Lunar Eclipse Gives Alien’s-Eye View of Earth

Of the more than 300 planets circling other stars we’ve found so far, only a handful have ever had their pictures taken directly.

Astronomers strongly suspect the vast majority of these so-called exoplanets exist based solely on indirect evidence, such as their gravitational effects on stars.

So the trick, then, is figuring out anything else about those planets beyond the fact that they’re there.

Is a given exoplanet the size of Jupiter or Mars? What’s it made of, and what’s in its atmosphere? And perhaps the most exciting question, is there liquid water?

Enric Pallé, of Spain’s Astrophysics Institute of the Canaries, and colleagues figured the best way to answer some of these questions would be to look no farther than home.


—Image courtesy Gabriel Perez Diaz/Nature

What’s more, the researchers decided to advance the frontiers of 21st-century astronomy using one of the oldest known astrophysical tools: a lunar eclipse.

Almost 60 of the known exoplanets have been found using the transit method—planets passing in front of their host stars periodically dim the light from those stars as seen from Earth.

Lunar eclipses do a pretty good job of mimicking a planetary transit, since the moon falls completely into Earth’s shadow. Any light hitting the moon is sunlight that is being filtered through Earth’s atmosphere.

By looking at this “earthshine” during a lunar eclipse on August 16, 2008, Pallé’s team recorded what Earth’s atmosphere would look like to aliens watching our home world transit the sun.

Reporting this week in the journal Nature, Pallé et al note that Earth as seen during a transit would not, after all, be a pale blue dot, but a pale red dot, because only the shorter red wavelengths of light make it through the atmosphere.

This makes sense, considering the way earthshine makes the moon turn a rusty red during a lunar eclipse.


—Image courtesy Daniel Lopez/Nature

What’s more, the team found that Earth’s “transit” light clearly shows atmospheric features that are key to life: molecular oxygen, ozone, water vapor, carbon dioxide, and methane.

The eclipse experiment also showed that astronomers can tell whether a transiting exoplanet has layers in its atmosphere. Pallé’s team saw the signature of Earth’s ionosphere, the upper layer of the atmosphere that is electrically charged by the sun’s radiation.

In a similar fashion, astronomers can glean a whole lot of information about the sun by watching total solar eclipses, Jay Pasachoff, of the California Institute of Technology, argues in another paper in this week’s Nature.

Pasachoff, a National Geographic grantee, has been studying the sun’s plasma atmosphere, or corona, for more than 30 years by following eclipses around the world.

Historically, solar eclipses have been vital to some major scientific discoveries. It’s thanks to eclipse chasers that we identified helium, took the temperature of the sun’s corona, and helped prove Einstein’s theory of general relativity.

And even with today’s advanced solar-watching satellites, such as STEREO and SOHO, certain aspects of the sun are still best viewed from the ground.

Solar eclipses, for example, provide us with the most detailed views of the nether regions of the sun’s corona.


—Image courtesy M Druckmuller/P Aniol/V Rusin

Space telescopes have to block out the sun to protect their sensitive instruments, and a good chunk of the lower atmosphere gets obscured by that blockage.

But during an eclipse, just enough of the sun gets blocked that we still have a clear view of the corona and its relationship with interesting solar activity.

Combining eclipse data with information from satellites therefore gives astronomers a more complete picture of the sun than either technique alone.

Want to take an active role in astronomy? The total solar eclipse due to happen on July 22 will be the longest lasting of the entire 21st century, clocking in at 6 minutes, 39 seconds.


—Image courtesy NASA

The path of totality, where you’ll be completely in the moon’s shadow, cuts across India, Nepal, Bangladesh, Bhutan, Myanmar (Burma), China, and some of the small Japanese islands in the Pacific.

So there’s plenty of scenic destinations for a vacation of astronomical proportions!

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