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BBC The Planets Different Worlds

orbit a single sun and are bound together by its gravity Closest to the sun lies Mercury, a tiny world of iron and rock, barely discernible in the glare. Then Venus, perhaps a second Earth, hidden beneath a blanket of cloud. Then Earth. And beyond us, Mars, the Red Planet. It has seasons, polar caps, and the possibility of life. Far beyond these rocky worlds are the distant giants. Jupiter, over 1,000 times bigger than the Earth, and Saturn, with its distinctive and dramatic rings. The two remaining planets are 15 times the size of the Earth, yet they are so distant that they appear as the faintest of stars. Uranus - an aquamarine mystery. And finally, Neptune, a world that moved unevenly across the sky. This irregular movement suggested the presence of a more distant planet, whose gravitational tug might be toying with Neptune's orbit - Planet X. February 18th, 1930. Clyde Tombaugh, sitting in an office very near to where we are sitting right now, looking at the photographs that he had taken of the night sky... Sitting with his eye at the eyepiece of that blink comparator back there. And he had been searching on the plates that were centred on a star in the constellation of Gemini, the Twins. He had started that morning. He had moved very closely, very slowly across, click, click, seeing one image, then the other, then the other, keeping on moving back. Planet X was soon named Pluto. It marks the end of the solar system. A tiny world of ice, smaller than our moon, now known to have its own satellite, Charon. But Pluto patrols the outer edge of the solar system, in the distant realm of giants. Worlds of swirling water, like the azure Neptune, and Uranus, which mysteriously orbits the sun spinning on its back. Pluto lies way beyond the gargantuan worlds, the gas planets that have no landscapes: Saturn, with wind reaching thousands of kilometres per hour, and Jupiter, that has an Earth-sized storm that has lasted for centuries. The closest worlds to the sun are small islands of rock and iron. Mars, with its faint atmosphere of carbon dioxide, and Venus, smothered in clouds of sulphuric acid. Then there is Mercury, boiling in sunlight, and freezing at night. Nine different worlds, with seemingly little in common, save that they orbit a single sun and are bound together by its gravity. And then there is the Earth. A small planet in the measure of the solar system. It has a thin atmosphere that clings to a rocky surface. But the Earth is different. It is special. It has life. What process could create such a variety of different worlds? Hal Levison is at the forefront of a branch of astrophysics that is still struggling with the mystery of how the planets formed. It's amazing, when you consider that all planets in the solar system: the Earth and the rest: the rocky planets, the cores of the giant planets, Jupiter and Saturn, and the majority of the outer planets, Uranus, Neptune and Pluto, formed from material that is very fine pieces of dust, much finer than the dust I'm holding in my hands. About the consistency or size of particles of dust in cigarette smoke. I was an astrophysicist, interested in, sort of, an obscure type of galaxy, when about five years ago I got the bug of trying to understand how material like this can form the planets that we see today. By the 18th century, astronomers had discovered that galaxies are filled with drifting clouds of gas called nebulae. Perhaps these clouds were the raw materials of the planets. Two men, the philosopher Immanuel Kant and the mathematician Simon de Laplace, looked at the uniform direction of the orbits of the planets in the sky. They suggested the planets were a relic of a cloud of dust and gas that circled the sun during its formation. In a single process, they concluded, the solar system was born. The idea was elegant, and quite brilliant, but the complex details of their theory lay centuries in the future. Its proof had to wait for the arrival of the space age. The arcing horizon was a humbling reminder that we were living on a gigantic ball of rock and iron. How such a world could have grown from a cloud of dust seemed more baffling than ever. George Wetherill has dedicated his career to the question of planet formation. When he started his work, the science was dominated by one man. No great scientist ever devoted his life to understanding this problem. It was sort of a hobby, something they did on the side. And he tried to identify what all the scientific problems are that you need to understand and need to solve, in order to understand the grand problem of the formation of the solar system. He set about structuring this complex process into comparatively simple stages. The first stage is still not fully understood. Remember, we're starting off with very fine pieces of dust, and the process of how you get from something like that to something the size of a boulder, or even the size of a mountain, is actually not very well understood. The party line of (the ... you know) what most people think actually happened, was that you had this disc of dust. Dust sort of settled into the mid plain of this protoplanetary nebula of this disc. And you got what's called gravitational instability that formed big clumps, things the size of maybe 100 metres in diameter. Safronov's second stage was less complex. It was called accretion. He calculated that in a remarkably