Science for kids: solar system for kids
Explanations and informations on the solar system for kids.
The center of the Solar System is our sun. The Sun is a star, and the largest and heaviest object in the Solar System. Everything else in the Solar System either moves around the Sun, or moves around something else that is, itself, moving around the sun.
Because astronomy deals so often with really huge numbers, astronomers have made up several special units for themselves so that they can discuss things like the mass of a star, or the distance between planets, without having use billions and billions of zeros. So, while the Sun is known to mass about 1,989,000,000,000,000,000,000,000,000,000 kilograms, it’s much easier to talk about it as having one Solar Mass. When astronomers talk about other stars, they use the Solar Mass as a quick way to refer to stellar mass. The reason that Solar Mass isn’t very useful for discussing the planets of the solar system is that even the largest of the planets in the solar system has less than one tenth of one percent of the mass of the sun. For this article I will compare all planet masses to that of earth, but while this is a useful comparison, it’s not an official unit.
Another special unit that astronomers do use within the Solar System is the Astronomical Unit, or AU. This is defined as the average distance between the sun and the Earth, or about 150 million kilometers. Thus, when someone mentions that Mercury averages 0.38 AU from the Sun that means that Mercury is only about a third of the distance from the sun than the Earth.
Our Sun is classified as a type G0 star. This is also sometimes referred to as a yellow dwarf star. As stars go, the Sun is actually a rather small star. This is actually good for us, since it means that the Sun will be burning for a long time to come. The Sun is considered to be about 5 billion years old, and will be burning for billions of years to come. Larger stars, like type O giants, can burn out in as few as a couple of millions of years.
Moving away from the Sun, the first planet we come to is little Mercury. Mercury is a small, rocky planet. It masses about 5% of what earth does. The planets in the Solar System come in two varieties: The small, rocky planets – also called Terrestrial planets – and the large planets that are made up of mostly hydrogen gas, called Gas Giants. Mercury has almost no atmosphere, and because it is so close to the sun it is very hot on the surface during the planet’s day. A planet’s day is that period that it takes the planet to rotate once around its axis. Mercury has one of the longest planetary days of any in the solar system: almost 59 earth days. Mercury also has the shortest year of any planet: it goes around the Sun once every 88 days.
None of the planets orbit the Sun in perfect circles. Each planet’s orbit actually is described as an ellipse. An ellipse can looks like what you’d get if you traced around half a watermelon, after cutting the watermelon in half the long way. Not all ellipses are that extreme. Some can look very close to circles.
The next planet out from the sun is Venus. Venus orbits the sun at about 0.72 AU. It is also in the orbit closest to being a real circle of any of the planets. Venus is sometimes called the twin of Earth. Venus, unlike most of the other terrestrial planets, has a very thick atmosphere. This isn’t a good thing, since Venus’ atmosphere keeps it so hot that lead would melt at the surface of the planet. Venus’ atmosphere is also very poisonous. Venus’ mass is about 80% of Earth. Venus has the longest planetary day of any planet in the Solar System: 243 Earth days. This is actually longer than Venus’ year: 224.7 Earth days.
Next we come to the Earth. In many ways Earth is unique in the Solar System. It is our home. Earth is also the only planet with liquid water on its surface. Earth’s moon is the largest moon, with respect to the planet it orbits, of any moon in the Solar System. The Moon masses less than 2% of what Earth does. The Earth orbits the Sun at an average distance of 1 AU. It takes 365.23 days for the Earth to orbit the Sun once.
Moving out from the Earth, the next planet from the Sun is Mars. Mars is another terrestrial planet, and like Earth, it actually has ice caps. Unlike Earth, though, the icecaps on Mars are Carbon Dioxide ice, not water ice. Mars has a very thin atmosphere, compared to Earth, but unlike Venus, the atmosphere isn’t poisonous. Mars has about 11% the mass of Earth. Mars also has a short day, like Earth: 1.026 Earth days. Mars’ year is very long, though: 687 Earth Days. Mars has two very small moons, which are believed to be captured asteroids. Recently scientists discovered evidence that suggests Mars may have had life on its surface at one point. Unfortunately, even if that is true, there seems to be no life on Mars, now.
