The orbit of Venus, the 2nd planet outward form the sun, comes closer to Earth than any other planetary orbit.

金星的轨道，围绕太阳外运行的第二颗行星，它比任何其它的行星轨道都靠近地球。

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Kepler went on to show that an elliptical orbit is sufficient to explain the movement of the other planets and to devise the laws of planetary motion that Newton built on.

接着开普勒证明了这个椭圆轨道也能很有效解释其他行星的运行轨迹。 然后他提出了行星运动定律，牛顿定律就是以此为理论基础的。

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PLANETRISE: In an artist's conception, rocky moons orbit a gas giant in a distant planetary system.

行星升起：在艺术家的想像图中，在一个遥远的行星系统内，一颗岩石卫星围绕一颗气体行星旋转。

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And he says five of the planetary candidates are both close to the size of Earth and orbit in the habitable area of their parent stars.

VOA: special.2011.02.09

But could there be intelligent life in another planetary system, in orbit around one of the 100 billion other stars in our Milky Way galaxy?

CNN: Three more homes for life in the universe?

This pending move back to planetary science from earth sub-orbit comes on the heels of improvements in pointing systems and instrument technologies that will allow ground-based controllers to fine-tune balloon payloads to include one-meter class optical and infrared telescopes.

FORBES: Budgets Nudge NASA Toward Balloon-Based Planetary Science: First Target? Comet ISON

The discovery of a true planetary system, as opposed to the solitary planets that are known to orbit a dozen or so other stars, is significant for two reasons.

ECONOMIST: Planet hunting