Simulating an Extreme TNO on the Road to Planet Nine

Title: Discovery and Dynamical Analysis of an Extreme Trans-Neptunian Object with a High Orbital Inclination

Authors: J. C. Becker, T. Khain, S. J. Hamilton, et al. (DES Collaboration)

First Author Institutions: University of Michigan

Status: Published in the Astronomical Journal, open access on arXiv.

Three years ago, Mike Brown and Konstantin Batygin published their seminal paper predicting a massive, undiscovered Planet Nine. The publication was quickly picked up by the media and reinvigorated the study of objects beyond the orbit of Neptune, appropriately called trans-Neptunian objects (TNOs). Among the TNOs are Kuiper Belt objects, which have well-behaved orbits, and “extreme” TNOs (ETNOs), which have highly eccentric and inclined orbits.

There are many open questions about ETNOs. Were they born extreme or scattered into extreme orbits later in life? How long will their orbits be stable? How many ETNOs even exist? Does the existence of Planet Nine explain or complicate the population of ETNOs? Few researchers were asking or trying to answer these questions until recently, when the search for Planet Nine uncovered a batch of ETNOs that are more exciting than previously thought.

The search for Planet Nine continues

Many telescope surveys have tried without success to find Planet Nine in the three years since the theory was published, leading some Astronomers and groups to question the hypothesis. Searching for Planet Nine requires special imaging techniques to identify slowly moving, cold, small objects from a trove of astronomical images, which has had the side effect of producing more discoveries of other types of TNOs.

This is where the Dark Energy Survey (DES) comes in. DES’ five years of high-resolution images have been put through a special pipeline designed to pick out only TNOs through a method called difference imaging. This technique is combined with machine learning to find the faintest TNOs possible and then “connect the dots” of each detection into complete orbits.

Extreme…in a good way

This paper reports on DES’ discovery of 2015 BP519, the “most extreme” TNO yet, because it has a highly elliptical orbit with an eccentricity of 0.92 and is inclined a whopping 54 degrees out of the plane of the planets.

The high inclination of this ETNO is puzzling because the solar system formed from a disk, so something would have had to severely disturb the orbit of 2015 BP519. To test that hypothesis, the authors ran a simulation of 2015 BP519 forward and backward in time, showing how it would change orbit by interacting with the current Solar System objects.

Figure 1. Numerical simulations of 2015 BP519 forward and backward in time interacting with the gas giant planets. Only the gas giants are simulated because the terrestrial planets contribute negligibly to the overall angular momentum of the Solar System. Each red line represents one simulation. Inclination (i), which is the tilt of the orbit, and eccentricity (e), which is the measure of how elliptical the orbit is, do not vary much over billions of years. Figure 8 in the paper.
Figure 2. Simulations of 2015 BP519 as Figure 1, but this time including Planet Nine. Inclination and eccentricity vary far more wildly here, showing how the presence of Planet Nine might explain how 2015 BP519 became so extreme. Figure 12 in the paper.

Figure 1 shows many individual simulations (each red line) of how orbital inclination (i), eccentricity (e), semi-major axis (a), and perihelion (q) vary over billions of years. Looking backwards in time from today, the simulated inclination and eccentricity do not vary significantly. To the authors, this suggests whatever perturbed this ETNO is missing from the model.

The authors then added Planet Nine into their simulation, which is shown in Figure 2, and was only run forward in time. Inclination, eccentricity, semi-major axis and perihelion all smear out over time, which means that in some simulations, interactions with Planet Nine could bring 2015 BP519 back into the plane of the planets and into a more circular orbit. By the same logic, when 2015 BP519 was born in the plane of the Solar System, interactions with Planet Nine over billions of years could be one potential method to scatter it into an extreme orbit. The authors only ran the simulations for Figure 2 forward because the history of Planet Nine is entirely uncertain. It is also possible, given the wide range of simulation outcomes, that Planet Nine had nothing to do with making
2015 BP519 so extreme; these results are merely consistent with the Planet Nine hypothesis.

A natural fit

Another analysis of interest from this paper is provided in Figure 3, which plots all known TNOs, where bluer color indicates more “extremeness.” Semi-major axis is plotted the x- axis versus the orbital elements on the y-axis. These quantities describe the orientation of the orbit in physical space. Figure 3 is quite detailed, but the important point is the higher density of dots in the shaded regions, which was the original impetus for the Planet Nine theory. 2015 BP519, marked by a star, adds another data point to support this theory.

Figure 3. Visualization of orbital elements on the y-axis versus semi-major axis on the x-axis for all known TNOs. Bluer dots represent more extreme orbits. BP519 coincides with the clustering of TNOs in the shaded regions that
inspired the Planet Nine hypothesis. Figure 13 in the paper.

The bottom line is that these simulations and results are consistent with a massive ninth planet but that is a far cry from requiring the existence of Planet Nine. Considering the difficulties in simulating the formation of planetary systems, the authors conclude that finding more TNOs and ETNOs would aid considerably in determining the conditions under which our solar system (and other planetary systems) formed. Regardless, 2015 BP519 is another in a growing set of bizarre objects that occupy the outer reaches of our Solar System.

About Will Saunders

I am a Ph.D. candidate at Boston University, where I study planetary atmospheres. My dissertation research involves using new and archival stellar occultations to measure the upper atmospheres of Uranus and Neptune. My work aims to better understand how the atmospheres of the ice giants are heated to hundreds of degrees. I received my Bachelors in Physics from the University of Pennsylvania. Be sure to check out astro[sound]bites, the only podcast combining Astrobites posts with lighthearted discussion about the latest astronomy research. Find us on, Apple Podcasts, Google Play, SoundCloud, and Spotify. In my free time, I enjoy cycling, exploring New England, and trying new wines.

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