The evidence for a mysterious planet at our solar system鈥檚 edge has become less convincing. The existence of this unseen world was inferred from unexpected clustering of objects that orbit the sun beyond Neptune, but new simulations suggest that clustering is not happening.
In 2016, Mike Brown and Konstantin Batygin at the California Institute of Technology suggested that six objects in the outer solar system called trans-Neptunian objects (TNOs) which聽have similar orbits may have been put on their paths because of the gravitational pull of a huge planet orbiting the sun between 13 and 26 times farther out than Neptune.
Since then, astronomers have spotted more TNOs, and further analysis has drawn into question the idea that their orbits are clustered in a way that is unlikely to happen randomly. Kevin Napier at the University of Michigan and his colleagues performed the largest analysis of TNOs yet, using the orbits of 14 TNOs from three different sky surveys.
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Their statistical analysis revealed no evidence of any clustering in TNOs that would point to an extra planet in the solar system. Instead, it seems that the apparent clustering was an effect of selection bias because of the directions in which these telescopes looked, which cover a relatively small area of the sky.
鈥淭he existence of this planet seems less likely than it did before,鈥� says Napier. 鈥淲e sort of took the wind out of the sails of its main argument.鈥�
However, Batygin disagrees that the effect is due to selection bias 鈥� just because the telescopes didn鈥檛 look in other areas of the sky does not mean that the TNOs in one particular area aren鈥檛 strange, he says.
鈥淪ay you were walking through a forest and noticed that there were lots of bears in the east, and not many elsewhere 鈥� that might make you think there must be a bear cave somewhere to the east,鈥� says Batygin. 鈥淏ut this analysis would argue that there is no directional preference to the bears, because the follow-up surveys have have not checked everywhere.鈥�
Napier acknowledges that a good next step would be to try looking for TNOs in other regions of the sky, although some of those areas are more difficult to check because of the density of stars there and the dimness of TNOs,聽caused by their small size and distance from the sun.
There are other oddities, such as asteroids that orbit the sun perpendicular to the disc of the solar system, that could potentially be explained by the existence of Planet Nine 鈥� although they do not constitute decisive evidence that聽such a planet exists.
鈥淧lanet Nine is not necessarily dead,鈥� says Napier. 鈥淚 didn鈥檛 set out to try and kill a planet, and I don鈥檛 think I did kill a planet, but I wanted to ask whether selection bias is an issue here, and it turns out that it is.鈥�
Even though the evidence for Planet Nine may appear to be getting聽weaker, Batygin and Brown will continue searching for it. 鈥淚 would argue that the Planet Nine hypothesis is still in good shape,鈥� says Batygin. The Vera C. Rubin Observatory, which is under construction in Chile and expected to start observing later this year, should find hundreds of TNOs all across the sky, which will help determine whether the clustering is real once and for all.
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