For the first time ever, scientists have detected water vapor in the atmosphere of a rocky exoplanet orbiting within the habitable zone of its star. In other words, this is the most likely world to host alien life out of the thousands of exoplanets that have been catalogued by astronomers.
The discovery is a major breakthrough for research into exoplanets, which are worlds that orbit other stars, and has important implications for the search for life beyond Earth, according to a study published on Wednesday in Nature Astronomy.
K2-18b, a planet located 110 light years from Earth, is “the only planet that we know of outside the solar system that has the correct temperature to support water and has an atmosphere [with] water in it,” said lead author Angelos Tsiaras, an astronomer at University College London, in a teleconference on Tuesday.
The planet is “the best candidate for habitability” known beyond our solar system, Tsiaras added. Because liquid water is such an essential ingredient for life on Earth, scientists judge the potential habitability of other planets based on the likelihood of surface water. It is not yet known whether K2-18b is covered in oceans or lakes, or if it currently hosts alien life.
Water has previously been discovered on large gassy exoplanets, but this is the first instance of a detection on a so-called “super-Earth” world. Planets in this category are bigger than Earth, but smaller than gaseous worlds like Uranus or Neptune.
K2-18b is about twice the size of Earth and eight times as massive, which means it is way too large to be considered “Earthlike.” The planet’s skies also contain a lot of hydrogen gas, in addition to the traces of water vapor, which is substantially different from Earth’s nitrogen-dominant atmosphere.
However, Tsiaras and his colleagues calculated that K2-18b is about as dense as Mars, which means it is highly likely to be a terrestrial planet with a rocky surface—another major indicator of habitability.
Scientists have been looking for traces of water in the atmospheres of rocky super-Earths for years with no luck. That suddenly changed when Tsiaras’ team used an algorithm to comb through Hubble Space Telescope observations of K2-18b during transits, which are periods when a planet passes directly in front of its star from our perspective on Earth. As its skies are backlit by the star, astronomers can sometimes detect chemical elements in its atmosphere by studying the filtered light.
According to study co-author Ingo Waldmann, who is also an astronomer at University College London, the discovery went down over the winter holidays of 2017.
“The algorithm ran on our cluster computer around Christmas, and on Christmas Eve, it popped out the signature of water in a habitable atmosphere that we publish today,” Waldmann said in an email. “Quite a nice Xmas present, I must say!”
“It was unbelievably exciting initially (and still is!),” he continued. “It took us over a year to do all the checks but whatever we threw at it, the feature would remain the same.”
Now that this milestone has been achieved, researchers hope to follow up on it with more detailed observations of K2-18b that might shed light on whether it hosts alien life.
A new generation of sophisticated space observatories will be essential to finding “biomarkers,” or chemical signatures of life, at the planet. NASA’s James Webb Space Telescope and the European Space Agency’s ARIEL are both due to launch within the next decade, and will be especially adept at resolving fine details about molecules in exoplanet atmospheres.
Waldmann said “the holy grail” of this quest would be to find an irregular molecular signal that cannot be explained by geological processes. “What these chemical signatures are, we don’t know yet,” he explained. “We know what it is for Earth, but life may well have evolved differently on that planet.”
The detection of water vapor at K2-18b is also likely to inspire scientists to redouble efforts to find water vapor in the atmospheres of other rocky exoplanets. NASA’s Transiting Exoplanet Survey Satellite (TESS), which launched in 2018, is designed to root out small terrestrial worlds within the habitable zones of their stars, so there will be plenty more candidates to investigate.
“These observations are the first direct evidence that these potentially habitable atmospheres exist, and that searching for habitable conditions on super-Earths orbiting small dwarf stars is indeed the way forward in finding water vapor on other terrestrial planets,” Waldmann said.
“We will find many more of these systems, so K2-18b is hopefully just the first of many exciting targets we will characterize.”