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The Earth Ordinary or Extraordinary. The Flat Earth.
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The Flat Earth The Flat Earth model is an archaic belief that the Earth's shape is a plane or disk. Many ancient cultures have had conceptions of a flat Earth, including Greece until the classical period, the Bronze Age and Iron Age civilizations of the Near East until the Hellenistic period, India until the Gupta period (early centuries AD) and China until the 17th century. http://www.ancient.eu.com/Bronze_Age/
Ancients-Aristotle, Ptolemy, medieval Christians-all thought we were at the center of the universe, sort of the throne of the cosmos, the most important place that everything revolved around. Then Copernicus and Kepler came along and said they can explain the movement of the planets better by assuming that the sun is at the center and that the planets including Earth-revolve around it.
Scientists later determined the sun isn't at the center of the universe; that we aren't at the center of the galaxy; and that the universe ultimately had no center, because scientists came to believe in the nineteenth century that it was infinite and eternal.
COPERNICAN PRINCIPLE • We revolve around a typical star in an average, mundane part of the universe, and that there's nothing particularly unusual or special about Earth • Known as the Principle of Mediocrity or the Copernican Principle
Fermi Paradox • Two scientists with this world view, Drake and Sagan beamed their message to the large concentration of stars called globular cluster M13. Their theory was that by transmitting their greeting toward a place packed with stars, there would be a higher chance of detection by an intelligent civilization. The Milky Way is home to fewer than 200 globulars. One of the best known is M13, the Hercules Cluster. It’s well up in the east at nightfall, in the constellation Hercules. One Million Advanced Civilization exist in our galaxy
Rare Earth Hypothesis The hypothesis argues that complex extraterrestrial life requires an Earth-like planet with similar circumstance and that few if any such planets exist. The term "Rare Earth" originates from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist from the University of Washington.Planets, planetary systems, and galactic regions that are as friendly to complex life as are the Earth, the Solar System, and our region of the Milky Way are very rare.
The Rare Earth Response: The Earth should already have been colonized, or at least visited. But no convincing evidence of this exists. Furthermore, no confirmed signs of intelligence elsewhere have been spotted, either in our galaxy or in the more than 80 billion other galaxies of the observable universe. Hence Fermi's question, "Where is everybody?”
More and more, astronomers are learning how the other planets tie into the habitability of Earth. For example, George Wetherill of the Carnegie Institution showed in 1994 that Jupiter-which is huge, more than three hundred times the mass of the Earth-acts as a shield to protect us from too many comet impacts. It actually deflects comets and keeps many of them from coming into the inner solar system, where they could collide with Earth with life-extinguishing consequences. "This was illustrated very nicely by the impact of Comet Shoemaker Levy 9 into Jupiter in July, 1994. This comet was attracted by Jupiter's tremendous gravitational pull and broke into fragments, with all of there hitting Jupiter. Even Saturn and Uranus participate in that kind of Comet catching.
Circumstellar Habitable Zone. That's the region around a star where you can have liquid water on the surface of a terrestrial planet. This is determined by the amount of light you get from the host star. You can't be too close, otherwise too much water evaporates into the atmosphere and it causes a runaway greenhouse effect, and you boil off the oceans. We think that might be what happened to Venus. But if you get too far out, it gets too cold. Water and carbon dioxide freeze (The Goldilocks Effect)
Other planets in our inner solar system protect us from getting bombarded by asteroids from the asteroid belt. The asteroids are mostly between the orbits of Mars and Jupiter. Our first line of defense is Mars, being at the edge of the asteroid belt. It takes a lot of hits for us. Venus does too. If you want to get an idea of the stuff that probably would have hit the Earth, look at the surface of the moon. The moon, unfortunately, has too little surface area to provide much protection, but it's a nice record."
As you go further out from the sun, you have to increase the carbon dioxide content of the planet's atmosphere. This is necessary in order to trap the sun's radiation and keep water liquid. The problem is that there wouldn't be enough oxygen to have mammal like organisms. It's only in the very inner edge of the Circumstellar Habitable Zone where you can have low enough carbon dioxide and high enough oxygen to sustain complex animal life.
