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Our Unknown Solar System
THE SIX BIGGEST MYSTERIES OF OUR SOLAR SYSTEM
Once upon a time, 4.6 billion years ago, something was brewing in an unremarkable backwater of the Milky Way；银河？. The ragbag of stuff that suffuses the inconsequential, in-between bits of all galaxies；星系？银河系？银河？ - hydrogen；氢？ and helium；氦？ gas
with just a sprinkling of；少量？一点儿？ solid dust - had begun to condense and form
molecules. Unable to resist its own weight, part of this newly formed molecular cloud collapsed in on itself. In the ensuing heat and confusion, a star was born - our sun.
We don't know exactly what kick-started this process. Perhaps, with pleasing symmetry, it was the shock wave from the explosive death throes of a nearby star. It was not, at any rate, a particularly unusual event. It had happened countless times since the Milky Way itself came into existence about 13 billion years ago, and in our telescopes we can see it still going on in distant parts of our galaxy today. As stars go, the sun is nothing out of the ordinary.
And yet, as far as we know, it is unique. From a thin disc of stuff left over from its birth, eight planets formed, trapped in orbit by its gravity. One of those planets settled into a peculiarly tranquil relationship with its star and its fellow planets. Eventually, creatures emerged on it that began to wonder how their neighbourhood came to be as it is - and could formulate the following six enduring mysteries of our familiar, and yet deeply mysterious, solar system. 1. THE BEGINNING
1: How was the solar system built?
Looking at the planets of the solar system, you could be forgiven for thinking that if they do belong to the same family, it is by adoption rather than kinship. Not so: the story of the solar system's birth reveals that they are blood siblings, all created from the same molecular cloud whose collapse formed the sun. You might also think that these disparate bodies are scattered across the solar system without rhyme or reason. But move any piece of the solar system today, or try to add anything more, and the whole construction would be thrown fatally out of kilter. So how exactly did this delicate architecture come to be?
When our sun formed, it swallowed about 99.8 per cent of the debris cloud；星云碎片？ around it.
According to the generally favoured picture, the lean pickings that remained were sculpted by gravity into a thin disc of gas and dust encircling；包围？环绕？ the newborn star's midriff (see
illustration). As the dust grains of this disc orbited the sun, they collided and progressively coagulated into ever larger bodies. In the disc's innermost region, the ignition and burning of
hydrogen in the sun made things very hot, so that only metals and silicate minerals with high melting points were present in solid form. Bodies in that region could only reach a certain size -
producing the four small rocky planets of the inner solar system: Mercury, Venus, Earth and Mars.
No such stringent；严格的？ limitations；限制？ existed further out, beyond the "ice line" where
methane；甲烷？沼气？ and water are also present as solids. Here, the developing planets could
grow bigger, and become large enough to start accreting；共生？ gas molecules - mainly hydrogen
- before energy from the sun's increasing glare ripped those molecules apart. That, ultimately, was
how the gas giants Jupiter and Saturn came to be and, further out in still colder climes, the ice
giants Uranus and Neptune. That is the reason astronomers；天文学家？ expect these planets too
to possess rocky hearts beneath their fluid coats.
So far, so straightforward. But when it comes to；当提到？ certain details, the accretion model
becomes rather hand-wavy, says Alessandro Morbidelli of the Côte d'Azur Observatory in Nice, France. For a start, no one really knows exactly how metre-sized boulders coalesced into bodies tens of kilometres across. Solid objects that small would have been buffetted around by the pressure of the gas surrounding them and sent spiralling into the sun before they could ever get together. A promising idea recently proposed is that local patches of turbulence in the gas provided
vortices of lower pressure in which the boulders could collect and coagulate.
A similar problem bedevils the gas giants, whose solid cores must have coalesced in the presence of gas they would later accrete. The risk of such planets being bounced towards the sun is illustrated by the "hot Jupiters" seen in other planetary systems. These are planets roughly the size of Jupiter but orbiting around their stars at the distance of Earth or closer (see "Is the solar system
unique?"). Had anything like that happened in the early years of our solar system, the Earth and other inner planets could well have been slingshotted out of the solar system altogether - although that's no certain conclusion.
According to Phil Armitage of the University of Colorado in Boulder, there's not much sign of any such drama in our neighbourhood. If evidence such as our overlarge moon is any indication, the inner solar system did remain a choppy place for its first 100 million years as the rocky planets consolidated (see "Why are the sun and moon the same size in the sky?"), but it soon settled down.
