New Study Casts Doubt on Planet Nine Hypothesis

Our image of the outer solar system in decades past was much simpler than it is today. Pluto was the ninth planet, and that was the end of it except for some scattered asteroids and comets. Now, science doesn’t consider Pluto a planet, but some believe there’s a still-undiscovered ninth planet out there tweaking the orbit of small planetoids. A new study calls into question that idea by showing that much of the orbital weirdness out there could also come from a collection of smaller objects.

In 2016, astronomers Konstantin Batygin and Mike Brown of Caltech noticed that some small objects in the outer solar system, known as trans-Neptunian objects or TNOs, had unusually eccentric elliptical orbits along similar alignments. The pair devised a model that showed those orbits could be the result of a ninth planet orbiting the sun about 20 times farther away than Neptune and offset 30-degrees from the planet of the solar system. They estimated this planet would have a mass about ten times that of Earth.

There was plenty of skepticism about this claim, as there always should be in science. However, only now do we have a competing hypothesis that seeks to explain the unusual alignment of TNOs in the outer solar system. Researchers at the University of Cambridge and the American University of Beirut have developed a new model that explains the 2016 Planet Nine observations using a multitude of tiny TNOs instead of an undiscovered planet.

The new study published in the peer-reviewed Astronomical Journal shows that Planet Nine isn’t necessary to explain the orbits of those anomalous TNOs. The team deleted Planet Nine from the model, replacing it with a cluster of objects with a combined mass of about 10 Earths. The data shows that small objects scattered across a wide area could have similar effects on TNOs as a single large planet.

This study doesn’t prove that there is no Planet Nine, but it offers a potential alternative for the observations we’ve made so far. No one has seen this mysterious potential planet, and it’ll be hard to detect it directly at such incredible distances. Proof could come gradually as astronomers map more TNOs with unusual orbits. We might reach the point where a ninth planet is a sure thing, but a few TNOs that fall outside the 2016 parameters could also blow the hypothesis apart.

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