| [00:00:00.04] | [Music] |
| [00:00:09.07] | Carolyn Crow: So far we've found more than 400 planets around other stars. |
| [00:00:13.10] | Unfortunately, for many years to come, we won't be able to see them as anything more than dim |
| [00:00:17.13] | points of light. If only the planets that we know best could help us |
| [00:00:21.20] | learn more about the real worlds behind those dim points of light. |
| [00:00:25.27] | Well it turns out they can. Remember the Deep Impact mission? |
| [00:00:29.45] | The NASA spacecraft that slammed a probe into a comet in 2005? |
| [00:00:33.48] | Well now it's headed for another comet. While on its way we used its |
| [00:00:37.50] | instruments to study the amount of red, green, and blue light reflected |
| [00:00:41.52] | by Earth, the Moon, and Mars. Combining this color information |
| [00:00:45.56] | with similar studies of the other planets in our solar system, we found an interesting |
| [00:00:49.58] | pattern. These crosshairs mark the spot where |
| [00:00:53.61] | a planet would perfectly reflect all the light from the Sun. The more |
| [00:00:57.64] | red light a planet reflects, the farther it moves to the right on this chart. |
| [00:01:01.66] | The more blue light it reflects, the higher it goes. And if the |
| [00:01:05.68] | planet reflects relatively little blue and red light, it falls in the dark |
| [00:01:09.86] | section. Viewed in this way, here's |
| [00:01:13.87] | where the planets lie. |
| [00:01:17.89] | |
| [00:01:21.92] | Not surprisingly, Mars, the reddest planet, occupies the reddest spot. |
| [00:01:25.94] | Mercury is also nearby on the plot because neither planet has a large |
| [00:01:29.98] | atmosphere that scatters blue light. Venus has a thick cloudy atmosphere |
| [00:01:34.00] | that reflects most of the red light and only a little bit |
| [00:01:38.04] | of the blue light, so it stays near the bottom. Jupiter and Saturn fall in the darkest |
| [00:01:42.07] | region of the plot. Their atmospheres have methane and ammonia in it, which |
| [00:01:46.12] | absorbs red light and other gasses that absorb blue light. |
| [00:01:50.13] | Here's what's really interesting. In this chart, Earth stands apart |
| [00:01:54.29] | from all the other planets. Earth is |
| [00:01:58.31] | really blue. Not because of its oceans, but because it has an atmosphere that reflects |
| [00:02:02.34] | a lot of blue light. It makes sense, that's why the sky is blue. |
| [00:02:06.37] | At the same time, Earth does reflect a little bit of red light. |
| [00:02:10.40] | Long before we have telescopes that will show us what extrasolar |
| [00:02:14.44] | planets really look like, we'll be able to measure their colors and put them on the plot. |
| [00:02:18.48] | This means we'll be able to tell the difference between alien |
| [00:02:22.52] | versions of Mars, Jupiter, and even Earth. And that's |
| [00:02:26.57] | something we're looking forward to. [Music] |
| [00:02:30.57] | |
| [00:02:33.58] | [Beeping] |