Glossary For Further Reading Index


Ancient people around the globe once believed the Moon, Sun, planets, and stars were gods and goddesses, demons and angels revolving around Earth, the centre of their universe. Little did they know that Earth is a mere speck in a vast universe. It took many centuries for people to realize that Earth isn't even the centre of our solar system. It is, however, one of the components of what's known as the inner solar system— comprised of the Sun, four terrestrial planets, and the moons that orbit them—which is examined thoroughly in this book.

With the development of modern astronomy and the advent of scientific tools, particularly the telescope, scientists began examining the solar system and theorizing about our place in it. The solar system and the space just beyond it form the extent of our physical reach in the universe. Even with manned and unmanned spacecraft, we may never explore farther than the outer boundary of the solar system. So scientists observe and study our solar system in order to learn more about the universe itself. Much of our knowledge comes from the celestial bodies within the inner solar system, but there is still much to learn.

Our solar system includes the Sun, eight planets (formerly nine), many moons, comets, asteroids, and traces of gas and dust known as interplanetary medium. Although the solar system still presents mysteries, scientists have made great strides over the past 400 years in explaining its origins. The commonly accepted explanation today says that billions of years ago a massive cloud of gas and dust began to collapse in on itself due to gravity. The cloud began to rotate as it collapsed, forming a disk shape. Nuclear reactions began to occur at the centre due to the immense heat and pressure, and the Sun began to form. Material in the rest of the disk slowly collided and merged together to form planets and moons.

The outer planets are much bigger than the inner planets and are largely made up of gases. From Jupiter out, the temperature is so cold that water exists mainly as ice. Large amounts of ice joined together when the outer planets formed, giving them more mass and greater gravitational pull. They drew in large amounts of hydrogen, helium, and other gases, creating "gas giants." However, water in the inner solar system remained liquid, allowing the inner planets to become much smaller and rockier. These ideas about the creation of the solar system are widely supported by many scientists.

Scientists believe the solar system was formed when a cloud of gas and dust (similar to the one in this image captured by NASA in 2004) collapsed upon itself, then compressed to form the Sun and the planets. HO/NASA/AFP/Getty Images

At the heart of the solar system is the Sun. This relatively large star makes up more than 99 percent of the solar system's total mass. The Sun's diameter is about 109 times larger than Earth's. The temperature of the surface of the Sun is about 5,500°C (10,000°F). However, that's nothing compared to the temperature at the Sun's core, which is closer to 15 million°C (27 million°F)!

Approximately 90 percent of the atoms in the Sun are hydrogen atoms. Helium makes up a much smaller amount. These amounts change very slowly over time because hydrogen is continually being converted to helium during a process called nuclear fusion in the Sun's core. The core becomes hotter as more helium is created, and the Sun becomes brighter. At its present rate, the Sun should shine for another 5 billion years, but it will eventually become much larger and brighter than it is now before its hydrogen is depleted.

The Sun's atmosphere is made up of layers. Closest to the Sun, the photosphere is the layer we see. Farther out are the chromosphere and the corona, which we can see during a solar eclipse. For centuries, scientists have observed and theorized about the Sun's many dazzling characteristics, including sun spots, prominences, and solar flares. Scientists continue to study the Sun and the many ways it affects life on Earth, including the solar wind it creates.

The innermost planet, Mercury, is the eighth largest planet in the solar system in terms of size and mass. Its small size—just 4,880 kilometres (3,032 miles) in diameter—and its closeness to the sun make it difficult to observe from Earth. Probes face nearly insurmountable forces to get close to the tiny planet. However, scientists are keen to study the planet further due to several notable differences between it and the other planets, such as its elongated orbit and peculiarities regarding the nature of gravity.

Mercury has an average orbital distance of 58 million km (36 million miles) and orbits the Sun once every 88 days. Although it is the smallest planet, it is also the densest. Its lead core makes up approximately three-quarters of the planet. The rocky, outer shell is dotted with craters caused by countless asteroid impacts. Between the larger craters are sparsely cratered surfaces called inter-crater plains and smooth plains. Caloris, Mercury's largest crater, is 1,550 km (960 miles) in diameter. Space probes have detected minute traces of atmospheric gases, but they are nearly nonexistent. Mercury's magnetic field resembles Earth's bipolar field, although it is much, much weaker.

