The Sun and most of the planets have a magnetic field that extends into space. The English physicist Sir William Gilbert determined, in the 16th century, that Earth’s magnetic field resembles the field of a large bar magnet.
The magnetic field of a planet directs charged particles from the Sun toward the magnetic poles of the planet. Often, swirls of light can be seen on a planet as the charged particles give off light when they interact with molecules in the atmosphere.
On Earth, these swirls of light are called the aurora borealis in the northern hemisphere and the aurora australis in the southern hemisphere.
The Terrestrial Planets • The terrestrial (or inner) planets have relatively high densities, slow rotations, solid metal cores (usually iron), solid surfaces, and no rings.
Mercury Mercury, the planet closest to the Sun, has daytime temperatures around 427 °C because it receives 10 times the sunlight that Earth receives. With no atmosphere to trap heat, however, its night-time temperatures fall to - 173 °C. Mercury is very close to the Sun, so it is rarely visible in the night sky. When it is visible, it is always just before sunrise or just after sunset.
• The many craters on its surface were caused by collisions with chunks of rock or meteorites.
• Mercury orbits the Sun at 50 km/s, which is faster than any other planet. • Despite its quick revolution, Mercury has a sluggish rotation. It completes only three rotations for every two revolutions around the Sun.
• Observers on Earth can watch Mercury pass in front of the Sun 13 times each century in an event called a transit (Figure 3(b)). The first two transits this century were on May 7, 2003, and November 8, 2006. The next transit is in 2016.
Venus • Venus’s atmosphere is composed mainly of carbon dioxide gas. This gas acts like the glass of a greenhouse, keeping the surface temperatures high enough to melt lead. The surface temperature averages 462 °C.
Lava from the 1.7 km high volcano flowed hundreds of kilometres to create the wild landscape of Venus.
Not including the Sun and the Moon, Venus is the brightest object we can see in the sky. This is due to Venus’s closeness to Earth and its thick atmosphere, which reflects light from the Sun.
Earth and Venus have more features in common than any of the other planets. Earth and Venus have a similar size, composition, and distance to the Sun.
Unlike Earth, however, Venus has no oceans and rotates in a clockwise direction.
Venus is so bright that it is sometimes called the evening or morning star. It is called the evening star because, when above the horizon at night, it is the first object seen in the sky after sunset. • It is called the morning star because, when above the horizon during the day, it is the last object seen after sunrise.
The harsh environment on Venus makes exploration of this planet difficult. The space probe Magellan used radar cameras that could penetrate Venus’s thick atmosphere and “see” the features on its surface, including its many impact craters.
Like Mercury, Venus can be seen from Earth as it passes in front of the Sun. Transits occur in pairs, eight years apart. The last transit took place on June 8, 2004, and the second transit in the pair will occur on June 6, 2012. The next pair of transits will occur in the next century.
The European Space Agency’s spacecraft Venus Express has been in orbit around Venus since April 2006. It was sent to study Venus’s atmosphere so that we can better understand Earth’s atmosphere.
Earth The conditions on Earth are ideally suited to promote the origin of life as we know it. Astronomers use Earth and our solar system as a model to search for life on other planets and even outside our solar system.
Earth’s unique atmosphere contains mostly nitrogen, oxygen, and water vapour. It also contains a small amount of ozone, which filters some of the damaging radiation from the Sun. The atmosphere is also responsible for keeping the temperatures on Earth relatively constant between day and night, although there is a wide variation of temperatures between the poles and the equator, ranging from about - 88 °C to 58 °C.
About 70 % of Earth’s surface is covered by water in the form of oceans, seas, lakes, rivers, and ice or snow. • Earth is often referred to as the blue planet because of the abundance of water over its surface.
Rich soils and moisture are an ideal medium for plant life to adapt and grow.
Weathering, erosion, active volcanoes, and earthquakes are changing the surface of Earth, although Earth is considered to be stable.
Mars • The surface temperature on Mars ranges from –90 °C to –5 °C. • Mars, the fourth planet from the Sun, is sometimes called the red planet because of its reddish soil. It is one of the brighter objects in the sky.
• Giant dust storms periodically engulf Mars, changing its wind-swept landscape. • Today, Mars is a dry and barren planet with many impact craters. However, there is evidence to suggest that Mars once had volcanoes, glaciers, and floods of water. For this reason, scientists believe that life may have existed there.
