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Around January of 2016, an AUB astrophysicist (Jihad Touma), and his then master's student (Antranik Sefilian, now a PhD candidate at Cambridge University), were hard at work exploring planet formation around binary stars, when they heard of a stunning proposition touching on rather delicate matters closer to home, in the solar system.
Caltech / R. Hunt (IPAC)
As fellow astronomers Konstantin Batygin and Mike Brown of Caltech would have it, a hitherto undetected ninth planet, lurking in the farthest reaches of the solar system, seemed essential to explain the curiously clustered orbits of thirty-odd bodies orbiting in trans-Neptunian realm, far beyond Pluto and the belt of debris which harbors it (aka the Kuiper belt).
The Planet Nine hypothesis, as commonly referred to since, would go on to grab the imagination of astronomers and the lay public alike, stimulating observational campaigns in search of the proposed planet, then theoretical works arguing for, or debating its existence.
Our local theorists were intrigued enough by the proposition to carefully examine it, along with the observations it was trying to explain. They took both seriously enough to want to consider alternatives. And a promising alternative they found in precisely the sort of models they had been studying. Their investigations were ultimately published in the Astronomical Journal, along with a press release from Cambridge University, both unleashing an avalanche of web-based reports and social media buzz on the serious challenge posed to the much cherished Planet Nine hypothesis.
So what is the fuss all about? What did Sefilian and Touma propose that was so disturbing, and where do things stand at this stage?
Well, to be sure, not every day does one get a chance to infer with mathematical models the presence of an additional planetary member of the solar system pantheon. The last time this was done was in 1846 when Urbain Le Verrier predicted the existence of (what he later named) Neptune to explain irregularities in the motion of Uranus. He predicted its existence mathematically, and further pinpointed a location in the sky where astronomers went looking for it and found it (within days). One can only imagine the excitement about that momentous discovery, and the process that made it possible.
It is surely the potential of similar such excitement that is keeping Batygin and Brown, alongside other astronomers, awake at night searching for that ninth Neptune-sized planet which they predicted to explain odd regularities in the spatial orientation of a pack of bodies orbiting the Sun at distances 5-20 times farther than Neptune's. Their determination aside, the hoped for discovery has not been as easy coming as Neptune's, with expectations that a few more years of deeper probing will be required to either discover “their" Planet Nine (or rule it out altogether).
Enter Sefilian and Touma who, having heard of a cluster of small bodies, preferred to think of a collection of many more such bodies contributing to the clustering. Instead of Planet Nine, and much to the alarm of its advocates, they considered a shattered and spread out substitute, a disk populated by icy shards which extend the known Kuiper belt farther out in the solar system. Doing so, they learned that such a disk, together with the known planets, can provide just what is needed to trap some of its members into clustered configurations.
In lieu of the alluring prospect of a ninth planet, Sefilian and Touma gave astronomers what they believe is a more natural process, which delivers desirable clustering in our solar system, and is likely at play in various other astrophysical settings. Ironically, deeper surveys in search of Planet Nine are likely to further secure evidence of the disk they propose, whether through direct observations of disk members, or the more difficult detection of the dust that is expected to accompany it.
So which shall it be? A dim, though relatively sizable, source of reflected light, or a spreading of faintly glittering shards? As we wait for refinements and updates from proponents of Planet Nine (very soon we are told), Sefilian reminds us that their study suggests that “a combination of a lighter planet and a massive disk is likely to be optimal for clustering," while Touma will surely remain “seduced by the elegance of our basic process, likely ubiquitous at work, whether or not Nature agrees with us in the context which brought it to light."