New clues emerge in the hunt for Planet Nine
Ever since we kicked Pluto out of the planet club in 2006, the solar system has been known to house eight planets. But in the last few years there's been talk of that tally rising back up to nine, because evidence suggests there's a gigantic rocky world lurking on the fringes. The astronomers who originally proposed the Planet Nine hypothesis have now published two more papers analyzing the case, highlighting a few new clues.
In 2016, Caltech astronomers Mike Brown and Konstantin Batygin put forward the idea that there was a large, as-yet-undiscovered planet orbiting the Sun far beyond Pluto. The team had noticed that six Kuiper Belt objects in particular had strange orbits – they were all tilted 30 degrees off-kilter from the plane of the eight known planets, and were clustered together. Computer simulations suggested that this formation could come about as a result of the gravity of an unknown planet tugging on them from the shadows.
But the researchers acknowledge that this apparent clustering could potentially just be an observational bias, meaning that perhaps the team found evidence of a ninth planet in those objects' orbits because that's what they were looking for. They began by revisiting this through a new method of quantifying the potential for bias in their observations which basically ruled it out, concluding the probability of bias to be just one in 500.
But of course, just because that clustering is happening, doesn't necessarily mean a new planet is to blame. Other studies have found that the same result could be reached if known distant objects are jostling each other like "gravitational bumper cars," or if there's a large disc of rock and ice out there instead.
"Though this analysis does not say anything directly about whether Planet Nine is there, it does indicate that the hypothesis rests upon a solid foundation," says Brown.
The second study revised the likely properties of a Planet Nine, based on thousands of new computer models of how the distant solar system evolved. Originally, the astronomers had said the evidence pointed to a rocky planet that was 10 times more massive than Earth, and may be orbiting at a distance of 600 Astronomical Units (AU), where 1 AU is the average distance between the Earth and the Sun.
But the new study suggests that if it's there, the mysterious planet may be smaller and closer than previously thought. The researchers are now saying Planet Nine could be "only" five times bigger than Earth, and orbits about 400 AU from the Sun. Interestingly, recent observations of other planetary systems show that it's actually weirder if our solar system doesn't have one of these so-called Super-Earths.
"At five Earth masses, Planet Nine is likely to be very reminiscent of a typical extrasolar Super-Earth," says Batygin. "It is the solar system's missing link of planet formation. Over the last decade, surveys of extrasolar planets have revealed that similar-sized planets are very common around other sun-like stars. Planet Nine is going to be the closest thing we will find to a window into the properties of a typical planet of our galaxy."
Whether Planet Nine is really out there or not, the search will no doubt continue. The researchers on the new study acknowledge that they may be wrong, but are pretty confident that we'll find it soon.
"My favorite characteristic of the Planet Nine hypothesis is that it is observationally testable," says Batygin. "The prospect of one day seeing real images of Planet Nine is absolutely electrifying. Although finding Planet Nine astronomically is a great challenge, I'm very optimistic that we will image it within the next decade."