In fact, in May 2002, the spacecraft Mars Odyssey discovered a large quantity of ice only 1 m below the surface.
The Gas Giants The atmospheres of the gas giants (outer planets) consist mainly of hydrogen and helium gases, so these planets have low densities. The cores of these planets may be denser and may contain metals, much like the terrestrial planets do.
Although the gas giants are less dense than the terrestrial planets, their immense sizes result in masses that are far greater than the masses of the terrestrial planets. Their large masses result in stronger gravitational fields, which can pull more debris and moons into orbit.
Jupiter The average temperature on Jupiter is -148 °C. Jupiter is the largest planet in our solar system. It has a diameter that is 11 times greater than Earth’s diameter, and its mass is greater than the masses of all the other planets combined.
Due to its immense size and the light reflected off its clouds, Jupiter is easily visible in the sky at certain times of the year. However, we are unable to see its orbiting ring of rocks from Earth.
Jupiter has over 60 known moons, four of which are visible from Earth with only binoculars. These moons (Io, Europa, Ganymede, and Callisto) are commonly referred to as the Galilean moons because they were discovered by the famous astronomer Galileo Galilei.
Jupiter is famous for its coloured bands and the Great Red Spot, a huge hurricane fed by constant high winds. This hurricane, which is the size of two Earths, shows no signs of dying since it was first discovered by Galileo almost four centuries ago.
The atmosphere on Jupiter resembles the atmosphere of the Sun. Immense pressure and high temperatures cause hydrogen gas to become a liquid metal and give rise to electric currents, which result in the planet’s strong magnetic field. Jupiter’s magnetic field is 20,000 times stronger than Earth’s.
Saturn The cloudy atmosphere is windy due to Saturn’s high speed of rotation. Saturn is colder than Jupiter (approximately - 178 °C on average). With a diameter that is about five-sixths the diameter of Jupiter, Saturn is the second largest planet in our solar system. Saturn is the least dense of all the planets, and it may not have a solid core.
Saturn is mainly composed of hydrogen and helium and has a volume that is 755 times greater than Earth’s.
Saturn has nearly one dozen rings. Astronomers are not certain whether the rings formed from the crumbled remains of Saturn’s many moons and other nearby objects or whether they formed around the same time as the planet did.
Saturn currently has over 50 moons with undoubtedly many more waiting to be discovered. One of Saturn’s moons, Titan, is actually larger than Mercury.
Conditions on Titan are very similar to the conditions on early Earth. Titan
Uranus The average surface temperature on Uranus [pronounced YUR-uh-nuhs] is - 216 °C. The atmosphere consists mainly of hydrogen, with traces of helium and methane. Uranus has winds that blow up to 500 km/h.
Despite appearing to us on Earth like a faint star, Uranus’s diameter is four times greater than Earth’s diameter. Uranus is so far away that it is difficult to see in the night sky. Uranus was thought to be a star until 1781, when its motion around the Sun was discovered.
In 1986, the space probe Voyager 2 passed near Uranus and gathered much of the data we still use today.
Uranus is unusual in that it spins on its side. Like Venus, Uranus also rotates in the opposite direction from Earth. The unusual rotation of Uranus may be due to a collision that knocked it out of its original rotation early in its formation. • The white patch in the digitally enhanced image is a polar cap over the south pole. • Uranus has a weak, irregular magnetic field.
Neptune • Neptune’s average surface temperature is -214 °C.
Computer-enhanced images sent from Voyager 2 in 1989 revealed that Neptune has bright blue and white clouds and a dark region, called the Great Dark Spot, that appears to be the centre of a huge storm. The cause of Neptune’s brilliant blue colour is not fully understood, although the methane in its atmosphere is known to reflect blue light.
Neptune was discovered through a feat of scientific reasoning. Neptune is so far from Earth that it can barely be seen, even with powerful telescopes. • Astronomers noted that Uranus’s orbit was irregular, almost as if it was being tugged into an uneven orbit by the gravitational attraction of another object. • Using detailed calculations, they predicted where this hidden object might be and consequently discovered Neptune in 1846.
• Neptune has six rings that are thought to have formed much later than the planet itself. • One of Neptune’s 13 moons, Triton, actually revolves around Neptune in the opposite direction to the planet’s rotation.