A recent discovery by two independent teams of scientists has confirmed that an exoplanet atmosphere contains water. The transiting planet, called K2-18b, orbits in what’s called the “habitable zone” of a star – close enough that starlight (or what we call sunlight) provides enough warmth for water to flow and gather on the planet’s surface.
Angelos Tsiaras, an astronomer at University College London, was the lead author of one of the studies published in Nature Astronomy. Using computer models and the data from the Hubble Space Telescope, Tsiaras and his team found evidence of water vapor on K2-18 b. He notes that this is the only planet currently known outside our solar system to have an atmosphere with water in it.
The amount or volume of water is unknown. Water vapor could exist at a hundredth of a percent to half of K2-18 b’s atmosphere, according to the researchers. Larger, broadband observations from more advanced telescopes and space facilities will be needed to estimate the amount of water on the planet.
More than twice the size of Earth, K2-18 b is theorized to be a solid core of rock or ice surrounded by a thick cloud of hydrogen and other gases. The planet was discovered by Kepler in 2015. Scientists have determined that the planet takes 33 days to orbit a cool red dwarf star approximately 110 light from Earth.
Because the planet passes in front of the star, known as transiting, starlight passes through its upper atmosphere and is picked up by telescopes on Earth, providing clues about the makeup of K2-18 b’s atmosphere.
Between 2016 and 2017, researchers at the University of Montreal were able to measure the planet’s transitory shimmer on eight different occasions with the Hubble Space Telescope, along with the Kepler and the Spitzer telescopes. This separate analysis by the Montreal team provided additional evidence for the existence of water on K2-18 b. The Montreal researchers hypothesized that liquid-water droplets existed – in other words, there was evidence of clouds and rain on the planet.
The two independent studies make the evidence stronger. “Finding water vapor is great, but what is so special about K2-18 b is that our models suggest parts of its atmosphere have sufficient temperature and pressure for that vapor to form droplets of liquid water. And these, like in Earth’s atmosphere, will form clouds and fall as rain,” said Benneke.
Studies of this kind of intermediate planet offer insights into how planets are formed. It is believed that planets around this size experience a change from rocky to gaseous states. “K2-18 b represents a great step on the path to probing cooler and smaller planets,” she says. “It has the potential to inform us about how atmospheres form and evolve for planets at or near the habitable zone around red dwarf stars, which will be necessary for understanding the potential habitability of smaller ‘Earth-sized’ planets,” said Nikole Lewis, an astronomer at Cornell University.
The most exciting possibility suggested by the water vapor evidence on K2-18b is that other small planets may possess an atmosphere similar to Earth. Earlier Hubble studies offered little substantive results, noting that some worlds were subjected to intense radiation or had no atmospheric air at all. Others appear to be dry rocks with little evidence of water vapor.
That’s what makes this transiting planet a big deal for scientists and astronomers. It offers the possibility that other planets exist with water, and that may indeed be habitable.