New Study Knocks Theory of Planet Nine’s Existence Out of Orbit

It was previously thought the object’s gravitational pull was responsible for the strange orbits of six dwarf planets that whiz beyond Neptune

A picture showing simulated dwarf planets clustred near the theoretical "Planet Nine". There are a total of 6 pictured dwarf planet orbits in the photo and one orbit that belongs to Planet Nine.
The orbits of the Extreme Trans-Neptunian Objects (ETNOs) were tilted and elongated towards the sun, leading researchers to suspect the ETNOs were bunched together because of Planet Nine's gravitational pull. Caltech/R. Hurt (IPAC)

The hidden Planet Nine first made headlines in 2016 when Caltech researchers Mike Brown and Konstantin Batygin found evidence of a massive object ten times the size of Earth orbiting 20 times farther away from the sun than Neptune. Using computer simulations and modeling, Planet Nine was found based on observing six "extreme" Trans-Neptunian Objects (TNOs) that appeared clustered together. The orbits of the TNOs were tilted and elongated towards the sun, leaving Brown and Batygin to suspect the TNOs were bunched together because of Planet Nine's gravitational pull, reports Victor Tangermann for Futurism.

But a recent study carried out by Kevin Napier, a physics Ph.D. student at the University of Michigan, and his colleagues could challenge Brown and Batygin's analysis.

Napier and his team suggest that selection bias led Brown and Batygin to hypothesize Planet Nine's existence, and the "cluster" of TNOs may not have been caused by the gravitational pull of Planet Nine. Instead, Napier's team suggest the objects appeared clumped together because Brown and Batygin only observed a small portion of the sky, during a specific part of the year, at a specific time of day, reports Daniel Van Boom for CNET.

"[The clustering] is a consequence of where we look and when we look,” Napier tells Science's Daniel Clery. Napier's paper was published on the preprint server arxiv and recently accepted by Planetary Science Journal, so it is still awaiting peer-review by experts not involved with either study.

TNOs are challenging to spot because they are only seen when their orbit is closest to the solar system. Once TNOs orbit away from the sun, they are almost impossible to spot, reports Michelle Starr for Science Alert. Searching for TNOs is also restricted by the limited and variable sensitivity of existing telescopes. Those technical challenges should be elliminated when the powerful Vera C. Rubin Observatory, currently under construction in Chile, is complete in 2023. The Vera C. Rubin Observatory will have well-defined selection biases, which could allow astrophysicists to spot hundreds of new TNOs without issue, Science reports.

The original Planet Nine study observed only six TNOs gathered from various sky surveys. Furthermore, the selection functions Brown and Batygin used in their initial survey were unpublished, Science Alert reports.

To rule out the possibility of selection bias, Napier and his team selected 14 TNOs that were not included in Brown or Batygin's study. All selected TNOs were from the Dark Energy Survey (DES), the Outer Solar System Origins Survey, and a third that used various telescopes, reports Science. (Batygin tells Science that the DES survey used in Napier's analysis was in the same area of sky as his initial analysis.)

Napier's team observed the TNOs' movements using a computer simulator that was programmed to remove selection bias, like the timing and positioning of the telescopes, reports Adam Smith for the Independent.

If Planet Nine's gravitational pull caused clustering TNOs, then when Napier's team analyzed new surveys, their results should have affirmed that conclusion, reports Science Alert.

To assert that TNOs are clustering implies that these objects are usually distributed evenly around the solar system and have somehow been pulled out of their typical placement.

But Napier's team did not find enough evidence to support the idea that TNOs have uniform positions in the solar system to begin with, which would upend the conclusion that these objects were pulled out of place at all, Science reports. Essentially, Napier's team negated the foundational evidence that must be present to support the existence of Planet Nine.

"The existence of this planet seems less likely than it did before. We sort of took the wind out of the sails of its main argument," says Napier to Leah Crane for New Scientist.

Some clustering was still observed in the 14 new TNOs Napier's team observed, which means that the TNOs may behave this way independently and may not influenced by gravity. This information makes Batygin remain confident in their Planet Nine theory. “I would argue that the relevant [Planet Nine] data set is in pretty good shape,” Batygin tells Science.

Ultimately, Batygin disagrees that selection bias and argues that just because other parts of the sky were not looked at does not mean TNOs can't behave strangely in other areas of space, reports New Scientist.

"Say you were walking through a forest and noticed that there were lots of bears in the east and not many elsewhere – that might make you think there must be a bear cave somewhere to the east," Batygin tells New Scientist. "But this analysis would argue that there is no directional preference to the bears, because the follow-up surveys have not checked everywhere."

Napier and his team recognize the small sample size of 14 TNOS does not tell the complete picture, and it's difficult to draw a conclusion with this limited dataset, Science reports. Still, others have tried and failed to repeat Batygin and Brown's conclusions, like astronomer Samantha Lawler of the University of Regina, who tells Science that Napier's study is “a more uniform analysis.”

“Every survey has biases,” Lawler says. “Some are aware of them, some are not.”

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