The Search for Planet X

With more than 1,600 planets detected around other stars, surely we know how many planets live closer to home in our own solar system, right? Not exactly! Let's sort through the tumultuous history, including a recent intriguing discovery, of the search for the mysterious, distant Planet X.

Sabrina Stierwalt, PhD
5-minute read


In 1999, three astronomers (Maltese, Whitman, and Whitmire of U of Louisiana) once again resurrected the debate over a Planet X by predicting that a so-far undetected Jupiter-sized gas giant planet was orbiting the Sun in the Oort Cloud. Their prediction was based on irregularities they observed in the orbits of comets that they decided could be explained the the gravitational pull of this mystery planet.

So as not to be confused with the deadlier Nemesis, the predicted planet was called Tyche, the Greek goddess of prosperity and the “good sister” of Nemesis. Although such a planet could conceivably faint enough to have gone undetected in optical searches, its emission in the infrared should be well above the sensitivity limits of the WISE mission, NASA’s latest infrared space probe. WISE has failed to detect such a planet, making its existence highly unlikely.

Planet Nine

Over the past few years, the debate over a Planet X has been revived once again, this time by scientists studying the orbits of KBOs. In 2003, planetary scientists Brown, Trujillo, and Rabinowitz found Sedna, a minor planet that is the most distant object in our solar system and thus named after the Inuit goddess of the sea, thought to live in the depths of the Arctic Ocean. Since then a few more objects with odd, elliptical and distant orbits have been discovered in the outer solar system, including VP113, announced in 2014, by planetary scientists Scott Shepphard and Chad Trujillo. Shepphard and Trujillo noted that at its closest approach to the Sun, VP113, along with Sedna and four other distant minor objects, all passed very close to the ecliptic, the term for the two-dimensional plane in which the other planets orbit.

Then, in January 2016, astronomers Konstantin Batygin and Mike Brown announced a finding that took the orbital calculations a step further: not only did all six objects pass by near the ecliptic, but they made this passage at the same spot. Statistics dictate that there should only be a 0.007% chance that such an unusual clustering would happen randomly, without something massive to hold them there. The math further suggests that the hypothetical planet, so far dubbed “Planet Nine” would be roughly ten times the mass of the Earth and would orbit the sun every 15,000 years. Batygin and Brown suggest the planet was knocked out of its original, closer orbit during the formation of our solar system.

Will this latest hypothesis finally turn up a mystery planet? Only time (and more research) will tell. If you ask Mike Brown however, he is quoted as saying, “Why is this different? This is different because this time we’re right.”

Until next time, this is Sabrina Stierwalt with Ask Science’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com


Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.