快猫短视频

THE WORLD beneath your feet is no more than 20 kilometres across, its gravity
so weak that you could ride a bicycle up a ramp and into space. But the view of
Saturn is breathtaking. It appears over twice as large in the sky as the full
Moon does from Earth, and that鈥檚 not including its magnificent ring system.

The anonymous rock you鈥檙e standing on was spotted in early August last year
by Brett Gladman, perhaps the most successful moon hunter ever. Based at the
Observatory of the C么te d鈥橝zur in southern France, and with a team of
collaborators all over the world, Gladman has discovered five new satellites
orbiting Uranus, confirmed one orbiting Jupiter, and found no fewer than 12 new
moons of Saturn鈥攁ll in the space of a few years. Meanwhile, a team led by
David Jewitt of the University of Hawaii in Honolulu has found another 11
satellites of Jupiter.

Never before have so many moons been discovered in such a short period, and
the rush isn鈥檛 over. The race is on to find out just how many satellites circle
the outer planets. The numbers could be astonishing. But there鈥檚 more to moon
hunting than bagging trophies. Gladman and Jewitt hope to use this swarm of
moons to find out more about the early days of the Solar System and the
formation of the giant planets.

After Galileo trained his home-made telescope on Jupiter and saw four specks
of light circling the giant planet, new moons appeared only gradually over the
centuries. By 1950, only around thirty had been spotted in the entire Solar
System.

Then NASA鈥檚 two Voyager spacecraft discovered a clutch of new, tiny
satellites during their close passes by the four giant planets between 1979 and
1989. By 1996, 61 planetary satellites were known, and the rate of discovery
seemed to have slowed down again. But now everybody seems to have lost count
after a sudden glut of new finds. In the past couple of years, the total has
risen to at least 90.

Why this deluge of discoveries? It鈥檚 simply that detectors have
improved. 鈥淚n the past, discoveries were made more or less at random,鈥� says
Jewitt. Sometimes, when an astronomer was observing one of the outer planets, he
would stumble across a speck of light that was moving against the background of
stars, indicating that it was either a moon or a stray asteroid or comet.

But then in 1997, Gladman realised that technology could take the
chance out of moon hunting. Modern solid-state cameras can capture faint objects
over relatively large patches of sky鈥攍arge enough to let him trawl for the
satellites of giant planets. 鈥淭he development of large detectors has advanced to
the point where it鈥檚 now possible to search large areas for very faint objects,鈥�
he says.

Gladman started on Uranus and Neptune, the outermost giants in the Solar
System. In some ways, this is easier than looking for companions of Jupiter or
Saturn. Every planet has its own sphere of gravitational influence that might
harbour satellites, called the Hill sphere. Jupiter and Saturn are much more
massive than Uranus and Neptune, so their Hill spheres are much larger. And as
those two planets are closer to the Earth, their Hill spheres look bigger still.
鈥淭he Jovian Hill sphere has an apparent diameter of five degrees,鈥� says Jewitt.
That鈥檚 ten times the diameter of the full Moon. For Uranus and Neptune, however,
the Hill sphere is small and distant, so there isn鈥檛 so much sky to scour.

The drawback is that satellites of Uranus and Neptune are much fainter than
those of Jupiter and Saturn. If you double the distance of an outer planet鈥檚
moon from the Sun, it only gets a quarter of the light. But at the same time you
have also, roughly speaking, doubled its distance from the Earth鈥攕o its
brightness decreases by another factor of four. Overall, it has dimmed by a
factor of 16. Because of this, Gladman says, it鈥檚 no wonder his team didn鈥檛 find
anything orbiting Neptune. 鈥淭ake any of the known irregular satellite systems
and put it at Neptune鈥檚 distance, and it is well below the limit of
observability,鈥� he says.

Gladman鈥檚 team had more luck with Uranus. In September 1997, they found two
new satellites, and then three more in July 1999. These newcomers are not at all
like our Moon, or the other large satellites of the Solar System, such as the
four large Galilean moons of Jupiter. Instead of tracing out nice, circular
orbits around the equator of their mother planet, they move erratically, looping
along strongly elongated, highly tilted orbits
(see Diagram).

Eight such 鈥渋rregular鈥� satellites had already been found around Jupiter, and
another, called Phoebe, around Saturn. But these new discoveries of irregulars
around Uranus made it look as though irregular satellites might be found in
orbit around all giant planets.

Between August and November 2000, Gladman鈥檚 team surveyed the whole Hill
sphere of Saturn, and found 12 new irregulars. With 30 confirmed satellites, the
ringed planet is now the record holder among the giant planets鈥攂ut
probably not for long.

Last year, Jewitt set out to search for fragments of comets torn apart by
Jupiter鈥檚 gravity. Shoemaker-Levy 9, which crashed into the giant planet in July
1994, was just such a fragmented comet. But instead of finding doomed comets,
Jewitt and his colleagues discovered a whole bunch of Jovian satellites. One of
them, labelled S/2000 J1 (for the first satellite of Jupiter found in the year
2000), had in fact been spotted back in 1975 and hadn鈥檛 been seen since. But 10
were completely new.

By late last year, Jewitt鈥檚 team had brought the number of Jovian satellites
to 28, and he expects many more discoveries to follow. 鈥淚 believe Jupiter might
turn out to have something like a hundred small satellites,鈥� he says.