And according to a theory developed by Morbidelli and colleagues, there was a rearrangement and
expansion of the outer solar system a few hundred million years after the sun was born, when a particular conjunction of the orbits of Jupiter and Saturn gave a gravitational shove that propelled Uranus and Neptune out to the distant orbits they occupy today. Some of the small bodies that scattered on the way fell back towards Jupiter, whose immense gravity may have ejected some of them from the solar system. Deep in space, these unaccreted fragments collected as the hypothetical Oort cloud ("Where do comets come from?").
The knock-on effect of this last gravitational twitch of the solar system may have been a disturbance in the asteroid belt between Jupiter and Mars, creating the Late Heavy Bombardment that showered Earth with meteorites some 4 billion years ago, 500 to 600 million years after the sun formed. Since then, however, the objects that constitute our solar system have settled into a tranquil, if sensitive, balance - to our own inestimable advantage.
2.Why are the sun and moon the same size in the sky?
It is one of the most glorious pieces of natural theatre. Assuming you spend your life on the same part of the Earth's surface, you might witness it once - if you are particularly lucky or very long-lived, perhaps twice. But a total solar eclipse is worth the wait. At the height of totality, the fit of sun and moon is so perfect that beads of sunlight can only penetrate to us through the rugged valleys on the lunar surface, creating the stunning "diamond ring" effect.
It is all thanks to a striking coincidence. The sun is about 400 times as wide as the moon, but it is also 400 times further away. The two therefore look the same size in the sky - a unique situation among our solar system's eight planets and 166 known moons. Earth is also the only planet to harbour life. Pure coincidence?
Almost undoubtedly, say most astronomers. But perhaps it is not so much of one as the bare numbers suggest. Our moon is different. The many moons of the large outer planets - Jupiter, Saturn, Uranus and Neptune - are thought to have originated through one of two processes: from the accretion of a disc of material in the planet's gravity field, in a miniature version of the formation of the solar system's planets, or through the later gravitational capture of passing small bodies. The second possibility is also thought to account for Mars's two small satellites, Deimos and Phobos, the only other moons in the inner solar system.
But our moon is simply too big relative to Earth's own size to have formed easily by either of these processes. Planetary scientists believe there can be only one explanation: in the first 100 million years of the solar system, when unattached debris was still zinging around the inner solar system, a Mars-sized object smashed into Earth. That impact radically remodelled our planet, expelling a huge amount of debris that eventually congealed into our oversized moon.
And here's the best bit. Such a big moon is a big boon for life on Earth. As Earth spins on its own axis, it has a natural tendency to wobble, owing to the varying pull on it from other bodies such as the sun. The unseen hand of the moon's gravity gently damps that wobble, preventing rotational
instabilities which would otherwise have caused dramatic changes in Earth's climatic zones over time. Such instabilities would have made it much more tricky for life to get started on our planet. Earth's position in the "habitable zone" around the sun where liquid water is abundant is undoubtedly the single most important factor in its fecundity. But the presence of a large moon - one large enough to cause total eclipses - might also have been crucial. If so, that has important consequences for the search for life on other planets.
Since the impact that created it, the moon has been moving steadily away from us, currently about 3.8 centimetres per year. The dinosaurs did not see eclipses like we do: the moon was too close 200 million years ago, more than big enough in the sky to block out the entire sun. Equally, any occupants of Earth in a couple of hundred million years' time will not see total eclipses at all, as the moon will appear too small.
Our luck seems the result of two coincident timescales: that of the recession of an impact-formed
moon, and that for the evolution of intelligent life. If you should be fortunate enough to experience a total eclipse in your lifetime, consider this intriguing possibility: that large moon might be the reason you are there.
3. Is there a Planet X?
If we know enough to say the solar system is a filigree construction, we might reasonably assume we know where all its bits are. But lurking in the solar system's dark recesses, rumour has it, is an
unsighted world - Planet X, a frozen body perhaps as large as Mars, or even Earth. Planet X would be the most significant addition to the solar system since the discovery of Pluto, the now notorious non-planet, in 1930. When the International Astronomical Union voted to downgrade Pluto to dwarf planet status in 2006, they established three criteria for a fully blown planet in our
solar system: it must orbit the sun; its gravity must suffice to mould it into a near-spherical shape; and it must be massive enough to have ploughed its orbit clear of other bits and bobs. Pluto falls down on this third point. It is just one of many Kuiper belt objects (KBOs), icy bits of debris that pepper space from Neptune's orbit at 30 astronomical units out to around 50 AU, where 1 AU is the distance between Earth and the sun.
