A new method of exoplanet detection can go beyond standard measurements like mass and distance to tell us crucial details about the planet's atmosphere. But astronomers needed to make sure the method worked - so they tried it on Earth.
As it stands, telescopic observation has proven to be fantastic at finding exoplanets - we're well into the hundreds at this point - but it's not nearly as good as telling us what the planet is actually like. To be sure, we can figure out the planet's mass, distance from its star, and whether it's rocky or gaseous. But even all that doesn't really get you close to knowing whether the planet in question could support life - after all, alien astronomers many light-years away would get pretty much identical readings in those categories for Earth and Venus, and those two couldn't be much more different in their ability to support life.
So what astronomers really need is a way to deduce the nature of a planet's atmosphere using little more than the manner in which it orbits its star. There's a phenomenon known as stellar occultation that allows us to do exactly that. Basically, certain elements in a planet's atmosphere interact with the wavelengths of the star's emitted light in very specific ways, and it's possible to reconstruct the entire atmosphere from the addition and subtraction of these wavelengths. This was first tested back in 2001 on the Jupiter-sized planet HD 209458b, an artist's conception of which you can see up top.
But could this method work on a much smaller, Earth-like planet? Astronomers decided to test this by using it to figure out one of the most complex atmospheres in the known universe - that of Earth itself. Obviously, we can't send a telescope to another solar system to test it out on Earth, but by making measurements during a lunar eclipse they were able to closely replicate what occurs during a stellar occultation.
The test was hugely successful - the spectrograph used in the experiment was able to detect ozone, oxygen, nitrogen, and sodium using only light reflected from Earth's atmosphere. Obviously, it's a very huge jump from successfully using this method on a planet you're standing on to one tens or even hundreds of light-years away, but its ability to detect minute amounts of various elements during what the astronomers described as less than ideal conditions suggests that this really could be a powerful new tool in the search for Earth-like planets.