Journey through the Solar System: Planet Earth

by Carolyn Collins Petersen
Updated February 19, 2018
In the range of solar system worlds, Earth is the only known home to life. It's also the only one with liquid water flowing across its surface. Those are two reasons why astronomers and planetary scientists seek to understand more about its evolution and how it came to be such a haven. 

Our home planet is also the only world with a name not derived from Greek/Roman mythology. To the Romans, the goddess of Earth was Tellus, meaning "the fertile soil," while the Greek goddess of our planet was Gaia or Mother Earth. The name we use today, Earth, comes from Old English and German roots. 
Humanity's View of Earth

 Earth As Seen From Apollo 17. The Apollo missions gave people their first look at Earth as a round world, not a flat one. Image Credit: NASA
It's not surprising that people thought Earth was the center of the universe only a few hundred years ago. This is because it "looks" like the Sun is moving around the planet each day. In reality, Earth is turning like a merry-go-round and we see the Sun appear to move.

Belief in an Earth-centered universe was a very strong one until the 1500s. That's when Polish astronomer Nicolaus Copernicus wrote and published his grand work On the Revolutions of the Celestial Spheres. In it pointed out how and why our planet orbits the Sun. Eventually, astronomers came to accept the idea and that is how we understand the position of Earth today.  
Earth by the Numbers

Distant Earth and Moon
 Distant Earth and Moon as viewed from a spacecraft. NASA
Earth is the third planet out from the Sun, located at just over 149 million kilometers away. At that distance, it takes slightly over 365 days to make one trip around the Sun. That period is called a year.  

Like most other planets, Earth experiences four seasons each year. The reasons for the seasons are simple: Earth is tilted 23.5 degrees on its axis. As the planet orbits the Sun, different hemispheres get more or fewer amounts of sunlight depending on whether they are tilting toward or away from the Sun.

The circumference of our planet at the equator is about 40,075 km, and 
Earth's Temperate Conditions

Earth's atmosphere seen from the ISS
 Earth's atmosphere looks very thin when compared to the rest of the planet. The green line is airglow high in the atmosphere, caused by cosmic rays striking the gases up there. This was shot by astronaut Terry Virts from the International Space Station. NASA
Compared to other worlds in the solar system, Earth is incredibly life-friendly. That's due to the combination of a warm atmosphere and a large supply of water. The atmospheric gas mixture we live in is 77 percent nitrogen, 21 percent oxygen, with traces of other gases and water vapor The affects Earth's long-term climate and short-term local weather. It's also a very effective shield against most of the harmful radiation that comes from the Sun and space and swarms of meteors our planet encounters. 

In addition to the atmosphere, Earth's has abundant supplies of water. These are mostly in the oceans, rivers, and lakes, but the atmosphere is water-rich, too. Earth is about 75 percent covered with water, which leads some scientists to call it a "water world."
Habitat Earth

earth and the life it carries
 Views of Earth from space show evidence of life on our planet. This one reveals streams of phytoplankton along the California Coast. NASA
Earth's abundant water supplies and temperate atmosphere provide a very welcome habitat for life on Earth. The first life forms showed up more than 3.8 billion years ago. They were tiny microbial beings. Evolution spurred more and more complex life forms. Nearly 9 billion species of plants, animals, and insects are known to inhabit the planet. There are likely many more that have yet to be discovered and cataloged.
Earth from the Outside

Earthrise - Apollo 8
 Earthrise - Apollo 8. Manned Spacecraft Center
It's clear from even a quick glance at the planet that Earth is a water world with a thick breathable atmosphere. The clouds tell us that there's water in the atmosphere as well, and give hints about daily and seasonal climate changes.

Since the dawn of the space age, scientists have studied our planet as they would any other planet. Orbiting satellites give real-time data about the atmosphere, surface, and even changes in the magnetic field during solar storms.

Charged particles from the solar wind flow past our planet, but some also get entangled in Earth's magnetic field. They spiral down the field lines, collide with air molecules, which begin to glow. That glow is what we see as the aurorae or the Northern and Southern Lights
Earth from the Inside

earth cutaway
 A cutaway showing Earth's interior layers. The motions in the core produce our magnetic field. NASA
Earth is a rocky world with a solid crust and a hot molten mantle. Deep inside, it has a semi-molten molten nickel-iron core. Motions in that core, coupled with the planet's spin on its axis, create Earth's magnetic field. 
Earth's Long-time Companion

Picture of the Moon - Moon Color Composite. JPL
Earth's Moon (which has many different cultural names, often referrerd to as "luna") has been around for more than four billion years. It is a dry, cratered world without any atmosphere. It has a surface that is pockmarked with craters made by incoming asteroids and comets. In some places, particularly at the poles, the comets left behind water ice deposits.

Huge lava plains, called "maria," lie between the craters and formed when impactors punched through the surface in the distant past. That allowed molten material to spread out across the moonscape.

The Moon is very close to us, at a distance of 384,000 km. It always shows the same side to us as it moves through its 28-day orbit. Throughout each month, we see different phases of the Moon, from crescent to quarter Moon to Full and then back to crescent. 
Thanx    Carolyn Collins Petersen
Knight  Jenny  C.