How many planets are there in our solar system? Depending on your opinion the answer is eight or nine. In 2016, two Caltech researchers presented evidence of a new, ninth planet.
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The announcement divided the space community with many stating the astronomers had not presented enough evidence for the claim. Now Mike Brown and Konstantin Batygin and published a pair of papers analyzing the research done in the last three years.
Twin papers reveal new evidence
The newspapers cast more light onto the suspected location and makeup of the planet. The first paper is titled "Orbital Clustering in the Distant Solar System," it deals with the theory that clustering of objects in the Kuiper Belt beyond Neptune is influenced by the gravitational pull of an unseen planet.
However, this theory has been questioned by experts who suggest that this gravitational tugging is biased by how and where the objects are observed from. This first paper was published in The Astronomical Journal on January 22.
Computer models provide a solid base for further research
To understand if this observational bias is present the two researchers quantified the amount of bias in each individual observation, then calculated the likelihood that the clustering is spurious. That probability, they stated, is around 1 in 500.
"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 Richard and Barbara Rosenberg Professor of Planetary Astronomy.
The second paper, titled "The Planet Nine Hypothesis," is an invited review that will be published in the next issue of Physics Reports.
Planet closer to the sun than previously thought
This paper outlines the makeup of the planet including an estimate that it is smaller and closer to the sun than previously suspected. The paper provides thousands of new computer models of the evolution of the distant solar system.
Based on these models, a group of researchers from the University of Michigan suggests that Planet Nine has a mass of about five times that of the earth.
"At five Earth masses, Planet Nine is likely to be very reminiscent of a typical extrasolar super-Earth," says Batygin, an assistant professor of planetary science and Van Nuys Page Scholar.
"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."
In 2016 Batygin and Brown first presented the idea that a giant planet may be orbiting through the outer solar system.
They stated at the time, that this planet could explain many phenomena observed in the solar system, such as why some Kuiper Belt objects have a perpendicular orbit with respect to the plane of the solar system.
After the announcement on January 20, 2016, astronomers from all over the globe began searching for more evidence of the mysterious planet.
Researchers optimistic they'll find a planet within the decade
Although Brown and Batygin have always graciously accepted that the planet may not exist, they say the more evidence they find, they surer they are that Planet Nine is out there.
My favorite characteristic of the Planet Nine hypothesis is that it is observationally testable," Batygin says.
"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."