Does Planet 9 Exist? Why Scientists Are Puzzled by Potential Missing Planet

Revisiting data collected in the 1980s, an astronomer has discovered possible evidence for a suggested "Planet 9" at the edge of the Solar System.

The planet has been a source of speculation ever since it was first suggested there could be a planet even further from the sun in 1846 after the discovery of Neptune.

Hypothesized to exist out beyond even the furthest body of the solar system, this massive, frozen planet, is believed by many astronomers to be shrouded by the icy cometary bodies of the Kuiper belt.

Despite the fact that its influence seems to be present in these bodies, as well as some of the solar system's other smaller bodies, and even in the axial tilt of the sun, Planet 9 exists it remains frustratingly elusive.

Every so often, a piece of research crops up, such as a 2015 study by California Institute of Technology (CalTech) astronomers Michael Brown and Konstantin Batygin, that reignites the whole debate about the solar system's hidden planet.

This has left some astronomers intently hunting for Planet 9 while others simply dismiss its existence.

Michael Rowan-Robinson had initially conducted the search for what would have been at the time the tenth planet of the solar system, this being before Pluto was demoted to a dwarf planet. This investigation turned up nothing, and by 1991 Rowan-Robinson had concluded that Planet 9 did not exist.

This changed when Rowan-Robinson re-examined the decades-old data he first studied in the 1980s with an eye on advances made in our knowledge of planetary systems. This revealed three infrared sources that Rowan-Robinson suggests could be the long-hypothesized world.

Rowan-Robinson's discovery could potentially reopen the debate surrounding Planet 9 once again, moving the solar system's most mysterious object into focus once more.

The Case For Planet 9

Located beyond Neptune, the supposed home of Planet 9, the Kuiper belt, consists of icy bodies ranging in size from large boulders to objects with diameters greater than 1,000 miles. These objects are material left over from the earliest era of the solar system that was never incorporated into the forming planets.

Most objects in the Kuiper belt, which also hosts the dwarf planet, Pluto, have extreme orbits that carry them far from the sun. Many of these orbits seem to be both clustered into the same quadrant of the solar system and tilted at the same angle.

Brown and Batygin produced a mathematical model that could indicate that there is some large, unseen body exerting its gravitational influence on these bodies stretching out and titling their orbits.

"Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we become increasingly convinced that it is out there," Batygin said in 2016. "For the first time in over 150 years, there is solid evidence that the solar system's planetary census is incomplete."

Kuiper Belt Objects Orbits
A diagram that shows the clustered orbits of some Kupier Belt objects. Could these object's orbits be stretched out by an unseen Planet 9? Caltech

Theoretical astrophysicist at Harvard University, Amir Siraj, said has been the best evidence of Planet 9 ever since.

"Planet 9 appears to be a potentially compelling explanation for the observed clustering of some objects beyond the orbit of Neptune," Siraj told Newsweek.

"If the existence of Planet 9 is confirmed through a direct electromagnetic search, it will be the first detection of a new planet in the Solar system in nearly two centuries."

Siraj explains that the reason Planet 9 could be so difficult to detect is that it would be extremely dark and therefore would only be visible in the electromagnetic spectrum, including visible light, with the most powerful telescopes.

He added that as the planet could be anywhere in the sky, a huge search would be needed to find it.

The Case Against Planet 9

Assistant professor of astronomy, University of Regina, Samatha Lawler, addressed the clustering objects beyond Neptune in the Kuiper belt, currently the best line of evidence for Planet 9.

"There are dedicated searches happening by at least two teams that I know of. They need to use pretty large telescopes from excellent observing sites because this undiscovered planet is predicted to be very faint and far away," told Newsweek. "None of the dedicated searches has found a new planet."

Lawler is part of the OSSOS team that used the Canada-France-Hawaii Telescope over course of five years to discover over 800 new Kuiper belt objects, doubling the amount of these bodies cataloged by astronomers.

Lawler explained that when these distant Kuiper belt objects' orbits are plotted the original pattern of "clustering" used to suggest the existence of Planet 9 becomes weaker and weaker.

Kuiper Belt Objects Orbits Revised
A more recent plot of Kuiper belt objects produced by Samantha Lawler. The clustering that indicated the existence of Planet 9 sees to have disappeared we have discovered more Kuiper Belt objects. Samantha Lawler

In addition to the work of Lawler's OSSOS team, astronomers have used data from the Dark Energy Survey to discover a further 300 Kuiper belt objects. This also showed none of the clustering orbits associated with evidence for Planet 9.

She points out that this means that years of dedicated searching for Planet 9 have turned up no evidence of a planet at the edge of the solar system.

Lawler added: "My own research has shown that the evidence really isn't there to support this particular theory ."

Should We Stop Looking For Planet 9?

Lawler concedes that while the search for Planet 9 has thus far turned up no planet on the edge of the solar system, that doesn't mean the search is completely fruitless.

She continues: "The theory of Planet 9 has sparked a huge amount of research into the Kuiper belt: loads of astronomers were really excited to try to test this theory in many different ways. It's such a cool idea.

"So while I think we shouldn't be searching for Planet 9 directly because the evidence isn't there, there are a lot of incredible discoveries just waiting in the outer solar system, and I think that's a great use of telescope facilities."

Siraj has another suggestion that would involve not Planet 9 not being a planet at all.

He has begun working on the possibility that this body at the end of the solar system in the Kuiper belt may actually be a grapefruit-sized black hole with a mass five to ten times that of Earth. He concedes this is very much an outside chance.

"There is no unambiguous evidence showing that black holes less massive than the about the mass of the Sun exist, so Planet 9 is most likely a planet, not a black hole," Siraj concedes.

"But, if it were, the discovery of a black hole in the outer solar system would prove the existence of primordial black holes and serve as a testing ground for fundamental physics."

Working with Havard theoretical physicist, Avi Loeb, Siraj calculated that if Planet 9 were indeed a black hole it would produce electromagnetic flares when comets in the Kuiper Belt pass by it. The two produced a paper on the subject earlier this year.

He added: "The final result was rather striking: the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST), which will come online in the near future, will be the perfect tool for detecting or ruling out a black hole in the outer Solar System."

That means that if Siraj is correct, just as Lawler suggests, searching for Planet 9 may eventually turn something even more significant to the progress of science.

Planet 9
An illustration of a possible ninth planet at the edge of the solar system, Neptune's orbit can be seen as bright ring in the background. The recent discovery of a possible sign of Planet 9 in archival data has reignited debate about the Solar System's most mysterious body. Tom Ruen/nagualdesign/ESO