The solar system

All planets in the solar system orbit our own star, the Sun.

The Sun – the star that is the center of our solar system – is located in one of the arms of the Milky Way, which makes up 99.8 percent of the mas of the Solar System – The planets therefore only constitute 0.2 percent of the mass in our solar system. Although the Sun is a so called ‘yellow dwarf star’ the star of our solar system is among the 5% of the Milky Way’s largest stars and is 333,000 times the size of the Earth.

Like all other stars, the Sun is a sphere of glowing gases. This is mainly hydrogen and helium. In the 15 million degree heat and violent environment of the Sun’s core, hundreds of millions of tons of hydrogen nuclei are transformed into helium every second. This is how the Sun produces energy. On average, it takes hundreds of thousands or even millions of years for energy to reach the Sun’s surface.

The sun radiates energy within all wavelengths of the electromagnetic spectrum – both the light we can see but also the UV, infrared, radio and gamma radiation that we humans cannot see. However, we can see the Sun at different wavelengths with a variety of telescopes that are sensitive to each part of the electromagnetic spectrum.

When Galileo Galilei pointed his telescope at the Sun, he discovered sunspots  – areas of the visible surface, which are approx. 2000 degrees colder than the rest of the surface. The sunspots look like dark areas on the solar disk, but they only look dark because they are colder than the surroundings. The sun is an important driver of the Earth’s environment, and the most important condition of life on our planet. Therefore, the study of the Sun is an important branch of astronomy and can, for example, give us more knowledge about the rich activity in the Sun’s atmosphere, where millions of tons of material are regularly thrown into solar explosions.

Venus is named after the Roman love goddess and has a surface temperature of about 480 ℃.

Venus’ hot temperature is because Venus’ atmosphere consists largely of CO2, which is a greenhouse gas. That is, the heat is kept within the atmosphere just like in a greenhouse. The dense cloud layer around Venus consists of concentrated sulfuric acid, and it also counts for sulfuric acid, but because Venus is so hot, the rain never reaches the planet’s surface.

Planets do not shine by themselves, but they reflect light from the Sun. Venus is the planet in our solar system that shines brightest because its cloud layers reflect the light of the Sun. It shines so clearly that it has led to several misunderstandings throughout history.

During World War II, hunter pilots sometimes fired shots at Venus because they believed the planet to be a hostile aircraft and Venus’ clear light has given rise to many rumors of UFOs. While the surface of Venus cannot be seen directly because the planet is covered by a thick layer of clouds, NASA’s spacecraft Magellan from 1990-1994 created images of what was hidden behind the clouds using radar technology. The pictures revealed a golden, flat stone landscape, sometimes covered by mountains, valleys and extinct volcanoes.

Venus is our closest neighbor in the Solar System and you can find several similarities between Venus and Earth. The two planets are almost the same size, and there is only a 2 percent difference in the material they each comprise.

The moon’s landscape is golden and covered with craters caused by meteor showers. Most craters dating from early in the Moon’s history, from the period which we call the Great bombardment (Late Heavy Bombardment, about 4.1 to 3.8 billion years ago). Because the Moon has no atmosphere, the meteors encounter no resistance before colliding with the Moon’s surface, and there is no wind and weather to blur the craters. Large meteoric strikes happen far more rarely today. One of these craters, Tycho, is named after the Danish astronomer Tycho Brahe. Although the Moon’s gravity is only 1/6 of the Earth’s, both the Moon and the Earth’s shape change slightly due to each other’s gravity. The tide here on Earth is a result of the attraction of the Moon.

Satellites are not just man-made constructions that can help us observe and collect data, watch TV and listen to radio, or communicate. A satellite is defined as something orbiting a celestial body. The moon is orbiting the Earth and we call it a natural satellite. Our Moon is one of the Solar System’s over 170 known moons. From Earth we always see the Moon from the same side. It has what is called a bound rotation. In connection with the six Apollo landings from 1969-1972, astronauts have collected large amounts of data on the Moon.

Mercury is the smallest among the planets of our solar system – it is not much larger than the Moon! The ancient Greeks believed that Mercury was two planets – when Mercury was east of the Sun they called the planet ‘Apollo* and when it was west of the Sun they called it *Hermes’. However, the Greek philosopher and mathematician Pythagoras discovered that there was only one planet. Today, science faces many challenges when it comes to learning more about Mercury. From Earth it is difficult to see Mercury, and the Hubble Space Telescope, which is in orbit around the Earth, cannot look directly at Mercury either, because the heat and the light of the Sun will damage the precious telescope.

However, NASA has launched missions to gain more knowledge about the inner planet of the solar system. The Mariner 10 space probe passed the planet three times in the mid – 1970s and from 2008-2009 the Messenger space probe flew past Mercury three times and in 2011 was orbited around the planet. Messenger crashed on Mercury in 2015, after the space probe ran our of fuel. Mercury is named after the Roman god, reminiscent of the Greek Hermes. Just as the winged messenger of the gods was moving fast, Mercury’s orbit of the Sun only lasted 88 days – twice as fast as Mercury’s day, on 176 earth days. Mercury has almost no atmosphere, and among the

planets of the Solar System, Mercury is the planet with the largest temperature fluctuations – from as much as 430 degrees C during Mercury’s day to – 183 degrees C during Mercury’s long night.

