Astronomers at the Center for Space Exochemistry Data at the University College London in the United Kingdom used data from NASA’s Hubble Space Telescope to find water vapor in the atmosphere of K2-18b, an exoplanet around a small red dwarf star about 110 light-years away in the constellation Leo. If confirmed by further studies, this will be the only exoplanet known to have both water in its atmosphere and temperatures that could sustain liquid water on a rocky surface. Liquid water would only be possible if the planet turns out to be terrestrial in nature, rather than resembling a small version of Neptune.
Given the high level of activity of its red dwarf star, K2-18b may be more hostile to life as we know it than Earth, as it is likely to be exposed to more high-energy radiation. The planet, discovered by NASA's Kepler Space Telescope in 2015, also has a mass eight times greater than Earth's. That means the surface gravity on this planet would be significantly higher than on our planet.
The team used archive data from 2016 and 2017 captured by Hubble and developed open-source algorithms to analyze the host star’s light filtered through K2-18b’s atmosphere. The results revealed the molecular signature of water vapor, and also suggest the presence of hydrogen and helium in the planet’s atmosphere.
The authors of the paper, published in Nature Astronomy, believe that other molecules, including nitrogen and methane, may be present but they remain undetectable with current observations. Further studies are required to estimate cloud coverage and the percentage of atmospheric water present. A paper from a different team of scientists using Hubble observations has been submitted to the Astronomical Journal.
K2-18b is one of hundreds of "super-Earths" — exoplanets with masses between those of Earth and Neptune — found by Kepler. NASA’s TESS mission is expected to detect hundreds more super-Earths in the coming years. The next generation of space telescopes, including the James Webb Space Telescope, will be able to characterize exoplanet atmospheres in more detail.
The Hubble Space Telescope is a project of international cooperation between ESA (the European Space Agency) and NASA.
K2-18b’s orbit also takes it seven times closer to its star than Earth gets to the sun. But because it circles a type of dim red star known as an M dwarf, that orbit places it in the star's potentially life-friendly zone. Crude models predict that K2-18b’s effective temperature falls somewhere between -100 and 116 degrees Fahrenheit, and if it is about as reflective as Earth, its equilibrium temperature would be roughly the same as our home planet’s.
The fact that researchers have detected water on this type of planet bolsters hope for finding habitable worlds beyond our solar system.
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 right now,”
Seeing the light
In the past two decades, astronomy has undergone a revolution. Since the first detection of exoplanets in 1992, scientists have cataloged thousands of alien worlds orbiting distant stars—some of which show signs of having atmospheres.
For a handful of these planets, astronomers have even spotted signs of atmospheric water vapor. But previously, worlds with confirmed water were uninhabitable for life as we know it. For instance, in 2018, NASA announced the discovery of water vapor in the atmosphere of WASP-39b, an enormous Saturn-size planet where the day side reaches a scorching 1,430 degrees Fahrenheit.
Researchers had been hoping to spot water vapor in the atmosphere of a more Earth-like planet, especially one in its star’s habitable zone, but these worlds are relatively small, making observations of their atmospheres extraordinarily difficult. The larger the planet, the easier the detection, so researchers focused their efforts on super-Earths: planets with masses up to 10 times that of our home planet, which means they might also sport rocky surfaces. (See some of the most alien landscapes on Earth.)
To put K2-18b to the test, University of Montréal astronomer Björn Benneke asked to use the Hubble Space Telescope to watch the super-Earth pass in front of its home star, which it does every 33 days.
As it makes each transit, the star’s light shines through K2-18b’s atmosphere. But not all that starlight makes it through, since chemical compounds in the atmosphere absorb light at telltale frequencies. Water, in particular, absorbs near-infrared light at specific wavelengths, creating a visible sign of water vapor.
Between 2015 and 2018, Benneke’s team used Hubble to monitor nine K2-18b transits. When they stacked up the transits’ data sets and cleaned them up, they found that telltale sign of water vapor.
Chance of rain
Independently, a team led by Tsiaras and Ingo Waldmann at University College London used the same Hubble data to do an analysis of their own
In addition to bolstering the search for life-friendly exoplanets, the discovery may open the door to understanding alien weather. Benneke’s team points out that conditions in the atmosphere of K2-18b might allow for the formation of liquid water droplets—and maybe even rain. Water-vapor clouds have been found before in brown dwarfs, hulking objects that hover on the border between planet and star. If the results hold, K2-18b would be the first confirmed exoplanet with clouds of water vapor.
Both research teams say that the studies make K2-18b an ideal target for follow-up missions, including NASA’s upcoming James Webb Space Telescope and the European Space Agency’s future ARIEL space telescope. Unlike Hubble, these telescopes will be able to see other atmospheric gases, such as methane, ammonia, and carbon dioxide—and potentially even chemical markers for life.
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