The size of Pluto pinned down

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Data from New Horizons has allowed scientists to more firmly determine, for the first time, Pluto’s precise size.

Mission scientists have found Pluto to be 1,473 miles (2,370 kilometers) in diameter, somewhat larger than many prior estimates. Images acquired with the Long Range Reconnaissance Imager (LORRI) were used to make this determination. This result confirms what was already suspected: Pluto is larger than all other known solar system objects beyond the orbit of Neptune. “The size of Pluto has been debated since its discovery in 1930. We are excited to finally lay this question to rest,” said mission scientist Bill McKinnon, Washington University, St. Louis.

Pluto’s newly estimated size means that its density is slightly lower than previously thought, and the fraction of ice in its interior is slightly higher. Also, the lowest layer of Pluto’s atmosphere, called the troposphere, is shallower than previously believed.

This means that Pluto is at this moment the largest Kuiper Belt object so far known, bigger than Eris, the Kuiper Belt planet discovered in 2005 that had been thought to be bigger than Pluto and whose existence was used by some to demote Pluto’s status as a planet.

I say, they are both planets, because they are both heavy enough for gravity to have forced them to become spherical.



  • Can New Horizons be reoriented to capture photos of Eris after its Pluto fly by?

  • I am 99% sure the answer is no. Based on its speed and direction, it has a very small cone or region of territory outward to where it can travel. They have been searching for other Kuiper Belt objects in that cone, but so far have not announced any reachable candidates.

  • Ken Young

    Thanks for your informative post. I don’t think the fact that Pluto is massive enough to have become spherical should be enough to qualify it as a planet. After all, Ceres is spherical, and Vesta is nearly spherical. All of the largest planetary moons (including Pluto’s largest moon, Charon), are spherical. The IAU made a good call when they “demoted” Pluto – there’s a clear physical distinction between the planets, which are large enough to have cleared out all objects in similar orbits, and other solar system objects that happen to be spherical.

    Thanks also for your continued coverage of the Thirty Meter Telescope issue.

  • I have interviewed numerous planetary geologists who say that as far as they are concerned moons like Titan, Ganymede, Io, Europa, etc are planets. Some are larger than Mercury. They are vast and from a scientific perspective must be researched from the same perspective.

    And yes, I would consider Ceres a planet. Vesta, because it isn’t quite spherical, rates as a failed planet, which by the way is exactly what scientists concluded after Dawn was finished there.

    As for the IAU, do a search on BtB for IAU. Their definition for planets is faulty and unusable. And once again, that simply isn’t my opinion, but the opinions of many planetary geologists.

    Anyway, the bottom line will not be what an elite academic gathering of scientists declares, but what people who actually live in space decide. We already have a hint of that by the research into exoplanets. You will note that this is what people call them, no matter what their size.

  • Richard M

    Unfortunately, Eris is far out of position for any possible rendezvous – New Horizons would have to make a full left turn, to speak, and then further adjust its trajectory 44 degrees below the eclipitic. It’s also nearly at aphelion, almost 97 AU’s from the Sun. New Horizons carried little fuel to begin with, so as to keep the weight down – only minor trajectory adjustments are possible.

    Fortunately, JPL has identified two sizable Kuiper Belt objects in the 30-55km diameter range that are within reach, sometime in 2019 – next month, they will decide which to pursue after looking over the initial data from Pluto. The adjustments have to be made by this fall in order to make the flyby. That assumes that Congress will provide four years of additional funding for operations, but that seems likely…so cheer up: New Horizons should give us a look at at least one more Kuiper Belt object, so long as it stays healthy.

  • Very good response. Thank you. Hopefully these future kuiper belt objects get some attention. It will be interesting to see what else is out there.

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