The next closest planet to the Sun, Venus, is very similar to Earth in size, mass, density, and gravity. It has an average orbital distance of 108 million km (67 million miles). Of all the planets its orbit is the most circle-shaped. Unlike most planets, Venus rotates in a clockwise direction when viewed from its north pole.

The planet also spins very slowly, taking 243 Earth days to complete a rotation.

Venus's dense, swirling cloud cover circles the planet in just four days. The planet has the most massive atmosphere of all the inner planets. Carbon dioxide is the most abundant gas, but it also contains highly concentrated sulfuric acid and solid sulfur, among other ingredients. The atmosphere is so thick it creates a powerful greenhouse effect similar to the one on Earth. As a result, the temperature at the surface can be about 464°C (867°F), which is higher than the melting point of lead.

Little was known about Venus's surface until the 1960s, when scientists used radar and spacecraft to study the planet. Most of the surface is rocky and sandy, dominated by vast, rolling plains. Some areas feature rifts, mountains, volcanoes, lava flows, and craters. Scientists hope for clearer images of Venus's surface as technology improves.

Earth—the only planet known to harbor life—orbits the Sun at a mean distance of 150 million km (93 million miles), completing one orbit in about 365.25 days. It makes one revolution every 23 hours, 56 minutes, and 4 seconds. Earth is the largest of the inner, rocky planets, with a diameter of 6,378 km (7,926 miles). It has one satellite, which we simply call the Moon.

Earth's atmosphere is mainly comprised of nitrogen (78 percent) and oxygen (21 percent), but contains tiny amounts of other substances, including argon, methane, carbon dioxide, and water vapour. Most of the atmosphere is contained in the troposphere, a layer that extends up to an altitude of about 10-15 km (6-9 miles). The ozone layer is found in the upper layer of the stratosphere and protects the Earth from harmful ultraviolet rays from the Sun. Above the stratosphere are the mesosphere, thermosphere, and ionosphere. Unlike the other planets, Earth also has a hydrosphere, which is a layer of water at or near the planet's surface.

Earth's crust is a rigid layer of rock that "floats" on a softer layer of hot rock called the mantle. The landmasses on Earth's surface are in constant motion, often bumping into each other or drifting apart. About one-third of Earth's mass is contained in its hot, liquid iron core. Fluid motions within Earth's core create a powerful electromagnetic field around the planet. This field, called the magnetosphere, deflects dangerous solar winds and keeps them from striking Earth.

Earth's satellite is one of the largest moons in the solar system and the brightest object in the sky other than the Sun. Some scientists consider Earth and the Moon "twin planets." It is the only object in the solar system besides Earth that human beings have visited so far. Scientific observations of the Moon and the way it acts have helped scientists make breakthroughs about the mechanics of the solar system.

The gravitational fields of Earth and the Moon have a great effect on each other. The Moon causes the cycle of tides in Earth's larger bodies of water. Because the rate at which the Moon orbits Earth matches the rate at which the Moon rotates on its axis, the same side of the Moon always faces Earth. Seen from Earth, the Moon is lighted by the Sun and takes on different shapes, called phases, depending on the day of the month.

The Moon orbits Earth at a mean distance of about 384,000 km (238,600 miles).

Sunlight illuminates specific portions of the Moon as it orbits Earth. Consequently, the Moon is visible from Earth in phases, changing from partially illuminated (top) to fully illuminated (bottom) and back again.

It has a diameter of 3,476 km (2,160 miles). Its surface features many impact craters separated by flat, rocky plains and is covered with rocks and dust caused by the countless objects that have hit it. Because of the Moon's small size and mass, its gravity is about one-sixth of Earth's. It has very little atmosphere.

Thanks to the Moon, people on Earth are treated to occasional celestial shows called eclipses. An eclipse occurs when one object in space partially or totally blocks another. Scientists have been studying eclipses for centuries and have used them to make many discoveries about the solar system.

A solar eclipse occurs when the Moon passes in front of the Sun, casting a shadow over part of Earth's surface. To someone standing in this shadow on Earth, the Sun appears to be growing dimmer. Those lucky enough to witness a total eclipse will see all but the Sun's corona totally disappear for several minutes, creating a dazzling natural light show. The diameter of the Sun is about 400 times larger than that of the Moon. However, it is also about 400 times farther away from Earth. Due to this fascinating coincidence, the Moon and the Sun appear to be about the same size from the perspective of someone standing on Earth. This coincidence is what makes solar eclipses possible. However, the Moon is slowly moving away from Earth. Millions of years from now, total eclipses will no longer be possible.

