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[Music]

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Carolyn Crow: So far we've found more than 400 planets around other stars.

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Unfortunately, for many years to come, we won't be able to see them as anything more than dim

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points of light. If only the planets that we know best could help us

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learn more about the real worlds behind those dim points of light.

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Well it turns out they can. Remember the Deep Impact mission?

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The NASA spacecraft that slammed a probe into a comet in 2005?

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Well now it's headed for another comet. While on its way we used its

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instruments to study the amount of red, green, and blue light reflected

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by Earth, the Moon, and Mars. Combining this color information

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with similar studies of the other planets in our solar system, we found an interesting

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pattern. These crosshairs mark the spot where

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a planet would perfectly reflect all the light from the Sun. The more

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red light a planet reflects, the farther it moves to the right on this chart.

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The more blue light it reflects, the higher it goes. And if the

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planet reflects relatively little blue and red light, it falls in the dark

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section. Viewed in this way, here's

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where the planets lie.

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Not surprisingly, Mars, the reddest planet, occupies the reddest spot.

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Mercury is also nearby on the plot because neither planet has a large

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atmosphere that scatters blue light. Venus has a thick cloudy atmosphere

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that reflects most of the red light and only a little bit

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of the blue light, so it stays near the bottom. Jupiter and Saturn fall in the darkest

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region of the plot. Their atmospheres have methane and ammonia in it, which

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absorbs red light and other gasses that absorb blue light.

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Here's what's really interesting. In this chart, Earth stands apart

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from all the other planets. Earth is

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really blue. Not because of its oceans, but because it has an atmosphere that reflects

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a lot of blue light.  It makes sense, that's why the sky is blue.

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At the same time, Earth does reflect a little bit of red light.

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Long before we have telescopes that will show us what extrasolar

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planets really look like, we'll be able to measure their colors and put them on the plot.

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This means we'll be able to tell the difference between alien

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versions of Mars, Jupiter, and even Earth. And that's