The two planets closest to the sun are Mercury and Venus. Next comes Earth. We know more about Earth than about any other planet.
Living things, as we know them, need sunlight, water, and certain gases. We know that Earth has what living things need. It has the right amount of sunlight. It has water — about three times as much water as land. And it has gases that plants and animals need to live.
Just above Earth is its atmosphere. Atmosphere is the air around an object in space. Not all objects in space have an atmosphere.
Earth does. Its atmosphere is made up mostly of nitrogen and oxygen. Dust particles float inside the atmosphere. So do clouds and microbes. (Microbes are living organisms so small you need a microscope to see them.) The clouds are water droplets that form rain.
Earth has a magnetic field all around it. The magnetic field begins inside Earth's core and goes out way beyond the atmosphere. A magnetic field acts like a magnet.
Earth's atmosphere and the magnetic field above it block most of the deadly rays from the sun and other stars. Sometimes chunks of rock in space, called meteorites, attracted by Earth's gravity, enter our atmosphere. The atmosphere burns up most meteorites before they can hit Earth. Earth's atmosphere and magnetic field protect life on Earth.
Scientists believe there is probably life somewhere else in the universe. But they don't know where, yet.
The planet Earth spins around — rotates — once in 24 hours. With one 24-hour rotation, most of the planet has a night and a day.
A planet rotates as if there were a long rod through its center. But there is no rod. We make believe there is and call it an axis. The axis always points toward the North Star, tilting Earth as it orbits around the sun. Because of the tilt, parts of Earth are either closer to or farther from the sun at certain times of the year. These times are our seasons.
During winters at the North and South Poles, the nights last much longer than they do anywhere else on the planet. A winter night in the polar regions lasts all winter long. Summertime at the Poles is one very long day.
Earth has many kinds of climates. Temperatures on Earth can be as high as 60 degrees Celsius (140 degrees Fahrenheit). It gets that hot in some deserts. At the North and South Poles, it can be as cold as minus 90 degrees Celsius (130 degrees below zero Fahrenheit).
There may be some places on Earth where people may not be able to live. But some form of life is possible almost anywhere on Earth.
The moon is Earth's natural satellite. It takes the moon 27 days and 8 hours to circle Earth. Earth and moon orbit the sun together.
The moon's surface is a lot like Earth's. But the moon has no atmosphere, no magnetic field, no water, and no sign of life.
The moon, like Earth, is lit by the light of the sun. Now and then there are certain places on the moon that seem to glow with more light. Astronomers are not sure what causes this.
Earth and moon revolve around the sun in 365 days, plus part of another day. We make up for the part of a day by adding one extra day every four years. That year is called leap year and it has 366 days.
Mercury is the planet closest to the sun. It is also the smallest of all the known planets except for Pluto.
The nearby sun's glare makes it hard to see Mercury in the sky, even with a telescope.
More than three billion years ago, huge meteorites crashed into the planet. They left deep scars on its surface.
Mercury's atmosphere is much, much thinner than Earth's. There is also a very weak magnetic field around Mercury.
Mercury orbits the sun much faster than Earth does. But Mercury spins completely around on its axis much more slowly than Earth. While Mercury makes one spin, Earth has had 59 days and nights!
On Mercury, the days are terribly hot and the nights are terribly cold. There is no water. There is not much atmosphere. And the gases are not the kinds living things need. Scientists say there can't be any kind of life on Mercury.
When you look up at the sky, the brightest objects you can see are the sun and the moon. The next brightest is Venus.
Venus is about the same size as Earth — just a little smaller. It is closer to Earth than any other planet. But it is very different from Earth.
Earth has lots of water, mostly in the form of oceans, rivers, and lakes. The only water we know of on Venus is in the form of water vapor. (Water vapor is a gas, which you cannot see.)
Earth turns on its axis in 24 hours. Venus turns on its axis very slowly. More than half a year passes on Earth before Venus has a new day.
Venus's clouds swirl around fast. Winds blow at about 350 kilometers (250 miles) an hour, stronger and wilder than any hurricane on Earth.
Two countries have explored Venus. The USSR has landed spacecraft on Venus. The United States has sent out probes that orbit the planet and can radar-map the surface. Its surface is rougher than Earth's and has many more craters.
Venus is nearly twice as far from the sun as Mercury, yet it is almost as hot as Mercury. Unlike Mercury, nights and days on Venus have about the same very high temperatures.
The reason for this is because Venus's atmosphere has carbon dioxide gas that stays all around the planet. It's like a blanket of smog that we sometimes get over some cities on Earth.
The sun's rays come through this atmosphere and heat the planet. The planet gives off heat rays, but those rays cannot pass back through the atmosphere to outer space. The heat is trapped under the blanket of carbon dioxide gas.
Something like this happens to a car parked in the sun with the windows shut. The inside gets much hotter than the outside because heat cannot escape.
On Venus, nothing we know of can live.
Mars is the inner planet farthest from the sun. It seems a lot like Earth. Nights and days on both planets take about the same time.
Though it's a bit colder on Mars, there are clouds and fog there. And it has volcanoes, lava fields, canyons, and cracks in its crust, like Earth.
Its North and South Poles also have ice caps.
As Mars orbits, the ice cap closer to the sun shrinks because the sun's rays melt some of the ice.
About the same time each Mars year, part of the surface seems to change color. Mars seems to have seasons, as Earth does.
But there are differences between Mars and Earth. Mars has many moonlike craters, billions of years old. Its atmosphere is mostly carbon dioxide and much thinner than Earth's. Its sky is not blue, but creamy pink, because of red dust in the air.
Some geologists who study planets believe that once there used to be large, deep lakes on Mars under a much denser atmosphere. Now, dust storms sometimes cover the whole planet, and water on Mars is not liquid. It's in the form of vapor, ice clouds, or surface ice.
Mars has two satellites — tiny moons that revolve around it.
Many scientists think some kind of life may be possible on Mars. So far, we have no proof of it. We hope to find answers with future space probes — and someday, even with a visit from Earth people.
Adapted from Scholastic's "A Book About Planets and Stars" by Betty Polisar Reigot.