Characteristics of Earth That Support Life
Eight planets orbit the Sun, but as far as we know, only one has a school bus, a pizza place, and a forest. Why Earth? In this page you will find three features that work together to make our planet a home for living things: the right distance from the Sun, liquid water, and an atmosphere with the right gases.
The habitable zone
Think about sitting near a campfire. Too close and you burn. Too far and you freeze. Somewhere in between, it feels just right.
Planets work the same way. The Sun sends out energy, and a planet that is closer receives more of it. The habitable zone is the range of distances from a star where a planet's surface can be warm enough for liquid water, but not so hot that the water boils away.
The figure is a simple sketch, not drawn to scale. Earth is about 150 million km from the Sun. Venus is closer (about 108 million km) and Mars is farther (about 228 million km).
Earth is sometimes said to be in the "habitable zone" around the Sun. In your own words, define what this means, and state what form of water is most important for life.
Earth is sometimes said to be in the "habitable zone" around the Sun. In your own words, define what this means, and state what form of water is most important for life.
Liquid water
Every living thing we know needs liquid water. Cells are mostly water, and the chemical reactions inside them happen in water. Water also carries nutrients through plants, blood, and oceans.
Water can be solid (ice), liquid, or gas (water vapor). Only the liquid form lets cells do their work. On Earth, the average surface temperature is about $15\,^\circ\text{C}$ ($59\,^\circ\text{F}$), and water freezes at $0\,^\circ\text{C}$ and boils at $100\,^\circ\text{C}$. Most of Earth's surface stays between those two numbers, so oceans, lakes, and rivers stay liquid.
Venus is closer to the Sun than Earth, and its surface temperature is about $465\,^\circ\text{C}$. Mars is farther from the Sun, and its average surface temperature is…
Venus is closer to the Sun than Earth, and its surface temperature is about $465\,^\circ\text{C}$. Mars is farther from the Sun, and its average surface temperature is about $-60\,^\circ\text{C}$. Explain why liquid water is common on Earth's surface but not on the surfaces of Venus or Mars.
Look at this data table. | Planet | Average distance from Sun (million km) | Main gas in atmosphere | Liquid water on surface | |---|---|---|---| | Venus | 108 | Carbon…
Look at this data table.
| Planet | Average distance from Sun (million km) | Main gas in atmosphere | Liquid water on surface |
|---|---|---|---|
| Venus | 108 | Carbon dioxide | No |
| Earth | 150 | Nitrogen | Yes |
| Mars | 228 | Carbon dioxide (very thin) | No |
(a) How much farther from the Sun is Mars than Earth, in million km? (b) Using the table, name two differences between Earth and the other two planets that help explain why life exists on Earth.
The atmosphere
Earth's atmosphere is the layer of gases around the planet. It is about 78% nitrogen and about 21% oxygen, with small amounts of argon, carbon dioxide, and water vapor.
The atmosphere helps life in several ways:
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Oxygen is what most living things, including you, need to breathe.
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Carbon dioxide is used by plants and algae to make food (photosynthesis).
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The ozone layer absorbs much of the Sun's harmful ultraviolet (UV) radiation.
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The greenhouse effect: some gases trap thermal energy, keeping Earth warmer than it would be otherwise, so nights are not extremely cold.
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The air pressure from the layer of gas helps hold liquid water on the surface.
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The atmosphere also burns up many small meteoroids before they hit the ground.
Earth's gravity holds the atmosphere in place. A planet with very weak gravity cannot keep its gases, and they drift off into space.
List three characteristics of Earth that help living things survive. Choose from the following ideas: distance from the Sun, liquid water, and the gases in the…
List three characteristics of Earth that help living things survive. Choose from the following ideas: distance from the Sun, liquid water, and the gases in the atmosphere.
Name the two most abundant gases in Earth's atmosphere. Then tell which one most living things need in order to breathe.
Name the two most abundant gases in Earth's atmosphere. Then tell which one most living things need in order to breathe.
Mateo says, "The atmosphere does not matter for life. Only sunlight and water do." Give two reasons why Earth's atmosphere is important for living things. Include the…
Mateo says, "The atmosphere does not matter for life. Only sunlight and water do." Give two reasons why Earth's atmosphere is important for living things. Include the role of the ozone layer and of gases such as oxygen or carbon dioxide.
How the features work together
Each feature is necessary, but none is enough alone. Use the planets as evidence.
| Planet | Distance | Atmosphere | Result |
|---|---|---|---|
| Venus | closer | very thick carbon dioxide traps thermal energy | surface far too hot, no liquid water |
| Earth | in the habitable zone | nitrogen and oxygen, ozone layer | liquid water, life |
| Mars | farther | very thin carbon dioxide | too cold, water frozen |
| Mercury | closest | almost none | extreme heat by day, extreme cold at night |
Venus shows that distance is not the whole story: Venus is hotter than Mercury even though it is farther from the Sun, because its thick atmosphere traps thermal energy. Mercury shows that a good atmosphere is needed too: with almost none, temperatures swing wildly and nothing shields the surface.
Sofia and Marcus are arguing about which feature matters more for life on Earth: its distance from the Sun or its atmosphere. Sofia says distance is more important.…
Sofia and Marcus are arguing about which feature matters more for life on Earth: its distance from the Sun or its atmosphere. Sofia says distance is more important. Marcus says the atmosphere is more important. Write a short argument that uses evidence from at least two planets (for example, Venus, Mars, or Mercury) to show how the two features work together. State a conclusion about whether either feature alone is enough.
Scientists discover a new planet that orbits a star similar to the Sun. The planet is at nearly the same distance from its star as Earth is from the Sun, but it has no…
Scientists discover a new planet that orbits a star similar to the Sun. The planet is at nearly the same distance from its star as Earth is from the Sun, but it has no atmosphere and a very weak gravitational pull. (a) Predict whether liquid water could stay on its surface and explain your reasoning. (b) Describe two ways the surface conditions would differ from Earth's, such as temperature changes between day and night and exposure to harmful radiation. (c) Name one additional piece of evidence scientists would look for before saying the planet could support life.
Back to the start: Earth is the home of life because it has the right distance, liquid water, and a protective atmosphere, all at once.