Astronomers have discovered thousands of planets outside our solar system. Many people still have a soft spot for Earth.
It might be biased, but a combination of several different factors seems to make Earth unique.
Don Brownlee, an astronomer at the University of Washington, told Space.com in 2008 that if you were taken to Mars, you wouldn't feel happy there. It's not like that.
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Size and location are important.
Earth has established itself in a nice neighborhood.
The star isn't too disruptive to Earth even at its peak activity because it has an 11-year activity cycle. Red dwarfs, small stars known for their large amounts of radiation, are a marked contrast to that.
Earth is a long way away from the sun, at an average distance of nearly 150 million miles. Earth has liquid water on its surface because of the planets and stars.
The diagram shows Earth's position around the sun. The image is from NASA/JPL-Caltech.
Diana Valencia, a planetary scientist at the University of Toronto Scarborough in Canada, told Space.com in 2008 that a planet that is far out would quickly freeze if it received too much energy from the sun. Earth's size allows it to hold onto our atmosphere.
Friendly neighbors.
Earth is shaped by its partnership with the moon.
Our planet's rotation is stable because of the moon's influence on the poles. The moon creates the ocean's tides.
Jupiter is far away from Earth, but it is still a key player. Jupiter is so large that it acts as a sort of vacuum cleaner to clean up the debris in the solar system that is small and large.
The moon would have snuck past Jupiter if the Mars-size object had collided with the early Earth.
A watery world.
Earth is unique because of its vast oceans. The image is from the Historic NWS Collection.
70% of the planet's surface is covered by the oceans, which are perhaps the most unique feature of Earth. Earth is the only planet in the solar system with liquid water.
The oceans of Mars and Venus are long gone. Scientists have found signs of water ice on a number of planets, but they have not been able to find its liquid form.
The plates are tectonics.
The system of plate tectonics and the slip-sliding movements of Earth's crust are thought to have shaped the planet's towering mountain ranges and plunging ocean depths.
Many scientists think water is the reason plate tectonics happen. In 2008, Mike Brown, a planetary scientist at Caltech, told Space.com that water lubricates plate tectonics, which leads to the extreme difference between continents and seafloors.
The planet's temperature is regulated by plate tectonics, remaining in temperature ranges at which liquid water can survive.
Habitability and beauty.
The Apollo 11 mission saw humans land on the moon for the first time. The image is from NASA/ JSC.
Earth is the only world known to host living beings because of the many features that make it unique. Examining how that life is possible isn't just about sharing the planet with everything from amoebas to elephants and from oaks to zebrafish.
Understanding what is unique about Earth is important for understanding what other planets might be like. Even though humans have traveled to other worlds, we still have first-hand knowledge of Earth.
It's doubly unique that Earth hosts intelligent life and life capable of studying the galaxy around us. In 2008 Gregory Laughlin told Space.com that he believes these achievements should be credited to our planet.
"During the last half century, the planet Earth has fashioned together tiny pieces of the metal in its crust, and has flung these delicately constructed objects to all of the other planets in the solar system." If one adopts the point of view of an external observer, it is the 'planet' that has done these remarkable things.
There are different worlds.
Artist's depictions of the planets discovered by NASA. The image is from NASA/JPL-Caltech.
We haven't seen a planet outside the solar system that looks like Earth. Scientists have discovered nearly 5,000 exoplanets to date, and while they can't know much about each of these worlds, nothing matches Earth's unique set of characteristics.
Instead, scientists have discovered a treasure trove of planets each unique in their own way: hot gas giants zipping around their stars in just hours, planets where one side is eternally hot enough to vaporize iron, planets with no star at all.
Many planet hunters hope to find Earth's twin as technology improves. Scientists are debating whether Earth is as unique as they think. Many scientists believe that life may be found on some of the other planets if only sceptics could see it.
He said there will be other planets that support life. I think life is very common. I think we're going to find a lot of them.
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