What do you like about space? All sorts of ideas come to mind. Beauty, serenity, and cultural significance could describe the stars. Power, innovation, and courage are fair for human and robotic exploration. Let me add another idea to that list: perspective. A lot of what humanity has gained in its study of the sky is perspective. An idea of how we came into this universe, where we sit within it, and in which ways we are special and we are ordinary.
Earth is called “The Blue Planet” for good reason. It has a great blue vibrancy—and frankly a green vibrancy too—that is unparalleled in our solar system. The green comes from the great range of photo-synthetic life on the surface, which I’m sure you know is a somewhat unique feature. But the blue water that we also associate with our little home is less identifying than terms like “Pale Blue Dot” would have us believe. Water, it turns out, is all over our solar system. For starters, our close neighbour the Moon is known to have surface ice in its polar regions. On Mars, it’s hard to pick which instances of water to share: There is a variety of robotic and satellite data suggesting that briny water flows on the surface at night, more water ice exists at the poles, and Curiosity has been wandering through a landscape of water-birthed sedimentary rocks for years. If you want to be picky and say that Earth’s water is significant because it’s liquid, you need only look at the Jovian moon Europa; Europa is suspected to have twice as much liquid water as Earth hidden under its icy surface, kept from freezing by the gravitational forces of giant Jupiter kneading the moon. So that’s an interesting perspective: Earth isn’t special for having water or even liquid water. What it is special for is having liquid water on its surface.
Perhaps a more revealing perspective is the size of Earth’s moon. Have you ever seen Apollo 13? Then you probably remember Tom Hanks standing on Earth and using his thumb to cover the moon. While in orbit around the moon, Hanks then does the same trick to cover Earth. To be pedantic, this couldn’t happen because the Moon is only one quarter the radius of the Earth, and thus Earth would appear four times larger than Hanks’ thumb. But even a product of four is surprisingly small. If you take a look at Mars, for instance, its larger moon—Phobos—is only 11 km across. Now, some other moons are larger than the Moon, but they’re all orbiting gas giants. Ganmede is the largest moon in the solar system and it is slightly larger than Mercury, but it is 26 times smaller than the titanic Jupiter that it orbits. So the size of the Moon relative to Earth remains a relative novelty (with notable parallels in the Pluto-Charon and Eris-Dysnomia dwarf planet-moon pairs).
These are just a few of the lessons that astronomy has taught us over the centuries about our place in the cosmos. The ways in which our planet is really unique, and the ways in which our planet is very ordinary. Humans are now embarking on a new odyssey of uncomfortable-but-fascinating research into the typicality of our home. The growing search for exoplanets, championed by the Kepler Space Telescope, is revealing new ways in which our solar system is a rather boring one. Every few months, a new story comes out of some discovery that would have been ridiculous just months before, like gas giants so close to their host star that they orbit in a day. There is no knowing what is or isn’t possible, and what curiosity will be found next. But suffice it to say that our perspective of little green Earth will change again and again as long as astronomy is studied.
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