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Because they don't have any moons. The easiest way to determine the mass of a planet is the measure the distance of its moon and the time that moon takes to orbit around the planet.

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14y ago

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How long does it take venus and mercury to rotate one time?

Venus : about 243 Earth days. Mercury about 58.65 Earth days.


Is Venus' or Mercury hotter?

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Which planets day is more than half as long as its year?

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Why Venus have no satellite?

Since Earth's Moon was likely formed from a chance collision, the formation of moons may not have been a stable process in the inner solar system. Any moons that formed around Venus or Mercury could have been swept away by the stronger gravity of the Sun, as might smaller moons around Earth. If Mercury or Venus ever had satellites, they were lost long ago.


Which planets have days that are longer than 24 hours?

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Which planet has the longest cycle of day and night?

The planet is Mercury. (Some people think it is Venus, but it is Mercury.)


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Which planet can get colder Venus or Mercury?

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Which of the inner planets does NOT show evidence of volcanic activity?

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