Is Planet Nine Pulling Us Closer?

It’s not time to update the posters, rulers, books, felt sets, lollipops, and mnemonics just yet, but astronomer Michael Brown anticipates that it will be by the end of next winter. Planet Nine, a predicted gas giant orbiting the sun far beyond Neptune, explains so many mysteries of the solar system, he says, that’s it’s hard to believe it doesn’t exist. The latest of these is the curious case of the six-degree tilt of the sun.

Predicted earlier this year, a ninth planet about ten times as massive as the Earth and located far beyond Neptune could explain the strange behavior of some objects in the outer solar system. These objects are in the Kuiper Belt, a region of space home to many comets and minor planets (the designation Pluto now has). Most of these objects behave as expected, considering the gravitational pull of Jupiter and Neptune. However, astronomers have discovered a handful of outliers with strange, distant orbits that cluster in a way not likely to be random.

According to an analysis by Brown and Konstantin Batygin, both Caltech astronomers, Planet Nine would explain this clustering. Their research builds on 2014 work by Chadwick Trujillo and Scott Sheppard, and resulted in an article succinctly titled Evidence for a distant giant planet in the solar system that laid out the case for Planet Nine last January. Since then, they and other researchers have made significant progress in narrowing down its probable location and characteristics, work that is guiding the efforts of several search teams.

In a press briefing yesterday at the joint meeting of the American Astronomical Society’s Division for Planetary Sciences and the European Planetary Science Congress in Pasadena, CA, Brown outlined fascinating new work indicating that Planet Nine would explain the mysterious tilt of the sun relative to the orbits of the planets in the solar system. The work was led by Elizabeth Bailey, a graduate student at Caltech who works with Brown and Batygin.

Since the 1850s, scientists have wondered why the sun is slightly tilted with respect to the well-aligned orbits of the planets. Based on what we know about the formation of the solar system, this is unexpected and has remained unexplained. However, Bailey’s simulations show that if Planet Nine has the mass and tilted, highly elliptical orbit around the sun that other types of evidence suggest it does, Planet Nine would gradually pull the other planets toward its orbital plane over billions of years. The result? At this point in history, the planets would be tilted away from the sun by six-degrees in exactly the direction we see. Their findings were announced at this week’s meeting and will be published in an upcoming issue of the Astrophysical Journal.

Back in January, we featured Batygin and Brown’s prediction in a blog post and podcast. You can check them out for more research details and the story of our ever-changing number of planets. I’ll summarize that story with a line taken directly from the post. “Since the discovery of Neptune, our solar system has had eight planets, then maybe nine (Vulcan), then eight again, then nine (Pluto), then eight once more, and—soon perhaps—nine again.”

So, here we are, possibly on the eve of a monumental discovery whose physics significantly affects the solar system we thought we knew. Not everyone agrees that Planet Nine is likely to exist, but as the evidence grows a number of teams are looking for the faint signal that would offer confirmation—some through telescopes such as Hawaii’s Subaru, an optical telescope run by the National Astronomical Observatory of Japan. Others are using clever techniques to search sky survey archives or locate the planet in other ways.

Detecting the planet is not as straightforward as it sounds. Although its probable location has been narrowed down, we don’t know where along its estimated 15,000-year path around the sun Planet Nine is right now (if it’s out there), and the farther it is from us the dimmer it will be. Brown and others hope that understanding the influence of Planet Nine on the solar system will help us narrow down the search field even more.

The new planet may not replace the nostalgia many of us have for Pluto, but its discovery would be one for the ages. (Coincidentally, the same Michael Brown whose work predicts Planet Nine is also author of How I Killed Pluto and Why It Had It Coming). Eight-planet solar system memorabilia may soon have a place next to the lunchboxes and t-shirts featuring Pluto, on the shelves of collectors.

—Kendra Redmond