The discovery of a tenth planet orbiting our Sun was announced today by Dr. Michael E. Brown of the California Institute of Technology in Pasadena, California, USA. The planet is at least the size of Pluto and perhaps twice as large. It has been designated 2003UB313 for now while the name suggested by its discoverers is reviewed by the International Astronomical Union.

The announcement came just hours after news of another object discovered in the outer solar system hit the media. That object, designated 2003 EL61, is slightly smaller than Pluto in size and has a tiny moon. The team that discovered the tenth planet had also been actively observing 2003 EL61 but was scooped by astronomers from the Sierra Nevada Observatory in Spain.

Both objects were originally observed in 2003 but new analysis with updated observations has allowed astronomers to confirm the discoveries. Such distant objects move very slowly against background stars and require sequential observations over long durations to pin down their orbits. Estimates for their size are based on brightness. If Planet 2003UB313 reflected all sunlight it would be the size of Pluto, which is 2,300 kilometers (1,429 miles) across. Because most objects do not reflect all sunlight that hits them, the object must be larger than Pluto.

The newly discovered objects, like Pluto, are all members of the Kuiper belt, a band of objects orbiting the sun past Neptune. These remnants from the formation of the solar system are not well understood. NASA’s New Horizons mission to Pluto and its moon Charon will provide the first opportunity to view Kuiper belt objects up close. The spacecraft is expected to be launched January 11, 2005 and will travel for nearly ten years before flying by Pluto. If successful, the spacecraft may be targeted toward other Kuiper belt objects.

More Information

• Planet 2003UB313
  • NASA Press Release, July 29, 2005 – “Planetary Scientists Discover Tenth Planet“
  • California Institute of Technology, Division of Geological and Planetary Sciences – Dr. Michael E. Brown
• Object 2003 EL61
  • New Scientist Article about 2003 EL61, July 19, 2005 – “New world found in outer solar system“
  • Consejo Superior de Investigaciones Científicas, Instituto de Astrofísica de Andalucía – “Bright Trans-Neptunian Object“
  • MPEC 2005-O36 : 2003 EL61
• New Horizons – NASA’s Pluto-Kuiper Belt Mission