Water vapor is found in the atmosphere of a distant planet that is just over twice the size of the earth. It is the smallest world ever found with water in the surrounding atmosphere, and it is possible that it even rains liquid water there. That makes this world a tempting candidate in the constant search for extraterrestrial life outside of our cosmic neighborhood.
Finding water around an exoplanet (a world outside of our solar system) is particularly exciting for scientists because water is a critical ingredient for life on our planet. It can be just as crucial for the life that exists elsewhere in the universe. Researchers have previously found this precious molecule around exoplanets, but these worlds were not suitable places to live. They have been large gas balls, similar in size to Jupiter or Neptune, that lack any surface for life as we know it.
This planet, detailed in a study accepted by the Astronomical Journal, is a bit more unique. Called K2-18b, it is about nine times the size of our own earth, a type of world that is often referred to as a mini Neptune. Worlds of this magnitude abound in our Milky Way but are missing in our own solar system. K2-18b also spins in a sweet spot around its guest star, known as the habitable zone where temperatures are just right for water to collect on the surface of a planet. That means that this planet shares some very important properties with our planet. "For the first time, a planet in this temperature regime – a regime that is very similar to the Earth – let's see that there is actually liquid water," Björn Benneke, an exoplanet researcher at the University of Montreal and lead author on the study published in the Astronomical Journal, tells The edge.
Although finding water is a major problem for exoplanet researchers, it is unlikely that this world is flooded with oceans. In fact, it seems unlikely that the surface of the planet is rocky because of its size. "These planets will not look like Earth," says Sara Seager, an exoplanet expert and professor at MIT who was not involved in this study. The edge. "It's definitely not rocky like we know a rocky planet." K2-18b also revolves around a star, very different from our sun. All in all, these factors significantly reduce the chances that life there could survive.
Researchers found K2-18b for the first time, thanks to the Kepler space telescope from NASA, a spacecraft that was nearly 100 million miles from Earth and spent most of the last decade hunting for exoplanets. When an exoplanet passes directly between its parent star and the earth, it dims the star's light slightly, which is a tiny change that Kepler could detect. By observing these transits, Kepler discovered more than 2,000 exoplanets. In 2015, the spacecraft caught K2-18b, which is 111 light-years from the earth.
Then, in 2016 and 2017, Benneke and his team used NASA & # 39; s Hubble Space Telescope, currently orbiting the Earth, to learn more about the atmosphere of K2-18b. The chemicals and molecules that surround an exoplanet can tell us a lot about what's lurking on the surface of the distant world. The substantial amounts of methane in the Earth's atmosphere are, for example, a by-product of the many biological organisms that live here.
Studying the atmosphere of an exoplanet is particularly difficult for worlds that are similar in size to the earth. The light from these distant space rocks is easily overwhelmed by the light from their parent stars, making them incredibly difficult to see. And to find out what is in the atmosphere of an exoplanet, researchers must observe light from the star as it passes through the & # 39; outer edges & # 39; of the world. When the light passes through the gas, the whole light is warped, indicating which types of molecules are present. It is an incredibly delicate – and challenging – measurement to take.
Researchers have been lucky with K2-18b because it has an atmosphere that extends far into space, making it a little easier to observe. "It looks a bit like a hybrid planet that may have a rocky ice core, but most of the volume is actually gas," says Benneke. Moreover, it revolves around a kind of weak, small star known as a red dwarf that does not have as much light as a star like our sun. That makes it easier to study planets that may circle in the neighborhood.
Benneke and his team observed the planet while they were in transit eight times, allowing them to detect water vapor in the atmosphere. They then did a number of climate modeling and discovered that the vapor probably forms in clouds where water condenses and then rains on the planet – just like on Earth. "If you talk to a biologist, she doesn't care about vapor; they care about liquid water," says Benneke. "Because biology only works if you have liquid water."
Yet there is still a lot that we do not know about this planet, especially about the composition of the surface. Seager notes that exoplanets that are believed to be rocky such as the Earth, Venus, or Mars are typically less than 1.6 times the size of our planet. This is 2.3 times the size of the earth, which means that the surface is probably not rocky. "It's a bit above that threshold," says Seager. Either it is a rocky core surrounded by a giant envelope of an atmosphere, or it is possible that half of it consists of water ice. Both options are not very conducive to having oceans on rocky ground like here on Earth. "These objects are mini-Neptunes, they are very common and we do not know what they are," says Seager.
So although today's findings are great, the search is still for the biggest prize of all: a rocky exoplanet with water in its atmosphere. When that happens, it will be a big day for the exoplanet community, producing the closest analogy to the earth that we have found so far.
The next step is to learn more about what K2-18b looks like and what other gases can live in the planet's atmosphere. Scientists could get this information if more powerful telescopes come online in the coming years. In particular, the upcoming James Webb Space Telescope from NASA, which will be the most powerful observatory after its launch in 2021, can tell us more about the surface and atmosphere of K2-18b. The telescope will be able to study worlds smaller than K2-18b and more related to our planet, in addition to mini-Neptunes and worlds circling in the habitable zones of their stars.
"This is the first step towards really exploring planets that are at the right temperature around other stars to see what's going on there," says Benneke. For now we know that everything that is going on is probably wet, but it may not be very lively.