Cosmic water

 By Michael Belfiore

Cool, refreshing water: it’s the basis of everything we eat and drink. We build our houses near it and even vacation on it. We cannot survive without it, nor, for that matter, could any life on Earth, from the biggest tree to the smallest single-cell organism. It comprises more than half of our bodies and covers nearly three-quarters of the Earth’s surface

That is why researchers reacted with excitement in September 2019 to the discovery of water on a planet 110 light years away, K2-18 b. “It is the first time that water vapor [has been] discovered in the atmosphere of a small planet in the habitable zone”, explains Giovanna Tinetti, director of the Centre for Space Exochemistry Data (CSED) at University College London and one of the scientists who made the discovery.

Water for life

In other words, K2-18 b is the first potentially Earth-like planet outside our solar system or “exoplanet”, orbiting its star at just the right distance to allow liquid water that is confirmed to actually contain water. The discovery follows the first confirmation of water on any planet beyond the solar system—in that case, a massive, Jupiter-like world—by a team led by Tinetti in 2007. “The presence of water does not guarantee life, but its absence does suggest life might struggle to exist”, says Elizabeth Tasker, an astrophysicist at Japan Aerospace Exploration Agency’s Institute for Space and Astronautical Sciences and author of “The Planet Factory”. “Life as we know, needs the presence of liquid water”, Tinetti agrees. She also believes that K2-18 b is likely only the first of many small, water-bearing worlds we’ll find beyond the solar system. “As far as we can tell, there is nothing profound in being exactly the size of the Earth. We know the most common planets in the galaxy are planets smaller than three times the radius of the Earth”.
Could the discovery of life beyond Earth follow? And why else should we care about water in space?

A representation of the planetary system around K2-18, indicating the orbits of two known exoplanets and the star's habitable zone; K2-18 b is now the only super-Earth exoplanet known to host both water and temperatures that could support life (Exoplanet Exploration Program, JPL/NASA)

Finding signs of water

To find water on K2-18 b, Tinetti and her colleagues at CSED, and in a separate team of researchers at the Université de Montréal, MIT, and other institutions, examined data from the Hubble Space Telescope. Light shining or reflecting from different substances produces unique wavelengths, or spectra. This allows observers, even at a great distance, to confirm the presence of those substances. As distant planets pass in front of their stars relative to Earth, starlight shines through their atmospheres, producing tell-tale spectra. Spectrographic analysis of light passing through the K2-18 b’s atmosphere revealed the presence of water vapor within it. Tasker, who wasn’t part of either team that found the water, calls the discovery remarkable. She’s among the experts who believe the planet probably doesn’t have a habitable surface. K2-18 b is eight times the mass of Earth, leading to speculation that it may not even have a rocky surface. It also orbits a red dwarf star. Red dwarfs produce more solar flares than our sun does, potentially sterilizing the surfaces of their planets. Still, K2-18 b is probably a preview of similar discoveries to come, Tasker says.

Our sun has a temperature of about 5800K. For stars cooler than our sun (M dwarfs at 3000-4000K) the habitable zone is closer in. For hotter stars (A dwarfs at 10,000K) the region is much farther out (NASA)

Continuing the search

New telescopes like the James Web Space Telescope (JWST), for example, due for launch in 2021, will bring more powerful instruments to bear on the challenge of finding water and other signs of habitable planets.
As for finding Earth-size planets with water, Tasker says that might have to wait for yet another generation of space telescopes. “This is a difficult detection, even for JWST, as these planets are so tiny. For a really good analysis of what is out there, we may have to wait for the successor to JWST”. And finding life itself? That’s also the job of space-based instruments still on the drawing board. The proposed Habitable Planet Observatory (HabEx), for example, will aim to capture the first direct images of planets like Earth orbiting other stars. It will also seek signs of not just water but also products of biological activity such as oxygen and ozone. Tasker believes that, although it’s impossible to predict the world’s reaction to the discovery of life elsewhere in the universe, it will be far-reaching. “I do think scientists and science communicators need to be acutely aware that such a discovery may have a profound effect on many people”.

K2-18 b proportions compared to Earth and Jupiter (

Closer to home

There’s another way water on other worlds could influence future society: by enabling settlements in space. Astronomers have already found water on the moon, Mars, asteroids, and elsewhere in our solar system. That means future astronauts can draw water from local sources to drink, grow crops and even breathe, by splitting water into its constituent elements of oxygen and hydrogen. Astronauts can even convert water in hydrogen and oxygen to be used as high-powered rocket fuel. And that makes water from space a potentially valuable source of energy for future space exploration. Exoplanets such as K2-18 b, are too far for us to reach in the foreseeable future. But they are fascinating all the same to scientists such as Tinetti. Speaking of a space telescope slated for launch in the late 2020s by the European Space Agency, she said, “ARIEL observations will allow [us] to understand how planets form and evolve in our galaxy, and place the Earth and the Sun’s planets in a broader galactic context”. And when future generations enjoy a tall, cold glass (or bulb) of water on the moon, Mars, or in space, they will have today’s astronomers to thank for finding it.

READ MORE: Using the Sun to orbit the Earth by Peter Ward

about the author
Michael Belfiore