This figure represents the surveys' sensitivity for detecting ETNOs, with brighter white spots corresponding to more sensitive areas. The radial quantity is an object's distance from the sun, and the azimuthal quantity roughly corresponds to direction. The ETNOs actually detected by the three surveys, shown in red, are perfectly consistent with the surveys' sensitivity.
Michigan Physics graduate student Kevin Napier is lead author on a paper, recently accepted by the Planetary Science Journal and posted on arXiv.org, which has gained international attention as it casts doubt on the existence of an unseen giant planet in the outer solar system dubbed ‘Planet 9’. This planet was proposed in 2016 by Caltech scientists Konstantin Batygin and Mike Brown to explain the apparent orbital clustering of a population of small icy bodies beyond Neptune called the Extreme Trans-Neptunian Objects (ETNOs), which take thousands of years to complete one orbit around the sun.
Most of the known ETNOs follow long elliptical orbits that are roughly co-planar and point in the same direction. Given current models of our solar system, it is rather unlikely at any given time for the population of ETNOs to be as clustered as our observations show. It is thus enticing to invoke a mechanism to cause persistent clustering, which Planet Nine seems to do beautifully. However, this is not the whole story.
It is not a foregone conclusion that the ETNOs are even clustered in the first place! Because these objects are on such long skinny orbits, they are only observable for a (relatively) brief period when they make their closest approach to the sun. This makes the discovery of ETNOs highly susceptible to an effect called selection bias, which takes into account factors like where you pointed your telescope, when you took the observation, and how faint of an object the telescope was able to detect. The biases are particularly strong in the parameter that describes where they make their closest approach to the sun. In simplest terms, this parameter basically describes the direction of the orbit (one of the parameters that is supposedly clustered!). Before this work, the biases in a large sample of the ETNOs had not been sufficiently accounted for. We set out to determine whether we could say with high confidence that the ETNOs are clustered by robustly accounting for the selection biases.
We performed a meta-analysis on all of the ETNOs discovered by surveys with calculable biases. We found that when one properly accounts for these observational biases, the population of ETNOs we observe is fully consistent with a uniform (not clustered) underlying distribution. While we cannot rule out the existence of Planet Nine, we also cannot rule out the possibility that there is a uniform underlying distribution of ETNOs awaiting discovery. So, while Planet Nine may be out there, it is not required to explain our observations of the ETNOs because the simpler model (no Planet Nine) explains the ETNOs just as well. When you hear hoofbeats, you should think horses; not zebras.
Below is the link to the original paper and a listing of press generated from this paper.
Link to original paper: https://arxiv.org/abs/2102.05601