Dwarf Planets Pluto and Ceres Make Night Sky Appearance

This month two dwarf planets — Ceres and Pluto — come into opposition with the sun, offering a rare view of these tiny worlds in the night sky.

What, you may ask, is a "dwarf planet"? Basically it is a solar system object which is too small to qualify as a planet.

Interestingly enough, both Ceres and Pluto were, at the time of their discovery, considered to be planets. When Giuseppe Piazzi discovered Ceres on the first night of the nineteenth century, he thought he had discovered a new planet circling in an orbit between Mars and Jupiter. Later Ceres was demoted to become the largest of the asteroids, but in 2006 was elevated to the newly created status of dwarf planet.

Ceres is named for the Roman goddess of plants, the same source from which our word "cereal" comes. It is a small world, just 580 miles (940 km) in diameter.

Similarly, when Clyde Tombaugh discovered Pluto in 1930, he thought it was a new planet in an orbit beyond Neptune. However, its small size and peculiar orbit led astronomers to doubt its status; in 2006 Pluto too was reclassified as a dwarf planet.

Pluto was named by an 11-year-old English girl, Venetia Burney, after the Roman god of the Underworld. It is a little more than twice the size of Ceres, 1,430 miles (2,300 km) in diameter, slightly smaller than Eris, the largest dwarf planet. All of these dwarfs are tiny compared to our moon, 2,170 miles (3,500 km) in diameter.

Ceres comes to opposition on Friday, June 18, and Pluto a week later on June 25. However, the moon is Full on June 26 and quite close to Pluto in the sky on the 25th, so it would be better to try hunting it down at the same time as Ceres. The graphic shows the location of both dwarfs on June 19, a bit past midnight, against the backdrop of the constellation Sagittarius.

On June 18, Ceres will reach magnitude 7.2, so binoculars will be necessary to spot it. (On this astronomers' scale, larger numbers represent dimmer objects. The faintest object visible to the naked eye under perfectly dark skies is about magnitude 6.5) Sagittarius is one of the most crowded areas of the sky, so a star atlas or planetarium software would be helpful to distinguish Ceres from the background stars. Try to observe Ceres again a night or two later, so that you can confirm, by its motion, that you actually saw the asteroid, and not a star.

Finding Pluto will be a much greater challenge, since it is about seven magnitudes fainter than Ceres, magnitude 14.0! You will need a telescope with at least 10 inches aperture, a fairly high magnification, and a very detailed finder chart. Again, you will need to plot Pluto?s position over two or three nights to be certain of its location.

If you manage to observe these two objects this month, you will be one of the very few people on Earth to have seen two dwarf planets. Admittedly, they aren?t very exciting objects to look at, nothing more than pinpoints of light, but when you think of them as tiny worlds in the vastness of space, they are quite awesome.

• More Night Sky Features from Starry Night Education
• Image Gallery: The New Solar System
• Telescopes for Beginners

This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions.