Earth is the third closest planet to the Sun, and completes one orbit about every 365 days. Earth is larger, denser, and more massive than Mercury, Venus, and Mars, and has one natural satellite, the Moon.
Astronaut Mark C. Lee above the Earth. Image credit: NASA/Public domain.
Earth is the only place in the universe where life is known to exist, and almost all life on Earth is fuelled by energy from the Sun. It takes light just over 8 minutes to travel to Earth, across a distance of about 150 million km. This distance is defined as one Astronomical Unit (AU).
Over 70% of the surface of the Earth is covered with salt-water oceans. The rest is composed of continents and islands. The interior of the Earth contains a thick layer of solid mantle, a liquid outer core that generates a magnetic field, and a solid inner core of iron. The Earth's axis is tilted by about 23.4°, and this is what causes us to experience seasons.
Shortly after its formation, about 4.54 billion years ago, the Earth had an atmosphere of hydrogen and helium, which soon escaped the gravity of Earth, and were replaced by heavier molecules like water, carbon dioxide, and ammonia. These were released from the Earth by volcanoes. During the Earth's first 700 million years, the surface was bombarded with comets, which brought water and other elements and molecules, including amino acids, the 'building blocks of life'[3a].
The four inner planets to scale. Image credit: NASA/Public domain.
2. Life on Earth ↑
The first life on Earth consisted of simple cells that may have evolved about 3.8 billion years ago, with bacteria evolving shortly after[3b]. Photosynthesising bacteria began producing oxygen about 2.5 billion years ago. This changed the atmosphere, and poisoned almost all life that had evolved before.
Eukaryotic cells evolved over 1.5 billion years ago, and divided into three groups - the ancestors of plants, fungi, and animals - within a few hundred million years. There have been at least five mass extinction events since then. The latest occurred about 65 million years ago, and wiped out 76% of species, including most of the different species of dinosaurs.
Australopithecus evolved about 4 million years ago. These were the first hominids to create stone tools. Homo neanderthalensis, Neanderthals, evolved about 400,000 years ago, and died out about 30,000 years ago, and Homo sapiens, human beings, evolved about 200,000 years ago.
Astronaut Bruce McCandless II above the Earth. Image credit: NASA/Public domain.
Humans began to form cities about 11,000 years ago, and have had a significant effect on the environment since the industrial revolution, which began in the late 1700s. This is because the industrial revolution led to an increase in amount of coal that was burnt, and this produces carbon dioxide. Carbon dioxide is too heavy to escape into space and so builds up, forming a layer in the atmosphere that traps the light of the Sun. This is causing the average temperature of the Earth to rise.
As the temperature of the Earth increases, glaciers will melt, there will be more extreme weather events such as droughts and heavy rainfall, and many species will be wiped out. It's hoped that human intervention can prevent this from happening.
NASA observes the Earth using the Earth Observing System. This is composed of a series of satellites that orbit, and monitor the Earth. The first of these satellites was launched in 1997, and there are currently about 25 satellites in operation.
Satellites in the Earth Observing System. Image credit: NASA/Public domain.