Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2009 March 25
See Explanation.  Clicking on the picture will download
 the highest resolution version available.

Orcus of the Outer Solar System
Credit: M. Brown (Caltech), C. Trujillo (Gemini), D. Rabinowitz (Yale), Samuel Oschin Telescope

Explanation: A newly discovered object in the outer Solar System moves like an anti-Pluto. 90482 Orcus was first discovered in 2004 and is slightly smaller than Pluto, although still one of the largest Kuiper belt objects known. Orcus may one day have the same IAU designation as Pluto: a dwarf planet. Orcus and Pluto have similar orbits: each achieves nearly the same maximum and minimum distances from the Sun, each orbits on a similarly shaped ellipse, and each orbital ellipse is tilted toward the other planets' orbital ellipse by roughly the same angle. The great mass of Neptune causes each to circle the Sun twice for every three Neptune orbits. Orcus is like an anti-Pluto, however, because the two objects always remain across the Solar System from each other. Orcus can be found as the spot near the center of these discovery frames moving slightly down from the top. Until the end of next week, the discoverers of Orcus ask for your help in naming its newly discovered moon.

Tomorrow's picture: pixels in space

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