Harvard University
Cambridge, Massachusetts

18 November 1999

21 Moons 'Swarm' Planet Uranus

By William J. Cromie, Gazette Staff

Three new moons have been discovered orbiting Uranus, a great gasball of a planet about 2 billion miles from Earth. The discovery, last July, raises the number of Uranian moons to 21, the most, as far as is known, in the skies of any planet.

"We were excited to find these newest satellites," says Matthew Holman, an astronomer at the Harvard-Smithsonian Center for Astrophysics. "We think they are cometlike objects captured by Uranus when the solar system was billions of years younger."

The evidence for such capture is in the strange orbits of the moons. Most moons circle the mother planet in the same plane as planets circle the sun, called the ecliptic plane. Uranus boasts 16 moons that move this way. But the "newest" moons travel along irregular paths, at a steep angle to the ecliptic plane.

Two such moons, called "irregulars" by astronomers, were found rotating around Uranus two years ago. The three new moons bring the total number of irregulars to five. They range between 6 and 12 miles in diameter and orbit about 10 million miles from Uranus. (Earth's moon circles approximately 240,000 miles away.)

Astronomers felt more relieved than surprised to find them because faint, reddish irregulars seem to be the rule rather than the exception in the outer solar system. Jupiter, the largest planet, has four irregulars as well as 12 regular moons. Saturn, the second largest, has at least one irregular among its 18 moons. It would be curious if Uranus, the third largest planet, didn't have irregular companions.

Neptune, smaller and farther away than Uranus, has at least one irregular in addition to eight regulars.

Most astronomers believe the small unordered moons come from what is known as the Kuiper belt, a region of icy planetesimals circling between Neptune and Pluto, the farthest-out planet. In fact, some experts think Pluto, which is about the size of Earth's moon, is a Kuiper object.

Kuiper objects seem to be chunks of ice, dust, and gas left over from the beginning of the solar system, some 4 billion to 5 billion years ago.

During the solar system's first billion or so years, much more material whirled around the outer solar system than does there today. "By studying these irregular satellites, we should reach a better understanding of what the solar system was like at that time," Holman points out.

Origin of Comets

Today, some Kupier objects travel highly elliptical orbits that bring them close to the sun. When solar heat begins to melt them, they produce long, luminous tails, and we call them comets.

Billions of years ago, such objects are thought to have swarmed like bees through the outer solar system. Uranus and its big companions consisted of rocky cores that attracted huge envelopes of gas around themselves. (Uranus is 4,000 times the diameter of Earth.) The gravity of such massive planets pulled in any Kuiper objects that wandered close enough.

Meanwhile, in the inner solar system, there were less flying objects and gas. The smaller, rockier planets, like Earth and Mars, didn't have enough gravitational pull to capture what gas and flying objects were around. (Many astronomers believe that the close approach of an object the size of Mars ripped the moon from Earth.)

This is the story as Holman and others believed it happened. It fits with the evidence but is subject to changes as more observations are made.

Goofy Discoveries

Uranus today appears -- through the lens of a good telescope -- as a featureless blue-gray globe. It is strangely tilted on its side; that is, the north and south poles are in almost the same relative position as the Earth's equator. Finding faint objects, less than 20 miles in diameter and orbiting Uranus about 1,700,000,000 miles from Earth, is enough to make an astronomer feel proud.

The first two Uranian irregulars were discovered in 1997 with the Hale Telescope on Palomar Mountain in California, which has a lens 16 feet across. Brett Gladman and two companions had some spare time after a session of looking for Kuiper objects, so they decided to point the telescope in the neighborhoods of Uranus and Neptune. After finding two previously unknown moons around Uranus, they decided that a more thorough search would reveal even more satellites.

Two years later, Gladman, Holman, J.J. Kavelaars of McMaster University in Canada, and Jean-Marc Petit of the Observatory of Nice, France, continued the search from atop a 14,000-foot extinct volcano on the island of Hawaii.

"At that altitude, the lack of oxygen causes people to act a little goofy," Holman notes. "We didn't get sick, but it took a lot longer to do anything."

