The exoplanet, which orbits a relatively benign red dwarf star, is “the best candidate for habitability that we know right now.”
As portrayed by this artist’s idea, the exoplanet K2-18 b orbits a red dwarf star and has a thick atmosphere that contains at least some water vapor. Another exoplanet in the system is closer to the star, but it is still within the habitable zone of the star.
Astronomers have detected water vapor in the atmosphere of a super-Earth exoplanet that orbits within the habitable zone of its star. The discovery suggests that liquid water may exist on the surface of the rocky planet, potentially forming a vast ocean.
The discovery, made by the Hubble Space Telescope of NASA, is the first detection of water vapor in the atmosphere of such a planet.
The newly observed water vapour makes K2-18 b one of the most attractive prospects for follow-up research with next-generation space telescopes, given that the planet’s temperature is believed to be comparable to Earth’s.
“This is the only planet right now that we know outside the solar system that has the correct temperature to support water, an atmosphere, and water, making this planet the best candidate for habitability that we know right now,” said lead author Angelos Tsiaras, an astronomer at University College London, at a press conference.
K2-18 b – The essentials
K2-18 b is 110 light-years away in the constellation Leo and orbits a small red dwarf star with roughly one-third the mass of the Sun. Red dwarfs are famed for being active stars that release tremendous flares, but this star, according to the astronomers, is remarkably docile.
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. “Because the star is much cooler than the Sun, the planet receives similar radiation to Earth,” Tsiaras explained. “And, according to calculations, the temperature of the planet is also similar to that of the Earth.”
The report states that the temperature range for K2-18 b is between -100 °F (-73 °C) and 116 °F (47 °C). Temperatures on Earth range from -120 °F (-84 °C) in Antarctica to greater than 120 °F (49 °C) in Africa, Australia, and the Southwestern United States.
Although K2-18 b boasts some of the most Earth-like characteristics ever observed in an exoplanet — water, habitable temperatures, and a rocky surface — scientists emphasize that the planet is still very different from Earth. K2-18 b’s diameter is nearly double that of Earth, making it roughly eight times as massive. This brings K2-18 b close to the upper boundary of a super-Earth, which normally refers to planets with masses between one and ten Earth masses.
However, K2-18 b’s density is what classifies it as a rocky planet. K2-18 b’s composition most closely resembles that of Mars or the Moon, and its density is nearly twice that of Neptune. Because the planet is believed to have a solid surface and a thick atmosphere containing at least some water vapor, researchers believe K2-18 b may be a water world with an ocean covering its whole surface.
However, they cannot be assured.
Hubble’s inability to explore the atmospheres of distant exoplanets in great detail is the source of the ambiguity. Using a sophisticated technique, the researchers were able to extract the irrefutable signature of water vapour in the atmosphere of K2-18 b, but they were unable to determine the amount of water vapour there. As a result, they took a conservative approach and presented a broad estimate of water abundance in their study, ranging from 0.01 to 50 percent.
Before we can tell precisely how much water is on K2-18 b, the researchers say we must wait for the next generation of powerful space telescopes to be operational. NASA’s James Webb Space Telescope, which is scheduled to launch in 2021, and the European Space Agency’s Atmospheric Remote-sensing Infrared Exoplanet Large Survey (ARIEL), which is scheduled to launch in the late 2020s, are ideally equipped for the mission.
The new study was published on September 11 in Nature Astronomy. The preprint of the paper is accessible at arXiv.org.