Is Planet Nine tilting our Solar System?

The existence of a ninth planet in our Solar System could provide the solution to an age-old problem concerning the orbits of the planets.

An artist's impression of the hypothetical Planet 9.
Published: April 23, 2019 at 2:11 pm
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An artist’s impression of Planet Nine, looking back towards the Sun. Image Credit: Caltech/R. Hurt (IPAC)


As astronomers look to confirm the existence of a ninth planet in the outer edges of our Solar System, a new study suggests evidence for the distant body could be found right at the centre, in the tilt of our Sun.

Planet Nine was predicted by Konstantin Batygin and Mike Brown from Caltech at the beginning of 2016, and is thought to be about ten times the size of Earth with an orbit that takes it 20 times farther from the Sun on average than Neptune’s.

The existence of the planet has been strengthened by a new study that suggests it could be responsible for adding a wobble to the Solar System that causes the Sun to appear tilted.

The planets in the Solar System orbit on a flat plane that rotates at a 6° tilt with respect to the Sun, which makes it appear that our host star sits at an angle.

However, no conclusive answer as to why this happens has ever been found. From Batygin and Brown’s calculations, we know that Planet Nine appears to orbit about 30° off compared to the orbital plane of the other planets, and that its orbit has an influence on objects in the Kuiper Belt on the edge of the Solar System.

This new study suggests that, while the other eight planets in the Solar System should exist on a flat plane with respect to the Sun, their orbits are being tilted by the existence of a ninth planet.

According to Brown, the calculated size and distance of Planet Nine would fit the 6° tilt of the orbital plane.

"Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the Solar System has no choice but to slowly twist out of alignment," says Elizabeth Bailey, a graduate student at Caltech and lead author of the study.


Although it is not yet known how such a planet would develop such an unusual orbit, one theory is that Planet Nine could have been pushed to the outer edges of the Solar System by Jupiter, or else pulled away from the centre by other bodies in the past.

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Iain Todd, BBC Sky at Night Magazine
Iain ToddScience journalist

Iain Todd is BBC Sky at Night Magazine's Content Editor. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.


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