First Heliocentric Model
Our Solar System
First Heliocentric Model



The Planets
Row of planets

Orbiting the Sun are nine planets. In the order they are out from the sun, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Of these, the five bright naked eye planets: Mercury, Venus, Mars, Jupiter and Saturn have been known since antiquity, but until the invention of the telescope, their nature was unknown. Our home, the Earth, is the third planet out from the Sun, but only relatively recently in human history was it realized that it is one amongst several other planets orbiting a star that we call the Sun. The three outermost planets, Uranus, Neptune and Pluto were only discovered after the telescope was invented. Pluto is the last planet to be discovered, which took place in 1930, and is very faint and difficult to see. Because of its small size, there has recently been some controversy whether Pluto should even be called a planet. Between 1990 and now (2005), a large number of icy bodies have found beyond the orbit of Neptune in what is called the Kuiper Belt, which is a sort of outer icy asteroid belt. It appears that Pluto is possibly the largest member of this group of bodies, at least known so far, so perhaps Pluto should be demoted. Nevertheless, we will consider Pluto to be a planet here.

Below we give a brief introduction of the planets in order out from the sun, with some of the information coming from the Nine Planets website.



Mercury
Mercury
This is the closest planet to the sun, and completes one orbit in 88 days. It is about 1/3rd the diameter of the earth and is the only planet with no atmosphere. It also has no moons. Because it is well inside the earth's orbit, it can never be seen far away from the sun in the sky, in fact never more than 28°, so can only been seen with the naked eye in the western twilight sky after sunset or the easterm twilight sky before sunrise.


 
Venus
Venus
This is the second planet from the sun, and in terms of size, mass and bulk composition, is more like the earth than any of the other planets, being only very slight smaller in size and mass than the earth. However, in other respects it is quite different from the earth with oven like temperatures and a mostly carbon dioxide atmosphere with a surface pressure of 90 times that of the earth's atmosphere at sea level. Like Mercury, as it is inside the earth's orbit, it can never get very far from the sun, but with a maximum elongation of 47°, it can be seen for several hours after sunset or before sunrise when favorably placed. Because it is larger than Mercury, comes closer to the earth, and has brilliant highly reflective clouds, it is very bright, and is the brighest object in the sky after the sun and moon, indeed it can often be seen in the middle of the day with the naked eye when it is favorably places, the sky is transparent, and you know where to look. Also like Mercury, it has no moons.
 


Earth
Earth
This is our home and is the third planet out from the sun. As far as we know, it is the only place in the solar system where there is life. The earth is unusual in a number of other respects, such as being the largest planet with a solid surface, the giant planets like Jupiter have no solid surface that we know of, it has plate techtonics, and most of the earth's surface is covered with liquid water. The earth has one moon as follows:
Name
Semi-major axis (Km)
Diamter(Km)
Moon
Moon 384,400 3476


 
Mars
Mars
This is the first planet beyond the earth from the sun, so can be seen opposite the sun in the sky, unlike Mercury or Venus. It is about half the size of the earth, but in other respect is more like the earth than Venus, with an axial title that is 25° compared to the earth's value of 23.5°, and has obvious seasons, as well as a day that is only about 37 minutes longer than the earth's. Mars is a considerable source of fascination, as there has been long speculation as to whether there is, or has been, life there, and many spacecraft have, and still are, investigating it. Mars has a very thin atmosphere of mostly carbon dioxide, and has two very small moons as follows:

Name
Semi-major axis (Km)
Diamter(Km)
Phobos
9378 22.2
Deimos
23,459 12.6



 
Jupiter
Jupiter
This is the largest planet in the solar system with over 300 times the mass of the earth and over 10 times its radius. It is the prototype giant planet consisting of mostly of the gases hydrogen and helium. At great depths these gases are highly compressed into liquid forms, and there may be a solid core in the center. It has four large moons discovered by Galileo in 1610, and a large number (about 60) small moons, with new ones being discovered every few years. The four large moons are the following:
 
Name
Semi-major axis (Km)
Diamter(Km)
Io
422,000 3630
Europa
670,900 3138
Ganymede
1,070,000 5262
Callisto
1,883,000 4800


                                             
Saturn
Saturn
This is the second giant planet, and is in a number of respects similar to Jupiter, being only slightly smaller and about 100 times the mass of the earth. However, Saturn is best known for its magnificent ring system, which can be easily seen an a small telesope. Like Juputer, Saturn has a large number of moons, there is one big moon, Titan, which is the only moon in the solar system with a thick atmosphere, and is larger than our moon, a number of medium sized moons, and many small moons. Saturn is currently being studied by the Cassini spacecraft. The four largest moons are:

Name
Semi-major axis (Km)
Diamter(Km)
Tethus
294,660 1060
Dione
377,400 1120
Rhea
527,040 1530
Titan
1,221,830 5150



 
Uranus
Uranus
 
This is the seventh planet out from the sun and is a scaled down version of a giant planet, being about four times the radius of the earth and about 15 times its mass, and is sometimes called an ice giant, because it formed mostly from ices, such as frozen water in the outer solar system, although there is no ice in its interior in the way we would understand. Uranus is unusual in having an axial tilt of 98°, so it is orbiting on its side with at some periods of the Uranian year with the north or south poles pointing towards the sun. Because the axial title is more than 90° means that it is rotating retrograde. Uranus has several medium sized and many small moon. The four largest moons are:
Name
Semi-major axis (Km)
Diamter(Km)
Ariel
190,930 1158
Umbrial
265,980 1170
Titania
436,270 1578
Oberon
583,420 1523


 
 
Neptune
Neptune
This planet in most respects is almost an identical twin to Uranus, being nearly the same size and mass, but with a more "normal" axial tilt. It has one large moon called Triton, not to be confused with Titan, that has a very thin atmophere, and many much smaller moons. Voyager 2 flew past Neptune in 1989 and took many spectacular pictures. The details on Triton are as follows:
 
Name
Semi-major axis (Km)
Diamter(Km)
Triton
354,760 2700



 
Pluto
Pluto
This is the outermost of the planets with an orbital period of 248 years and was discovered in 1930. It is very small, smaller than our moon, and there have been some arguments as to whether it really is a planet or just a large Kuiper belt object. It has a low density and probably consists mostly of ices with some rocks; a giant dirty snowball. For about 20 years on each orbit it comes inside the orbit of Neptune, but because its orbit is inclinded by about 17° to the plane of most of the planets, there is no chance of a collision. Indeed, at times when Pluto is closer to the sun then Neptune, Neptune is far away on another part of its orbit. Its orbit is the most eccentric, i.e. most non-circular, of the planets. Pluto has a moon that is half its size, Charon, which makes it the largest moon in the solar system relative to its parent planet, with our moon coming in second. Pluto has a thin atmosphere consisting of methane and nitrogen. The details on Charon are:
 
 
Name
Semi-major axis (Km)
Diamter(Km)
Charon
19,640 1172



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