Just where are we in the Galaxy? The ancients were convinced that the Earth was the center of the universe and that everything revolved around it, but today we know that our planet is not at the center of the vastness of space and that it revolves around the Sun just like the other planets. In addition, the solar system is in a rather outlying position in one of the spiral arms of stars which forms the Galaxy, approximately 30 000 light-years from its center and about 20 000 from its farthest edge. Though part of an integrated system, held together by the attraction from the Sun, the nine planets are very different from each other: for example, Jupiter's diameter is thirty times greater than that of Mercury, Pluto takes a thousand times longer than Mercury to make a complete revolution around the Sun, the Earth is the only planet that has water in the liquid state, Mercury has no atmosphere, while the atmosphere on Venus is so dense that it is similar to the conditions found at the bottom of the Earth's oceans. However, in their entirety, the planets can be grouped into two large categories: the Earth types, which are relatively small and have a solid core (Mercury, Venus, the Earth, Mars and perhaps Pluto) and the Jovian types, larger but a large part of which are made with ice, dust and gas (Jupiter, Saturn, Uranus and Neptune). The Earth can be considered to be a minuscule piece of rock and metal occupying one of the inner parts of the solar system. Its average distance from the Sun is 149 600 000 kilometers, and this value is used in astronomy as one of the fundamental measurement units, at least for the space that surrounds us. The planets closest to the Earth revolve around their own axis very slowly and take many days to complete one full revolution (the "record" is held by Venus, with a day of no less than 243 Earth days). On the contrary, the outer planets complete a full revolution in less than one day, with the exception of Pluto which takes a little more than six days. However, while the outer planets take years to complete their revolution around the Sun, the inner planets take much less time, with a maximum, for Mars, of 687 days. An observer outside the system, looking at the Sun "from the top", would see that all the planets move along orbits which lie on the same plane as the Earth, except Pluto, whose orbit is inclined by 17 degrees. Moreover, the planets move around the Sun in what are almost perfectly circular orbits; only Pluto has an elliptical orbit: this is why, during certain periods, it is not the outermost planet. Starting in 1987 its orbit, in the segment closest to the Sun, will be closer than that of Neptune and will remain there until the end of the century. We should also say a few words about the asteroids and comets. Asteriods are relatively small celestial bodies which, as we have already seen, orbit between Mars and Jupiter. Some are no larger than a pebble, but the biggest, Ceres, has a diameter of 760 kilometers. Asteroids may be the remains of a collision between two or more planets in the embryonic stage, but they are more likely the residue of the original solar nebula, with insufficient mass to generate a real planet. Comets, on the other hand, are bodies which appear periodically, like Pilgrims of interplanetary space. Composed of a solid core of ice and dust, they may have originated from a type of cloud that surrounds the solar system like a shell, about one light-year from the Sun. When their orbit brings them close to our star, the outer layers of the ice sublimate into very small particles, forming the coma and tail that the light from the Sun makes visible to create one of the most amazing sights that can be observed on the face of our planet. But how did all this originate? What happened to make each body in the solar system different than the other one even though they all obeys precise physical laws? When did all this happen? And what will happen in the next few millions of years?
Just where are we in the Galaxy? The ancients were convinced that the Earth was the center of the universe and that everything revolved around it, but today we know that our planet is not at the center of the vastness of space and that it revolves around the Sun just like the other planets. In addition, the solar system is in a rather outlying position in one of the spiral arms of stars which forms the Galaxy, approximately 30 000 light-years from its center and about 20 000 from its farthest edge. Though part of an integrated system, held together by the attraction from the Sun, the nine planets are very different from each other: for example, Jupiter's diameter is thirty times greater than that of Mercury, Pluto takes a thousand times longer than Mercury to make a complete revolution around the Sun, the Earth is the only planet that has water in the liquid state, Mercury has no atmosphere, while the atmosphere on Venus is so dense that it is similar to the conditions found at the bottom of the Earth's oceans. However, in their entirety, the planets can be grouped into two large categories: the Earth types, which are relatively small and have a solid core (Mercury, Venus, the Earth, Mars and perhaps Pluto) and the Jovian types, larger but a large part of which are made with ice, dust and gas (Jupiter, Saturn, Uranus and Neptune). The Earth can be considered to be a minuscule piece of rock and metal occupying one of the inner parts of the solar system. Its average distance from the Sun is 149 600 000 kilometers, and this value is used in astronomy as one of the fundamental measurement units, at least for the space that surrounds us. The planets closest to the Earth revolve around their own axis very slowly and take many days to complete one full revolution (the "record" is held by Venus, with a day of no less than 243 Earth days). On the contrary, the outer planets complete a full revolution in less than one day, with the exception of Pluto which takes a little more than six days. However, while the outer planets take years to complete their revolution around the Sun, the inner planets take much less time, with a maximum, for Mars, of 687 days. An observer outside the system, looking at the Sun "from the top", would see that all the planets move along orbits which lie on the same plane as the Earth, except Pluto, whose orbit is inclined by 17 degrees. Moreover, the planets move around the Sun in what are almost perfectly circular orbits; only Pluto has an elliptical orbit: this is why, during certain periods, it is not the outermost planet. Starting in 1987 its orbit, in the segment closest to the Sun, will be closer than that of Neptune and will remain there until the end of the century. We should also say a few words about the asteroids and comets. Asteriods are relatively small celestial bodies which, as we have already seen, orbit between Mars and Jupiter. Some are no larger than a pebble, but the biggest, Ceres, has a diameter of 760 kilometers. Asteroids may be the remains of a collision between two or more planets in the embryonic stage, but they are more likely the residue of the original solar nebula, with insufficient mass to generate a real planet. Comets, on the other hand, are bodies which appear periodically, like Pilgrims of interplanetary space. Composed of a solid core of ice and dust, they may have originated from a type of cloud that surrounds the solar system like a shell, about one light-year from the Sun. When their orbit brings them close to our star, the outer layers of the ice sublimate into very small particles, forming the coma and tail that the light from the Sun makes visible to create one of the most amazing sights that can be observed on the face of our planet. But how did all this originate? What happened to make each body in the solar system different than the other one even though they all obeys precise physical laws? When did all this happen? And what will happen in the next few millions of years?
