Representation of our solar system, in Astronomical Units (AU). Blue dots represent known KBO's as of January 2015.

Photo By: CFEPS Project and the Minor Planet Center

For those upset by the recent downgrading of our solar system from nine to eight planets, there is a new hope! Research by scientists at the California Institute of Technology (Caltech) suggests that there may, after all, be a ninth planet in our solar system.

As many of us remember, Pluto was eliminated as a planet about ten years ago. This change in Pluto’s planetary status occurred after the International Astronomical Union (IAU) released an official definition of a planet. Their Resolution B5 stated that the three necessary qualifications for a planet are:

  • It must revolve around the Sun.
  • It needs to have enough mass and gravitational force to maintain a spherical shape.
  • It needs to have enough mass to clear its orbital region of space debris.

Although Pluto fits the first two criteria, it does not fit the third.

But if a large mass is a necessary for an object to be considered as a planet, then how have we not observed the proposed ninth planet before?

Well, the proposed ninth planet is expected to lie in the Kuiper belt, a disc region beyond all the other planets, home to billions of comets and space bodies (including Pluto!) known as Kuiper Belt Objects (KBOs). Leading up to this paper, other astrophysicists have studied KBOs and found that their orbits cluster together in statistically significant way. In other words, this clustering is extremely unlikely to be due to chance and likely has an alternate explanation.

Konstantin Batygin and Michael E. Brown, the authors of this paper, first sought to clarify the previous research. They looked through already collected data and found that not only do the KBOs share perihelia, the points at which an object’s orbit is closest to the sun, but are also physically clustered in space. Through statistical modeling and analysis, they determined that these results are free from obvious sources of bias and that there is a 0.007% likelihood that they are due to chance. Furthermore, they found that the previously proposed models do not explain the physical clustering of KBOs. What does explain it, then?

The authors developed an analytical model based on the assumption that an external body, i.e. a planet, is responsible for the KBOs’ orbital behavior. The physics underlying their analysis is complicated but the takeaway is this: the presence of a massive planet is consistent with their model. This finding provides further evidence for the existence of the ninth planet, also referred to as Planet X or Planet 9, about the size of Neptune.

The CalTech researchers do caution their readers, however. While their analyses show that the existing data is consistent with the presence of Planet X, there is still more data to be collected. The model therefore should be consistently evaluated as more KBOs are observed and their orbits characterized. There also remain existing observations that have yet to be accounted for. The findings outlined in this article are promising but, at this point, more of a working model than a true discovery of a ninth planet.


It's hard to say what a model of the solar system would look like - visualizing this potential planet will require a couple more leaps in technology.

Photo By: Lsmpascal, Wikimedia Commons

Years from now, we may again be able to claim that nine planets exist in our solar system. Interestingly, however, the potential planet is very different from the other eight. It is in the outer solar system, about 20 times farther away from the sun than Neptune. Such a distance complicates the type of research that can be done on the planet. Along this line, we do not and may not have — for a while at least — an image of the planet. How will this impact classrooms and diagrams that depend on showcasing the planet to eager and curious students?

These findings remind us of how little we know and how much more there is to learn about the Universe — not just in galaxies far away but also in our closest point of reference: our solar system. What Planet X represents is exciting, not only for our conceptualization of the solar system but also as a reminder of the amazing and increasingly sophisticated work that astrophysicists are accomplishing.

For more information about Planet X, including a discussion by the lead authors of the paper mentioned, see NASA’s page.

About The Author

Maddy Russell

I'm very interested in science and health, especially their ethical aspects. Basically anything within this realm will make me excited, so there's not one specific research interest I have, but if I had to pick, I'd choose the topic of informed consent in the health world and cellular and tissue-level processes in the biology world. I'm proud to be writing for Innovation and glad to have the opportunity to communicate these subjects to people with different backgrounds.