Earth, 3rd rock from the sun, 150 million kilometers-or one astronomical unit-away.

Earth is the densest body in the solar system, with a core of rock and iron.

It's the only planet with plentiful liquid water, which covers more than two thirds of its surface.

And earth is-so far-the only world where we know for sure that life began and flourished.

Claudia Alexander
U.S. Project Scientist, ROSETTA Comet mission
Jet Propulsion Laboratory, NASA/Caltech

...over the long term the Earth has changed so frequently in geologic time and we want to be prepared for what's coming next. Is there global warming? Is there going to be another Ice Age? And what other kinds of changes are there that we don't know about? Are there a special kind of volcanic explosions that could completely change our climate that we're not aware of?

It's by studying other planets like Venus that we take a fresh look at the Earth and we say "Holy Mackerel", you know, we saw this... what we call a "super volcano", and now when we look down on Earth... it's giving us a whole totally fresh idea about the potential changes that might happen on Earth that could be ...have very significant effects on our lives.




Distance From the Sun

365 Earth days

1 Earth day

6,378.14 km

149,597,890 km



Surface Temperature


5.9742 x 1027 grams

5.515 gm/cm3
Earth is the densest large body in the solar system

The average temperature is 15C






Composed of 77% nitrogen, 21% oxygen and traces of argon, carbon dioxide and water

Most meteors burn up in Earth's atmosphere.

There are many volcanoes which are still active.

3/4 of the surface is covered in water.

A greenhouse on the surface of the Earth helps plants grow by allowing the heat and light of the Sun in through a glass roof, but reducing the amount of energy re-radiated back to space. That's why a greenhouse is warmer in winter than the air outside.

A planetary greenhouse effect is caused when gases in the atmosphere drive up surface temperatures by allowing in solar energy, but cutting back its re-emission.

Just as in a greenhouse, the balance between incoming and outgoing heat is critical for living creatures.

It's possible to make things too hot... or too cold.

Chris Chyba
Carl Sagan Chair for the Study of Life in the Universe
SETI Institute & Stanford University

What does all that mean? One thing that means is that the Earth is still replenishing its atmosphere. Volcanoes on the Earth are still belching carbon dioxide into our atmosphere, so we actually have a carbon dioxide cycle on the Earth where the volcanoes put the CO2 into the atmosphere, rainfall washes it back out, and we kind of maintain a balance of the right amount of carbon dioxide, and incidentally the right amount of water vapor, to give us an amount of Greenhouse Effect that's just right for liquid water to exist.

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