Uranus orbits the Sun once about every 84 Earth years. Uranus has a mass about 14 times the mass of Earth, but it is much less dense than Earth. Like Jupiter and Saturn, Uranus is composed mainly of hydrogen and helium, with an outer gas layer that gives way to liquid on the inside. Uranus has a higher percentage of icy materials, such as water, ammonia NH 3 , and methane CH 4 , than Jupiter and Saturn.
When sunlight reflects off Uranus, clouds of methane filter out red light, giving the planet a blue-green color. There are bands of clouds in the atmosphere of Uranus, but they are hard to see in normal light, so the planet looks like a plain blue ball. Most of the planets in the solar system rotate on their axes in the same direction that they move around the Sun.
Uranus, though, is tilted on its side so its axis is almost parallel to its orbit. In other words, it rotates like a top that was turned so that it was spinning parallel to the floor.
Scientists think that Uranus was probably knocked over by a collision with another planet-sized object billions of years ago. Uranus has a faint system of rings Figure below. This image from the Hubble Space Telescope shows the faint rings of Uranus. The planet is tilted on its side, so the rings are nearly vertical. Uranus has 27 known moons and all but a few of them are named for characters from the plays of William Shakespeare. These Voyager 2 photos have been resized to show the relative sizes of the five main moons of Uranus.
Scientists predicted the existence of Neptune before it was discovered because Uranus did not always appear exactly where it should appear. Neptune was discovered in , in the position that had been predicted, and it was named Neptune for the Roman god of the sea because of its bluish color. This image of Neptune was taken by Voyager 2 in The Great Dark Spot seen on the left center in the picture has since disappeared, but a similar dark spot has appeared on another part of the planet.
In many respects, Neptune is similar to Uranus Figure below. Neptune has slightly more mass than Uranus, but it is slightly smaller in size.
Neptune is much farther from the Sun at nearly 4. When Voyager 2 visited Neptune in , there was a large dark-blue spot that scientists named the Great Dark Spot, south of the equator. When the Hubble Space Telescope took pictures of Neptune in , the Great Dark Spot had disappeared but another dark spot had appeared north of the equator.
Astronomers think that both of these spots represent gaps in the methane clouds on Neptune. The changing appearance of Neptune is caused by its turbulent atmosphere. This extreme weather surprised astronomers, since the planet receives little energy from the Sun to power weather systems. Neptune is also one of the coldest places in the solar system. Temperatures at the top of the clouds are about o C o F.
Neptune has faint rings of ice and dust that may change or disappear in fairly short time frames. And the conditions needed for life to exist have to be just right. Earth is inside our Solar System, along with the other planets like Mars, Mercury, and Jupiter orbiting a star we call the Sun. But our Solar System is just one of many inside the huge Milky Way galaxy. And the Milky Way is just one of many, many galaxies in the Universe.
Plus, we have no way of knowing exactly how big the Universe is beyond what we can directly see. So while there may be life on other planets, it could be in another solar system in a different part of the Milky Way galaxy. Or in another galaxy far, far away. Read more: Curious Kids: Where are all the other galaxies hidden? Much of the search for life has focused on trying to find liquid water, because it is essential for all life forms here on Earth. Cells are mostly made up of water.
Then solar wind and radiation stripped most of its atmosphere away. Its minimally active core ceased to generate a protective magnetic field. Its surface became forbiddingly cold and dry even as it was bombarded with radiation. Is anything alive on Mars, perhaps beneath the surface, or in the frozen polar caps? Two strikes against Mars, Voytek said, are its lack of available water and the absence of plate tectonics — the process on Earth that moves continents over eons and recycles buried nutrients back up to the surface.
Strikes in its favor might include detection of methane in the Martian atmosphere. On Earth, methane, otherwise short-lived in the atmosphere, is replenished by the metabolic action of life forms. Methane also can be produced through reactions of water and rock, but microbial life beneath the surface is another possibility. Some of these moons could well be habitable worlds; one of them, Titan, has a thick atmosphere, rain, rivers and lakes, though composed of methane and ethane instead of water.
We first glide toward Europa, a moon of Jupiter with an icy shell. Beneath the frozen surface, however, space probes have detected evidence of a vast ocean of liquid water. Two other Jovian moons, Ganymede and Callisto, also are likely to host subsurface oceans, though these might be sandwiched between layers of ice. That makes life less likely, Cable says. A potentially more accessible example can be found among the moons of Saturn, the next planet out. Now the origin of planets must be dependent on the temperature of the central star.
If it is too cold, the atmosphere of the protoplanets will not be blown away, resulting perhaps in the formation of a system of planets similar to Jupiter, but even larger and more massive. On the other hand, if the star is too hot, radiation pressure will disperse the solar nebula rapidly, leaving, if anything, small atmosphereless planets, or a system of millions of tiny asteroids. For planets to be formed, the temperature of the star must be between these extremes.
There is another reason to believe that hot stars do not have planets. If the formation of planetary systems and the slowing down of stellar rotation both arise from the existence of solar nebulae, then we should expect the hot stars which dissipate their solar nebulae and do not form planets to rotate faster.
This is exactly what is observed! The hotter the star, the faster the rotation. Cooler stars rotate more slowly than would otherwise be expected. At a temperature of about 7, degrees, characteristic of what are called F stars, there is a sudden large decrease in average rotational velocities, and it is possible, perhaps, that below this temperature all stars retain enough of their solar nebulae to form planets, provided they have not used up their solar nebulae in forming double or multiple sun systems.
The number of such stars is between one and ten per cent of the total number of stars, suggesting that there are as many as ten billion solar systems in our galaxy alone. Of these, perhaps one per cent, or million have planets like the earth. What is the probability of life on these worlds? Since the most abundant element, cosmically, is hydrogen, the atmosphere of the early protoplanets of any system must contain much hydrogen and hydrogen compounds.
The hydrogen compounds of carbon, nitrogen, and oxygen are probably the most abundant hydrogen compounds in the proto-atmosphere. Urey showed that when hydrogen, methane, ammonia, and water vapor are mixed together, and supplied with energy, some fundamental organic compounds are produced.
The energy source in protoatmospheres is probably ultraviolet light from the sun about which the protoplanet revolves. These compounds are almost all amino acids, the biochemical building blocks from which protein is constructed. There is also some reason to believe that amino acids lead to the formation of purines and pyrimidines, which are in turn building blocks for nucleic acids. Proteins and nucleic acids are the two fundamental constituents of life as we know it on earth; hereditary materials such as genes and chromosomes are composed perhaps exclusively of nucleic acids and proteins.
In addition, enzymes, which catalyze slow chemical reactions and thereby make complex life forms possible, are always proteins. Experiments of comparable importance to those of Miller have been performed by S.
0コメント