Earth as an Exoplanet

Last week a group of Spanish scientists published some unique observations of the Earth's atmosphere, as it might be seen by an alien civilization looking for signs of life. The idea was to see what they could learn about our home from the type of data that may soon be available for distant Earth-like planets orbiting other stars. The results were impressive. The researchers identified several of the dominant molecules in the Earth's atmosphere -- and they could even tell that the sunsets are red, and the skies are blue.

More than 350 planets have now been discovered around other stars, with nearly 60 of them passing directly in front of their Suns from our vantage point on Earth. These "transiting" planets provide a special opportunity to study the atmospheres of alien worlds. As they pass in front of their host star, they block some of the starlight from reaching us. Just from the amount of missing light, we can determine the relative size of the planet compared to the star. So far, most of the planets we know about from such measurements are big -- like the outer planets in our solar system: Jupiter, Saturn, Uranus and Neptune. But with more precise measurements from satellites, we will soon be able to find smaller ones. During the transit a small fraction of the starlight passes through the planet's atmosphere, and we can use this light to learn about its composition and other properties.

Something similar happens during a lunar eclipse. As our Moon passes into the shadow of the Earth, the only sunlight that reaches it passes through our planet's atmosphere. By measuring the light on the Moon during an eclipse, we can see what a transit of the Earth would look like to a distant observer. The Spanish scientists measured a lunar eclipse last August, passing the moonlight through a prism to separate it into its spectrum of colors. Individual molecules in the Earth's atmosphere absorb specific colors of light, creating dark lines in the spectrum like a fingerprint that identifies the molecule. The lunar eclipse spectrum revealed signatures of oxygen, water vapor, carbon dioxide and methane -- as well as traces of many other molecules. But the most dramatic feature of the spectrum was immediately obvious -- nearly all of the blue light was missing.

As sunlight passes through the Earth's atmosphere, some of the light that isn't absorbed by the individual molecules is scattered by them instead. The typical size of air molecules makes this process, known as "Rayleigh scattering", much more effective for blue light than it is for red light. As a consequence mostly the red light makes it through the atmosphere to the Moon during a lunar eclipse, while the blue light is scattered and makes the sky appear blue. This is also why sunsets look red, since the sunlight passes through much more atmosphere when it is near the horizon than when it is high in the sky.

The most amazing thing about these new observations is that nobody had ever thought to look at the Earth in this way before. In the near future, after NASA has discovered transiting Earth-like planets around other stars, satellites such as the James Webb Space Telescope will be able to obtain spectra if their atmospheres. And now we know what it might see... blue skies.