Ceres — Planet #5?

Today’s Astronomy Picture of the Day is about Ceres, which will become the fifth planet from the sun (between Mars and Jupiter) if the International Astronomical Union goes ahead with its proposed definition of “planet.” As you can see from the picture at APOD, we don’t know much about Ceres, but that will change when/if NASA’s Dawn mission orbits the asteroid/planet in 2015.

The reason Ceres would be a planet, according to the proposal, and that the next largest asteroid, Vesta, would not has to do with shape. Ceres, with a diameter of 466 km, is massive enought to be spherical. On the other hand, Vesta has a diameter of 265 km and is somewhat nonspherical, as can be seen on yesterday’s APOD, and is therefore at the borderline of what would be classified as a planet.

Grace and Peace

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