How Do Planets Get Water?

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The Webb telescope will trace the origins of water in developing planetary systems.

Earth’s abundant water makes all the life we know possible. Yet we don’t know how that water got here. Did it arrive via comet collisions, or was it already part of our planet’s ancient origin material? Astronomers disagree, and hope that the James Webb Space Telescope will bring them closer to a resolution. While there is no way to travel back in time to Earth’s early days, astronomers will use Webb to look at other planets forming around other stars, and discover how water is working its way into those systems.  

Earth straddling the limb of the Moon, LRO, October 12, 2015
Did comets deliver water to our “blue marble”? CREDIT: NASA, Goddard, Arizona State University. GET THE FULL IMAGE IN RESOURCE GALLERY >

NASA’s Spitzer Space Telescope, which like Webb studies infrared light, found that at least half of all protoplanetary disks around Sun-like stars contain water vapor. Webb will use spectroscopy to examine Spitzer’s findings in greater detail, studying the chemical composition of the disks, including water. This research will also extend to lower-mass stars and dim brown dwarfs.

Where water is found in a protoplanetary disk is an important clue to the mystery of how habitable worlds form. Water close to a star would be in a gaseous state and drift away from a forming rocky planet. This is our understanding of why the gas giant planets in our solar system are found further from the star, where water took the form of solid ice, contributing to greater mass in forming planets that then was able to attract gas through its greater gravitational pull. However, this still leaves the unanswered question of how Earth’s liquid water oceans came to be; they are unique in our solar system, but other systems may provide us crucial clues to our own origins.

A Recipe for Water

A water molecule consists of two hydrogen atoms bonded to one atom of oxygen. Earth’s atmosphere contains a lot of oxygen, but hydrogen gas is scarce. Hydrogen in our atmosphere that isn’t bound in a molecule tends to escape into space, too light and energetic for Earth’s gravity to hold. So when we find hydrogen on Earth, it’s usually part of a molecule—most often a water molecule.

Protoplanetary disk surrounding TW Hydra
This illustration depicts the protoplanetary disk surrounding the young star TW Hydra, where an icy planet was found forming in 2016, using the Atacama Large Millimeter/submillimeter Array (ALMA). CREDIT: NAOJ.

For hydrogen in Earth’s early history to have arrived and stayed put in great enough amounts to bond with the oxygen in Earth’s atmosphere, it must have been attached to a carrier—another atom that bound it into a molecule. That’s why some astronomers look at comets rich in water ice as possible sources of Earth’s oceans. These icy comets would have impacted the planet, releasing their water into the atmosphere.

Other planets, depending on where they formed in their star’s planetary disk, may have different carriers. Ammonia consists of nitrogen and hydrogen, for example, and that combination is stable enough to deliver these two light, energetic elements.

With these types of spectroscopic data, the Webb telescope will enable astronomers to make leaps in understanding planetary formation, and fill in some of the blanks in our own story here on Earth.

Last Updated: May 31, 2018

Keywords: Exoplanets, Solar System

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