Astronomers say they've studied a Neptune-sized exoplanet just 100 light-years from the Earth that may share at least one similarity with our own planet: a blue sky.

A global network of telescopes used to detect light scattered by tiny particles in the atmosphere on a Neptune-size transiting exoplanet suggests the nearby world — nearby in cosmic terms, at least — may have an atmosphere of a more familiar hue to us on Earth.

When exoplanets pass in front of their parent star, astronomers can gather clues to the spectrum of any atmosphere present by measuring the planet's size at different wavelengths and by looking for light scattered by tiny particles, a phenomenon known as Rayleigh scattering.

That spectrum can reveal the substances present, and therefore, the composition of the atmosphere.

The majority of exoplanets with well-understood atmospheres found to date have been hot Jupiters, around 10 times the size of the Earth.

However, the planet studied in the new research, GJ 3470b, is only about four times the size of our Earth, closer to that of Neptune, researchers report in the Astrophysical Journal.

It is thus the smallest exoplanet where Rayleigh scattering has been detected, they say, and the measurements suggests a thick hydrogen-rich atmosphere underlying a hazy layer scattering blue light — so the sky is blue on GJ 3470b.

The planet is in orbit around a small, red dwarf star, meaning GJ 3470b blocks a significant amount of the star's light during its transits, making them easy to detect and providing ample opportunity to characterize its atmosphere, even with smaller telescopes on Earth, the researchers point out.

In fact, many of the telescopes in the LCOGT global network used to study the exoplanet are small 1.0- or 2.0-meter instruments.

"[T]his detection brings us closer to understanding the nature of increasingly smaller exoplanets through the use of a novel approach which allows us to probe the atmospheres of exoplanets even if they are cloudy," says astronomer Diana Dragomir of the University of Chicago.

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