While observing the atmosphere of a Neptune-sized exoplanet, an international team of astronomers have discovered water vapor and clouds. This is impressive in its own right, but it has also allowed the researchers to uncover how the planet, known as HAT-P-26b, might have formed.

In a paper, published in Science, the researchers discussed how they used observations of the atmosphere to constrain the planet's properties. The team used water as a proxy for other heavy elements and established when this exoplanet acquired its atmosphere. 

"A few dozen exoplanets have been studied in the same way we looked at HAT-P-26b, but only a handful are in the same mass range and all showed muted (smaller) or absent atmospheric signatures," lead author Hannah Wakeford told IFLScience. "Our analysis of HAT-P-26b is one of the most detailed studies of a 'warm Neptune' and the first with such a strong absorption signature of water measured from its atmospheric transmission. From this, we were also able to approximate the amount of heavy elements in the atmosphere, which can lend clues as to the formation of planets."

The composition suggests that HAT-P-26b has an atmosphere very similar to the primordial disk it formed from. This suggests that it acquired its atmosphere late in its formation and that there has been very little contamination from smaller, metal-rich planetesimals that might have flown by the star system.

A well-known relationship exists in the Solar System between the mass of gas giants and the amount of heavy elements in their atmosphere. The smaller the mass, the more heavy elements. Not only that, but Neptune-size worlds can acquire an atmosphere in a variety of ways, which makes them an intriguing target for planet-hunters. They could be rich in hydrogen and helium, or they could be water worlds, or even rocky with an intense outgassing.

The scientists think that its location is the key to its low abundance of heavy elements, technically known as metallicity. HAT-P-26b formed closer to its star, where it was too hot for many rocky planetesimals to form. This is a good explanation for why the exoplanet has not been contaminated much.

"This is the first measurement of diversity from the expected evolutionary path, but I would not be surprised if in the next decade with more studies and accurate measurements of larger and smaller planets, we are learning something new about planet formation and our own solar system in return," Wakeford added.

The team used NASA's Spitzer and Hubble telescopes to study the atmosphere in infrared. HAT-P-26b is a transiting planet, meaning it passes directly in front of its star from our point of view. It orbits its star every four days, making it ideal for observations. The star system is located 437 light-years away.