How We Might Look to the Neighbors*
*If They Had a Good Set of Instruments and a Clear View
Astronomers are enjoying an unprecedented time of planetary discovery. As of May, 2008, here have been 293 planets found around other stars. That number will change, and in fact, it already has, and it’s only the 2nd of June.
These planetary discoveries have the potential to teach us a lot about how and where planets are scattered around the galaxy, and perhaps other galaxies as well. But, here’s what’s also cool–at least one mission out there, the former Deep Impact Mission to a comet–is now spending its time looking for other planets. And, once in a while it turns its eye back to Earth to look at our home planet as if it were an exoplanet. It’s not the first time a spacecraft has looked at Earth, but it is the first time one has done it with an array of instruments tuned to study the light from our planet (and others) in high detail.
The image here was taken by the spacecraft on May 29, 2008, and shows the Moon in its orbit around Earth. It shows our planet from the spacecraft-eye view; if a nearby civilization had a similar spacecraft and looked at us with it, they’d likely see a bright spot (if they could manage to separate us out from the glare of the Sun). If they had a REALLY good instrument, they’d probably notice that our brightness would dim occasionally and periodically, almost like something is passing in front of us.
The idea behind looking at our home world as if it was an alien planet is so that we can use what we find to compare it to other planets we might find. If we can understand how we look to the neighbors (i.e. our color, atmospheric properties, distribution of continents and oceans) then it will help us recognize similar properties when we see them on other planets.
At the summer meeting of the American Astronomical Society, being held this week in Saint Louis, astronomers working on the EPOXI mission (using the Deep Impact craft) talked about their work. They said that later this summer they’ll release a time-lapse movie of the Moon transiting Earth. As it passes between Earth and the point of view of the spacecraft, it will cover up places like the deserts of North African, which reflect a lot of light.
To a distant observer, this kind of transit would show up as a dimming of the planet (which would actually appear pretty bright since its surface areas and clouds reflect light). This well-known transit effect is also how astronomers can “see” or actually infer the existence of larger planets around stars. The light blockage is what what helps them detect the existence of a planet. So, the story of planetary discovery keeps on ticking, like that well-known battery-operated bunny rabbit. There are, in fact, more results from the meeting that I’ll write about in another post. But for now, enjoy the pretty sight of Earth from “alien eyes” and think about what it will be like when we find another Earth out there.