A hypothetical mystery planet thought to be responsible for strange orbits in the outer Solar System just got dealt one of its biggest blows yet.
According to a comprehensive analysis of extremely distant objects, led by physicist Kevin Napier of the University of Michigan, Planet Nine may not exist - because the evidence for its existence doesn't exist. Rather, what astronomers took to be the influence of a planet's gravity is instead selection bias in the observations.
Planet Nine burst onto the scene in 2016, when astronomers Konstantin Batygin and Michael Brown of Caltech published a paper in The Astronomical Journal laying the case for an as-yet undiscovered planet in the outer reaches of the Solar System. The evidence, they said, lay in other objects from far beyond the orbit of Neptune.
These objects are called Extreme Trans-Neptunian Objects (ETNOs). They have huge elliptical orbits, never crossing closer to the Sun than Neptune's orbit at 30 astronomical units, and swinging out farther than 150 astronomical units.
Batygin and Brown found that these orbits have the same angle at perihelion, the point in their orbit closest approach to the Sun. They ran a series of simulations, and found that a large planet could cluster the orbits in this way.
Planet Nine, according to their calculations, should be around five to 10 times the mass of Earth, orbiting at a distance somewhere between 400 and 800 astronomical units.
Because this hypothetical planet would be so far away, and because the sky is so big, it would not be easy to find. So the search for it has been ongoing.
Just as Planet Nine itself would be hard to find, however, so too are ETNOs. These bodies are smaller than a planet, and therefore fainter. When they move away from the Sun, we have just about zero chance of seeing them. And this is where some astronomers believe that there is a selection bias.
"Because ETNOs follow highly elliptical orbits, and their brightness decreases like 1/r4, they are almost always discovered within a few decades of perihelion," the researchers wrote in their paper.
"Moreover, telescopic surveys observe a limited area of the sky, at particular times of year, to a limited depth. These effects result in significant selection bias."
The difficulty involved in seeing ETNOs means we haven't found many. The initial simulations run by Batygin and Brown were based on just six ETNOs, which were gathered from an assortment of surveys with unpublished selection functions; in other words, any selection biases were unclear.
More recent surveys have been meticulous about their selection functions. And, while no single survey has found enough ETNOs to constitute a comprehensive statistical population, combining surveys can lead scientists to a stronger conclusion. This is what Napier and his team did.
They took five objects from the Outer Solar System Origins Survey (OSSOS) (which previously found no evidence of clustering), five objects from the Dark Energy Survey, and four objects found by astronomers Scott Sheppard, Chad Trujillo, and David Tholen, who have been leading the search for Planet Nine.
Since all three surveys had quite different goals, they therefore had different selection functions. The challenge was to resolve these differences so that the objects could be effectively combined into one large survey. To do this, the team designed a survey simulator.
"In essence," they wrote in their paper, "a survey simulator simulates detections of a model population of Solar System bodies by using a survey's pointing history, depth, and tracking criteria. This allows for the computation of a survey's selection function for a given population, which enables us to account for bias, and therefore understand the true underlying populations."
If the ETNO clustering was caused by a physical effect, then it should have remained consistent with the larger sample of objects Napier's team analysed. Instead, their results suggested that the ETNO sample was consistent with a uniform distribution of parent bodies in space.
This doesn't mean, the researchers were careful to note, that there is no Planet Nine. It just means that the planet's existence cannot be inferred from ETNO data. There's not enough information to either confirm it or rule it out.
Other lines of evidence point to its existence, too. For example, the strangely tilted orbits of outer Solar System Kuiper Belt objects such as Sedna - although astronomers have also proposed other explanations for these behaviours.
A stronger ruling will be possible with a larger population of ETNOs and Kuiper Belt objects to analyse, which may mean waiting for objects from a more powerful telescope, such as the Vera Rubin Observatory that is expected to commence operations sometime this year.
In the meantime, the avid hunt for the elusive planet is resulting in some really amazing discoveries, including some possible dwarf planets slingshotting way out to the Solar System's outer limits, and a whole bunch of gas giant moons.
So whether Planet Nine exists or not, the debate itself is amazing for science – leading to discoveries we may not have otherwise come across.
The study has been published on pre-print website arXiv.org.
Editor's note (17 Feb 2021): Brown has now published a rebuttal to the researchers' study, which is available to read here.