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| A brief survey of the solar system |
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A number of celestial bodies, 8 planets, thousands of small planets, smaller stones, comets, dust and other material circulate round the Sun.
All together this is called the solar system.
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Illustration: NASA
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All these objects shine because they reflect sunlight. The enormous radiation of the Sun illuminates the innermost planets quite strongly. They are therefore extremely hot (up to 450 ºC); while less light and heat are left for the planets further away form the Sun.
The illustration to the right shows the planets and the Sun in correct relative sizes. But the distances between them are completely wrong here. |
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| The inner parts of the solar system. |
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In the inner parts of the solar system the planets Mercury (inner circle), Venus, the Earth, Mars, Jupiter and Saturn move. Click on the figure in order to see the placement of the planets more clearly. Then try to click the right-hand button and choose “Zoom In” in order to see the details better.
The outermost planet that is known, Pluto, lies almost 6 billion kilometres from the Sun. It receives little light from the Sun and reflects even less. It is therefore very dim and we need a rather large telescope just to be able to catch a glimpse of it. For this reason it was not discovered until 1930.
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| | | Eight planets in our solar system |
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| Pluto with moons, 24th June 2006. |
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As a planet, Pluto is small and strange. Like a large, stony lump of ice, Pluto is, however, quite normal in a region of the solar system scattered with similar objects.
There has been much disagreement as to whether Pluto is a planet or an asteroid. Up to the present time, there has been no good definition as to what a planet really is. Furthermore, there is also relatively recent information that Pluto is not at all like the other planets, but much more like loads of other objects whirling around out there. Pluto is only one of many of the so-called Kuiper Belt Objects (named after Gerad Kuiper the astronomer).
These objects are a type of mini planets not unlike the asteroids in the asteroid belt. We are aware of more than 800 Kuiper Belt Objects, most of them discovered after 1992. Since 2000 we have constantly discovered several "large" objects.
For a long time it was the case that that all the Kuiper Belt Objects that were discovered were smaller than Pluto, the ninth planet, and in the scientific world, Pluto was placed as the largest of these objects. But then, in July 2005, a Kuiper Belt Object was discovered that was larger than Pluto, about 2 400 kilometres in diameter. This formed the basis for a definition of the concept “planet”.
The International Astronomic Union, IAU, has for a long time been working on an official definition of the term "planet". IAU set up a working group of 19 persons to consider this question.
This was the subject of the IAU general meeting in Prague 14th to 25th August 2006. On the 24th August IAU arrived at a new definition of a "planet".
The following criteria are necessary in order to deserve the designation planet:
- it must orbit round the Sun
- it must be large enough to make sure that its own gravitation into an approximately circular form
- it must have cleaned its orbit of other objects
Pluto was thus automatically disqualified, as Pluto’s strongly elliptical orbit overlaps with Neptune’s orbit. Pluto is now classified as a "dwarf planet", a designation that is different from "planet". Therefore there are, at present, officially eight planets in our solar system. |
| | | The planets |
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| The outermost planets: Uranus and Neptune with the dwarf planet Pluto. |
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Starting from the innermost one, the planets of the solar system are as follows: Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The dwarf planet Pluto is also included.
All planets are affected by the sunshine and even if the innermost planets are bathed in sunshine that gives temperatures of more than 400 ºC, the impact on the outermost planets is also very interesting. |
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| Illustration: Nine Planets/NASA |
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Among other things the impact of the sunlight on these planets have been studied in order to find out how the radiation of the Sun varies in the long run. This may give us the answer as to how much human beings contribute to changing the climate on our planet and how large a part of the climatic changes are due to natural changes.
The illustration to the left shows the nine most known planets in the solar system in mutually correct size.
The distances between the planets are not correct. Click on the picture in order to get a larger version. |
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| Our own planet. Photo: NASA. |
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Our own planet is also called “The blue planet”. When the Earth is photographed from space such as here, we understand the reason. 70 % of the surface of the Earth is covered with water.
The Sun contains more than 99.8 % of all the mass of the solar system. The enormous gravitational force of the Sun keeps all other bodies – the planets included – in place in their respective orbits.
If you want to know more about the planets and the other celestial bodies in the solar system, you may click on the link to the website of “The nine planets”.
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| | | Transit of Venus June 8 2004 |
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| The planet Venus from Mariner 10 5th February 1974. |
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On June 8, 2004 we will experience the most extraordinary and important celestial phenomenon in our lifes. For more than 6 hours the planet Venus will be visible in front of the Sun. No living persons anywhere on the Earth have witnessed this before! Actually, this will be the first time in history that everyone can follow the transit with their own eyes using eclipse glasses. The transit is expected to be seen by more people than any other celestial phenomenon ever! It will help scientists to find life on remote planets.
Since the planets Mercury and Venus orbit the Sun inside Earths orbit, they may from time to time pass between the Sun and Earth. On very rare occasions they may even line up with the Sun and the Earth so that they transit the Sun when observed from the Earth.
Transits of Venus are extremely rare - the last one was i 1882 and Venus has transited the Sun 6 times since the invention of the telescope in 1610. In Norway the phenomenon has not been seen since 1769!
Read more about the transit of Venus 8th June at the website astronomi.no. |
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