Despite identifying thousands of exoplanets, the search for life beyond Earth still continues. Several scientists across the globe are scouring the cosmos to pinpoint biosignatures — indicators of life — or the conditions required to support life on any of the known worlds.

While most of the exoplanets sit between a few hundred to thousands of light-years away from Earth, a new study suggested some of these worlds, particularly those two to four times bigger than Earth, could have one most critical element required for life – water.

For years, scientists have known water is the most basic pre-requisite for the existence of life as we know it. The presence of carbon, oxygen and other biomarkers is also important, but water tops the list and is always given priority while exploring the dynamics of planets — whether within or beyond our own solar neighborhood.

This is why researchers looked at thousands of exoplanets by considering the aspect of water. They used data from NASA’s Kepler Space Telescope and European Space Agency’s Gaia satellite to take exoplanetary radius and masses into account.

Most of the exoplanets identified till date have radiuses averaging around 1.5 or 2.5 times that of Earth. The smaller worlds are five times heavier than Earth, while the bigger ones carry nearly 10 times more mass than our planet.

The researchers used these size and weight measurement to develop a model detailing the internal structure and composition of different exoplanets. The results of the work revealed smaller planets are rocky in nature, while the bigger ones — essentially those two to four times the size of Earth — are likely to be water worlds. 

Presence of water was perhaps the best explanation for the increased mass of these worlds. In fact, according to the work, nearly 35 percent of all known exoplanets bigger than Earth fall into the water-world category, with as much as 50 percent (weight) of water content. By comparison, this is much more than 0.02 percent water content on Earth.

"This is water, but not as commonly found here on Earth," Li Zeng, the lead researcher behind the work, said in a statement. "Their surface temperature is expected to be in the 200 to 500 degree Celsius range. Their surface may be shrouded in a water-vapor-dominated atmosphere, with a liquid water layer underneath. Moving deeper, one would expect to find this water transforms into high-pressure ices before we reaching the solid rocky core.”

Zeng believes the core of most of these worlds probably formed in the same way the giants of our solar system — Jupiter, Saturn, Uranus, Neptune — came to be.

"It was a huge surprise to realize that there must be so many water-worlds," he added.

That said, further observations from NASA’s TESS exoplanet hunter and upcoming James Webb Telescope will play a critical role in this field. TESS, as NASA has already described, could add more potentially habitable planets into the list, while Webb’s follow-up spectroscopic analysis of these, and another previously identified worlds, could provide more insight into their atmosphere, composition as well as on the possibility of life.

The study was presented at the Goldschmidt 2018 conference in Boston.