via the Discovery Channel
There are billions of stars in the Milky Way galaxy. One of them is our own Sun. And around the Sun, orbits a system of planets and moons: a solar system.
“Our solar system is clearly a precious planetary system. And it begs the question: are there other solar systems like ours in the universe?
“The marvelous reality is that our own Milky Way galaxy contains some 200 billion stars or so. And many of those stars have their own planetary systems. Our solar system with its eight major planets is not alone; there are other brethren planetary systems out there—by the billions.”
Of course, astronomers hope to find another solar system with a planet like Earth. And they’re off to a good start. So far astronomers have discovered over 360 stars with orbiting planets.
“[…]Stars tend to be orbited by not just by one planet, by usually two, three, four, or a multitude of planets. Planets come in families. Not unlike the family of planets we enjoy here, around our own Sun. […] Planetary systems offer a wide diversity of different architectures sizes, masses of the planets, and so on, rendering our solar system just one type of a planetary system out of thousands. It’s bizarre at the least, if not completely frightening.”
- Prof. Geoff Marcy, Astronomer
It could be that each and every solar system is one-of-a-kind. But they all have one thing in common: each one begins with a star. First, a star is born in a cloud of dust and gas, called a nebula. These are the pillars of creation.
What scientists have been trying to figure out is, “What triggers the star-making process?”
One possibility is that a nearby supernova explosion took place, and rammed into this otherwise innocuous cloud, pushing it, smushing it, compressing it down so that gravity can take over. Once gravity takes over, the cloud begins to shrink, sucking in more and more gas, into a giant, spinning disk. Gravity, at the center, crushes everything into a dense, super-hot ball. It gets hotter, and hotter. Suddenly, atoms in the gas begin to fuse, and the star ignites.
The leftover dust and debris forms a disk spinning around the new star. It contains the seeds of planets, moons, comets, and asteroids.
“We have this marvelous, first-ever tool, by which we can take pictures of planets caught in the act of formation. It’s quite a marvelous opportunity to see the planets around other stars forming, thereby giving us a glimpse as to how our own solar system must surely have formed.”
Scientists understand not only where stars come from, but also how planets grow from the disk of gas and dust that remain. An astronaut in the International Space Station using grains of salt and sugar discovered that in the zero gravity of space, particles of dust don’t float apart; they clump together.
“The dust particles would collide, and stick, and grow into ever-larger dust particles, and eventually rocks, and eventually boulders.”
The bigger the boulder, the more gravity it has. It becomes larger, and heavier, and consumes bigger and bigger rocks. Eventually, some of these rocks grow into planets. This is what happened in our solar system, 4.6 billion years ago. There were about a hundred young planets orbiting around the new Sun. Collisions were inevitable.
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At the beginning, solar systems are violent. So it was with our own. As some of the planets grow larger, and so would the collisions. Only the largest survived—the rest are smashed to pieces. Something very large crashed into young planet Mercury. It blew the crust off, and left behind just the iron core. A similar collision on Earth resulted in our own Moon.
This demolition derby raged for 500 billion years.
“What we see now, Mars, Venus, Earth, Mercury… these planets from the inner solar system, they’re the survivors, they’re the ones who lived through these giant impacts.”
Debris from smashed infant planets ended up in the asteroid belt, a junkyard of rocky, leftover planet parts.
Most of the big impacts happened in the inner solar system. But, one of the planets, Uranus, was hit and knocked on its side. A mystery, since the outer planets formed mainly from gas, and escaped most of the ferocity of the inner solar system’s collisions.
“These rocky cores formed, and gas coalesced and accumulated around them. And this process actually happened fairly rapidly in astronomical terms—in only about a million years. And those are the giant planets we see today.”
Beyond the gas giants Jupiter and Saturn, are Uranus and Neptune. These two are made of gas—and ice. And beyond them, lies the Kuiper Belt, a band of icy rocks, and dwarf planets. We used to think that one Kuiper Belt object, Pluto, was the ninth planet. We’ve since decided that Pluto is, in fact, a dwarf planet—one of many orbiting more than 3 billion miles from the sun.
“There are millions of these out there. They are so far away and so faint that they are hard to see. These are leftover from the formation of the solar system itself.”
The Kuiper Belt marks the edge of the Sun’s influence. There’s not much light, or warmth, out here. But it’s not the end of our solar system. A shell of trillions of icy objects, called the Oort Cloud, is even further out. The Oort Cloud is so far away, light from the Sun takes a full year to reach it.
From the cold, icy edge, to hot star at the center, our solar system seems stable. But something isn’t right: Uranus and Neptune are in the wrong place!
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The planets of the solar system grew from a giant disk of dust and gas. The four inner rocky planets closest to the Sun, and the giant gas planets further out. But Uranus and Neptune seem out of place—there wasn’t enough stuff this far from the Sun to make such big planets.
One theory suggests that Uranus and Neptune formed closer to the Sun and were pushed violently outward from it. So what could have shoved two massive planets clear across the solar system?
“We believe that Jupiter and Saturn got into this funny configuration, where Jupiter went around the Sun twice every time Saturn went around once. And that configuration allows the planets to kick each other more as they pass one another. And that caused the whole system to go nuts.”
The combined gravity of Jupiter and Saturn yanked hard on Uranus and Neptune, and pulled them away from the Sun. As they got pulled outward, the two planets plowed through asteroids and other debris left over from the formation of the other planets. This sent billions of chunks of rock flying in all directions. Some rocks form the asteroid belt. But most were thrown out to create the vast Kuiper Belt.
But the gravitational push from Jupiter and Saturn was so strong that it may have reversed the original position of the two planets (Uranus and Neptune).
“It’s possible that Uranus and Neptune formed in the opposite order—Neptune was closer to the Sun than Uranus. But these gravitational interactions actually swapped their positions.”
It was the blizzard of rocks that Uranus and Neptune ran into that acted like a brake, and slowed them into the orbits they keep today. The idea of planets changing orbits may sound crazy, but scientists have seen it happening in other solar systems. So now, they think it’s just the way all solar systems work.
In one far off system, scientists have spotted a planet as big as Jupiter orbiting its central star.
“Some of these giant planets are found orbiting very close to the host star, taking only days, a few days, to orbit the star. Obviously, such close-in massive planets are blow-torched by the star, which pushes the temperature of the planet to over 2000° C.”
There’s no way a gas giant, could have formed that close-in. It’s way too hot. The only explanation is that it must have formed out there, and moved in here.
The same thing could have happened in our own solar system. Scientists have found large amounts of the element Lithium on the surface of the Sun. Lithium doesn’t normally exist in stars—but it is found in gas planets. Maybe there was another gas giant in our own solar system that spiraled in, and crashed into the Sun. That would explain how the lithium got there.
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In the beginning, solar systems are violent, messy. But, over time, they settle down and become more stable. But stability is just an illusion: Any planet in the solar system is always in danger of total annihilation.
There are all kinds of solar systems in the Milky Way galaxy. Most seem strange compared to our own. Some planets have crazy orbits. Some smash into each other. Others dive into their stars. So, why are the orbits of our own planets so regular and stable?
“Well, that’s because all of the planets have motion left over from the formation of the solar systems. When the nebula collapsed around the Sun as the Sun was forming, there was an intrinsic motion. That gave our planet a velocity. Literally, we are free-falling towards the Sun at all times—but we’re going so fast we keep missing it. That’s what an orbit is.”
-Dr. Michelle Thaller, Astronomer
Think of a merry-go-round. The faster it spins, the farther you are thrown from the center. When it slows down, you lose momentum, and fall back inwards. It’s something like that with the planets. The disk that gave birth to them was spinning—and the momentum left over from that keeps everything going around to this day. Moving at 66,000 mph, the Earth takes almost 365 days—one year—to orbit the Sun. Other planets further from the Sun, have bigger orbits, move slower, and take longer. Saturn orbits the Sun once every 29 years. Neptune takes 164 years. Each planet stays on a precise path around the Sun—which for us is a good thing.
“Our solar system has a somewhat fortunate spacing of the planets, with nearly circular orbits. If our solar system did not have such neat and orderly spacing and orbits, the Earth would not be here—and we wouldn’t be here talking about it.”
The planets are on safe, stable orbits—but billions of comets and asteroids are not. Many come streaking into the inner solar system—and when they do, watch out!
“A meteor crater which we see here today formed as the result of a hundred-and-fifty-foot rocky, iron object coming and slamming into the Earth roughly 50,000 years ago.”
Some of the objects coming our way can be much bigger. Look at the Moon: it’s covered with large impact craters. Earth has been hit too—a lot. But the craters have eroded. We know that a huge asteroid smashed into the Earth off the eastern coast of Mexico 65,000,000 years ago. It was going 45,000 mph, and when it hit, it released more energy than 5 billion Hiroshima bombs. It wiped out 70% of life on Earth. A few more impacts like that could destroy all life on Earth. But, believe it or not, Earth has a giant bodyguard.
“Jupiter is actually probably really important for life on Earth. Jupiter’s gravity is so huge, and it’s just in the right place in the solar system, that it protects the Earth from comets that come from deep in the solar system and swing by the Sun and could possibly hit the Earth. As these comets come by, most of them get knocked out of solar system by Jupiter.”
In 1994, the comet Schumaker-Levi 9 raced toward the inner solar system. But it never got past Jupiter. Astronomers watched as Jupiter tore it to pieces, and dragged its remains down to the planet’s surface.
“We have seen planets smash into Jupiter and create fireballs that were bigger than the Earth. If Jupiter wasn’t there, the impact rate for the Earth would be something like 1000 times more than we see today.”