For four nights they stared intently at computer screens attached to the Canada-France-Hawaii Telescope on the mountain. Attached to the telescope was a powerful camera that could survey large areas in the vicinity of Uranus.

The team took three images of each field of view, one hour apart. Then they used two different computer programs to spot any objects that had moved during that time.

"Things got exciting when we finally saw something move at the right speed and in the right direction," Holman recalls. "Then it was time to shout 'Victory!' "

They shouted it twice during those four days. Later, Holman discovered a third moon while re-examining data from the telescope in his office in Cambridge, Mass. Thus, Uranus entered science and history books as the planet with the most moons. At the pace of astronomical discoveries, it may not stay there too long.

To see what Holman and his colleagues saw on those nights, tune to http://cfa-www.harvard.edu/~mholman.


Matthew Holman takes a close look at three images of Uranus with the help of projection equipment at Boston's Museum of Science. The images, taken by the Voyager 2 spacecraft in 1986, show that the planet is encircled by rings, similar to, but much more tenuous than, those of Saturn. Photo by Kris Snibbe.

Harvard-Smithsonian Center for Astrophysics

September 20, 1999


Until just a few years ago, many astronomers believed the planet Uranus was a bit strange. That's because, unlike the other giant members of the Solar System, Uranus did not appear to have any so-called irregular satellites, or, distant moons with unusual orbits. However, recent observations have found what appear to be three new irregular moons around Uranus, thus suggesting that the seventh planet from the Sun is just one of the gang after all.

Using the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii, an international team of astronomers made very careful observations over the summer to find these extremely faint objects. If confirmed, and tallied with two other irregular satellites discovered in 1997, Uranus would have 16 regular and five irregular moons, making it the most populated planetary satellite system known.

Irregular satellites do not follow the normal, near-circular orbits of most satellites, such as the Earth's Moon. Instead, these irregular objects either travel in highly elliptical orbits, or follow paths that are severely tipped to the plane of the planet's equator.

"The discovery of these irregular satellites is very important because it means that Uranus is not some oddball, but rather is just like Neptune, Saturn, and Jupiter," says Matthew Holman, a planetary scientist at the Harvard-Smithsonian Center for Astrophysics and a member of the team that made the discovery. "It might also help us better understand how the irregular satellites of the giant gas planets originated and how they've evolved."

These newly discovered objects are being referred to as "candidate" irregular satellites because further observations are necessary to absolutely confirm that these bodies are not comets or asteroids on planet-encountering orbits. However, based on the data so far, the team is confident these are true moons of Uranus.

"Given how these bodies are following the planet exactly, it is highly unlikely that these are some sort of Solar System interlopers," says Brett Gladman of the Observatory of Nice, France, and leader of the team. Gladman and his colleague J.J. Kavelaars of McMaster University, Canada, were both members of the team that found Uranus's first two irregular moons in 1997.

The three new candidate satellites were discovered in a search using the world-class wide-field imaging camera, known as CFH12K, which is a mosaic of CCD detectors covering a very large patch of sky (currently 35x28 arcmin, or roughly the area of the full moon). This instrument allowed the team to explore more than 90 percent of the region around Uranus in which satellite orbits are stable and to find these extremely faint objects, which are no more than 20 kilometers in diameter and orbit Uranus at a distance of 10 to 25 kilometers.

Other members of the discovery team include Jean-Marc Petit and Hans Scholl (Observatory of Nice, France), and P. Nicholson and J. A. Burns (Cornell University.) Follow-up observations were obtained at the Mount Palomar 5-meter and Kitt Peak 4-meter telescopes, the latter in conjunction with D. Davis and C. Neese of the Planetary Science Institute in Tucson, AZ. Brian Marsden and Gareth Williams of the International Astronomical Union's Minor Planet Center computed preliminary orbits for the reported objects.

McMaster University
Hamilton, Ontario L8S 4L8

Astronomers Discover Two New Moons around Uranus

Hamilton, July 29, 1999 -- A team of astronomers including McMaster University's JJ Kavelaars has discovered two new moons orbiting around the planet Uranus. The moons were revealed after four nights of observation using the world's largest electronic camera which was mounted on the Canada-France-Hawaii telescope located in Hawaii. The electronic camera , known as CFH12k, allowed the research team to cover the entire region around Uranus searching between five million and 100 million kilometres out from the planet.

