Jupiter stabilizes Earth climate and preserves our planet from being hit by comets and asteroids serving as a space vacuum cleaner. The Earth's orbit is an ellipse. The eccentricity is a measure of the departure of this ellipse from circularity. If Jupiter was not in orbit around the Sun and Earth were the only planet orbiting our star, the eccentricity of its orbit would not vary over time. The Earth's eccentricity varies primarily due to interactions with the gravitational fields of Jupiter and Saturn.

Quote from "astrobio.net"

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In our solar system, Jupiter plays two roles believed to be important to life on Earth. It helps to stabilize the orbits of the inner planets, which in turn helps to stabilize Earth's climate. And it keeps the inner solar system relatively free of comets and asteroids that could cause devastating impacts.
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http://upload.wikimedia.org/wikipedia/commons/thumb/d/da/Eccentricity_zero.svg/661px-Eccentricity_zero.svg.png
Circular orbit, eccentricity cero. The Earth without Jupiter influence.

http://upload.wikimedia.org/wikipedia/commons/thumb/1/13/Eccentricity_half.svg/581px-Eccentricity_half.svg.png
Elliptical orbit, eccentricity 0.5. The Earth today with Jupiter influence.
Source(s):
http://astrobio.net/index.php?option=com_exclusive&task=detail&id=1...

http://en.wikipedia.org/wiki/Milankovitch_cycles

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