Astronomers have found two of the smallest moons ever spotted around Uranus, brining the distant planet's satellite tally to 24, the third most in the solar system.
The moons are 8 to 10 miles across (12 to 16 km) -- about the size of San Francisco -- and were discovered with the Hubble Space Telescope.
With greater technology, astronomers are finding smaller moons around the giant planets with remarkable frequency, especially over the past two years. Jupiter leads the way with more than 50 known moons, Saturn has more than 30.
More moons are sure to be found as the search continues. Jupiter could have about 100 down to 0.62 miles (1 kilometer) in diameter, one expert has said.
The newly detected moons orbit closer to the planet than the five major Uranian satellites, which are each several hundred miles wide. The International Astronomical Union (IAU) will announce the finding today.
"It's a testament to how much our Earth-based instruments have improved in 20 plus years that we can now see such faint objects 1.7 billion miles (2.8 billion km) away," said Mark Showalter of Stanford University and NASA's Ames Research Center.
The newly discovered moons are temporarily designated as S/2003 U1 and S/2003 U2 until the IAU formally approves their discovery. S/2003 U1 is the larger of the two moons, measuring 10 miles (16 km) across. The Hubble telescope spotted this moon orbiting between the moons Puck, the largest satellite found by the Voyager spacecraft, and Miranda, the innermost of the five largest Uranian satellites.
Astronomers previously thought this region was empty space, according to a statement issued today by Hubble officials. S/2003 U1 is 60,600 miles (97,700 kilometers) away from Uranus, whirling around the giant planet in 22 hours and 9 minutes.
The smallest Uranian moon yet found, S/2003 U2, is 8 miles (12 kilometers) wide. Its orbital path is just 200 to 450 miles (300 to 700 km) from the moon Belinda. S/2003 U2 is 46,400 miles (74,800 km) away from Uranus and circles the planet in 14 hours and 50 minutes. The tiny moon is part of a densely crowded field of 11 other moons, all discovered from pictures taken by the Voyager spacecraft.
"The inner swarm of 13 satellites is unlike any other system of planetary moons," says co-investigator Jack Lissauer of Ames. "The larger moons must be gravitationally perturbing the smaller moons. The region is so crowded that these moons could be gravitationally unstable. So, we are trying to understand how the moons can coexist with each other."
One idea is that some of the moons are young and formed through collisions with wayward comets.
"Not all of Uranus's satellites formed over 4 billion years ago when the planet formed," Lissauer said. "The two small moons orbiting close to [the moon] Belinda, for example, probably were once part of Belinda. They broke off when a comet smashed into Belinda."
The astronomers hope to refine the orbits of the newly discovered moons with further observations.
"The orbits will show how the moons interact with one another, perhaps showing how such a crowded system of satellites can be stabilized," Showalter explained. "This could provide further insight into how the moon system formed. Refining their orbits also could reveal whether these moons have any special role in confining or 'shepherding' Uranus's 10 narrow rings."
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