During a full Moon—when the Moon and the Sun are on opposite sides of

Earth—the Moon may pass through Earth's shadow, resulting in a lunar eclipse. The Moon dims considerably but remains partially visible, often with a red glow. Although total solar eclipses rarely last more than a few minutes, a total lunar eclipse can last several hours.

Mars, the fourth planet from the Sun, is the second-smallest planet in the solar system. With a diameter of 6,792 km (4,220 miles), it is about half the size of Earth. It has an average orbital distance of 228 million km (140 million miles). One day on Mars is slightly longer than a day on Earth. One year on Mars equals 687 Earth days. Due to its elongated orbit, Mars is less than 56 million km (35 million miles) from Earth at its closest, but almost 400 million km (250 million miles) away when they are on opposite sides of the solar system.

The Martian atmosphere is comprised mainly of carbon dioxide, and is much thinner than Earth's. Water vapour in the atmosphere sometimes forms low-lying clouds and fog in valleys or craters. Some of the water vapour forms ice caps at the north and south poles. Many scientists now think water also exists beneath Mars's surface. Dust storms are common and can last for weeks. "The red planet," as it is often called, has deep rifts, some of which can be seen with the aid of a telescope. It also has the tallest volcano in the solar system—Olympus Mons. Mars has two small, irregularly shaped moons, Phobos and Deimos, which may have once been asteroids captured by Mars's gravitational field.

Mars was one of the first planets to be studied with telescopes, and it continues to intrigue scientists. Some think it once supported life and may one day be colonized by humans. More than 50 meteorites that originated on Mars have been found on Earth. We are constantly learning new things about Mars thanks to several space probes and rovers that have been sent to explore the planet.

These are the planets and moons of the inner solar system, but other objects are lurking out there. They include comets, asteroids, and meteors, many of which have their own orbits around the Sun. Scientists use the term interplanetary medium to describe thinly scattered matter drifting between the planets. Neutral hydrogen atoms and plasma come from the Sun in the form of solar wind. Cosmic rays are high-speed, high-energy atomic nuclei and electrons. Some come from the Sun itself, while most come from outside the solar system. Very small dust particles, sometimes called micrometeor-oids, orbit the Sun much like the planets. This dust is probably the result of collisions between asteroids and material shed from comets.

The inner solar system is our local neighbourhood in a limitless, continually expanding universe. It provides scientists with some of the best chances for studying the forces that govern the cosmos. This book will guide you through the inner solar system and give you a close-up look at many of the topics discussed in this introduction and more. So strap in, the countdown has begun. Let's explore the inner solar system!

Composition and Origin of the Solar System

Our home planet, Earth, floats as a tiny oasis of life in the immensity of the universe. However, Earth does not wander space alone. From the earliest ages of history, humanity watched the skies and studied the motions of the Sun, which lit the day, and the Moon, which shone at night. The stars were thought fixed to the vault of heaven, but some stars were wanderers, or planetes, as the Greeks called them. Earth, the Sun, the Moon, and the planets are part of one assembly of nearby astronomical bodies called the solar system.

Located at the centre of the solar system and influencing the motion of all the other bodies through its gravitational force is the Sun, which in itself contains more than 99 percent of the mass of the system. The planets, in order of their distance outward from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The first four planets, Mercury, Venus, Earth, and Mars, constitute the inner solar system.


All the planets move around the Sun in elliptical orbits in the same direction that the Sun rotates. This motion is termed prograde, or direct, motion. Looking down on the system from a vantage point above Earth's North Pole, an observer

would find that all these orbital motions are in a counterclockwise direction.

The shape of an object's orbit is defined in terms of its eccentricity. For a perfectly circular orbit, the eccentricity is 0; with increasing elongation of the orbit's shape, the eccentricity increases toward a value of 1, the eccentricity of a parabola. Of the four planets of the inner solar system, Venus has the most circular orbit around the Sun, with an eccentricity of 0.007. Mercury, the closest planet, has the highest eccentricity, with 0.21.

Another defining attribute of an object's orbit around the Sun is its inclination, which is the angle that it makes with the plane of Earth's orbit—the ecliptic plane. Again, of the inner planets,

Mercury has the greatest inclination, its orbit lying at 7° to the ecliptic.

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