Exoplanets and Water

How Is Water Delivered to Forming Planets?

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The Earth straddling the limb of the Moon (LRO, October 12, 2015). Credit: NASA/Goddard/Arizona State University

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Comets may have delivered water to planets in the solar system. Webb will trace the origins of water in developing planetary systems.
We still don't know where the water on Earth originated. Were we bombarded with water-bearing comets, or were water molecules wrapped up in the primordial material of our planet? Some studies suggest that much of Earth's water could not have come from comets, but models also imply that water present on the debris that made up the planet-forming disk around our Sun would have evaporated out into space before Earth even formed.

Astronomers expect Webb to make strides toward solving this conundrum. We can't travel back in time to witness our own drenching, but we can look at planets forming around stars elsewhere, and discover how water is working its way into those systems.

The Spitzer Space Telescope, which like Webb studies the infrared light from the universe, found that at least half of all protoplanetary disks around stars roughly the size of our Sun contain water vapor. The Webb telescope, using spectroscopy to study the chemical composition of such disks, will be able to examine this finding in greater detail, pinpointing just how much water is present in the region close to the star, and looking at whether the discovery extends to lower-mass stars and even dim "brown dwarf" stars.

Webb will identify where water exists in forming planetary disks and in what amounts. It won't be able to tell directly how it reaches planets, but the first step in that discovery is pinpointing where the components of water are originally located, and what brings them to a planet. Astronomers have observed and suspect that large amounts of water are always present in planet-forming material, but Webb will determine whether that's accurate across the board.

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This large disk of dust and gas encircles the star Beta Pictoris. (Beta Pictoris is blotted out in the center of the image, its light obstructed in order to capture the dimmer disk.) A giant planet is known to be embedded in the disk. This is the first and best example of what a young planetary system looks like.

A Recipe for Water

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

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 astronomers look at comets rich in water ice as a possible source of Earth's oceans. These icy comets would have impacted with the planet, releasing their water into the atmosphere.

Other planets, depending on where they formed in their 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 to other planets.

Finally, Webb will look at ratios of heavy water — water molecules that contain a deuterium atom instead of hydrogen — to water in the planet-forming disks, helping trace how the water formed and under what conditions. The more heavy water, the colder the environment was in which the water formed, meaning it likely came from farther away in the disk — or may even pre-date the disk, since it's easier for heavy water to form in the molecular cloud that spawned the star and planetary system than in a dust disk.

Updated: May 10, 2016