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A hypothetical mysterious planet believed to be responsible for strange orbits in the Outer Solar System has just received one of its biggest hits to date.
According to a comprehensive analysis of extremely distant objects, conducted 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 taken to be the influence of a planet’s gravity is rather a selection bias in observations.
The pre-print paper has been uploaded to arXiv and is awaiting peer review.
Planet Nine burst onto the scene in 2016, when astronomers Konstantin Batygin and Michael Brown from Caltech published an article in The astronomical journal placing the case for a still unknown planet in the far reaches of the solar system. The evidence, they said, was in other objects far beyond Neptune’s orbit.
These objects are called extreme trans-neptunian objects (ETNOs). They have huge elliptical orbits, never intersecting closer to the Sun than Neptune’s orbit at 30 astronomical units, and swaying at more 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 group orbits this way.
Planet Nine, by their calculations, is expected to be about five to ten times the mass of Earth, orbiting at a distance of between 400 and 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. Research is therefore ongoing.
Just as Planet Nine itself would be hard to find, so are ETNOs. These bodies are smaller than a planet, and therefore weaker. When they move away from the Sun, we hardly have a chance to see them. And this is where some astronomers think there is a selection bias.
“Because ETNOs follow highly elliptical orbits and their luminosity decreases as 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 particular times of the year, at a limited depth. These effects result in significant selection bias.”
The difficulty in seeing ETNOs means we didn’t find many. The initial simulations performed by Batygin and Brown were based on only six ETNOs, which were assembled from an assortment of surveys with unpublished selection functions; in other words, the selection biases were not clear.
More recent surveys have been meticulous about their selection functions. And, while no survey has found enough ETNOs to form a complete statistical population, the combination of surveys may lead scientists to a more solid conclusion. That’s what Napier and his team did.
They took five objects from the Outer Solar System Origins Survey (OSSOS) (which had 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 led the research for Planet Nine.
Since the three surveys had very different objectives, they therefore had different selection functions. The challenge was to resolve these differences so that the objects could be combined effectively into one large survey. To do this, the team designed a survey simulator.
“Essentially,” they wrote in their article, “a survey simulator simulates detections of a body model population of the solar system using the pointing history, depth and tracking criteria of a survey. . This makes it possible to calculate the selection function of a survey for a given population, which allows us to account for biases, and therefore to understand the real underlying populations.
If the ETNO clustering was caused by a physical effect, it should have remained consistent with the larger sample of objects analyzed by the Napier team. Instead, their results suggested that the ETNO sample was consistent with a uniform distribution of parent bodies in space.
This does not mean, the researchers were careful to note, that there is no new planet. It just means that the existence of the planet cannot be inferred from ETNO data. There is not enough information to confirm or exclude it.
Other evidence also indicates its existence. For example, the oddly tilted orbits of objects outside the Solar System’s Kuiper Belt such as Sedna – although astronomers have also offered other explanations for these behaviors.
A stronger decision 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 which is expected to start. its operations this year.
In the meantime, the avid search for the elusive planet leads to some truly astonishing discoveries, including some possible dwarf planets that launch into the outer limits of the solar system and a whole bunch of giant gas moons.
So whether or not Planet Nine exists, the debate itself is astonishing to science – leading to discoveries we might not otherwise have encountered.
The study was published on the preprinted website arXiv.org.
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