The discovery brings the total number of moons around Uranus to 20 which now surpasses Saturn as the planet with the largest number of known moons. The two new moons are smaller and fainter than the others measuring about 20 kilometres in diameter and orbiting some 10 to 25 million kilometres from the planet's centre.

Kavelaar says he is anxious to complete more research and analysis to determine if there is a common formation history between the new moons and two others he and Brett Gladman discovered in 1997.

The astronomy team includes Brett Gladman, Jean-Marc Petit and Hans Scholl of the Observatoire de la Cote d'Azur and Matt Holman of the Harvard-Smithsonian Centre for Astrophysics.

Additional information.

UA News Services

May 19, 1999


An Arizona scientist has discovered an 18th moon orbiting the planet Uranus, the International Astronomical Union announced today. Until now, Saturn has been the only planet in our solar system known to have as many as 18 satellites.

Erich Karkoschka, a researcher at the Lunar and Planetary Lab of The University of Arizona in Tucson, made the discovery. The newly found moon is the first satellite of Uranus discovered in 1999 but will still be designated as Satellite 1986 U 10 (short S/1986 U 10).

"This discovery is very unusual," Karkoschka said. "Typically, satellites are found within days after the discovery image has been taken. In this case, the discovery image is more than 13 years old."

The interplanetary spacecraft Voyager 2 took seven images of the new atellite when it flew by Uranus in late January, 1986. These images have been publicly available in digital format. However, nobody recognized the satellite until Karkoschka investigated these images recently. He has studied the Uranian satellites based on images taken with the NASA/European Space Agency Hubble Space Telescope (HST) and found the new satellite when he compared his HST results with images taken by Voyager 2.

The discovery image can be viewed at the WWW site: http://science.opi.arizona.edu/pics/umoons.jpg

The image shows Uranus with its ring system and the 10 innermost satellites. All but S/1986 U 10 were known at the time the image was taken, based on Voyager images taken in January 1986. Arrows at the edge of the image point to the 10 satellites. The other dots of light are background stars of the constellation Sagittarius. (If you know enough astronomy to want to look this up on a star chart, the bright star next to Juliet is Kaus Borealis, Lambda Sagittarii.)

Belinda and S/1986 U 10, imaged near the upper right hand corner, circle Uranus in almost identical orbits. They pass each other once a month. This is the first example of two satellites in nearby orbits passing each other so slowly.

Astronomers began discovering moons around other planets in our solar system in the 17th century. Throughout the approximately 60 satellite discoveries made over the four centuries since, either Jupiter or Saturn has had the most known satellites. Saturn is known to have 18 moons. Jupiter has 16 known satellites, not counting the Galileo spacecraft, anartificial satellite around that planet. Many scientists had thought that these two largest planets in our solar system would have more moons than smaller planets. Smaller Uranus now dispells that belief.

The other known satellites orbit Neptune (8 satellites), Mars (2 satellites), and Earth, Pluto, and asteroids Ida and Eugenia (1 each).

Soon after England's William Herschel discovered Uranus in 1781, he found its two largest satellites, which are about half the size of our moon. In 1851, English astronomer William Lassell detected two more Uranian satellites. In 1951, Gerard Kuiper of the University of Chicago -- later founder of the UA Lunar and Planetary Lab -- discovered Uranus' fifth satellite. The Voyager team found 10 more Uranian satellites in 1985-86.

Two years ago, Brett Gladman of the Canadian Institute for Theoretical Astrophysics and Philip Nicholson of Cornell University discovered the 16th and 17th satellites. These two satellites are some 100 times farther away from Uranus than are the satellites discovered by the Voyager team.

Gladman, Nicholson and Karkoschka have been the only scientists to find new moons around a major planet in the 1990s. However, during this decade, the first two satellites of minor planets (Ida and Eugenia) were imaged.

