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.

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.

Instead, they found that the assumptions about Planet 9 that they were working from explained the tilt of the Sun. "The amazing thing is, for these very standard parameters that we like to talk about for Planet 9, it tilts [the Sun] nearly exactly 6 degrees," said Brown. The 6 degrees is important—but the calculations reveal something even more significant: it tilts the Sun in the right direction. Brown described this as "kind of amazing."

But here's the somewhat confusing thing: Planet 9, if it exists, hasn't actually tilted the Sun—it's tilted the plane of the entire solar system. From our vantage point on Earth, the solar system seems straight up and down, and it's the Sun that appears tilted. But the reverse is actually true.

This insight also tells the astronomers that they are on the right track to find the planet. "Our estimates of the orbit have to be essentially correct or we'd be getting the wrong value for where the Sun is and where Planet 9 is," Brown noted. Armed with this knowledge, and using other simulations they're working on, Brown's team has considerably narrowed the already small search area. "We have about 400 square degrees of sky," said Brown. "I think that by the end of next winter—not this winter, but next winter—I think there will be enough people looking for [Planet 9] that somebody is going to actually track it down."

From a technology standpoint, it shouldn't even be that hard to find. They know which swath of sky to focus on, and they know how bright the new planet should be. If it is 1000 AU—that is, 1000 times the distance of the Earth to the Sun—an object four times the size of Earth—or roughly Neptune sized—would at that distance be what astronomers call "25th magnitude." That's faint, but it's also kind of perfect in that an object of the 25th magnitude is the limit of what astronomers can conveniently see with the biggest telescopes on Earth. That means spotting the new planet is well within reach.

"At this stage," said Brown, "we have so many different lines of evidence that there's a massive planet out there that if there's not a massive planet out there, then it has to be that there was one there yesterday that has disappeared. It is really very hard for me to think of how the solar system could be doing all the things it's doing out there without there being a massive planet."

The implications for the discovery of a massive, Neptune-sized planet would be an astronomy game-changer. Renu Malhotra, of the University of Arizona, tells mental_floss that new kinds of celestial mechanics can be simulated by such a planet—things like certain resonant dynamics (the recurring gravitational influence of orbiting bodies on each other) that have not been previously studied.

In the hunt for Planet 9, Malhotra is studying the current observations of the distant minor planets to find anomalies that would point to an unseen planet. "Actual discovery of such a planet would stimulate a whole new view—indeed a major revision—of current theory of the formation and historical development of the solar system," she says, cautioning, "until the putative planet is observed and its properties are determined, it is rather impossible to predict where our theories will go."