Claim for giant ‘Planet Nine’ at edge of solar system takes a hit | Science

By Daniel CleryFeb. 15, 2021 at 15:32

For planetary scientists, this was the boldest claim in a generation: an additional invisible planet, up to 10 times the mass of Earth, lurking on the edge of the solar system, beyond Neptune. But the claim looks increasingly uncertain, after a team of astronomers reported last week that the orbits of a handful of distant pieces of rock are not clustered by the gravity of ‘planet nine,’ as its supporters believe, but only seem clustered because where the telescopes were looking.

Planet Nine supporters aren’t backing down yet, but a skeptic not involved in the new work says she is “very happy” to see it. The study performed “a more uniform analysis” than previously of distant rock bodies known as trans-neptunian objects (TNOs), says astronomer Samantha Lawler of the University of Regina, who tried and failed to simulate grouped orbits in computer models with an additional planet.

Mike Brown and Konstantin Batygin of the California Institute of Technology made headlines around the world in 2016 with their prediction for a distant new planet. They based their conclusion on a study of six TNOs, each smaller than Pluto, in extremely elongated and tilted orbits around the Sun. The orbits of these “extreme” TNOs were clustered together, Brown and Batygin said, because the gravity of planet nine had pushed them there for billions of years. Several more extreme TNOs discovered since also appeared to be regrouping. “I would say that [Planet 9] the dataset is in fairly good condition, ”says Batygin.

Lawler and other astronomers were concerned about selection bias, however. Given the extreme size and darkness of the NWT, they are not visible – if at all – when approaching the closest to the inner solar system, and often only if not seen in the light context. of the Milky Way disk. Critics of Planet Nine’s claim said the apparent clustering of the discovered TNOs could only be due to the fact that this is where the telescopes looked or were most sensitive. “Every investigation has biases,” Lawler says. “Some are aware of it, others are not.”

A team led by Kevin Napier of the University of Michigan decided to test whether selection bias played a role. They gathered 14 TNOs from the same distance discovered by three different surveys: the Dark Energy Survey (DES) which uses the Blanco telescope in Chile, the Outer Solar System Origins Survey on the Canada-France-Hawaii telescope in Hawaii, and a third which used a variety of telescopes. All three had well characterized selection biases. None of the 14 NWT was among the original six cited by Brown and Batygin.

Napier says the team took into account when and where the telescopes were pointing and their sensitivity to pale objects. With this data, the team calculated a “selection function” that varies across the sky. And of course, the extreme TNOs found by all three surveys were in or near areas with the highest selection function, the team reported on Feb.11 in an article posted on arXiv and accepted by Journal of Planetary Sciences. As a result, according to Napier, the team was unable to reject the null hypothesis that the extreme TNOs are evenly distributed around the solar system, depriving Planet Nine of its fundamental evidence. Grouping “is a consequence of where we look and when we look,” he says. “Another model does not need to fit the data.”

Batygin does not accept this conclusion. He notes that the DES investigation largely focused on the area of ​​the sky where the TNO group resides that he and Brown identified and found to be more extreme TNOs. So excluding clustering is “not logical,” he says. “The most relevant question to ask is, can their analysis distinguish between a clustered and uniform distribution, and the answer appears to be ‘no’,” he says.

Napier acknowledges that it is difficult to try to draw conclusions from a sample of 14 NWT. “There is only a limited amount of statistical power that you can draw with so few objects,” he says. The issue is unlikely to be settled, he adds, until the Vera Rubin Observatory – a powerful new observing telescope under construction in Chile – begins observing in 2023. Its investigation will be biased. well-defined selection and is likely to detect hundreds of new extreme TNOs. This, Napier says, “will be like Christmas morning.”