The new satellite is about 25 miles (40 km) in diameter, similar in size to comet Hale-Bopp, and it may also have similar composition as the comet, Karkoschka said.

"Hale-Bopp was a spectacular sight when it crossed the inner part of the solar system two years ago," he said. "On the other hand, the new satellite will never get spectacular since it will remain in the dark, frigid parts of the solar system. It will remain a tiny speck of light."

Uranus may well have more than 18 satellites, Karkoschka noted: Jupiter and Saturn have satellites of about half the size of the new Uranian satellite. No such small satellite has yet been discovered around Uranus since the dim sunlight at Uranus makes the detection of such small satellites very hard.

Based on the detection in seven images, Karkoschka concluded that the satellite orbits Uranus once every 15 hours and 18 minutes. This is similar to the rotation period of Uranus. The satellite hovers 32,000 miles (51,000 km) above the clouds of Uranus, or the same distance as the diameter of Uranus. The motions of satellites of Uranus can be viewed at the Space Telescope Science Institute WWW site: http://oposite.stsci.edu/pubinfo/pr/1999/11/animations.html

When Voyager 2 took the discovery image on January 23, 1986, it was 650,000 miles (1 million km) from Uranus. This is 2,500 times closer than the Earth ever gets to Uranus. Nineteen hours later, Voyager 2 flew by Uranus.

The 10 satellites shown in the new discovery picture have been in continuous sunlight during the last 25 years. Starting next year, Cordelia, the satellite inside the ring system, will enter the shadow of Uranus during every orbit. The other satellites will follow. By 2002, all 10 innermost moons will enter the shadow of Uranus every orbit. These eclipses occur during two intervals within the 84-year long Uranus-year. Similarly, lunar eclipses on Earth occur twice within each Earth-year, currently in January and July.

The new discovery image is a mosaic of 10 exposures, Karkoschka said. The exposures of Uranus had shorter exposure times than the exposures of the surrounding area containing the rings and satellites. Since Uranus is a million times brighter than its satellites, Karkoschka retained the darker planet image so the satellites would be visible.

"To an astronaut on board the Voyager spacecraft, the satellites would have appeared as faint stars while Uranus in the center would have been blazingly bright, as bright as the full moon on Earth," Karkoschka said. "In visible light, Uranus seems to be a bland, quiet place. Only two faint little cloud features can be found upon close examination of the image. The true activity in the atmosphere of Uranus is only revealed in infrared light." While the Voyager camera was not sensitive to infrared light, the Hubble Space Telescope has imaged atmospheric activity. The HST image can be viewed at the WWW site: http://oposite.stsci.edu/pubinfo/pr/1998/35/index.html

The colors in the image are close to realistic, he added. The newly discovered moon won't be stuck with the name "S/1986U10" forever, Karkoschka noted. The International Astronomical Union names satellites and asteroids a year or more after discovery. In the past, the IAU has often adopted a name suggested by the discoverer if that name fits in the context of the system's previously named satellites. However, anyone can suggest what the new moon might be named, Karkoschka added.

Brush up your Shakespeare. Uranus' other nine innermost moons are Belinda, Bianca, Juliet, Rosalind, Ophelia, Cressida, Portia, Cordelia and Desdemona.

Cornell University News Service

May 1, 1998

Caliban and Sycorax: Astronomers propose names for their two recently discovered icy moons of Uranus

ITHACA, N.Y. -- Cornell University astronomer Philip Nicholson and his colleagues have proposed names for the two recently discovered moons of the planet Uranus. They are Caliban and Sycorax, both characters in Shakespeare's play "The Tempest." The names are likely to be approved by the International Astronomical Union.

The astronomers detail their discovery of the two moons in a report in the April 30 issue of the magazine Nature. They confirm that Caliban and Sycorax are the faintest planetary moons yet imaged by ground-based telescopes. The discovery of the two moons was reported on Oct. 31 by Nicholson and colleagues Joseph Burns, professor of engineering and astronomy at Cornell, Brett Gladman of the Canadian Institute for Theoretical Physics at the University of Toronto, and J.J. Kavelaars of McMaster University, Canada.

