Before we jump into this one, you need to know why the sky is blue. Ultimately, our sky looks this color because of something known as "Rayleigh scattering." In short, molecules in the air scatter blue light from the Sun more than they scatter red light. Since the blue light travels more, it's what we see when we look up on a clear, sunny day.
The light we see in the atmosphere is called "diffuse sky radiation," but that's probably jumping into things a little too much.
Now, the same phenomenon was detected by astronomers on a Neptune-size exoplanet (an "exoplanet" is an alien world found in a solar system other than our own). The planet in question is known as GJ 3470b, and it is only 100 light-years away. As an aside, if you are wondering, 100 light-years is just 950,000,000,000,000 km away—about 950 trillion km (588 trillion miles)—a distance that would take us hundreds of thousands of years to travel.
The discovery was made using the Las Cumbres Observatory Global Telescope Network.
One way of detecting exoplanets is through transit photometry. From our line of sight, as an exoplanet passes its parent star, it minutely reduces the amount of light we see from the star. Nevertheless, using this tiny change in information, astronomers can measure the planet's size at different wavelengths in order to generate a spectrum of its atmosphere. The spectrum then reveals the substances present in the planet's atmosphere, and therefore its composition
The planet, termed GJ 3470b, is the smallest exoplanet for which a detection of Rayleigh scattering exists. The planet orbits a red dwarf, allowing it to block a large amount of light during every transit. This makes transit detection easier and the planet more easily characterized.
The obtained information suggests that the planet has a thick hydrogen-rich atmosphere below a layer of haze that scatters blue light.
Notably, this measurement is the first clear detection of a spectroscopic feature in the atmosphere of an exoplanet using only small (1.0m and 2.0m) telescopes. This finding allows further understanding of the nature of increasingly smaller exoplanets. Furthermore, the result highlights the role that meter-size telescopes can play toward characterizing the atmospheres of these worlds.