In an incredibly exciting finding, for the first time, astronomers have discovered water in a planet orbiting a distant star outside our solar system with Earth-like temperatures that could sustain life. The discovery is the first successful atmospheric detection for an exoplanet orbiting in its star’s ‘habitable zone’, at a distance where water can exist in liquid form, they said. The Kepler space telescope discovered K2- 18b in 2015. The planet is bigger and heavier than Earth: about 2.5 times Earth’s radius and eight times its mass.
The planet orbits the cool dwarf star K2-18, which is about 110 light-years from the Earth in the Leo constellation. In 2016 and 2017, astronomer Benneke, of the University of Montreal led a group using the Hubble Space Telescope to probe K2-18b for signs of an atmosphere, as it passed in front of its star.
The molecules in the planet’s atmosphere absorbed certain wavelengths of the star’s light hinting astronomers of their existence. The team found that the planet has an atmosphere and that the atmosphere imprints the telltale signature of water vapour molecules. The team reported that its atmosphere also contained hydrogen and helium.
According to astronomer Björn Benneke “Water vapour exists everywhere in the universe, but it’s not so easy to make liquid water; you need the right pressure and the right temperature. That’s what makes this planet special.” “Until now, the planets for which we had the atmosphere observed and found water were gas giants, planets more similar to Jupiter, Saturn or Neptune,” said Tsiaras from the University College London (UCL) in the UK. K2-18b’s location in the habitable zone, size and watery atmosphere mean that “this is the best candidate for habitability that we now have.”
Benneke and colleagues took the work a step further and observed K2-18b with the Spitzer space telescope. The combined results of Hubble, Spitzer and Kepler observations suggest that clouds form at a certain level in the planet’s atmosphere, absorbing more starlight than at other levels.
They discovered that the region where the clouds condense could have the right pressure and temperature for liquid water to form. That means liquid water droplets could condense out of the clouds and rain down. Benneke says, “It’s quite likely that this planet has liquid rain on it which is actually one of the most exciting findings from this data.”
Liquid water, even if it exists on K2-18b, doesn’t imply the assured survival of anything on the planet. The size is such that it places the exoplanet somewhere between the girth of Earth and Neptune. It’s difficult to tell if they’re rocky super-Earths, gassy mini-Neptunes or sodden water worlds.
K2-18b is one of the hundreds of super-Earths, planets with a mass between Earth and Neptune found by NASA’s Kepler spacecraft. With so many new super-Earths expected to be found over the next couple of decades, it is likely that this is the first discovery of many potentially habitable planets.