Pluto’s Moons

“Charon” Photo Credit:, NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team

It’s been 32 years since the discovery of Charon, one of Pluto’s moons. Today, we know that Pluto is a dwarf planet—an object orbiting the sun, large enough to resemble a planet but too small to pick up objects along its orbit. But, that doesn’t mean Pluto is sans-lunar. Pluto has not just one but three moons: Charon, Nix and Hydra.Closer to Earth, IKAROS, a solar spaceship, is en route to Venus on the other side of the sun. Using a 66-foot wide aluminum sail, IKAROS is being pushed through space by solar wind—electromagnetic discharge from the sun—and is guided by a solar-powered navigation system. Plus, how to determine a star’s temperature based on its color. Here’s a hint: “red-hot” might be a misnomer out in space.

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  • Laurel Kornfeld

    Dwarf planets are planets too, in spite of the controversial ruling by 424 members of the IAU. Only four percent of the IAU voted on the controversial demotion, and most are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. One reason the IAU definition makes no sense is it says dwarf planets are not planets at all! That is like saying a grizzly bear is not a bear, and it is inconsistent with the use of the term “dwarf” in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies. Also, the IAU definition classifies objects solely by where they are while ignoring what they are. If Earth were in Pluto’s orbit, according to the IAU definition, it would not be a planet either. A definition that takes the same object and makes it a planet in one location and not a planet in another is essentially useless. Pluto is a planet because it is spherical, meaning it is large enough to be pulled into a round shape by its own gravity–a state known as hydrostatic equilibrium and characteristic of planets, not of shapeless asteroids held together by chemical bonds. These reasons are why many astronomers, lay people, and educators are either ignoring the demotion entirely or working to get it overturned.

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