Any new object would have to be well clear of the Kuiper belt to qualify as a planet. Yet intriguingly, it is studies of the belt that have suggested the planet's existence. Some KBOs travel in extremely
elongated orbits around the sun. Others have steep orbits almost at right angles to the orbits of all the major planets. "Those could be signs of perturbation from a massive distant object," says Robert Jedicke, a solar system scientist at the University of Hawaii.
That is by no means a general consensus. An early, slow outward migration of the giant planets (see "How was the solar system built?") could also explain some of these strange KBO orbits -
although it has difficulty explaining all of the belt's observed properties.
Over the past 20 years, huge swaths of the sky have been searched for slowly moving bodies, and well over 1000 KBOs found. But these wide-area surveys can spot only large, bright objects; longer-exposure surveys that can find smaller, dimmer objects cover only small areas of the sky. A Mars-sized object at a distance of, say, 100 AU would be so faint that it could easily have escaped detection.
That could soon change. In December 2008, the first prototype of the Panoramic Survey Telescope
and Rapid Response System (Pan-STARRS) was brought into service at the Haleakala
observatory on Maui, Hawaii. Soon, four telescopes - equipped with the world's largest digital cameras, at 1.4 billion pixels apiece - will search the skies for anything that blinks or moves. Its main purpose is to look out for potentially hazardous asteroids bound for Earth, but inhabitants of
the outer solar system will not escape its all-seeing eyes.
Jedicke and his team are busy developing software to spot objects automatically using Pan-STARRS. The discovery of a further planet would be thrilling, he says. The only explanation for its presence there would be that large bodies coalesced very early in the solar system's history, only to be ejected by the gravity of the giant planets later on. That would firm up our ideas about how the solar system must have developed, and perhaps be a stepping stone towards its even more distant recesses (see "Where do comets come from?").
4.Where do comets come from?
Few cosmic apparitions have inspired such awe and fear as comets. The particularly eye-catching Halley's comet, which last appeared in the inner solar system in 1986, pops up in the Talmud as "a
star which appears once in seventy years that makes the captains of the ships err". In 1066, the comet's appearance was seen as a portent of doom before the Battle of Hastings; in 1456, Pope Callixtus III is said to have excommunicated it.
Modern science takes a more measured view. Comets such as Halley's are agglomerations of dust
and ice that orbit the sun on highly elliptical paths, acquiring their spectacular tails in the headwind of charged particles streaming from the sun. We even know their source: they are Kuiper belt objects (KBOs) tugged from their regular orbits by Neptune and Uranus.
But there's a problem. Certain comets, such as Hale-Bopp, which flashed past Earth in 1997,
appear simply too infrequently in our skies. Their orbits must be very long, far too long to have an origin in the Kuiper belt. The conclusion of many astronomers is that the known solar system is surrounded in all directions by a tenuous halo of icy outcasts, thrown from the sun's immediate vicinity billions of years ago by the gravity of the giant planets.
This celestial Siberia is known as the Oort cloud, after the Dutch astronomer Jan Oort, who proposed its existence in 1950. This diffuse sphere of material encircling the solar system has
never been seen, but if the longest-period comets are anything to go by, it must be vast, reaching out about 1000 times further than the outer edge of the Kuiper belt. At such huge distances, it would not be passing planets that throw the comets sunwards - it would be the tug of the Milky Way and nearby stars. The Oort cloud would truly be where our solar system meets the void. Unfortunately, if looking for Planet X is difficult, spotting the Oort cloud is a nightmare. It is simply too dim and distant, and its pieces too small, for telescopes to spy. That is unfortunate, as counting and estimating the size of such objects could help in reconstructing a picture of the sun's birthplace, and perhaps provide us with a glimpse of the unadulterated material from which the giant planets were pieced together.
So far, the only information about this primordial rubble comes from stray comets and the largest KBOs, which should have a similar composition. "That's like trying to figure out what a whale looks like from the exposed blowhole on one side and the tip of the fin on the other," says Hal Levison, a planetary scientist at the Southwest Research Institute in Boulder, Colorado.
Even so, mapping the rest of the whale might be only a few decades off. Oort cloud objects should dim and diffract the light coming from distant stars. These occultations last just a fraction of a second, but astronomers can use them to measure the size and distance of the intervening body, a technique already being put to work on KBOs. Flickers induced by turbulence in Earth's
atmosphere make the subtle detections of the more distant Oort objects impossible from ground-based detectors, but future space-telescope surveys should be able to detect them in great numbers.