The team used light-sensitive semiconductors, called charge-coupled devices, attached to the 5-meter Hale telescope on Mount Palomar, Calif., to track the irregular, or non-circular, orbits of the two moons. Regular satellites orbit near a planet's equatorial plane. The two moons are the first irregular satellites discovered around Uranus.

Both Caliban and Sycorax, the astronomers write, are unusually red in color, which suggests a link with the recently discovered populations of comet-like bodies called trans-Neptunian objects, which orbit the sun beyond the orbit of Neptune, and Centaurs, which cross the orbits of the outer planets.

Both trans-Neptunians and Centaurs, say the researchers, have a wide range of reddish colors, perhaps resulting from the bombardment of their organic-rich icy surfaces. Nicholson says this bombardment could be from cosmic rays or from the sun's ultraviolet radiation. The methane on the moons' surfaces, he says, would be "cooked" by the radiation into hydrocarbons, showing up as a dark red through a telescope's filters.

The two moons, say the researchers, are presumed to have been captured by Uranus early in the history of the solar system. "My guess is that the moons were once trans-Neptunians and they became Centaurs and were captured by Uranus and became satellites," says Nicholson. Since the newly discovered moons are likely to have been captured by Uranus soon after its formation, the Nature article notes, "their physical properties may provide clues to conditions in the early solar system."

The process of capture could have taken two forms, Nicholson says. The moons could have been trapped by Uranus gravity as they came close to the planet. Another theory, he says, is that in the early days of the solar system Uranus might have been surrounded by a gaseous nebula that would have caused a drag on the objects' movement as they came close to the planet.

Nicholson estimates that Caliban, the smaller of the two moons, has a diameter of 60 kilometers (37 miles) and is orbiting Uranus at an average distance of about 7.2 million kilometers (4.5 million miles), taking 1.6 years to complete one revolution. Sycorax, he estimates, has a diameter of 120 kilometers (74.5 miles) and takes 3.5 years to complete one orbit of Uranus at a mean distance of about 12.2 million kilometers (7.5 million miles) from the planet. However, he says, Sycorax has a much more elliptical orbit than Caliban, bringing it as close as 6 million kilometers (3.7 million miles) to the planet.

The composition of the two moons, says Nicholson, "is probably a plum-pudding mixture of rocks and ice."

All 15 previously known satellites of Uranus lie on fairly evenly spaced, nearly circular orbits. Most recently Voyager 2, in 1985 and 1986, discovered 10 small, dark inner moons.

Jupiter has eight known irregular satellites, of which the last, Leda, was discovered in 1974. Saturn has one, Phoebe, discovered in 1898, and Neptune has one, Nereid, discovered in 1949.

To see images of the two newly discovered moons of Uranus, go to Gladman's page on the World Wide Web.

Auf Deutsch: Neue Uranusmonde entdeckt.

Astronomy Picture of the Day: Irregular Moons Discovered Around Uranus

October 31, 1997


Astronomers have discovered two possible satellites of Uranus. The objects are designated S/1997 U 1 and S/1997 U 2. They were detected on CCD images taken with the Hale 5-meter telescope at Palomar Observatory on September 6 and 7. Both objects have also been observed in late October. Thus, Uranus now has 17 known satellites. The five largest were discovered by ground-based observations: Oberon, Titania and Umbriel by Sir John Herschel in 1787-1801, Ariel by W. Lassell in 1851 and Miranda by Gerard P. Kuiper in 1948. Voyager 2 found 10 more satellites during its flyby in 1986. Preliminary orbital calculations reveal that the new satellites have highly eccentric and inclined orbits, taking them 6 and 8 million kilometers from the planet. This is more than 10 times as far away as the other moons. Based on their magnitudes, the objects are estimated to be 160 km and 80 km in diameter.

