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April 14, 2004 01:00 PM (EDT)

News Release Number: STScI-2004-14

Hubble Observes Planetoid Sedna, Mystery Deepens

April 14, 2004: Astronomers poring over 35 NASA Hubble Space Telescope images of the solar system's farthest known object, unofficially named Sedna, are surprised that the object does not appear to have a companion moon of any substantial size. This unexpected result might offer new clues to the origin and evolution of objects on the far edge of the solar system.

At a distance of over 8 billion miles, Sedna is so far away it is reduced to one picture element (pixel) in the image [at lower right] taken in high-resolution mode with Hubble's Advanced Camera for Surveys. This image sets an upper limit on Sedna's size of 1,000 miles in diameter.

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Q & A: Understanding the Discovery

  1. 1. Why was HST needed to look at Sedna?

  2. Hubble's sensitivity and sharpness gave astronomers an opportunity to look for a natural satellite orbiting Sedna and try and measure its diameter.

  3. 2. What did HST find?

  4. It's what HST did not find that keeps Sedna mysterious. There is no evidence for a moon, and the planetoid is even too small for Hubble to resolve.

  5. 3. Why did astronomers think Sedna had a moon?

  6. Sedna rotates very slowly. It takes at least 20 days to complete a rotation, maybe as long as 50 days. The gravitational pull of a moon is the best explanation for this slow rotation. Almost all other minor bodies in the solar system rotate in a matter of hours. Pluto has a six-day rotation because it has a satellite, Charon.

  7. 4. How do we know Sedna rotates?

  8. Measurements of Sedna's brightness from ground-based telescopes displays rhythmic changes in brightness indicative of a spinning body.

  9. 5. How small is Sedna?

  10. Sedna is so small it fits inside one picture element on Hubble's Advanced Camera for Surveys. This means the objects cannot be bigger than 1,000 miles, or three-quarters of Pluto's diameter. Sedna could be smaller still, depending on how bright and reflective the surface is.

  11. 6. Does this mean Sedna is not a planet?

  12. Moon or no moon, Sedna is too small and its orbit too elliptical for it to be considered a planet. It has been gravitationally battered around the outer solar system like a tennis ball. It is a primordial body from the early solar system, and its odd appearance and behavior may yield clues to the early solar system.

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Credit: NASA, ESA and M. Brown (Caltech)