Our own planet Earth is the third in the series of planets from the Sun.

The distance of the Earth from the Sun, its gravity, and the Earth’s atmosphere all help to create the conditions for liquid water. Ca. 2/3 of the Earth’s surface is covered with water, which was essential to life on Earth. Earth is still the only planet where we know for sure that life exists. How and when life on Earth was

Still being discussed eagerly. Life on Earth is approx. 4 billion years old, which Homo sapiens came about 200,000 years ago – human evolution is therefore only a very small part of Earth’s history.

The appearance of the Earth has changed a lot since the planet was formed about 4.6 billion years ago. The lithosphere, the crust of the Earth and the upper part of the mantle, is divided into a series of plates that have collided throughout the history of the Earth, which have pushed apart and have scraped up and down each other. The tectonic plates are moving only a few centimeters a year, but it has nevertheless been enough to Earth’s continents repeatedly formed one super continent – the last time it happened was about 200 million years ago, when our planet was dominated by super continent Pangea.

Under the crust is the Earth’ mantel – the largest part of the Earth’s interior, which consists mainly of solid rocks, yet has a gritty semi-liquid texture. At the very bottom of our planet is a core of nickel and iron that is as hot as the Sun’s surface, but because the pressure is so high, the innermost part of the core cannot melt. However, the inner part of the core revolves around a floating outer core, and it is the currents in the outer core that, together with the Earth’s rotation, cause the Earth’s magnetic field. It is not possible to drill down to the Earth’s core, but we know of the Earth’s internal structure from measurements of seismic movements within the Earth. For example, seismic measurements are made when an earthquake occurs. Then there are waves passing through the Earth, and these waves behave differently according to what material and where they are in the Earth’s interior.

Mars’ surface consists of volcanic basalt and has a rich content of iron – that is why it is red.

Small amounts of water have been found floating on the surface of Mars, which is an essential condition of life. However, there is almost no atmosphere on Mars, which is thought to be the reason why the water, which was probably once found in rivers and lakes on the surface of Mars, has long since evaporated.

Occasionally, dust storms may occur in the planet’s desert-like landscape, capable of covering the entire planet. Mars has ice caps at both poles. In the northern hemisphere there are flat lava plains and large, inactive volcanoes at the equator, while the southern hemisphere is dominated by many craters from meteor impact. Over the last 60 years, more than 40 missions have been sent to investigate Mars. Mars has been studied using spacecraft that have flown by, orbited or landed on the planet’s surface. One of them is Curiosity, which landed on Mars in 2012. Curiosity is a rover, a robot with wheels on it. It still travels around Mars’ surface and is equipped with cameras and measuring instruments that give us more knowledge about the planet. There are plans to send another Rover mission to Mars in 2020,

Many over the years have believed that the red planet housed intelligent life. In the 1800s and early 1900s, ideas flourished such that one should signal to Martians using a gigantic sign marked in Siberia’s snow landscape and that Martians had built an intricate canal system on the surface of Mars. Others, as late as the 1970s, were convinced that there were pyramids on Mars that were hundreds of times larger than Egypt’s pyramids.

The mass of Jupiter is two and a half times the mass of all the other planets together.

Jupiter is almost five times as far away from the Sun as Earth, and the length of Jupiter’s year equals 12 Earth-years. A gas giant like Jupiter consists mostly of gas, but it is possible that in the center of Jupiter’s interior is a solid core that is about the same size as Earth.

The only thing visible from Earth is the upper layers of Jupiter’s atmosphere. Jupiter’s thick atmosphere is dominated by strong winds, turbulent clouds and electric storms that could destroy the Earth. For example, Jupiter’s Great Red Spot is a storm three times as large as Earth. Due to Jupiter’s powerful magnetic field, there are also large, beautiful polar lights. The transition between the planet and atmosphere is literally fluid – below the atmosphere we encounter hydrogen in liquid form. This giant planet thus holds the largest system of the Solar System, but there are no waves and no surface. We know of 79 moons in orbit around the great planet. Jupiter’s moons resemble Earth more than Jupiter itself.

Io is the innermost of Jupiter’s moons, and here there is constant volcanic activity. Moon Europe is covered with water ice, and it is possible that liquid water could be found below the surface. In addition, there is a thin atmosphere consisting primarily of oxygen, so many believe that Europe acts as a bid for one of the places in the universe where humans could live in the future. Ganymede, the largest moon of the solar system, is larger than the inner planet of the solar system, Mercury. If Ganymede were in orbit around the Sun, instead of Jupiter, it could be categorized as a planet.

All the gas giants are surrounded by rings. These rings consist of small pieces of ice and rock.