News Service
Cornell University

October 31, 1997


'Irregular' satellites S/1997 U1 and S/1997 U2 discovered with Mount Palomar's 5-meter Hale telescope
"Discovery image" made Sept. 6, 1997, at the Palomar Observatory by Philip Nicholson, Joseph Burns, Brett Gladman and J.J. Kavelaars. The brighter of the two "new" moons of Uranus, S/1997 U2, is circled in the upper right portion of the frame. S/1997 U1 does not appear in this image. The bright light in the lower left is from the planet, which is out of the frame. The spikes extending from some stars are artifacts of the CCD electronic imaging process. To view a larger version of this photo, click here (126K JPEG).

ITHACA, N.Y. -- Astronomers using the 5-meter Hale telescope on California's Mount Palomar report the discovery of two "new" moons orbiting the planet Uranus.

The objects -- first observed Sept. 6 and 7 by Philip Nicholson and Joseph Burns of Cornell University, Brett Gladman of the University of Toronto and J.J. Kavelaars of McMaster University, and photographed again by the astronomers in late October -- bring to 17 the number of satellites known to orbit Uranus. After subsequent observations by telescopes in Hawaii and New Mexico, the discovery was confirmed today (Oct. 31) by Brian Marsden of the Central Bureau for Astronomical Telegrams at the Smithsonian Astrophysical Observatory. At the time of discovery, the fainter object was located approximately 6 arc-minutes east of the planet, and the brighter object, 7 arc-minutes to the west-northwest.

Estimated to be only about 50 and 100 miles in diameter, respectively, the two Uranian satellites are the faintest ever detected from the ground and are the first "irregular" moons discovered since 1974, when Jupiter's Leda was found.

The small satellites are termed "irregular" because they apparently follow eccentric and inclined paths, more than 6 million and 8 million kilometers, respectively, from Uranus, rather than the more regular paths followed by most larger satellites in equatorial-plane orbits around planets. They are the first irregular satellites discovered at Uranus; Jupiter is known to have eight irregular satellites, while Saturn and Neptune have at least one apiece.

"They also probably have an irregular -- rather than spherical -- shape," said Burns, the Irving P. Church Professor of Engineering and professor of astronomy at Cornell.

"The larger of the two satellites appears quite red in color, which may indicate a hydrocarbon surface that was bombarded by energetic particles from space," Burns said, adding that the color of the smaller moon will not be known until astronomers make further observations of the objects. He called them "enormous, irregular lumps of dark ice and gunk" that were once in their own orbits around the Sun until they were "captured" by Uranus.

"We'll get a rough idea of their shape by watching the objects as they rotate, but they really need to be visited by spacecraft to get fine details," said Nicholson, a professor of astronomy at Cornell. "We still know precious little about these irregular satellites."

For now, the two satellites are seen as fuzzy little dots on images made with CCD (charged-couple detector) camera on the Hale telescope. (The fuzziness is due to the light-distorting effect of Earth's atmosphere; had the photographs been taken by the Hubble Space Telescope, the moons would have appeared as more distinct, but smaller, dots.) The astronomers made the discovery by comparing photographs taken one hour apart and looking for objects that had moved through the stationary background field of stars.

The study was supported in part by funding from the National Aeronautics and Space Administration (NASA) and by Cornell University, which underwrites a 25 percent share of observing time at the Palomar Observatory. Gladman, who completed a Ph.D. in astronomy at Cornell in 1996, is a postdoctoral associate at the University of Toronto. Kavelaars is a postdoctoral associate at McMaster University in Hamilton, Ontario.

At present, the newly discovered moons are designated with astronomical code names S/1997 U1 (image, 295K GIF) and S/1997 U2 (image, 51K JPEG). Following additional observations, the objects will be certified by the International Astronomical Union (IAU) as true planetary satellites, and the discoverers will be allowed to suggest more lyrical names. By astronomical convention, the planet's previously discovered moons, including the last 10 found by the Voyager spacecraft in 1986, carry names, such as Cordelia, Ophelia and Ariel from works by Shakespeare and Pope.

"I guess that precludes my preference," Nicholson joked, "for naming a moon after my cat, Squeaker." Certification by the IAU of newly discovered planetary bodies can take up to two years, he said.

Meanwhile, Burns said, the discovery of two irregular objects orbiting far from a giant planet "gives us one more clue to what the environs of Uranus must have been like soon after its formation."

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