A hypothetical mystery planet thought to be responsible for strange orbits in the outer solar system has just received one of its biggest blows.
According to a comprehensive analysis of very distant objects, led by physicist Kevin Napier of the University of Michigan, Planet Nine may not exist – because the evidence for its existence does not exist. On the contrary, what astronomers have the influence of the gravity of a planet is rather the bias of selection in the observations.
The pre-printed paper has been uploaded to arXiv and is awaiting peer review.
Planet Nine erupted on the scene in 2016, when astronomers Konstantin Batygin and Michael Brown of Caltech wrote an article in The Astronomical Journal which is the case for an as yet undiscovered planet in the outer corners of the Solar System. According to them, the evidence lies in other objects from far beyond the orbit of Neptune.
These objects are called Extreme Trans-Neptunian Objects (ETNOs). They have large elliptical orbits, and never cross closer to the sun than Neptune’s orbit around 30 astronomical units, and swing out beyond 150 astronomical units.
Batygin and Brown found that these orbits have the same angle at the perihelion, the point in their orbit closest to the sun. They performed a series of simulations and found that a large planet could group the orbits in this way.
According to their calculations, the planet nine should be about five to ten times the mass of the Earth and orbit at a distance between 400 and 800 astronomical units.
Because this hypothetical planet would be so far away, and because the sky is so large, it would not be easy to find. The search for it therefore continues.
Just as Planet Nine itself would be hard to find, so are ETNOs. These bodies are smaller than a planet and are therefore fainter. 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 like 1 / r4, it is almost always discovered within a few decades of the perihelion,” the researchers wrote in their article.
“In addition, telescopic surveys observe a limited area in the air at certain times of the year to a limited depth. These effects have a significant selection bias.”
The difficulty of seeing ETNOs means that we have not found much yet. The initial simulations performed by Batygin and Brown were based on just six ETNOs, collected from a variety of surveys with unpublished selection features; in other words, any selection biases were unclear.
More recent surveys are careful about their selection features. And while no single survey has found enough ETNOs to form a comprehensive statistical population, the combination of surveys may 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 grouping), five objects from the Dark Energy Survey and four objects found by astronomers Scott Sheppard, Chad Trujillo and David Tholen , which led the search for Planet Nine.
Since all three surveys had completely different objectives, they 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 recording simulator.
‘Essentially,’ they wrote in their paper, ‘a survey simulator simulates the detection of a model population of solar system bodies using the survey history, depth and detection criteria of a survey. This allows the calculation of the selection function of a survey for a given population, which enables us to account for prejudice, and thus understand the true underlying populations. ‘
If the ETNO grouping was caused by a physical effect, it should have remained consistent with the greater number of objects that Napier’s team analyzed. Instead, their results indicated that the ETNO sample corresponds to a uniform distribution of parent bodies in space.
This does not mean, according to the researchers, that there is no Planet Nine. It just means that the existence of the planet can not be deduced from ETNO data. There is not enough information to confirm or exclude this.
Other evidence also points to its existence. For example, the strangely tilted orbits of outer Solar System Kuiper belt objects such as Sedna – although astronomers have also suggested other explanations for this behavior.
A stronger decision will be possible with a larger number of ETNOs and objects from the Kuiper Belt to analyze, which could mean you are waiting for objects from a more powerful telescope, such as the Vera Rubin Observatory which is expected to be sometime this year start.
Meanwhile, the zealous hunt for the elusive planet led to some amazing discoveries, including some possible dwarf planets moving to the outer limits of the solar system, and a whole bunch of gas-giant moons.
Whether Planet Nine exists or not, the debate itself is incredible for science, leading to discoveries we might not otherwise have encountered.
The study was published on the pre-print website arXiv.org.