If the Earth's distance from the sun were moved by, say, five percent either way, Disaster would happen. Animal life would be impossible. The zone for animal life in the solar system is much narrower than most people think. That's why you need a circular orbit like the one Earth has. You don't just want to be in the Circumstellar Habitable Zone part of the time; you want to be in it continuously. It doesn't do you any good to have melted water for four months and then have the whole planet freeze up again.
The sun really is unusual after all. For instance, it's among the ten percent most massive stars in the galaxy. In fact, if you pick a star at random, you're likely to pick one that's far less massive than the sun, usually red dwarfs, which make up about eighty percent of stars. Another eight or nine percent are called G dwarfs, most of which also are less massive than the sun. The sun is a yellow dwarf; technically, it has a G2 Spectral Type."
Red Dwarfs don't produce much ultraviolet light, which you need early on to build up oxygen in the atmosphere. Scientists believe that the oxygen in the Earth's atmosphere was built up at first by the ultraviolet radiation that broke up water into oxygen and hydrogen. The oxygen was allowed to build up in the atmosphere, while the hydrogen escaped into space, because it's lighter. But you get very little blue light from a red dwarf, so this phenomenon wouldn't occur as rapidly and you wouldn't get the build up of the oxygen you need to sustain life.
Our sun is not only the right mass, but it also emits the right colors-a balance of red and blue. As a matter of fact, if we were orbiting a more massive star, called an F dwarf, there would be much more blue radiation that would build up the oxygen and ozone layer even faster. any momentary interruption of the ozone layer would subject the planet to an immediate flood of highly intense ultraviolet radiation, which would be disastrous to life.
Johannes Kepler, the seventeenth-century astronomer who fanned the flames of the Copernican Revolution, gazed at the moon and believed he discerned caves that were populated by moon people. He even wrote a book in which he fantasized about what their lives might be like.;' A century later, William Herschel, who gained fame by discovering Uranus, thought he made out cities, highways, and pyramids on the lunar landscape. As scientific knowledge grew, dreams of finding lunar civilizations dissipated. Everyone came to agree that the moon cannot support life. Yet surprising discoveries in recent years have shown the opposite to be true: the moon really does support life-ours! Scientific evidence confirms how this parched, airless satellite actually contributes in unexpected ways to creating a lush and stable environment a quarter of a million miles away on Earth.
There was a remarkable finding that the moon actually stabilizes the tilt of the Earth's axis,” he said. "The tilt is responsible for our seasons. During the summer, in the northern hemisphere the north pole axis is pointed more toward the sun. Six months later, when the Earth is on the other side of the sun, then the south pole is more pointed toward the sun. With the Earth's tilt at 23.5 degrees, this gives us very mild seasons. So in a very real way, the stability of our climate is attributable to the moon."
The Great Moon Hoax • http://www.museumofhoaxes.com/hoax/archive/permalink/the_great_moon_hoax • http://www.npr.org/2011/11/18/142512088/is-a-moon-necessary-for-a-planet-to-support-life
Water World A terrestrial planet must have a minimum mass to retain an atmosphere. You need an atmosphere for the free exchange of the chemicals of life and to protect inhabitants from cosmic radiation. And you need an oxygen-rich atmosphere to support big-brained creatures like humans. "The bigger the planet, the higher the surface gravity, and the less surface relief between the ocean basins and the mountains
The rocks at the bases of mountains can only withstand so much weight before they fracture. The higher the surface gravity of a planet, the greater the pull of the gravity on the mountains, and the tendency would be toward creating a smooth sphere.