The content of the rings around planets can be as small as dust particles, while some objects are found to be several meters across. The rings of Saturn, which are the brightest of all the rings in our solar system, are particularly famous. When Galileo Galilei first observed the rings, he wondered about what looked like protruding ‘ears’ or ‘handles’ on each side of Saturn. However, the Dutch mathematician and astronomer, Christiaan Huygens, discovered that these ‘ears’ were, in fact, rings around the planet. Today we know that the rings of Saturn consist of countless fragments of ice and rock.

Saturn consists mainly of hydrogen and helium. The beautiful planet is almost the same size as Jupiter, but the density of Saturn is so low that if placed on a gigantic ocean, it would float on top of the water surface. Like Jupiter, there is constantly stormy weather in Saturn’s atmosphere. The rapid rotation of Saturn causes very high wind speeds and its storms can last for months.

At least 82 moons are orbiting Saturn. There are eight large moons, the largest of which are Titan, which was also discovered by Christiaan Huygens. Titan is the only moon in the Solar System that has a thick atmosphere – it is actually thicker than the Earth’s. The atmosphere on Titan consists mainly of nitrogen, and on the surface of the Titan it has been discovered that there are lakes of methane, ethane and propane, all of which are hydrocarbons. Hydrocarbons are organic substances which are compounds of carbon and hydrogen – all life here on Earth is made up of such chemical compounds. Therefore, the building blocks of life are present on Titan, though the moon is not an obvious place to find life.

The surface temperature is approximately -180 °C, meaning that the ice covering large parts of Titan’s surface is hard as stone.

Uranus and Neptune are the unexplored territories of our Solar System.

Uranus can just barely be glimpsed with the naked eye, and it was not until 1781 that German-British astronomer and composer Sir Frederick William Herschel, using his telescope, discovered that Uranus was not a star. Instead, Herschel claimed it to be a comet, but this sparked discussions in astronomical circles and more people began to observe Uranus. Two years later, Herschel accepted that it was a planet. After a variety of suggestions for names had been at stake, Uranus ended up being named after the Greek god of the sky.

Only one mission has examined the two gas giants, which are the outermost in the series of planets from the Sun: Voyager 2, which passed by Uranus in 1986 and Neptune in 1989. Uranus is of a blueish green colour because the methane in its atmosphere absorbs red light and reflects blue light. Uranus’ atmosphere is not quite as violent as Jupiter’s and Saturn’s. However, the atmosphere is the coldest we have found on planets in the Solar System, with temperatures as low as -224 °C.

Uranus differs from the other planets in our solar system in the way that Uranus’ axis of rotation is inclined nearly 97.8 degrees – almost as if the planet has been overturned and is rolling around the Sun in its orbit. There is disagreement as to why Uranus rotates so ‘crookedly’, but one explanation is that the unusual slope is due to a collision early in Uranus’ history.

Neptune takes 165 years to complete a single orbit around the Sun. The strongest winds in the Solar System are found on Neptune – with measured wind speeds up to 2400 km/h.

Neptune, the planet in our solar system that is farthest from the Sun, was not discovered directly – in the 19th century astronomers discovered that Uranus was attracted to an object, which they did not know what was. Neptune was therefore first predicted from the calculations of French astronomer, Urbain Le Verriers, and British astronomer and mathematician, John Couch Adams, in 1845. The existence of the eighth planet in our Solar System was confirmed in 1846 by German astronomer Johann Gottfried Galle, close to the position calculated by Adams and Le Verriers.

It is not possible to see Neptune with the naked eye, meaning that we need telescopes to observe the planet with the vibrant blue colour. The blue color comes from the fact that methane in Neptune’s atmosphere absorbs the red wavelengths of sunlight – but the reason Neptune has such a clear blue color is that the light passes through more of the atmosphere than what is the case on Uranus, which has a  pale teal appearance.

When Voyager 2 flew past Neptune in 1989, the data collected by the spacecraft confirmed the presence of 4-5 rings around the planet. The rings were named after five men who all played key roles in the discovery of Neptune: Adams, Le Verrier, Lassell, Arago, and Galle.

In 2006, a new designation was introduced for some of the objects in the Solar System

Dwarf planets are a term now given to globes that do not fulfill all three criteria for being a planet. One of these criteria is that a planet must have cleared its orbit. Pluto, for example, is located in an area called the Kuiper Belt, together with a lot of other material.

This meant that Pluto is no longer a planet – it became a dwarf planet.

If we move further out in the Solar System, we find the Asteroid Belt, which consists of metallic and rocky objects orbiting the Sun.

Asteroids are leftover material from when our Solar System were formed. They can be anywhere from a few millimeters in size to having a diameter of several hundred kilometers. Around 200 million of the asteroids in the Asteroid Belt are larger than 1 kilometer. There are not very many of the very large asteroids in the Asteroid Belt, but many asteroids are thought to be parts of bigger objects, which has been destroyed due to collisions.

The Asteroid Belt between the orbits of Mars and Jupiter marks the separation between the planets consisting mainly of rock and metal and those who are mainly composed of hydrogen- and helium gas. It is not possible to land a spacecraft on the surface of Jupiter, Saturn, Uranus or Neptune – the so-called gas giants, which only have solid cores.