Lucky for us, Earth has the perfect orbit. We’re close enough to Sun for liquid water—but not so close that it boils away: just the right combination for life.
Question is, if our own solar system could create the perfect conditions, could other solar system do it too? Planet hunters have spotted a solar system 20 light years, and it has a planet just the right size, in just the right place.
Astronomers around the world are looking for new planets, in new solar systems. So far, they’ve discovered 420. Most are huge gas giants, like Jupiter. But, they’re either very close to the star, or much farther away. Then, in 2005, astronomers discovered a system with rocky planets, like our own. These planets orbit a star called Gliese 581.
“This star, Gliese 581, and its four planets is, frankly, quite bizarre relative to our solar system. The four planets we know of all orbit very close to the star than any we know of—closer than the planet Mercury, our closest planet, orbits the Sun.”
But Gliese 581 is a small star—it doesn’t burn as brightly or give off as much heat as our Sun. The planets can orbit much closer without being vaporized.
“There’s one, about 8 times the mass of the Earth, that is getting far enough away from the Sun to be in the habitable zone.”
Like Earth, this planet orbits at a distance where water is a liquid. And where there’s water, there is life.
In March 2009, NASA launched the Kepler Space Telescope. Its mission: to search for planets similar to our own, in new solar systems.
“We may find planets that have methane atmospheres, that have ammonia atmospheres. We may find planets that are covered in heavy organics, or a tar-like material. We may find some covered entirely by water. I think the most glorious quests of the next decade or two, is to learn the full diversity of the family of Earth-like planets that might be out there in the universe.”
With Kepler, astronomers expect to discover hundreds, possibly thousands, of new solar systems.
“Think about our own Milky Way galaxy. The galaxy has roughly 500 billion to a trillion stars. Some fairly large percentage of that have planets. Now think about how many galaxies we know of. We certainly haven’t found all of the galaxies out there yet, but the ones we can take a picture of are about 60 billion galaxies. When you look up into the night sky tonight, simply in the path of your sight, even if you can’t see it, there are billions of solar systems all around you.”
And there could be a solar system with a planet just like Earth. If it happened once, it could happen again. Solar systems don’t last forever. Orbits fall apart. Planets collide. But, even if it doesn’t, in another 5 billion years, a catastrophe will end our solar system as we know it.
Nothing lasts forever. Not even solar systems. Ours may seem stable now, but actually, it’s falling apart. Solar systems begin and end with collisions. The gravitational pull enacted by the planets on each other could cause orbital disruptions. If two planets come close enough together, the resulting cosmic tug-of-war could fling one or both planets out of the solar system. But, one way or another, our solar system is doomed!
Like all solar systems, the end will come: when the star at the center dies. In another five billion years, our own star will run out of fuel and become a red giant. It will heat up, swell, and engulf the first three inner planets. The Earth’s surface will be scorched, the seas will evaporate, and the land will melt. Most of the entire Earth will be completely vaporized.
For a while, the red giant will fall apart, too, leaving behind a tiny corpse of star called a white dwarf.
“From the Earth, this dead, rocky planet that used to harbor an enormously vibrant civilization, there would be this fairly faint dot, which would be our Sun—now a white dwarf: a dying, almost dead, star.”
The remains of the inner planets will continue to orbit the white dwarf. But the giant outer planets will live on, untouched, albeit colder than they used to be. Even though this is 5 billion years in the future for our solar system, it may have already happened to many other systems throughout the universe.
Our solar system emerged from chaos to eventually support life. We were lucky. We’ve just the right amount of planets, at the right place, at the right distance from each—all orbiting the right type of star. But it could have been a very different story.
“There are so many things that are fortunate about our solar system, starting with the Sun. The Sun is a very stable, easy star—a perfect thing for life to evolve around. That’s probably not a coincidence that we’re here.”
An extraordinary chain of events, over billions of years, has made our solar system the perfect place for life to evolve.
“What we see today is not what has always been, and it is not what will always be. We are not unique. It’s just the way things worked out.”
Are we unique among the universe because all the pieces fit to create life? We don’t know. But every week scientists are discovering new galaxies, and it may only be a matter of time before we find out.
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check this out:
Interstellar Space Travel Could Be Possible In As Little As 200 Years
Check out this excellent paper from Marc G. Millis, arguing that interstellar space travel could be possible in as little as 200 years. The paper calculates the amount of energy required for two types of interstellar missions, and uses estimates of world energy output growth to determine when the required energy might be available to such missions. Millis bases his calculations on the fraction of energy made available to current space missions, accounting for various technological innovations and broader considerations that might accelerate or delay serious consideration of interstellar missions. The math suggests that an interstellar spacecraft colony could be achievable in approximately 200 years, and a probe to Alpha Centauri could be launched within 500 years.
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“The reason why the universe is eternal is that it does not live for itself; it gives life to others as it transforms.”
- Lao Tzu, from The Book of Tao
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wildcoyote
