Astronomers have discovered water vapor in the atmosphere of a super-Earth exoplanet orbiting within its star’s habitable zone. The discovery suggests that liquid water could exist on the rocky world’s surface, possibly forming a global ocean.
The discovery, made with NASA’s Hubble Space Telescope, is the first detection of water vapor in the atmosphere of such a planet. Because the planet, dubbed K2-18 b, is likely to have a temperature similar to Earth, the newly discovered water vapor makes it one of the most promising candidates for follow-up studies with next-generation space telescopes.
“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,” lead author Angelos Tsiaras, an astronomer at University College London, said in a press conference.
K2-18 b: The basics
K2-18 b is located 110 light-years away in the constellation Leo, and it orbits a small red dwarf star about one-third the mass of our Sun. Red dwarfs are notorious for being active stars that emit powerful flares, but this particular star appears to be surprisingly docile, according to the researchers.
This is good news for the water-bearing planet, whose 33-day orbit brings it roughly twice as close to its star as Mercury is to the Sun.”Given that the star is much cooler than the Sun, in the end, the planet is receiving similar radiation to the Earth,” said Tsiaras. “And based on calculations, the temperature of the planet is also similar to the temperature of the Earth.”
Specifically, the paper suggests K2-18 b has a temperature between about –100 °F (–73 °C) and 116 °F (47 °C). For reference, temperatures on Earth can span from below –120 °F (–84 °C) in regions like Antarctica to above 120 °F (49 °C) in regions like Africa, Australia, and the Southwestern United States.
Although K2-18 b has some of the most Earth-like features ever observed in an exoplanet — water, habitable temperatures, and a rocky surface — the researchers emphasize that the world is still far from Earth-like. To begin with, K2-18 b is roughly twice the diameter of Earth, making it nearly eight times as massive. This puts K2-18 b near the upper limit of what we call a super-Earth, which typically refers to planets with masses ranging from one to ten Earth masses.
The density of K2-18 b, on the other hand, firmly establishes it as a rocky planet. K2-18 b has a composition most similar to Mars or the Moon, with a density roughly twice that of Neptune. Because the planet is thought to have a solid surface and an extended atmosphere with at least some water vapor, researchers believe K2-18 b could be a water world with a global ocean covering its entire surface.
However, they cannot say for sure.
The uncertainty comes in part from Hubble’s inability to probe the atmospheres of distant exoplanets in great detail. For example, using a sophisticated algorithm, the researchers were able to extract the undeniable signal of water vapor in K2-18 b’s atmosphere, but they couldn’t tell how much water vapor is really there. As a result, in their paper, they took a conservative approach and provided a broad-range estimate of water abundance — somewhere between 0.01 percent and 50 percent.
The researchers say we’ll have to wait for the next generation of advanced space telescopes to come online before we can figure out exactly how much water is on K2-18 b. NASA’s James Webb Space Telescope, which is set to launch in December 2021, and the European Space Agency’s Atmospheric Remote-sensing Infrared Exoplanet Large Survey (ARIEL) telescope, launched on April 1st 2021, are both well-suited to the task.