Moving away from the Sun we come to a large gap between planets. In that gap there are a lot of small bodies orbiting the sun. Those small bodies are called Asteroids, and this group of asteroids between Mars and Jupiter is referred to as ‘The Asteroid Belt.’ This is a misleading label: It gives the impression that there is a band of small rocks orbiting the sun like Saturn’s rings. Instead it’s unlikely that you could see any other asteroids while standing on one of them. There are thousands of asteroids known today, and more are discovered every year. Only the largest ones, or those viewed as important for some other reason, have been given names. Ceres, Vesta, Pallas, and Juno are the names of the largest asteroids. Not all asteroids known in the Solar System are found in the asteroid belt. There are several families of asteroids. The two most interesting families are the Icarus objects, asteroids that cross the orbit of the Earth; and sungrazing objects, which come very close to the sun. These families are not exclusive, so some asteroids are both Icarus objects and sungrazing objects. All told, the aggregate of the mass of the asteroids is likely to be less than the mass of Earth’s Moon.
Further out from the asteroid belt is the largest planet in the Solar System: Jupiter. Jupiter masses 318 times Earth does, and is many more times larger than the Earth, too. Jupiter is the closest to the Sun of all the gas giant planets. Jupiter orbits around the Sun once every 11.86 Earth years, but has a planetary day of only 0.414 Earth days. Jupiter averages about 5.2 AU from the Sun. Jupiter also has over 15 moons, and more may be discovered. The four largest moons of Jupiter (Io, Europa, Callisto, and Ganymeade.) were discovered by Gallileo. Scientists have recently found evidence that there may be liquid water under the ice on the moon Europa. If that is true, it is very exciting because with liquid water the chances for life are greatly improved. Jupiter also has a very faint ring system, similar to Saturn’s rings.
Next from the Sun is the gas giant Saturn, orbiting an average of 9.53 AU from the Sun. Saturn is the second largest planet in the solar system, and masses about 95 times the Earth. Saturn also has a large number of moons, like Jupiter, and has a spectacular ring system of small moonlets orbiting around it. These rings are visible even with very small home telescopes. Saturn’s year is almost 30 Earth years, and its planetary day is 0.436 Earth days. Saturn is also the only planet in the solar system with a density less than water. Saturn’s largest moon is Titan, which has a very thick atmosphere of its own.
Next from the Sun is the gas giant Uranus. Uranus is the first planet to have been discovered by use of telescopes. All the planets closer to the sun were discovered and named by the ancients. Uranus masses about 14 times Earth. Uranus also orbits the sun with its poles almost flat to the plane of its orbit. It is believed that this is a result of a collision between Uranus and a large body very early in the formation of the Solar System. Uranus has a planetary year of 84 Earth years, and a planetary day of about 0.92 Earth days. Like Jupiter, Uranus also has five small, faint rings. Uranus’s has several moons, but all are rather small.
Neptune is the other gas giant planet to have been discovered after the invention of the telescope. Neptune orbits the Sun at an average of 30 AU, and has a planetary day of 0.67 Earth days. Neptune masses 17 times the Earth. Neptune has a set of 4 faint rings, like Uranus, or Jupiter. Neptune’s largest moon is Triton, and Neptune has several other smaller moons. Neptune’s year is 165 Earth years.
The last known planet in the Solar System is Pluto. Pluto is an anomaly in many respects. It is considered a terrestrial planet, and the only terrestrial planet further from the sun than Mars. Pluto is also actually a double planet system, since its moon, Charon, is so close to it, both in size and distance, that the two bodies actually orbit about a point in space between them, called the barycenter. Pluto and Charon also orbit the Sun backwards, or retrograde, compared to the other planets. Their orbit is also the most eccentric of all the planets as well. It is also the orbit furthest from the plane that the other planets orbit in. While the average distance for Pluto from the Sun is 39.5 AU, it is so eccentric that for a time each of its years, Pluto is closer to the Sun than Neptune is. Pluto’s year is 248 Earth years. Pluto’s day is 6.39 Earth days. Pluto is also the smallest of the nine planets, massing just 0.25% of Earth.
Orbiting in the same general region as Pluto is a class of recently discovered objects called Kupier belt objects. These are all smaller than Pluto. The Kupier belt is similar to the Asteroid belt in that it is a useful description for a body of objects, not a physical description of reality. The objects in the Kupier belt are even more spread out than those in the asteroid belt. There is some belief that Pluto shouldn’t be called a planet at all, and simply labeled the largest of the Kupier belt objects.
This survey of the Solar system isn’t meant to be exhaustive. There is much more detail available in your local library, and on the Web. NASA runs several websites that you will find interesting, including mission information about probes to all the planets listed here, except Pluto.
Interesting sites:
www.nasa.gov
www.universetoday.com