quick time, the clumps would gather together, building the embryos of planets. As they grew, a new force became significant - gravity. And that is, as these things start to grow, the bigger something gets, the more it can eat. So you end up with this runaway situation, where the bigger guys are getting bigger still faster than the other guys are, and it's sort of a race to eat up all the little guys. And so you start off with an uncountable number of objects that are the size of mountains. And you end up with maybe 100, in the inner part of the solar system, objects about the size of the Moon, or maybe going up to the size of Mars. Competing worlds sucked in the surrounding debris until there was simply no more to be had. How that army of worlds became just four was still a puzzle. But he had a hunch that the process would leave those planets spattered with the scars of impact. Was this what we could see on the moon? Unknown to the West, hehad taken a giant stride towards a theory of planet formation. Perhaps, somewhere in the solar system, there might be a planet bearing the hallmarks of his theory. In 1957, the Americans announced that they were preparing to enter the space age. They were about to launch the world's first artificial satellite. In the Soviet Union, Korolev acted immediately. For Korolev, it was the beginning of the race with Americans. And he wanted to be first, he wanted to be ahead of the Americans, like all of us. And I think he wanted to do this maybe 100 times more than any others. Then he called my father and told (him), "I want to launch this first satellite." "Let's do this before the Americans, as soon as we can." It would be a huge gamble, but finally Khrushchev agreed to let him try. Now Korolev had to convince his engineers that they could do it too. On October 4th, 1957, while the Americans were still finalising their plans, Sputnik was launched. 40 years on, and Korolev's achievement is still celebrated in Russia. That evening, he was very proud, he realised that it was a great achievement. And next day, he understood that the reaction of the outside world was much stronger than it was in our country, and really the feeling was much stronger than even his feeling, specially in the United States. Korolev's rockets had opened the door to space. The planets were beckoning. Bruce Murray is a veteran of the US space programme. When his career started, the planets seemed a very long way away. He still remembers the first time he saw Mars through a telescope. And it just blew me away. I was so taken with the fact that here was a real object, it was three-dimensional, or seemed to be three-dimensional. It was colourful, glowed, and really drove home to me there's another place out there, a real place, not just something I studied in school or somewhere. As a young man, Bruce Murray was taken under the wing of physicist Bob Leighton, who had developed a way to make time-lapsed films of the planets. The images were extraordinary because they could show the planet rotating, you could time-lapse it, take one frame, wait a minute, take another frame, and so forth and make this time-lapse. And it brought to everybody the image of Mars that the most dedicated astronomers only infer, because they don't see it that way either, they have to remember all those frames. It was an extraordinary achievement. And he did it for fun. Leighton's films brought the planets to life. For the first time, astronomers could see one of the moons of Jupiter orbiting its giant parent. The outer planets, the ones that are huge masses of gas, in the case of Jupiter and Saturn, you could actually see some beautiful structure. The first thing that strikes one, as in the inner solar system, is diversity - "My Lord, everything is different." But Mars, the Earth's smaller cousin, was always the most tantalising. Leighton could see mysterious dark patches rotating with the planet. But what would a close encounter with the surface reveal? In 1963, the American probe Mariner 4 set off to send back the first pictures from another planet. Bob Leighton was charged with bringing back the images. ..and blue clouds, oh, yes... He asked Bruce Murray to join him. I was dragged or sucked along, however you want to look at it, into this wonderful experience, of becoming the first experimenters to look at Mars through a close-up camera. This is Mariner Control Center at JPL. The spacecraft is 134.217 million miles from Earth and 50,142 miles from Mars. The first picture will cover an area of approximately 176 miles square on the sunlit bit of the planet. Craters were exactly what heexpected. Soon, Safronov's ideas were being discussed in the West, where superior technology allowed George Wetherill to take the accretion theory further. I'd called it the planetesimal problem. And it simply says that you've got a lot of objects, small planets, moving around the sun in orbits. And what you'd like to understand is how they accumulate together to form large planets. Wetherill's computers uncovered a terrifying period of planet formation. What you actually find if you do the problem with the computer is that, as they grow, they start to perturb one another into orbits which cross the orbit of another planet. Soon, the neat orbits of Safronov's army of planets became fatally disrupted. As they started tugging each other off course, the solar system was brimming with loose cannon. World-shattering collisions were inevitable. George realised that it was sort of like a wild frat party. All sort of hell breaks loose in the inner part of the solar system. Things are swung around, half the stuff is either... hits the sun or gets thrown out to Jupiter, which can then knock it out of the solar system. It's a very violent, happening party. If Wetherill was right, then during this period, the inner solar system must have been strewn with planetary debris. The four surviving planets would have had to endure a final stage of intense bombardment. In 1973, George Wetherill got the chance to test his work. Mariner 10 was on its way to Mercury. 