Hubble Finds Exoplanet That Could Mirror Planet Nine

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The Hubble Space Telescope has been spying on a very unusual exoplanet some 336 light years away. The planet, known as HD 106906 b, is 11 times the mass of Jupiter, and it orbits the binary stars at a distance of nearly 68 billion miles — 730 times greater than the distance between Earth and the sun. Astronomers believe this frigid world could serve as a proxy to help us understand the hypothetical Planet Nine in our own solar system. 

Astronomers first discovered HD 106906 b in 2013 using the Magellan Telescopes at the Las Campanas Observatory, but the team didn’t know anything about the planet’s orbit. The aging Hubble telescope was able to help there, spying on the exoplanet during the course of regular observations over years. The team used archived Hubble data to calculate HD 106906 b’s motion, and they were surprised to see such an extreme orbit. In fact, it looks a great deal like we’d expect Planet Nine to look, orbiting far outside the debris disk, which is the equivalent of the Kuiper Belt in our solar system. 

I want to stress, we are talking about a hypothetical ninth planet here — there is data that points to the possibility of a large planet in our outer solar system, but it’s far from a sure thing. The best evidence for Planet Nine is the unusual clustering of trans-Neptunian objects in the outer solar system. Scientists believe the gravity of Planet Nine could be shepherding space rocks into those orbits. 

HD 106906 b orbits so far from the binary stars at the heart of its solar system that Hubble can image it directly (see above). This allowed the team to understand its orbit, which is about 15,000 year years long. The planet’s orbit is also inclined and eccentric — that means the orbit is high on one end and much lower and closer to the parent stars on the other. That implies it formed elsewhere in the solar system and was pushed into its current orbit after falling in toward the stars and getting flung into the outer reaches. A passing star or some other gravitational force could then have stabilized HD 106906 b at the edge of the solar system. 

The HD 106906 system is only about 15 million years old. So, if the Planet Nine comparison holds up, that could indicate the world arrived at its current location very early in the history of our 4.5 billion-year-old solar system. Additional observations of HD 106906 b could help us determine how we might go looking for Planet Nine, too. The long-delayed James Webb Space Telescope should launch in less than a year, offering an even better tool for studying HD 106906 b. It may even be able to spot smaller exoplanets in the HD 106906 system that could help explain the bizarre exoplanet’s origins.

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