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Discovery of exoplanet’s bizarre orbit may help us track down Planet Nine

Published: 18th December, 2020 at 15:07
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14 years of Hubble Space Telescope data finally confirm HD 106906 b exoplanet’s orbit, with exciting parallels to what we expect from Planet Nine

Since 2014, scientists have debated whether the elusive Planet Nine really exists, as its proposed orbit is so different from the eight planets we know. It is thought that Planet Nine travels round the Sun with a highly elliptical motion. Now, a team of astronomers have provided evidence that the theorised orbit of Planet Nine exists for an exoplanet elsewhere in the Universe.


Analysing 14 years of data from NASA's Hubble Space Telescope, researchers led by Meiji Nguyen of the University of California, Berkeley, showed that such an elliptical orbit exists in an exoplanet, a planet outside of our Solar System, called HD 106906 b. Granted, it’s not the catchiest of names but this exoplanet’s observations could change our understanding of our Solar System and the beginning of the Universe.

All planets orbit at least one star and in our Solar System, this star is the Sun. Exoplanet HD 106906 b exists in another Planetary System and orbits a pair of twin stars. By analysing its pathway, researchers can draw parallels to the potential Planet Nine.

Thought to explain the phenomena of the outer Solar System, Planet Nine is likely 5 to 10 times more massive than Earth.

In our Solar System, the eight known planets travels round the Sun at different distances and speeds, but all follow an almost-circular motion. Predictions for Planet Nine indicate its orbit is highly elongated, swinging further out than other planets. In doing so, Planet Nine has remained out of sight. This is one theory researchers proposed for the orbit Planet Nine but, until recently, we had no evidence for this strange orbit.

Now, from a treasure trove of data, researchers have mapped out the orbit of exoplanet HD 106906 b. They describe its path as bizarre, with an incredibly inclined, elongated elliptical orbit, it travels at a much larger distance from its twin stars than any other planet we’ve observed.

“It's very widely separated from its host stars on an eccentric and highly misaligned orbit, just like the prediction for Planet Nine. This begs the question of how these planets formed and evolved to end up in their current state,” said Nguyen.

We've found proof that this far-flung motion can really occur by looking outside our Solar System to another Planetary System. This research lays the foundation for finding Planet Nine, helping us interpret other celestial objects and their motion.

By studying Planet Nine, we could gain insight into the very beginning of the Universe and how our Solar System and Earth came to be.

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However, researchers still need to explain the physical basis for this strange motion. It could be that the exoplanet was too close to its twin stars (or the dusty discs that surround them) when it first formed and a strong repulsion pushed it away. Or, another massive celestial body collided with the exoplanet in the early years of the Universe may explain its odd orbit.

“This is astronomy detective work, gathering the evidence we need to come up with some plausible storylines about what happened here," said Nguyen. Determining this will be key in continuing the search for Planet Nine.


While there are still many unknowns, these developments are steps towards better understanding our Solar System and the early Universe. Future work will reveal more, but these findings from the Hubble Space Telescope have renewed hope in this mysterious ninth planet.

What does it mean if an exoplanet is 'habitable'?

All forms of life that we know of depend on one critical component: liquid water. So, in the search for life, astronomers focus on planets where liquid water could exist, which they call 'habitable'.

Every star has a 'habitable zone', also called the 'Goldilocks zone', where it is not too hot and not too cold. A planet in the habitable zone gets the right amount of energy from the star to support liquid water. Any closer in to the star and water would boil, and any further out and it would freeze.

However, this doesn't guarantee that liquid water would exist on a planet in the habitable zone. The planet's atmosphere could be too thick, raising the temperature even higher. And even if liquid water does exist on the planet, habitable doesn't mean inhabited.

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Frankie MacphersonIntern Journalist, BBC Science Focus

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