What's New: May 2019

Ceres: A Planet, An Asteroid or A Dwarf Planet?

The object we call Ceres was discovered on the first day of the 19th century: January 1, 1801. Its discovery was the result of a systematic search for objects between the orbits of Mars and Jupiter; there were theoretical reasons (later discredited) for supposing that the orbits of those two planets would be more orderly if there was another planet in the "gap" between them, and the discovery of the "planet" Ceres fit the bill.

But trouble arose in the years and decades to follow, as other objects were found that had similar orbits. It was soon realized that Ceres was just the largest of numerous objects in what became known as the "asteroid belt." Even though Ceres was determined to be 587 miles across and contained a third of the mass of the entire belt, it was “demoted” to being an asteroid rather than a planet. That happened back in 1852.

Ceres will be in "opposition" on May 28; it will then be directly opposite to the Sun in our sky. It rises as the Sun sets, remains visible all night, and sets at sunrise. (This should also imply that Ceres will be at its closest to Earth; however, because the orbits of Ceres and the Earth are not precisely circular, the actual date of closest approach happens on May 26.) Ceres will then be 163 million miles from Earth. It will shine at magnitude 6.8 – just below the threshold of naked-eye visibility. If you know where to look, though, it will be easily visible in binoculars or a telescope.

Ceres is so small and distant that it appears as a small dot in even the largest telescopes on Earth. The best views from the Hubble Space Telescope revealed little more than a mysterious bright spot on its surface: