The grouped orbits of six distant worlds (purple) were invoked as evidence for Planet Nine (orange). But some think that grouping can only be an observational bias.
Caltech / R. Seer (IPAC)
By Daniel Clery
For planetary scientists, it was the boldest assertion in a generation: an invisible extra planet, as much as ten times the mass of Earth, hiding on the edge of the solar system, beyond Neptune. But the allegation seems increasingly shaky after a team of astronomers reported last week that the orbits of a handful of remote boulders were not joined by the severity of ‘Planet Nine’, as its proponents believe, but only grouped because it where telescopes happened to be looking.
Fans of Planet Nine are not back yet, but one skeptic who is not involved in the new work says she is very happy to see it. The astronomer Samantha Lawler of the University of Regina conducted a more uniform analysis than before of the distant rock bodies known as Trans-Neptunian Objects (TNOs). the grouped orbits in computer models with an extra planet.
Mike Brown and Konstantin Batygin of the California Institute of Technology made worldwide headlines in 2016 with their prediction for a distant Planet Nine. 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 merged, Brown and Batygin said, because the gravity of Planet Nine pushed them there over billions of years. Several more extreme TNOs discovered since then have also appeared. ‘I would argue that the [Planet 9] data set is pretty good, ”says Batygin.
Planet Nine is said to be nine to five times as massive as Earth, in an orbit far beyond Neptune.
Caltech / R. Seer (IPAC)
However, Lawler and other astronomers were concerned about selection biases. Given how small and dark extreme TNOs are, they are only visible – if at all – during their closest approach to the inner solar system, and often only if they are not observed against the bright background of the Milky Way’s disk. Critics of the Planet Nine allegation said the apparent grouping of the discovered TNOs may have been just because it was where telescopes looked or were the most sensitive. “Every survey has prejudices,” Lawler says. “Some are aware of it, others not.”
A team led by Kevin Napier of the University of Michigan has decided to test whether the choice of prejudice plays a role. They collected 14 TNOs at similar distances discovered by three different surveys: the Dark Energy Survey (DES) using the Blanco Telescope in Chile, the Outer Solar System Origins Survey on the Canada-France-Hawaii Telescope in Hawaii , and a third carrying a variety of telescopes. All three characterized choice biases well. None of the 14 TNOs were among the original six that Brown and Batygin invoked.
Napier says the team took into account when and where the telescopes showed, and how sensitive they were to faint objects. With the data, the team calculated a “selection function” that differs over the air. And sure enough, the extreme TNOs found by all three of the surveys were in or near areas where the selection function was highest, the team reported on February 11 in an article posted to the 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 uniformly distributed around the solar system, which would rob Planet Nine of the ground evidence. The grouping ‘is a result of where we look and when we look’, he says. “No other model is needed to match the data.”
Batygin does not accept the conclusion. He points out that the DES survey largely looked at the airspace where the TNO group he and Brown identified found more extreme TNOs. It is therefore not logical to exclude clustering, he says. “The more relevant question to ask is: can its analysis distinguish between a grouped and a uniform distribution, and the answer seems to be ‘no’, ‘he says.
Napier acknowledges that it is difficult to draw conclusions from a sample of 14 TNOs. “There’s just so much statistical power you can draw with so few objects,” he says. The matter is unlikely to be resolved, he adds, until the Vera Rubin Observatory – a powerful new telescope built in Chile – begins observing in 2023. The survey will have well-defined selection biases and is likely to detect hundreds of new extreme TNOs. . That, says Napier, “will be like Christmas morning.”