Context: A strange exoplanet, orbiting a double-star 336 light years away, has caught the interest of astronomers. 

• The oddball behaviour of the planet, HD106906 b, provide clues about our own mysterious Planet Nine - if it exists. 

Exoplanet

• HD106906 b is not a new discovery: It appears in archival images taken by the Hubble Space Telescope in 2004. 
• But at the time, people did not recognise that the object is a planet. It was not until follow-up observations were taken in 2013 of the system using a different telescope (specifically the Magellan Telescopes in Chile) that people figured out that this was actually a distinct planet. 

Using data on the object’s motion over 14 years, astronomers have now precisely calculated its orbit and other key details.

• Planet Nine is an elusive, distant planet in our own Solar System. 
• Although it has not been found yet, it has been predicted by a series of studies over the last few years, and has been described by astronomers as “hiding in plain sight”. If it exists, Planet Nine is 10 times as massive as Earth.
• These predictions arise from the peculiar behaviour and alignment of various objects in the Solar System. 
• Astronomers believe all this is happening under the influence of Planet Nine. For example, in the outer reaches of the Solar System, beyond Neptune, there is a region called the Kuiper belt, populated by icy debris. 

Some of the objects in this region have been found to be very peculiarly aligned, and Planet Nine is likely responsible for this.

• Then in 2018, astronomers reported the peculiar behaviour of another object in the Solar System, called 2015 BP519. 
• The object orbits our Sun - but at an extreme tilt (54°C) when compared to the orbits of Earth and the other seven planets. 
• Simulations showed that the influence of Planet Nine (if it exists) would explain this tilt. Without Planet Nine, the tilt would be unexplained.
• Both planets (assuming Planet Nine is real) reside far out in their respective stellar systems. Both orbit their respective stars at an extreme tilt. 
• And both are massive enough to influence the behaviour of other objects in their respective regions.
• The planet formed close to the binary star, then kicked out because of gravitational interactions with the star. 
• This stirred up the objects in the debris disc, and disturbed its symmetry with the comet ring. But when the planet was kicked out, it should normally have been sent out of the system altogether, and become a rouge planet. But it still orbits the star, from far away. 
• To explain this, the researchers suggest that a passing star interacted with the exoplanet and held it in place.