There might be rain in the forecast for at least one distant planet.
Far outside our solar system, it’s called K2 18b. Data from space telescopes and computer models suggest that this planet’s atmosphere hosts water vapor. If true, it could be the first exoplanet that has liquid water — an essential ingredient for life as we know it.
“Water vapor exists everywhere in the universe,” says Björn Benneke. He is an astronomer in Canada at the University of Montreal. On September 10, his team reported the potential discovery of water on K2 18b at arXiv.org. “It’s not so easy to make liquid water,” Benneke notes. “You need the right pressure and the right temperature,” he points out. “That’s what makes this planet special.”
Astronomers first spotted K2 18b in 2015. They were using the Kepler exoplanet-hunting space telescope. Kepler revealed that the planet orbits a dim star known as a red dwarf. The star and planet are some 110 light-years from Earth. The planet is bit more than twice as wide as Earth, and has eight times Earth’s mass.
So it’s “not an Earthlike planet,” Angelos Tsiaras explained in a September 10 news conference. He is an astronomer in England at University College London. His team also studies K2 18b. And they, too, have detected water vapor in K2 18's atmosphere. Tsiaras’ team reported that September 11 in Nature Astronomy.
Though not Earthlike, the planet sits in what astronomers call a habitable zone. This is a region around a star where planets could have just the right temperatures to host liquid water. And liquid water appears important for life.
Could this planet host life?
In 2016 and 2017, Benneke’s group studied K2 18b using the Hubble Space Telescope. They wanted to see if the planet had an atmosphere. So they studied the planet as it passed in front of its star. The astronomers were looking to see if the planet’s atmosphere absorbs some of its star’s light. Astronomers can tell which molecules are in a planet’s atmosphere based on which wavelengths of starlight they absorb.
Tsiaras and his team accessed the data Benneke‘s team had collected from a public archive. Then it used specially designed computer software to analyze those data. This revealed the planet indeed had an atmosphere. And the wavelengths of light it absorbed were a signature for water vapor. That meant the planet’s atmosphere held water. That atmosphere also contains hydrogen and helium, the team reports.
“Until now, the planets for which we had the atmosphere observed and found water were gas giants,” Tsiaras says. Those are planets “similar to Jupiter, Saturn or Neptune. Its size, watery atmosphere and location in its star’s habitable zone, make this planet unique, he says. In fact, he adds: “This is the best candidate for habitability that we now have.”
Rain may fall — but how far?
Benneke’s group took the work a step further. They observed the planet with the Spitzer space telescope. By combining observations from Hubble, Spitzer and Kepler, they now believe the planet’s atmosphere has clouds. They appear to form at a specific spot. Knowing where this is, Benneke and his team simulated the planet’s climate. And that spot where the clouds form, they now say, could have the right pressure and temperature for liquid water to form and condense out as rain.
“It’s quite likely that this planet has liquid rain on it,” Benneke concludes. “This is actually one of the most exciting findings from this data.”
However, he adds, those raindrops might never hit solid ground. Instead, they could reach a point in the planet’s thick atmosphere where the pressure and temperature are so great that the droplets just evaporate. That water vapor would then rise back up again to form clouds. That would suggest “there’s a bit of a water cycle,” Benneke says.
Other exoplanet experts aren’t so sure. “There is no definitive proof” of raindrops, says Sara Seager. She is an astronomer at the Massachusetts Institute of Technology in Cambridge. The idea of rain is “solid, but still speculative,” she believes.
She also points to another caveat. K2 18b may have liquid water, but that doesn’t mean anything lives — or can live — on this planet. One reason: K2 18b might not have solid ground. It’s not clear if the planet has a rocky surface. And that’s where life as we know would be expected to evolve. Most exoplanets in the Milky Way galaxy fall in the size range of this planet, she notes. But it’s hard to tell if those planets are rocky super-Earths, gassy mini-Neptunes or sodden water worlds.
From what astronomers can tell, this is “one of these really mysterious planets that are the most common type of planet in our galaxy,” says Seager. “We have no idea what they are.” Future observations with NASA’s planned James Webb Space Telescope may help. Those observations might be able to pin down how much water K2 18b contains. And that, Seager says, could help astronomers come to understand its composition.
archive (adj. archival) To collect and store materials, including sounds, videos, data and objects, so that they can be found and used when they are needed. The term is also for the process of collecting and storing such things. People who perform this task are known as archivists.
arXiv A website that posts research papers — often before they are formally published — in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics. Anyone can read a posted paper at no charge.
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
atmosphere The envelope of gases surrounding Earth or another planet.
caveat A potential exception to the general rule or to some general expectation.
climate The weather conditions that typically exist in one area, in general, or over a long period.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
condense To become thicker and more dense. This could occur, for instance, when moisture evaporates out of a liquid. Condense can also mean to change from a gas or a vapor into a liquid. This could occur, for instance, when water molecules in the air join together to become droplets of water.
data Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
evaporate To turn from liquid into vapor.
evolve (adj. evolving) To change gradually over generations, or a long period of time.
exoplanet Short for extrasolar planet, it’s a planet that orbits a star outside our solar system.
galaxy A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.
gas giant A giant planet that is made mostly of the gases helium and hydrogen. Jupiter and Saturn are gas giants.
habitable A place suitable for humans or other living things to comfortably dwell.
helium An inert gas that is the lightest member of the noble gas series. Helium can become a solid at -272 degrees Celsius (-458 degrees Fahrenheit).
hydrogen The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues. It’s made of a single proton (which serves as its nucleus) orbited by a single electron.
Jupiter (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).
light-year The distance light travels in one year, about 9.48 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
Milky Way The galaxy in which Earth’s solar system resides.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2); water is made of two hydrogen atoms and one oxygen atom (H2O).
NASA Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.
Neptune The furthest planet from the sun in our solar system. It is the fourth largest planet in the solar system.
orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
pressure Force applied uniformly over a surface, measured as force per unit of area.
range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.
red dwarf A type of smallish star that is relatively cool (and hence emits reddish light). Dwarfs are the most common size stars in the Milky Way.
Saturn The sixth planet out from the sun in our solar system. One of the four gas giants, this planet takes 10.7 hours to rotate (completing a day) and 29 Earth years to complete one orbit of the sun. It has at least 53 known moons and 9 more candidates awaiting confirmation. But what most distinguishes this planet is the broad and flat plane of seven rings that orbit it.
software The mathematical instructions that direct a computer’s hardware, including its processor, to perform certain operations.
solar system The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
super-Earth A planet (in a distant solar system) with between one and 10 times the mass of Earth. Our solar system contains no super-Earths: All of the other rocky planets (Mercury, Venus, Mars) are smaller and less massive than Earth, and the gas giants (Jupiter and Saturn) and ice giants (Neptune and Uranus) are all larger, containing at least 14 times the mass of Earth.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
unique Something that is unlike anything else; the only one of its kind.
universe The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).
water vapor Water in its gaseous state, capable of being suspended in the air.
wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.
Journal: B. Benneke et al. Water vapor on the habitable-zone exoplanet K2-18b. arXiv:1909.04642. Posted September 10, 2019.
Journal: A. Tsiaras et al. Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b. Nature Astronomy. Published online September 11, 2019. doi:10.1038/s41550-019-0878-9.