Data from the Hubble Space Telescope have revealed there is water vapor in the atmosphere of an Earth-size planet outside the solar system.

K2-18b

The exoplanet called K2-18b orbits a star smaller than the sun, but it is in the so-called "habitable zone" where it is warm enough for liquid water to exist on the planet's surface.

K2-18b is located about 111 light-years away and is nine times more massive than Earth. Its size means it is either an icy giant like Neptune or a rocky world with a thick and hydrogen-rich atmosphere.

Potentially Habitable Planet

In a study published in preprint journal arXiv.org on Sept. 10, researchers revealed they detected water vapor in the planet's atmosphere, making the planet a plausible candidate in the search for alien life.

The researchers also said that with its 33-day orbit around a cool 3 dwarf, the planet receives virtually the same amount of radiation from the star as the Earth receives from the sun.

"Thanks to our observations and our climate model of this planet, we have shown that its water vapour can condense into liquid water. This is a first," said study researcher Björn Benneke, from the Institute for Research on Exoplanets at the Université de Montréal,

Identifying Planets With Water

Researchers can study the chemicals in the atmosphere of planets by looking at the changes to the starlight as planets orbit their parent stars. Using this method, K2-18b appears to have the molecular signature of water, a vital ingredient for life on Earth.

As the planet makes a transit, its host star's light shines through K2-18b's atmosphere, but not all of the starlights makes it through. This is because the chemical compounds in the atmosphere absorb light at telltale frequencies. Water absorbs near-infrared light at specific wavelengths and thus create a visible sign of water vapor.

"We observed eight transits using HST/WFC3 in order to achieve the necessary sensitivity to detect water vapor," the researchers wrote in their study.

"Our observations demonstrate that low-mass habitable-zone planets with the right conditions for liquid water are accessible with state-of-the-art telescopes."