Edition: U.S. / Global

Puzzling Discovery About New Distant Member of Solar System

Published: April 15, 2004

Astronomers have pointed the sharp-eyed Hubble Space Telescope at Sedna, the newly discovered, most distant member of the solar system, and they are puzzled by what they do not see.

They saw the dot of light of Sedna, a frigid planetoid about eight billion miles away, three times farther than Pluto. They had expected to see two: Sedna's and a smaller companion moon's.

''Much to our surprise, there is no satellite,'' Dr. Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology and leader of the team that discovered Sedna, said yesterday in a telephone news conference.

That leaves astronomers at a loss to explain one of Sedna's peculiarities: why it spins so slowly.

Months of observations since the astronomers discovered Sedna in November indicated that it brightened slightly, then dimmed again, every 20 days, presumably the time for Sedna to complete one full rotation. That is slow compared with almost all other bodies in the solar system, whose rotations typically take from a few hours to a day.

The simplest explanation was that the gravitational tug of an orbiting moon had slowed Sedna, just as Pluto and its moon, Charon, leisurely twirl around each other once every six days.

When the astronomers publicly announced their discovery of Sedna a month ago, they said they expected that they would soon find a moon, too.

''We were so convinced that there had to be a satellite,'' Dr. Brown said, ''because there is no other good scientific explanation for why something will rotate as slowly as 20 days.''

The 35 photographs that the Hubble took of Sedna on March 16, however, showed only Sedna. Although Sedna is the largest object to be found in the solar system since Pluto in 1930, the Hubble photographs also could not make out enough detail to provide a direct estimate of the size. That means Sedna is less than 1,000 miles wide, Dr. Brown said. By comparison, Pluto's diameter is 1,400 miles.

Sedna may still have a moon, but if so it may be smaller and dimmer than expected. Or by unfortunate timing, the moon could have been passing in front or behind Sedna at the time of the Hubble photographs, blending into the larger dot.

The astronomers have also considered more outlandish explanations. Perhaps Sedna once had a moon, but it was later lost or destroyed. Maybe some other celestial physics is at work. (Venus, which is also moonless, has an even slower rotation rate, once every 243 days, and that, too, is a mystery.)

Or Sedna may not be spinning that slowly after all.

The astronomers have been looking at Sedna from ground-based telescopes once a day at the same time each day. Thus, Sedna may actually be spinning around once a day, but still look almost the same day after day because it has just completed a full revolution -- just as the Sun, if photographed once a day at noon, would appear almost motionless in the sky. Another possibility is the brightening and dimming are completely illusory, arising from random fluctuations in the instruments.

Sedna is currently almost out of sight of telescopes on the other side of the Sun, but will come back in view in six months.

With a highly elliptical, 10,500-year orbit, Sedna is now actually relatively close. In 72 years, it will reach its closest approach to the Sun and then swing back out, eventually reaching 84 billion miles.

Dr. Brown and his colleagues proposed the name Sedna after the Inuit goddess who created the sea creatures of the Arctic, and they suggest that other similar bodies discovered in the future be named for gods and goddesses from polar cultures. The official designation of the planetoid, for the time being, is 2003 VB12.