78 million kilometres from Earth, and way beyond the scope of even the most powerful telescopes, the surface of this planet was a total mystery. The first pictures of Mercury started coming and first that we saw was a fuzzy ball. And you could (sort of) imagine you saw craters, but after a while but it got closer and closer. Pretty soon, it started to look just like the Moon. But what of the Earth? Surely our planet could not have survived unscathed? In the field, Hal Levison gets a real sense of the violence that rained down on the planets, including our own. This hole in the ground was made in a matter of seconds. Despite being a very awesome sight, something that tells us that the solar system is still active and that things are still running into each other, It's a relatively small insignificant hole in the ground. 50,000 years ago, a 50-metre fragment of a world blown apart billions of years earlier careered into the Earth in what is now Arizona. Here is evidence of the final stages of accretion. But what of the worlds that dwarf the inner planets? How does the accretion theory account for the gassy giants that rule the distant regions of the solar system? We have very different planets, types of planets, as we get farther from the sun. That's because as you get farther from the sun, the temperatures drop. And particularly about four times more distant from the sun than the Earth is, we hit a point where water would condense and become a solid. With water turning to ice, the amount of material available to form the outer planets was far greater. Jupiter and Saturn grew so large that they started sucking in the primordial gases from the original dust cloud, swelling them to hundreds of times the mass of the Earth. Their orbits were also disrupted. We can find no traces of impacts in their gassy atmospheres, but evidence can be seen in their rotation.


  BBC The Planets Different Worlds orbit a single sun and are bound together by its gravity
 
This region was populated with many more worlds than exist today
This region was populated with many more worlds than exist today
  Jupiter grew a little bit during the week of July 16, 1994
Jupiter grew a little bit during the week of July 16, 1994
  In the inner solar system, where there are now four planets, there were once upwards of 100
In the inner solar system, where there are now four planets, there were once upwards of 100
  After a journey of eight months, Mariner 4 was homing in on its target
After a journey of eight months, Mariner 4 was homing in on its target
 
It is believed that a world the size of the Earth collided with Uranus. Today, Uranus still rolls around the sun on its back. When did these planet-building impacts come to an end? I've found a lot of comets. And I've been involved now in the discovery of 21 of them. There is nothing like the night we found the Shoemaker-Levy 9. We had no idea how important that discovery was going to be. We had made page 23 in the London Times, that Carolyn and Jean Shoemaker and I discovered this comet. Interest increased several months later, when it was announced that Shoemaker-Levy 9 was on a collision course with Jupiter. This was not page 23 of the London Times, this was now page 1. This was a different story. Shoemaker-Levy 9 was going to show us what it's all about. In all of civilisations, since Galileo first looked through a telescope, in 1609, and since he first looked at Jupiter, in 1610, this is the first time that we all ever have seen a comet strike a planet. July 16th, 1994. Impact day. And every available telescope is trained on Jupiter. This is how the solar system was built, comets hitting planets. Comets first hitting each other. Very slowly, it's kind of almost an embrace rather than a collision, and then these objects get bigger and their gravity gets bigger, the collisions get faster, and the speed gets higher, and it gets more violent. As the solar system reaches its teenage years it's become a little bit dysfunctional. And finally, when does it end? What Shoemaker-Levy 9 taught us is it hasn't happened yet. Right then, in the summer of 1994, around Jupiter, there's a big yellow police fence, that says danger, keep out, solar system under construction. It's still happening. Water was dumped on Jupiter. Here, then, are the gas giants. Jupiter and Saturn mark the current limit of the planet builders' theories. Far beyond these gargantuan worlds lie the ice giants, Uranus and Neptune. But out here, the accretion theory runs into trouble. The formation of Uranus and Neptune are the greatest mysteries in the formation of the solar system, because everything goes more slowly at greater distances from the sun, so all these processes slow down. When we try to run the same computer programs out there that we did in the terrestrial planet zone, we don't get planets forming.