Think what would happen if our planet were a smooth sphere. The Earth has a lot of water in its crust. The only reason we're not a water world right now is because we have continents and mountains to rise above it. If you were to smooth out all the land, water would be at a depth of two kilometers. You would have a water world-and a water world is a dead world
Water World Salinity We have life on Earth because we have the energy-rich sunlit surface of the oceans, which is teeming with mineral nutrients. Tides and weathering wash the nutrients from the continents into the oceans, where they feed organisms
Huge salt deposits accumulate on the continents, and the salt content of the ocean doesn't get out of control. But in a water world, eventually the excess salt would saturate the water and settle to the bottom. This would create a super-saturated salt solution that would be inhospitable to life."
Plate Tectonics Scientists over the last several decades have established the surprising centrality of plate tectonics, and the related continental drift, to the sustaining of life on Earth.
Continental drift refers to the movement of a dozen or more massive plates in the Earth's lithosphere, which is the outer, rigid shell of the planet. One crucial byproduct of plate tectonics is the development of mountain ranges, which are generally created over long periods of time as the plates collide and buckle.
Not only does plate tectonics help with the development of continents and mountains, which prevent a water world, but it also drives the Earth's carbon dioxide-rock cycle, This is critical in regulating the environment through the balancing of greenhouse gases and keeping the temperature of the planet at a livable level.
Greenhouse gases, like carbon dioxide, absorb infrared energy and help warm the planet. So they're absolutely crucial. The problem is that their concentration in the atmosphere needs to be regulated as the sun slowly brightens. Otherwise, the Earth would not be able to stabilize its surface temperature, which would be disastrous.
Plate tectonics cycles fragments of the Earth's crust-including limestone, which is made up of calcium, carbon dioxide, and oxygen atoms-down into the mantle. There, the planet's internal heat releases the carbon dioxide, which is then continually vented to the atmosphere through volcanoes. It's quite an elaborate process, but the end result is a kind of thermostat that keeps the greenhouse gases in balance and our surface temperature under control.
Eclipse total eclipses are possible because the sun is four hundred times larger than the moon, but it's also four hundred times further away. observers on Earth can discern finer details in the sun's chromosphere and corona than from any other planet. What's more, perfect solar eclipses have resulted in important scientific discoveries that would have been difficult if not impossible elsewhere, where eclipses don't happen.
Solar Eclipse Scientists welcome the total eclipse as a rare opportunity to study the Sun's faint corona. Why is the corona so hot? What causes it to spew massive bubbles of plasma into space through coronal mass ejections? Can solar flares be predicted and what causes them? These major mysteries may eventually be solved through experiments performed at future total eclipses.
Mediocre Earth • 100-200 billion galaxies • 10 24 Estimated planets10,000,000,000,000,000,000,000,000 • We’ve observed less than 1% of the universe
Right distance from a star; habitat for complex life; liquid water near surface; far enough to avoid tidal lock; right mass of star with long enough lifetime and not too much ultraviolet; stable planetary orbits; right planet mass to maintain atmosphere and ocean with a solid molten core and enough heat for plate tectonics; a Jupiter-like neighbor to clear out comets and asteroids; plate tectonics to build up land mass, enhance bio-diversity, and enable a magnetic field; not too much, nor too little ocean; a large moon at the right distance to stabilize tilt; a small Mars-like neighbor as possible source to seed Earth-like planet; maintenance of adequate temperature, composition and pressure for plants and animals; a galaxy with enough heavy elements, not too small, ellipitcal or irregular; right position the galaxy; few giant impacts like had 65 million years ago; enough carbon for life, but not enough for runaway greenhouse effect; Quality of oxygen and photosynthesis; biological adaptation.
The Privileged Planet • Earth's location, its size, its composition, its structure, its atmosphere, its temperature, its internal dynamics, and its many intricate cycles that are essential to life-the carbon cycle, the oxygen cycle, the nitrogen cycle, the phosphorous cycle, the sulfur cycle, the calcium cycle, the sodium cycle, and so on-testify to the degree to which our planet is exquisitely and precariously balanced.2
http://www.wisegeek.com/what-is-the-rare-earth-hypothesis.htmhttp://www.wisegeek.com/what-is-the-rare-earth-hypothesis.htm