Jewitt has one great advantage in the search for Jovian moons. Whereas
Gladman鈥檚 team must apply for observing time on professional telescopes around
the world, Jewitt can use his university鈥檚 own 2.2-metre instrument at Mauna
Kea, which is equipped with a CCD (charge-coupled device) camera that has a
staggering 64 million pixels. With this equipment, Jewitt should be able to
survey the whole of Jupiter鈥檚 gigantic Hill sphere. 鈥淪o far, we have searched
just a fraction of it.鈥�

So what will all these new irregular satellites tell us? Quite a lot, says
Jewitt. Studying their orbits, size distribution and physical properties should
confirm or disprove the popular theory of how the giant planets acquired these
moons鈥攁nd that might tell us more about the origin of the giant planets
themselves.

The irregular satellites can鈥檛 be recently captured asteroids. To switch from
a Sun-centred orbit into orbiting a planet, the satellite would have to lose a
lot of orbital energy. Something has to put the brakes on, and in the
present-day Solar System there is nothing out there that could do it.

Instead, the theory is that this 鈥渟omething鈥� was the forming planet, back
when the Solar System was growing out of a cloud of gas called the solar nebula.
According to one theory, the core accretion model, a solid core some 15 times
the mass of Earth would slowly pull in gas. Gas giant planets would then take
perhaps a few million years to grow to the size of Jupiter, says Alan Boss, a
theorist from the Carnegie Institution in Washington DC. According to Boss鈥檚
alternative scenario, called the disc instability model, Jupiter-sized planets
form in perhaps 100,000 years by direct gravitational collapse of a dense part
of the solar nebula.

In either case, the forming planet would be surrounded by a disc of material
from the solar nebula, and any asteroid venturing too close would be slowed by
drag from the gas and become trapped in an eccentric orbit with a random
orientation. If this is how it happened, the irregulars have circled their
planets for billions of years.

According to Gladman, the stresses of hurtling along through the
circumplanetary disc would probably break the original body into smaller
objects, which would end up in similar orbits. And the new findings back this
up: the irregular satellites do fall into groups with comparable orbital radii,
eccentricities and inclinations. 鈥淪aturn apparently has four different
groupings,鈥� says Gladman.

But another prediction of the theory seems to be contradicted by the new
discoveries. The smaller satellites within a particular group should end up in
smaller orbits than the larger members, because they would have been more
susceptible to gas drag. But the biggest two of the five new satellites of
Uranus are among the three innermost. 鈥淚t doesn鈥檛 seem to make a lot of sense,鈥�
says Gladman.

It may be that some smaller, inner moons have yet to be discovered. To decide
for sure whether the capture-and-fragmentation theory is right, more
observations are needed.

The theory predicts a specific distribution of object sizes, says
Jewitt, and the colours and spectral characteristics of the satellites should
match those of asteroid fragments. Moreover, he says, no satellites are expected
below a size of around a few hundred metres, as the smallest fragments would
have decelerated too much and burned up in the planet鈥檚 atmosphere. The full
survey of Jupiter鈥檚 system should let him test these predictions.

It might be harder to decide between the two models of how the
giants themselves formed. In both scenarios, circumplanetary discs exist that
could capture the irregular satellites. 鈥淗owever, 鈥� says Boss, 鈥渙ne obvious
difference is in time scales, as the disc instability process would form the
planet and its circumplanetary disc earlier.鈥� So if we knew whether the
asteroids were formed before the planets, we might be able to discriminate
between the two theories.

Right now, a different kind of problem is looming: how will astronomers name
all these new discoveries? The five Uranian satellites that Gladman鈥檚 team
discovered in 1997 and 1999 now have permanent designations鈥擴ranus XVI to
XX鈥攁nd the proper names Caliban, Sycorax, Prospero, Setebos and Stephano.
Like most of the other satellites of Uranus, they are named after characters
from Shakespeare鈥檚 plays.

So far so good. But this passion for tidiness poses problems
elsewhere. Brian Marsden of the Minor Planet Center in Cambridge, Massachusetts,
is on the committee that decides on naming, an international arbiter of taste in
these matters. Marsden says the orbits of S/1999 J1 and S/2000 J1 are also known
well enough to get rid of these preliminary designations. 鈥淚f the discoverers
were to produce acceptable names for these two satellites, they would surely
also at the same time receive their official designations as Jupiter鈥檚
seventeenth and eighteenth satellites,鈥� he says.

But all Jupiter鈥檚 satellites have been named after lovers of Zeus
(the Greek equivalent of Jupiter), or at least after mythological figures
somehow linked to this supreme deity. And, despite Zeus鈥檚 reputation, we are
quickly running out of useful names. It may be necessary to change tack, perhaps
naming them after figures connected to supreme deities from other cultures.

The different irregular satellite groups of Saturn might need to
have separate themes for their names. 鈥淥ne of the new satellites is in a similar
orbit to Phoebe鈥檚, so, preferably, it should also be named after one of the
Titans,鈥� says Gladman. 鈥淔or the other groups, different themes or even other
mythological cultures could be considered.鈥� If Shakespeare can be used for
Uranus, why not name other satellites after Tolkien鈥檚 hobbits, or characters
from Harry Potter?

Gladman has some ideas, but he鈥檚 keeping them to himself for now. Jewitt, on
the other hand, doesn鈥檛 much like the name game. 鈥淭he committee for Solar System
nomenclature is very interested in naming things,鈥� he says, 鈥渟o they probably
won鈥檛 give up on this. But personally, I hope it will one day stop.鈥� If Jupiter
turns out to have a hundred satellites or more, he thinks that a simple
numerical designation would be much more practical. If Jewitt gets his way, you
might one day be admiring the view from Jupiter LXXXVII.