No matter what we do, we can't form Uranus and Neptune using those kind of models. No matter how hard I try I can't make Uranus and Neptune go away. They're there, and our models can't make them. So we do indeed have a long way to go before we really figure all this out. How these worlds formed so quickly is a puzzle. Scientists don't know enough about early conditions this far from the sun. What kinds of worlds went into the formation of Uranus and Neptune? In 1992, two astronomers were surveying the space beyond Neptune when they found a substantial chunk of ice. Since then, they have found many more. Called the Kuiper Belt, it is now thought that they are the building blocks of ice giants that never were. The Kuiper Belt is a region where the small ice mountains that we've talked about actually started accreting and building into larger things. That's really to me the region we need to look at, because what happened there is planet formation started there, and it was frozen in at some intermediate state. And trying to understand that will let us know in detail how the accretion process started, but what shut it off is also going to be interesting, and will tell us something about the process as well. So to me, the future really lies in the outer part of the solar system. But there is a planet that lies at the inner edge of the Kuiper Belt. 70 years after its discovery, the strange, tiny world of Pluto may at last be making sense. Pluto was discovered in 1930, and it was the oddball of the solar system. Most of the planets are in nice circular orbits. Not Pluto. Most of the planets are set in this plane that represents the accretion disc. Not Pluto. And it was just an oddball, it was small and icy, and had no similarity to anything else that we really knew about. Could this small, icy world be one of the survivors of accretion? A world that somehow escaped being swallowed up by the growing Neptune, or being hurled out of the solar system? Could Pluto be the missing link in the formation of the ice giants? Turns out Pluto was just the largest known member of this population. So it went from being this lonely remote oddball, to being essentially the grandfather of a population. And we were talking about the Kuiper Belt probably has more objects in it than any other region in the solar system. So it's the most populous region in the solar system and yet we didn't know about it 10 years ago. In the 40 years since Mechta broke free from the Earth's gravity, we've sent probes to all the planets. We've sampled the corrosive clouds of Venus, and recorded planet-wide thunderstorms on its surface. We've survived dust storms on Mars, and seen canyons that could swallow countries. We've mapped the icy moons of Jupiter, and plunged into its atmosphere. We've skimmed the rings of Saturn. We've seen active geysers on the most distant and freezing moon in the solar system. But just as the first stage of our reconnaissance of the planets draws to a close, we have a new region to explore. In 1992, Clyde Tombaugh got a request from NASA - permission to visit his planet. Clyde was melted. He melted when he got that letter. He felt that all of his life's effort, with his work at White Sands, was coming to a head. He felt that that letter was really a sign that NASA, through their mission to Pluto, was finally acknowledging him as the man that he really was. Clyde Tombaugh died in 1997. Pluto Express is planned to launch in 2003. It will take 12 years to reach its goal. After analysing Pluto's composition, it will head out in search of a Kuiper Belt object. Perhaps something in their cratering record or their chemistry will provide the final piece of the creation jigsaw. Whatever the craft finds, Pluto's importance in the grand order of the solar system is assured. It will be a manned mission to Pluto in a very special sense. It's not going to have a real living person, but you can bet it's going to have Clyde's spirit on board on its way to Pluto, to see really what kind of a planet that little guy really is.