, which would have told them that they were entirely wrong. Or the results might have indicated that Planet 9 tilted the Sun by 0 degrees, leaving the mystery of the Sun’s tilt unanswered.

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