Scientists using NASA telescopes have detected "a strong water signature" in the atmosphere of a distant Neptune-sized planet, that could help understand more about the birth and development of planetary systems.

The study, combining observations from NASA's Hubble and Spitzer space telescopes, shows that the distant planet HAT-P-26b has a primitive atmosphere composed almost entirely of hydrogen and helium. Located about 437 light-years away, HAT-P-26b orbits a star roughly twice as old as our Sun.

The analysis is one of the most detailed studies to date of a "warm Neptune," or a planet that is Neptune-sized and close to its star.

The researchers determined that HAT-P-26b's atmosphere is relatively clear of clouds and has a strong water signature, although the planet is not a water world. This is the best measurement of water to date on an exoplanet of this size.

Compared to Neptune and Uranus, the planets in our solar system with about the same mass, HAT-P-26b likely formed either closer to its host star or later in the development of its planetary system, or both.

"Astronomers have just begun to investigate the atmospheres of these distant Neptune-mass planets, and almost right away, we found an example that goes against the trend in our solar system," said Hannah Wakeford, a postdoctoral researcher at NASA's Goddard Space Flight Centre in the US.

To study HAT-P-26b's atmosphere, the researchers used data from transits - occasions when the planet passed in front of its host star.

During a transit, a fraction of the starlight gets filtered through the planet's atmosphere, which absorbs some wavelengths of light but not others.

By looking at how the signatures of the starlight change as a result of this filtering, researchers can work backward to figure out the chemical composition of the atmosphere.

Researchers pooled data from four transits measured by Hubble and two seen by Spitzer. Together, those observations covered a wide range of wavelengths from yellow light through the near-infrared region.

"To have so much information about a warm Neptune is still rare, so analysing these data sets simultaneously is an achievement in and of itself," said Tiffany Kataria of NASA's Jet Propulsion Laboratory in the US.

Researchers were also able to use the water signature to estimate HAT-P-26b's metallicity - an indication of how rich the planet is in all elements heavier than hydrogen and helium. It gives more clues about how a planet formed. They determined its metallicity is only about 4.8 times that of the Sun.

"This analysis shows that there is a lot more diversity in the atmospheres of these exoplanets than we were expecting, which is providing insight into how planets can form and evolve differently than in our solar system," said David K Sing of the University of Exeter in the UK.

The research was published in the journal Science.