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Explained: In weird planet 336 light-years away, clues about our own Planet Nine

Like the presumed Planet Nine in our Solar System, newly described exoplanet orbits its star from a great distance, at an extreme tilt, and influences the behaviour of various objects in its system. What have scientists learnt about its formation and behaviour, and how does it mirror Planet Nine?

Written by Kabir Firaque , Edited by Explained Desk | New Delhi | Updated: December 21, 2020 3:05:17 pm
Artist’s impression of the exoplanet HD106906 b. Credit: ESA/Hubble

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. HD106906 b and its behaviour have been described in a study published in The Astronomical Journal on December 10.

What is this exoplanet, and what is Planet Nine?

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 the object to be 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,” lead researcher Meiji Nguyen, from the University of California-Berkeley, told The Indian Express by email. 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.

Why is Planet Nine believed to exist?

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, according to a paper published in 2016 by Konstantin Batygin and Michael Brown of the California University of Technology.

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°) 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.

Why is the new exoplanet being compared with Planet Nine?

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.

All that said, HD106906 b presents a more extreme case in these respects. While Planet Nine is assumed to be 10 times as massive as Earth, HD106906 b is 11 times the mass of Jupiter. HD106906 b is unusually far away from its pair of host stars — over 730 times the distance that earth is from the sun. That makes its orbit extremely long — 15,000 years. Its binary star is relatively young at 15 million years, compared to our Sun which is 4.6 billion years old. 📣 Follow Express Explained on Telegram

So, what clues does it offer?

The authors of the new paper investigated how HD106906 b could have reached such a distance from its star, and raised the question whether something similar happened with Planet Nine. The star of the exoplanet is surrounded by a disc of debris. This outer disc, observations and calculations showed, is out of symmetry with an inner comet ring. What could have disturbed the symmetry? Here is what the authors propose:

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. Today, it orbits its star at an eccentricity of 21° compared to the rest of the planetary system.

Did Planet Nine, too, form this way?

Indeed, a similar scenario has been proposed. Planet Nine, if it exists, formed early in the Solar System. Interactions with our giant planets, such as Jupiter or Saturn, then kicked it out of the inner Solar System. After that, passing stars stabilised its orbit.

Can Planet Nine’s location and orbit be predicted with measurements?

Indeed it can, Nguyen said. Brown and Batygin of Caltech, authors of the 2016 paper mentioned earlier, have made predictions for where they think Planet Nine is likely to be found in the sky. “Of course, the main challenge is actually trying to find such an object, because it will not only be very far away, but it will also be incredibly faint… So it would be like trying to find a tiny black speck against the pitch darkness of the already black night sky,” Nguyen said.

As such, many scientists are using indirect methods to track down the planet rather than spot it directly on a telescope.

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