Astronomers discover water on Earth-like planet sparking hopes of finding life on other planets
Astronomers have, in a remarkable discovery, found water in the atmosphere of a planet orbiting the "habitable" zone of a star. Dubbed K2-18b, the exoplanet is eight times the mass of Earth and is now the only planet orbiting a star outside the Solar System aka "exoplanet", that is known to have both water and temperatures that could support life, according to scientists.
The finding is the first successful atmospheric detection in an exoplanet orbiting in its star's "habitable zone", at a distance where water can exist in liquid form. The analysis, which comes from two independent studies, contributes to the understanding of habitable worlds beyond the Earth's Solar System and marks a new era in exoplanet research.
"Finding water in a potentially habitable world other than Earth is incredibly exciting. K2-18 b is not 'Earth 2.0' as it is significantly heavier and has a different atmospheric composition. However, it brings us closer to answering the fundamental question: 'Is the Earth unique?'," says the first author of one of the papers, Dr. Angelos Tsiaras from the University College London or UCL's Centre for Space Exochemistry Data (CSED).
Ever since the discovery of the first exoplanet in the 1990s, astronomers have made steady progress towards finding and probing planets located in the habitable zone of their stars, where conditions can lead to the formation of liquid water and the proliferation of life.
Results from the Kepler satellite mission, which discovered nearly two-thirds of all known exoplanets to date, indicate that "5 to 20% of Earths and super-Earths" are located in the habitable zone of their stars.
However, despite such abundance, probing the conditions and atmospheric properties on any of these habitable zone planets is very difficult and has remained elusive, until now. Super-Earths are planets with a mass between Earth and Neptune. The exoplanet K2-18b is also a super-Earth, which is located around 111 light-years from our Solar System.
"With so many new super-Earths expected to be found over the next couple of decades, it is likely that this is the first discovery of many potentially habitable planets. This is not only because super-Earths like K2-18b are the most common planets in our Galaxy, but also because red dwarfs — stars smaller than our Sun — are the most common stars," says co-author Dr. Ingo Waldmann from UCL CSED.
The results of the UCL study — based on data from the Hubble Space Telescope — has been published in Nature Astronomy.
K2-18b was found by NASA's Kepler spacecraft
Discovered in 2015, K2-18b is one of hundreds of super-Earths found by NASA's Kepler spacecraft. NASA's TESS (Transiting Exoplanet Survey Satellite) mission is expected to detect hundreds of more super-Earths in the coming years.
Using archived data (from 2016 and 2017) captured by the ESA/NASA Hubble Space Telescope, the team developed open-source algorithms to analyze the "starlight filtered through K2-18b's atmosphere."
"The results reveal the molecular signature of water vapor, also indicating the presence of hydrogen and helium in the planet's atmosphere. Other molecules, which includes nitrogen and methane, may be present. But, with current observations, they remain undetectable," the findings state.
The researchers say that further studies are needed to estimate cloud coverage and the percentage of atmospheric water present. The planet orbits the cool dwarf star K2-18, which is over 100 light-years from Earth in the Leo constellation. Given the high level of activity of its red dwarf star, K2-18b may be more hostile than Earth and is likely to be exposed to more radiation, says the team.
"Our discovery makes K2-18b one of the most interesting targets for future study. Over 4,000 exoplanets have been detected, but we do not know much about their composition and nature. By observing a large sample of planets, we hope to reveal secrets about their chemistry, formation, and evolution," says Professor Giovanna Tinetti (UCL CSED), co-author and principal investigator for the Atmospheric Remote-sensing Exoplanet Large-survey or ARIEL space mission.
According to scientists, the next generation of space telescopes, including the NASA/ESA/CSA James Webb Space Telescope and ESA's ARIEL mission, will be able to characterize atmospheres in more detail as they carry more advanced instruments.
ARIEL is expected to launch in 2028, and will observe 1,000 planets in detail to get a "truly representative picture of what they are like."
Similarities between K2-18b and Earth
The other study — by Professor Björn Benneke from the Institute for Research on Exoplanets at the Université de Montréal, and several of their collaborators — also reports the detection of water vapor and perhaps even liquid water clouds in the atmosphere of the planet K2-18b.
"This M-type star is smaller and cooler than our Sun, but due to K2-18b's close proximity to its star, the planet receives almost the same total amount of energy from its star as our Earth receives from the Sun," say researchers in their analysis, the preprint version of which was published in arXiv.org, and has been submitted to the Astronomical Journal.
Researchers say that the similarities between the exoplanet K2-18b and the Earth suggest that the exoplanet may potentially have a water cycle, possibly allowing water to condense into clouds and liquid water rain to fall.
This detection, says the team, was made possible by combining eight transit observations — the moment when an exoplanet passes in front of its star — taken by the Hubble Space Telescope.
"This represents the biggest step yet taken towards our ultimate goal of finding life on other planets, of proving that we are not alone. Thanks to our observations and our climate model of this planet, we have shown that its water vapor can condense into liquid water. This is a first," says Benneke.
Scientists currently believe that the thick gaseous envelope of K2-18b likely prevents life, as we know it, from existing on the planet's surface.
However, the study shows that even these planets of relatively low mass — which are, therefore, more challenging to study — can be explored using astronomical instruments developed in recent years.
The researchers contend that by studying these planets, which are in the habitable zone of their star and have the right conditions for liquid water, astronomers are one step closer to directly detecting signs of life beyond the Solar System.