Seeking the Ninth Planet

Since January this year everyone in the astrophysics community has been raving about the discovery of a potential ninth planet in our Solar System.

But if we’ve never actually seen this planet, how can we be so sure about the existence of such an object?  And not just any object… A huge one (about the size of Jupiter).

For years we’ve been observing an extensive region of space beyond Neptune in our solar system, filled with comets and other icy bodies, known as the Kuiper Belt.  Analyses have recently confirmed an irregular occurrence in the orbits of six Kuiper Belt Objects (KBOs).

Kuiper belt image courtesy
Kuiper belt image courtesy


“Recent analyses have shown that distant orbits within the scattered disk population of the Kuiper Belt exhibit an unexpected clustering in their respective arguments of perihelion.” (Batygin and Brown, 2016)

Perihelion refers to the closest point an object comes to the Sun within its orbit. So basically what Konstantin Batygin and Michael E. Brown, planetary scientists at the California Institute of Technology, are saying is that these KBOs are coming to their closest point to our Sun at about the same time, and this is not only extremely unusual, but extremely not likely due to chance.

But who’s to say that this occurrence isn’t just due to chance?

Batygin and Brown calculated elliptical trajectories for the six KBOs and found that not only do they “cross the ecliptic at a similar phase of their elliptical trajectories,” meaning they pass in front of each other and seem to cluster near perihelion, but “the orbits are physically aligned” (Batygin and Brown).

       This means that the orbits of the KBOs are at very different angles than the orbits of other KBOs, and are coincidently at extremely similar angles to one another.

Batygin and Brown used statistical methods to prove that it is near impossible for these conditions to occur simply because they can.

To do this they tested the statistical significance of the observed clustering. They randomly selected six objects from a sample of all objects in our Solar System that have orbits beyond the orbit of Neptune and semi-major axes of at least 50 astronomical units (AU, 1 AU = the distance between earth and the sun).

        They then calculated the “root mean square (RMS) of the angular distance between the perihelion position of each object and the average perihelion position of the selected bodies” (Batygin and Brown).

      Taking the RMS of data is basically a way to take an average of data that consist of both positive and negative numbers.

      So in simpler terms, what Batygin and Brown did was average the distances between perihelion positions of the KBOs that they have been observing and average the distances between the positions of perihelion of the randomly selected KBOs from the sample. They then repeated this process 100,000 times.

Batygin and Brown found that only 0.7% of the time were the sample objects getting as close to one another at their perihelion positions as the six observed KBOs.

The next step was to calculate the “RMS spread of the polar angles” (Batygin and Brown). More simply, this means that they took the average of the angles of the orbits of the randomly selected KBOs. The six KBOs being observed have orbits that are angled about 30 degrees downward in comparison to the average plane of the orbits of our eight well-known planets (Fesenmaier, 2016).

They found that a cluster as tight as the observed one in that of the six KBOs of interest only occurred 1% of the time. 

Now because these two measurements statistically don’t affect one another, Batygin and Brown simply multiplied them and found that the two probabilities together, the observation of both “the clustering in perihelion position and in pole orientation simultaneously,” result in a probability of only 0.007% (Batygin and Brown).

Even with a very small sample of only six, Batygin and Brown were able to determine a significance level of about 3.8 standard deviations. In the scientific community there is a 3-standard deviation threshold, meaning that any significance test resulting in a significance level greater than 3 standard deviations will be taken seriously.

      This makes the notion that the observed data cannot be due to chance extremely conceivable.

This fact drove the two to determine that there must be a massive object outside of the KBOs that is gravitationally “shepherding” the objects into the tightly aligned orbits that they follow. They calculated that the object must be about 10 times the size of Earth, similar to that of Jupiter.

Although their calculations prove that the extraordinary observations cannot be due to chance and that some massive object must be gravitationally influencing the orbits of the KBOs, we still have yet to observe this ninth planet.

Batygin and Brown’s next step is going to be to actually find this giant, icy planet. Although it would be ideal for the planet to be near perihelion (calculated to be about 200 AU) right now, that’s not the case. Batygin and Brown estimate that it is probably going to be closer to aphelion, the planet’s farthest position from the Sun. This means that it is probably somewhere between 600-1200 AU from the Sun.

Fortunately, Batygin and Brown are using the Subaru telescope, an 8-meter telescope owned by Japan and located in Hawaii, to search for this potential ninth planet.

Suburu telescope in Japan
Suburu telescope in Japan

However, they say that even teaming up with other planet-nine hunters and observatories in different locations, it will take about five years to search the regions of the sky where the planet could be hanging out (Hand, 2016).



Story and research by Marley Holder, student in Journalism and Astronomy

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