For planetary scientists, it was the bravest claim of a generation: an unseen extra planet, as much as 10 times the mass of Earth, lurking at the boundary of the solar system beyond Neptune. But the claim looks increasingly shaky after a team of astronomers last week reported that the orbits of a handful of distant clumps of rock are not gathered by the gravity of “Planet Nine,” as its spokesmen believe. but only seems the cluster because that’s where telescopes happened to look.
Planet Nine supporters are not returning yet, but a skeptic not involved in the new work says she is “very happy” to see it. The study has performed “a more uniform analysis” than previously done of the distant rock bodies known as Trans-Neptunian Objects (TNOs), says astronomer Samantha Lawler of the University of Regina, who has tried and failed to simulate the clustered orbits in computer models with an extra planet.
Mike Brown and Konstantin Batygin of the California Institute of Technology made headlines around the world in 2016 with theirs prediction for a distant planet nine. They based their conclusion on a study of six TNOs, each smaller than Pluto, in extremely elongated and oblique orbits around the Sun. The orbits of these “extreme” TNOs were assembled, Brown and Batygin said, because Planet Nine’s gravity had pushed them there for billions of years. More more extreme TNOs discovered since then also seemed to cluster. “I would argue that it is relevant [Planet 9] the data set is in pretty good shape, ”says Batygin.
Lawler and other astronomers, however, were concerned about selection perturbations. Given how small and dark extreme TNOs are, they are only visible – at all – during their closest approach to the inner solar system and often only if they are not observed against the light background on the Milky Way disk. Critics of Planet Nine argue that the apparent grouping of the detected TNOs may only be because it was here that telescopes looked or were most sensitive. “Every study has biases,” Lawler says. “Some are aware of them, others not.”
A team led by Kevin Napier of the University of Michigan decided to test whether selection disorder played a role. The total of 14 similar remote TNOs discovered by three different studies: Dark Energy Survey (DES), using the Blanco Telescope in Chile, Outer Solar System Origins Survey on the Canada-France-Hawaii Telescope in Hawaii and a third that used a variety of telescopes. All three had well-characterized selection disorders. None of the 14 TNOs were among the original six claimed by Brown and Batygin.
Napier says the team took into account when and where the telescopes pointed and how sensitive they were to weak objects. With this data, the team calculated a “selection function” that varies across the sky. And certainly, the extreme TNOs found by all three studies was in or near areas where the selection function was highest, the team reported on February 11 in a paper sent to arXiv and accepted by Planetary Science Journal. As a result, Napier says, the team could not reject the null hypothesis that the extreme TNOs are evenly distributed around the solar system, which would deprive Planet Nine of its basic evidence. The cluster “is a consequence of where we look and when we look,” he says. “There is no need for another model that fits the data.”
Batygin does not accept that conclusion. He pointed out that the DES study largely looked at the area of the sky where the TNO cluster he and Brown identified resides and found more extreme TNOs. So it does not make sense to exclude clusters, he says. “The more relevant question to ask is: can their analysis distinguish between a grouped and uniform distribution, and the answer seems to be ‘no’,” he says.
Napier acknowledges that it is difficult to try to draw conclusions from a sample of 14 TNOs. “There is only so much statistical power you can draw with so few objects,” he says. The case is unlikely to be settled, he adds, until the Vera Rubin Observatory – a powerful new research telescope built in Chile – begins observing in 2023. Its study will have well-defined selection perturbations and is likely to detect hundreds of new extreme TNOs. is. . That, Napier says, “will be like Christmas morning.”
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