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Is the solar system’s planetary count back up to nine?


By Tushna Commissariat

In August 2006 the distant world that is Pluto was “downgraded” from planet to “dwarf planet” by the International Astronomical Union. Now, 10 years on, it seems that a new planet may be joining the ranks of the other more familiar eight, thanks to two researchers from the California Institute of Technology in the US, who have uncovered evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system.

Although it has not been observed directly just yet, Konstantin Batygin and Mike Brown discovered the planet’s existence via mathematical modelling and computer simulations. The newly found “Planet 9” is about 10 times as massive as Earth and its orbit is nearly 20 times farther from the Sun on average than Neptune, placing it firmly within the Kuiper belt. If the duo’s calculations are correct, it means that it takes Planet 9 anywhere between 10,000 and 20,000 years to complete one orbit, making it a long year indeed. The research is presented in The Astronomical Journal (which is published by IOP Publishing, which also publishes Physics World).

“This would be a real ninth planet,” says Brown, adding that “there have only been two true planets discovered since ancient times, and this would be a third. It’s a pretty substantial chunk of our solar system that’s still out there to be found, which is pretty exciting.”

The team’s discovery came about thanks to previous work, done in 2014, that found that 13 of the most distant objects in the Kuiper Belt share some similar orbital properties – an unexpected feature as the gravitational pull of the giant planets should, in theory, have randomized any such orbits of small, icy bodies in the belt over the solar system’s lifespan. Planet 9’s conception arose as a way to explain these similarities. Batygin and Brown soon realized that the six most distant objects of the 13 all follow elliptical orbits that point towards a single direction in space. This was despite the fact that the outermost points of their orbits move around the solar system, and that they travel at different rates. Also, when viewed in 3D, they tilt nearly identically away from the plane of the solar system. These coincidences allowed the duo to show that an object just like Planet 9 is required to maintain this configuration.

“Although we were initially quite sceptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we have become increasingly convinced that it is out there,” says Batygin. “For the first time in more than 150 years, there is solid evidence that the solar system’s planetary census is incomplete.”

Brown, who rather ironically is know for his part in demoting Pluto, says that “all those people who are mad that Pluto is no longer a planet can be thrilled to know that there is a real planet out there still to be found”. “Now we can go and find this planet and make the solar system have nine planets once again,” he says. You can read more about this exciting finding and its implications over at AAS Nova.

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  1. Dileep Sathe

    Planetary count = 10:
    Actually, the decision on Pluto has not been globally accepted, still scientists are divided. I have questioned the third condition in the definition of a planet, adopted by the IAU in 2006. (Referenc: Dileep V. Sathe / Planetary trouble / IAU C46 / NL66 / p. 19 / 28/03/07). So I think planetary count will rise to 10, rather than 9.

  2. Anton Szautner

    Until this putative (~Neptune-mass) object is actually confirmed by imagery (an infrared spectrum can suggest its mass) and an orbit is computed, it is far too premature to claim a discovery of a new planet to our Solar System. The evidence, however intriguing, is still circumstantial. The anomalous groupings of the Kuiper Belt Object orbits cited can have arisen by other means, such as the ejection of a planet from our system or the near-encounter of a sibling star to our Sun during the early post-formation era, once the KBOs had been delivered there. The orbits of Sedna and the associated KBOs may be a ‘fossil’ of a single ancient perturbation event. In any case, it strains credulity that anything object of ~10 Earth masses has thus far eluded infrared detection by Spitzer and other space-based observatories, even if it is currently out beyond 10,000 AU. Let’s just keep looking and studying patiently and resist the temptation to degrade the definition of ‘discovery’ with insufficient evidence.

    • Dileep Sathe

      Is the distance of planet from Sun is in accordance with the Titius-Bode law, and is the velocity of planet in accordance with the equation, V^2 = GM / R. Answers to these two questions can help decide whether a body is planet or not.

  3. Jason Evans

    I’m sure astrologers were thrilled when the astronomy community downgraded Pluto. Perhaps they’ll be slightly less thrilled at this new news. :)

  4. Trackback: Is the solar system’s planetary count back up to nine? – MyPhysNet


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