Scientists have detected water vapor in the atmosphere of a potentially habitable planet for the first time.

At twice the size of the Earth and eight times our planet's mass, K2-18b is considered a super-Earth. It orbits within its red dwarf star's habitable zone, the range of distances in which liquid water can exist on a planet's surface.

"This is the only planet right now that we know outside the solar system that has the correct temperature to support water. It has an atmosphere, and it has water in it, making this planet the best candidate for habitability that we know," Angelos Tsiaras, the lead author of a paper on the finding, said in a press call. "We were extremely excited to see these results."

The Kepler space telescope discovered K2-18b, which is just 110 light-years away from us, in 2015. But the new study about the planet's water vapor, published Wednesday in the journal Nature Astronomy, is a major step toward understanding what habitable planets look like outside our solar system.

For now, scientists think super-Earths are the most common type of planet in the Milky Way galaxy — and red dwarfs are the smallest, coolest, longest-lived, and most common type of star — meaning there could be more habitable, water-containing exoplanets like this one out there. None have been confirmed yet, however.

Meet K2-18b

K2-18b orbits its tiny red dwarf star every 33 days. Researchers think it's either a rocky planet like Earth, or an icy planet with high concentrations of water in its interior. It might have clouds, but if so, scientists' observations suggest they aren't very thick.

Though K2-18b is much closer to its star than Earth is to the sun, the planet receives only slightly more radiation than Earth, since its star is much smaller and cooler than our sun. Researchers calculated that the planet has a similar temperature as Earth — somewhere in the range of about -99 degrees to 116 degrees Fahrenheit.

To determine the makeup of K2-18b's atmosphere, the researchers behind the new study used data from the Hubble Space Telescope to analyze the starlight filtering through its atmosphere. For the first time, they detected the signature of water molecules.

NASA's Hubble Space Telescope orbits Earth. NASA/Getty Images

The data also indicated the presence of hydrogen and helium in the atmosphere.

The importance of an atmosphere

An atmosphere keeps a planet's surface warm and protects it from its star's radiation. Life on Earth would not be possible without our atmosphere, which also provides many of the chemicals essential to life, like carbon and nitrogen.

K2-18b is ripe for continued study, since its water vapor is probably not going anywhere for a while, the researchers said. Its star is relatively inactive and hasn't put out any flares since scientists started observing the system.

Plus, the planet's atmosphere containing hydrogen also bodes well for the continued presence of water vapor — hydrogen is much lighter, so it would be the first molecule to go if K2-18b's atmosphere were stripped away by radiation.

This image taken aboard the International Space Station shows the Earth's orange-colored stratosphere, the lowest and densest portion of its atmosphere. NASA/Marshall Space Flight Center

"Water can stay quite long because super-Earths can retain their atmosphere for a longer time, because they have a bigger gravity," Giovanna Tinetti, a physicist who cowrote the paper, said in the press call.

She contrasted this planet's atmosphere with that of Mars. The red planet is much smaller and has a weaker gravitational pull than Earth, so though it once had an atmosphere (and probably liquid water), Mars was too small and weak to hang on to it. Over time, the flow of charged particles and plasma from the sun stripped away Mars' atmosphere.

Planets like K2-18b can help redefine habitability

NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

K2-18b's atmosphere is very different from Earth's, since it's made up mostly of hydrogen, unlike our own nitrogen-rich atmosphere. But the scientists behind the new study said life could still emerge in those conditions (at least based on our limited information right now).

"Habitability as we understand it is a very Earth-centric concept. So we know one world that is habitable, which is our own Earth now, and now we have a potentially second one that might be habitable," said Ingo Waldmann, an exoplanet researcher who cowrote the paper. "To actually understand how habitability evolves in a hydrogen-rich atmosphere is going to be a very intense subject of study over the next 10 years."

The researchers said scientists would need to start by determining the pressure of K2-18b's atmosphere and the amount of water within it. For now, there's no way to know what conditions on its surface are like.

"With the current data, we can only detect the existence of an atmosphere and the existence of water," Tsiaras said.

The James Webb Space Telescope is designed to observe faraway galaxies in detail with a 21-foot-wide folding beryllium mirror. NASA/Chris Gunn

However, the researchers behind another study, which was made available online Tuesday but has not been peer-reviewed, told Space.com that the data indicated the presence of liquid-water clouds that might produce rain on K2-18b. They also suggested that the planet's atmosphere is too thick and exerts too much pressure for life as we know it to exist on its surface.

Tsiaras said that NASA's James Webb Telescope, set to launch in 2021, and the European Space Agency's Ariel telescope, set to launch in 2028, would be able to gather much more data on K2-18b by observing light across a broader range of wavelengths.

That information could then improve scientists' search for more planets like K2-18b.

"This is hopefully just one of many that we will find — many super-Earths going around dwarf stars," Waldmann said. "Then we can actually start looking into studies of what habitability actually means compared to Earth or compared to other super-Earths."