Scientists have detected water vapor in the atmosphere of K2-18 b, a potentially habitable alien world about twice the size of Earth. The finding, announced today (Sept. 11), follows a separate study released yesterday announcing the discovery of water vapor, and likely clouds and rain, on the planet.

The far-off planet (it's about 110 light-years away) was discovered in 2015 by NASA's Kepler space telescope. The alien world is about two times the size of Earth and eight times as massive. K2-18 b orbits a red dwarf star in the "habitable zone" — the range of distances where liquid water could be stable on a world's surface.

Scientists with the study published today in the journal Nature Astronomy used data from Hubble Space Telescope observations made from 2016 and 2017 collected by the research team behind yesterday's study to determine that the exoplanet has water vapor in its atmosphere. K2-18 b is so far the smallest exoplanet that atmospheric water vapor has been discovered on.

Video: Water Vapor Discovery in Exoplanet K2-18 b Atmosphere

Related: These 10 Exoplanets Could Be Home to Alien Life

In a study published Sept. 11, 2019, researchers detected water vapor in the atmosphere of exoplanet K2-18b. This artist’s impression shows the planet K2-18b, its host star and an accompanying planet in this system. (Image credit: ESA/Hubble, M. Kornmesser)

Much remains unknown about the exoplanet K2-18 b, the researchers stressed. The exoplanet could be rocky with a large atmosphere, or a "water world" covered mostly or entirely by water, they asserted.

Still, K2-18 b is one of the best candidates to host alien life that we know about, lead researcher Angelos Tsiaras of the University College London (UCL) Department of Physics and Astronomy, told Space.com. The fact that it is a relatively small planet that lies within a habitable zone and has evidence of water "makes this target the best target for habitability that we know right now," he said.

The researchers studied Hubble data to analyze K2-18 b's transit, or its movement across its host star's face, using a technique known as transit spectroscopy. As the planet transits, "part of the stellar light is filtered through the atmosphere of the planet," Tsiaras said. "The atmosphere of the planet leaves a characteristic fingerprint on the light … this is what we try to observe."

The team analyzed the data from the planet's transits and from this analysis determined that K2-18 b's atmosphere likely contains between 0.01% and 50% water, and there could additionally be a significant amount of hydrogen.

This massive range stems from the fact that, with Hubble observations, researchers can only identify a water signature, the "fingerprint" observed using transit spectroscopy; they can't tell how much water is there, Giovanna Tinetti, a researcher on this study and a professor of astrophysics at UCL, said during a Sept. 10 news conference.

"The water signature is so strong that [with] even a tiny bit of water, you would immediately see the signature," Tinetti said. So, while the team knows there's water in the atmosphere, its abundance remains unknown.

Water vapor in air

As of now, the researchers have concluded that there is some quantity of water and likely hydrogen in the planet's atmosphere. They also know the size and mass of the cold planet, and that K2-18 b has much higher gravity than we do here on Earth (because of the much higher mass). K2-18 b has an orbital period of 33 Earth days — so one year on K2-18 b is equivalent to about one Earth month.

Additionally, K2-18 b orbits a red dwarf star alongside a second, smaller exoplanet (K2-18 c). So, if you were standing on the surface of the planet under clear skies, you would see a red star overhead instead of our yellowish-orange sun (though, because of the high gravity and higher UV radiation on the surface of this exoplanet, it might not be possible or safe to walk around).

This finding has sparked a host of new questions regarding what exactly this exoplanet and its atmosphere are like. According to this study, there are three equally likely atmospheric models for the exoplanet.

K2-18 b exoplanet possibilities

The first possibility described by these researchers is a cloud-free atmosphere that contains only water and hydrogen-helium. The second possibility is a cloud-free atmosphere that has water, hydrogen-helium and molecular nitrogen. Third, the researchers think that this planet's atmosphere could be cloudy with water and hydrogen-helium.

"These are, statistically, equally likely, given the data," Ingo Waldmann, one of the authors of this new study, said in the news conference. The "answer is probably somewhere in between those," he added.

Currently, it would be very difficult for researchers to study this planet and its atmosphere further using ground-based observations, the team said. "It's very difficult to observe an atmosphere with water through an atmosphere with water," Waldmann, who's also based at UCL, said about trying to observe K2-18 b through Earth's atmosphere.

However, the researchers think that they might be able to answer some of the many questions that have arisen about the planet, its atmosphere and its potential habitability with NASA's James Webb Space Telescope, which is set to launch in 2021.

"It's a step in the right direction," Sara Seager, a professor of planetary science, physics and aerospace engineering at the Massachusetts Institute of Technology who was not involved with this study, told Space.com. "We're trying to understand what the most common type of planet is. What's it made of? Is it a rocky planet with this giant envelope?"

"We have no planet like this in our solar system, yet they're incredibly common," Seager said about super-Earths. "We hope to get better data on this planet … with the upcoming James Webb Space Telescope, it could look at a much broader range of the spectrum and do much more detailed observations," she added.

These findings were published today (Sept. 11) in the journal Nature Astronomy.