A hypothetical mystery planet believed to be responsible for strange orbits in the outer solar system has just taken one of its biggest blows to date.
According to an extensive analysis of extremely distant objects led by University of Michigan physicist Kevin Napier, Planet Nine may not exist – because the evidence for its existence does not exist. What astronomers viewed as the influence of a planet’s gravity is instead selection bias in the observations.
The pre-print paper has been uploaded to arXiv and is awaiting peer review.
Planet Nine appeared in 2016, when astronomers Konstantin Batygin and Michael Brown from Caltech published a paper in The Astronomical Journal plea for an undiscovered planet in the outer reaches of the solar system. The evidence, they said, lay in other objects far beyond Neptune’s orbit.
These objects are called Extreme Trans-Neptunian Objects (ETNOs). They have enormous elliptical orbits and never intersect closer to the sun than Neptune’s orbit by 30 astronomical units, and swing further than 150 astronomical units.
Batygin and Brown found that these orbits have the same angle at perihelion, the point in their orbit closest 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 about five to ten times the mass of the Earth and orbit around 400 to 800 astronomical units.
Because this hypothetical planet would be so far away, and because the sky is so big, it wouldn’t be easy to find. So the search for it is ongoing.
Just as Planet Nine itself would be hard to find, so are ETNOs. These bodies are smaller than a planet and therefore weaker. If they move away from the sun, we have almost no chance of seeing them. And this is where some astronomers believe there is a selection bias.
“Because ETNOs follow highly elliptical orbits and their brightness decreases by 1 / r4, they are almost always discovered within a few decades of perihelion,” the researchers wrote in their paper.
“In addition, telescopic surveys observe a limited area of the sky, at certain times of the year, to a limited depth. These effects result in significant selection bias.”
The difficulty of seeing ETNOs means we haven’t found many. Batygin and Brown’s initial simulations were based on just six ETNOs, collected from an assortment of studies with unpublished selection features; in other words, any selection bias was unclear.
More recent studies have been scrupulous about their selection functions. And while no survey has found enough ETNOs to form 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 did lead the quest for Planet Nine.
Because all three surveys had very 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 study. 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 using a survey’s pointing history, depth, and tracking criteria. This allows a survey’s selection function to be calculated for a given population,” which allows us to explain bias, and thereby 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 analyzed. Instead, their results suggested that the ETNO sample was consistent with a uniform distribution of parental bodies in space.
This does not mean, the researchers noted, that there is no Planet Nine. It just means that the planet’s existence cannot be deduced from ETNO data. There is not enough information to confirm or rule it out.
Other evidences also point to its existence. For example, the oddly tilted orbits of outer Kuiper belt objects in the solar system, such as Sedna – although astronomers have suggested other explanations for this behavior as well.
Stronger verdict will be possible with a larger population of ETNOs and Kuiper belt objects to analyze, which may mean waiting for objects from a more powerful telescope, such as the Vera Rubin Observatory expected to become operational sometime this year.
In the meantime, the eager hunt for the elusive planet results in some truly astonishing discoveries, including some possible dwarf planets swinging towards the outer limits of the solar system, and a slew of gas giant moons.
So whether Planet Nine exists or not, the debate itself is astonishing to science, leading to discoveries that we might not have come across otherwise.
The study is published on the pre-print website arXiv.org.