Water vapor is discovered in an atmosphere on a distant planet that is just over twice as large as Earth. This is the smallest world that is still open with water in the surrounding atmosphere and may even flow with liquid water. This makes this world a tormenting candidate in the continued pursuit of extraterrestrial life beyond our outer space.
Finding water around the exoplanets (the world outside our solar system) is particularly exciting for scientists because water is a critical ingredient in the life of our planet. It could be just as important to life that exists elsewhere in the universe. Researchers have found this precious molecule around exoplanets before, but these worlds were not suitable places for life to flourish. They were large gas balls, similar in size to Jupiter or Neptune, lacking any kind of surface for life, as we know it exists.
The planet detailed in a study presented * in the Astronomical Journal and published in arXiv last night is a little more unique. Called K2-1
Researchers first discovered K2-18b, thanks to NASA's Kepler Space Telescope, a spacecraft located nearly 100 million miles from Earth that has been chasing exoplanets for most of the last decade, every time an exoplanet passing directly between its parent star and Earth, it slightly dims the light of the star, which is a minor change that Kepler could detect. Watching these transits, Kepler discovered more than 2,000 exoplanets. In 2015, the spacecraft caught the K2-18b, located 111 light-years from Earth.
Then, in 2016 and 2017, Benneke and his team use NASA's Hubble Space Telescope, which is currently orbiting the Earth, to learn more about the atmosphere of K2-18b. The chemicals and molecules that surround the exoplanets can tell us a lot about what may be lurking on the surface of the distant world. For example, significant amounts of methane in the Earth's atmosphere are a by-product of the many biological organisms that live here.
The study of the exoplanet's atmosphere is especially difficult for worlds that are similar in size to Earth. The light from these distant cosmic rocks is easily overcome by the light coming from their parent stars, which makes them incredibly difficult to see. And in order to understand what is in the exoplanet's atmosphere, researchers must observe the light from the star as it filters through the "outer edges" of the world. When light passes through the gas, it curves so lightly that it shows the types of molecules that are present. This is an incredibly delicate and challenging measurement that needs to be done.
Researchers were lucky with K2-18b because it has an atmosphere that extends far into space, which makes it a little easier to observe. "It looks a bit like a hybrid planet that may have a rocky ice core, but then most of its volume is actually gas," says Benneke. In addition, it orbits a type of weak, small star known as a red dwarf that does not have as much light as a star like our Sun. This makes it easier to study planets that can orbit nearby.
Benneke and his team observe the planet eight times in its passage, which allows them to find water vapor in the atmosphere. Then they did some climate modeling and found that steam was probably formed in clouds where the water condensed and then it rained down on the planet – just like on Earth. "If you talk to a biologist, they're not interested in fumes; they take care of the liquid water, "says Benneke. "Because biology only works when you have liquid water."
There are, however, quite a few things we don't know about this planet, especially the composition of its surface. Seeger points out that exoplanets that are thought to be rocky like Earth, Venus, or Mars are typically less than 1.6 times larger than our planet. This one is 2.3 times larger than Earth, which means that the surface is very rocky. "It's well above that threshold," Siger says. It's either a rocky core surrounded by a giant shell of atmosphere, or maybe half of it is made up of water ice. Both options are not very favorable for the presence of oceans with water on rocky ground, as here on Earth. "These mini-Neptunian sites are extremely common, and we don't know what they are," Sieger says.