Other mysteries remain. The numbers and trajectories of the long-period comets seen so far suggest that the Oort cloud contains trillions of objects a kilometre across or larger, with a combined mass several times that of Earth. That is more material than our current ideas about the solar system's formation can explain - which means that our models might need a fundamental overhaul, says Levison.
5. Is the solar system unique?
Since the first discovery of a planet orbiting another star in 1992, some 280 alien solar systems
have been identified. Most look quite unlike ours, and for good reason. Planets are mainly spied by the way their gravity makes their host star wobble as they orbit. The smaller the planet, the smaller the wobble: lightweight planets like Earth produce effects too small to detect with current technology.
Most known extrasolar planets, or exoplanets, are gas giants similar in size to Jupiter or Neptune, but orbiting close in, within a few AU (Earth-sun distances) of their stars - 6 or 7 per cent of sun-like stars seem to have such satellites. The prevalence of giant planets orbiting at greater, Jupiter-like distances from their parent star is unknown. They would take a decade or more to complete an orbit, and few gravity-wobble surveys have been watching long enough to detect them.
According to the standard picture of solar system formation (see "How was the solar system built?"),
gas giants should not form that close to their host stars, as the heat means not enough solid material is present to make a sufficiently large rocky core. However, while the orbits of the planets in our solar system are almost circular, those of many of these giant exoplanets are often highly elliptical. This might provide a solution to the mystery: most solar systems seem to have had more
chequered histories than our own, with initially distant giant planets competing for living space and bouncing each other into strange orbits closer in.
Definitive conclusions are difficult until we know the limitations of our observations. "It might be that we see solar systems with very violent histories because those are the only ones we can see," says
Phil Armitage of the University of Colorado, Boulder. Results from two sensitive space-based planet hunters should help reduce the uncertainty: the French-led COROT mission, which launched in
December 2006, and NASA's Kepler, scheduled to blast off in March this year.
A foretaste of what they might find is given by the 10 or so known "super-Earths" - planets with just a few times Earth's mass. If the picture of planet formation gleaned from our solar system is correct, these are rocky worlds like our own. Two of them, Gliese 581 c and d, are at the sort of distance
from their parent star at which liquid water might exist on their surfaces, depending on the warming effects of greenhouse gases and cooling effects of clouds in any atmospheres they might possess. There are other hints that rocky planets are more common than our first observations suggest. Dust close in to young stars, reported in 2008 by NASA's Spitzer Space Telescope, points to collisions
connected to planet formation, and suggests that rocky worlds form around 20 to 60 per cent of stars.
But other evidence from Spitzer of dust circling much older stars dampens the prospects for tranquil rocky worlds that could harbour life. Nine in 10 solar systems seem to be more dusty than our own, in some cases by a factor of 20 or more. As planet formation is expected to be a relatively cursory process within the first 100 million years or so of a star's existence, that dust is probably the remnant of catastrophic comet collisions later in the life of these solar systems.
Fortunately, our inner solar system is an exclusive club with heavyweight bouncers on the door. The powerful gravity of the more distant giant planets - Jupiter in particular - often ejects comets before they have a chance to penetrate the solar system's inner sanctum.
That's another reason to be glad our solar system is how it is. Ultimately, whether it is uniquely so will remain a mystery until we get down to seeing Earth-sized exoplanets, as well as giant planets farther out from their stars, says Jonathan Lunine at the University of Arizona in Tucson. "The simple, honest answer is that we still don't know."
6.How will the solar system end?
We live in uninteresting times. Since the ructions that created the planets in the solar system's first 100 million years (see "How was the solar system built?") - and apart from an early migration of the
giant planets and the odd colliding comet not swept safely aside by Jupiter - nothing much has really been happening. The planets circle like clockwork, the sun burns steadily, and even delicate life has survived on at least one world.
It cannot last. Something unpleasant is bound to shatter this comfortable calm.
Our sun will die, of course, about six billion years from now. But things could get ugly long before that. The steady gyrations of the solar system today may conceal the seeds of chaos. Even the tiniest of irregularities can build up over time, gradually altering the paths of the planets. Between now and final sundown, it has been calculated, there is a roughly 2 per cent chance of catastrophe. Mars might drift too close to Jupiter and be thrown out of the solar system. If we're very unlucky, hot-headed Mercury could run wild and smash into Earth.