We cannot repeat experiments with just one planet.
我们只有一个地球，无法重复进行实验。
However, as one planet exists around this star, it is possible that others will be discovered — perhaps in more habitable orbits — as observations continue.
但是，既然恒星周围存在一颗行星，那随着观测的进展，很可能会发现其他的行星——也许位于更适宜居住的轨道上。
Earth is just one planet in the universe.
地球只是宇宙中的一颗行星。
We now know, for example, that every star in the sky likely hosts at least one planet.
举例来说，我们现在已经知道，夜空中的每一颗星星都很 有可能拥有至少一颗行星。
When I got there, I didn't even know one planet in our solar system.
当我到达那里时，我甚至不知道我们太阳系里的任何一颗行星。
The theme of this year’s World Environment Day, “Many Species. One Planet.
今年世界环境日的主题为“众多物种·同一个星球·同一个未来”。
One planet, about the size of Mars, probably had an orbit which crosses Earth's, and eventually a collision occurred.
一个大小与火星相当，轨道可能与地球轨道交叉的行星，最终与地球发生了碰撞。
One planet, about the size of Mars, probably had an orbit which crosses Earth's, and eventually a collision occurred.
一个大小与火星相当，轨道可能与地球轨道交叉的行星，最终与地球发生了碰撞。
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