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Planet Nine could be five times the size of the Earth and closer than we thought

Planet Nine is believed to be approximately five times the mass of Earth, in a mildly eccentric orbit with a period of about 10,000 years. James Tuttle Keane/Caltech

For the last three years, the astronomy community has been debating the idea that there could be a mysterious ninth planet in our solar system. Known as “Planet Nine,” a pair of astronomers from Caltech have argued for the existence of the unseen planet based on the movements of other objects in the solar system.

Now the same astronomers have published a pair of papers discussing the data in support of the Planet Nine hypothesis. The data used to argue for the existence of the planet relates to the orbits of Kuiper Belt Objects which appear to be clustering due to the gravity of an unseen planet, although there was debate as to whether the clustering observed was real or was merely an artifact of bias in the way the objects are observed. The new paper investigates whether this bias could be responsible for the clustering observed and finds that the probability of this is just one in 500.

This means the clustering is well supported, making the hypothesis that there is a planet affecting the objects stronger. “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,” Mike Brown, professor of planetary astronomy at Caltech and author of the paper, explained in a statement.

An illustration showing the orbits of Kuiper Belt objects and Planet Nine. Orbits in purple are primarily affected by Planet Nine’s gravity and exhibit tight orbital clustering. Orbits in green are influenced by Neptune. James Tuttle Keane/Caltech

The pair also made predictions about the properties of Plant Nine. They believe it has a mass around five times that of the Earth, making it a type of planet called a super-Earth which is bigger than our planet but much smaller than a gas giant like Jupiter. They now think that it is closer than previously believed as well — suggesting that it orbits at a distance of around 400 times the distance between the Earth and the sun, or 400 astronomical units.

Co-author of the paper Konstantin Batygin says that Planet Nine’s size makes it an ideal candidate for understanding more about how planets develop. “At five Earth masses, Planet Nine is likely to be very reminiscent of a typical extrasolar super-Earth,” he said in a statement. “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.”

The new findings are published in The Astronomical Journal and the review is published in Physics Reports.

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