Experts say that compared to the sun’s equator, our Sun is tilted by about 6 degrees. Since the 1970s, astronomers have not been able to figure out why this is happening. Now, experts think they may have found a solution: Planet Nine. Elizabeth Bailey works at the California Institute of Technology in Pasadena as an astrophysicist and planetary scientist.

She says that planet nine tilted our sun by six degrees and that the missing planet may have also tilted the other planets over the course of the solar system’s history. Planet Nine is a celestial body that hasn’t been found yet but is thought to be at the very edge of our solar system. Konstantin Batygin and Mike Brown predicted its existence in January 2016.

Experts say that all of the planets in our solar system move around the sun in a flat plane that is within a few degrees of each other. But experts say that plane rotates at a six-degree angle to the sun, which makes it look like the sun isn’t in the right place.

No one could figure out how something like this could happen for years.

What could make the sun, which is the biggest object in our solar system, tilt by six degrees? In fact, the sun is the only thing in our solar system that isn’t facing the same way as everything else.

Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy, says, “It’s such a deep-rooted mystery and so hard to explain that people just don’t talk about it.”

Bailey and his team of researchers ran computer simulations to figure out what was going on. They found that the tilt of the eight planets could be explained by Planet nine’s gravitational pull. In an interview with Space.com, Bailey said that this is just one theory and that there may be other reasons for the strange tilt our solar system has taken over its roughly 4.5 billion-year history.

One possibility is that there was an imbalance in the mass of the core of the young sun. According to Konstantin Batygin and Mike Brown, our sun is probably circled by a massive, unseen planet that is about ten times the size of Earth and has an orbit that is about 20 times farther from the sun than Neptune’s.

Batygin says, “It keeps surprising us. Every time we look closely, Planet Nine explains something about the solar system that had been a mystery for a long time.”

Astronomers haven’t been able to figure out why our sun and the other planets in our solar system tilt in such a strange way for a long time. Experts think that planet nine’s angular momentum—in physics, angular momentum is the rotational equivalent of linear momentum—is having a huge effect on the solar system because of where it is and how big it is.

Even though this is one reason why our solar system tilts in a strange way, scientists still need to find the rogue planet.