A ‘Goldilocks’ Star Reveals a Previously Hidden Step in How Water Gets to Earth

A ‘Goldilocks’ Star Reveals a Previously Hidden Step in How Water Gets to Earth

Without water, life on Earth could not exist as it does today. Understanding the history of water in the universe is critical to understanding how planets like Earth formed.

Astronomers commonly refer to the journey that water takes from its formation as individual molecules in space to its resting place on the surfaces of the planets as “the water trail.” The trail begins in the interstellar medium with hydrogen and oxygen gas and ends in the oceans and ice caps of the planets, with icy moons orbiting gas giants and icy comets and asteroids orbiting stars. The beginnings and ends of this trail were easy to see, but the middle remained a mystery.

I am an astronomer which studies the formation of stars and planets using observations from radio and infrared telescopes. In a new paper, my colleagues and I describe the first measurements made its once hidden central water trail and what these findings mean for water found on planets like Earth.

The development of a star system from a cloud of dust and gas to a mature star with orbiting planets.
Star and planet formation is a coherent process that begins in a cloud of molecules in space. Photo Credit: Bill Saxton, NRAO/AUI/NSF, CC BY

How Planets Form

The formation of stars and planets are interrelated. The so-called “emptiness of space”—or the interstellar medium—in fact contains large amounts of hydrogen gassmaller amounts of other gases, and grain of dust. Due to gravity, some pockets of interstellar medium will be denser as the particles attract each other and form clouds. As the density of these clouds increases, the atoms begin to collide more often and form larger moleculesincluding water that forms in the dust grains and coated the dust in the ice.

Stars begin to form when parts of the collapsing cloud reach a certain density and become hot enough to start fusing together hydrogen atoms. Since only a small fraction of the gas initially collapses into the newborn protostar, the rest is gas and dust forming a flat disk of material revolving around the spinning, newborn star. Astronomers call it a proto-planetary disk.

As icy dust collides within a proto-planetary disk, they begin to cluster. The process continues and eventually forms familiar space objects such as asteroids, comets, rocky planets like Earth and gas giants like Jupiter or Saturn.

Two Theories for the Source of Water

There are two potential paths that water could have taken in our solar system. The first, called chemical inheritanceis when the water molecules originally formed in the interstellar medium were delivered to the proto-planetary disks and all the bodies they created without any changes.

The second theory is called chemical reset. In this process, the heat from the formation of the proto-planetary disk and newborn star dissociates water molecules, which then reform as the proto-planetary disk cools.

To test these theories, astronomers like myself look at the ratio between normal water and a special type of water called semi-heavy water. Water is basically made of two hydrogen atoms and one oxygen atom. Semi-heavy water is made of one oxygen atom, one hydrogen atom and one atom of deuterium—a heavier isotope of hydrogen with an extra neutron in its nucleus.

The ratio of semi-heavy to normal water is a guiding light on the water trail—measuring the ratio can tell astronomers a lot about the water’s origin. Chemical models and experiments showed that about 1,000 times more semi-heavy water would be produced in the cold interstellar medium than the conditions of a protoplanetary disk.

This difference means that by measuring the ratio of semi-heavy to normal water in an area, astronomers can tell whether that water has gone through the chemical inheritance or chemical reset pathway.

A star surrounded by a ring of gas and dust.
V883 Orionis is a young star system with a rare star at its center that makes it possible to measure water in the proto-planetary cloud, shown in the cutaway. Image Credit: ALMA (ESO/NAOJ/NRAO), B. Saxton (NRAO/AUI/NSF), CC BY

Measuring Water During Planetary Formation

Comets have a ratio of semi-heavy to normal water that is almost entirely consistent with chemical inheritance, meaning water has not undergone a major chemical change since it was first created in space. Earth’s ratio is somewhere between the inheritance and reset ratios, making it unclear where the water came from.

To truly determine where water on planets comes from, astronomers need to find a goldilocks proto-planetary disk—one that’s the right temperature and size to allow observations of water. Doing so has proved incredibly difficult. It is possible to see semi-heavy and normal water when water is a gas; unfortunately for astronomers, most proto-plantary disks are very cold and contains almost iceand it is near water ratios are impossible to measure from ice at interstellar distances.

A breakthrough came in 2016, when my colleagues and I were studying proto-planetary disks around a rare type of young star called a FU Orionis star. Most young stars eat matter from the proto-planetary disks around them. FU Orionis stars are unique in that they consume matter about 100 times faster than typical young stars and, as a result, releasing hundreds of times more energy. Because of this higher energy output, the proto-planetary disks around the stars of FU Orionis are heated to higher temperatures, turning the ice into water vapor at large distances from the star.

Using Atacama Large Millimeter/submillimeter Arraya powerful radio telescope in northern Chile, we discovered a large, hot proto-planetary disk around the young sun-like star V883 Ori, about 1,300 light years from Earth in the constellation Orion.

V883 Ori emits 200 times more energy than the sun, and my colleagues and I identified it as an ideal candidate to observe the relatively heavy to normal water ratio.

A radio picture of the disk around V883 Ori.
The proto-planetary disk around V883 Ori contains gaseous water, shown in the orange layer, allowing astronomers to measure the ratio of semi-heavy to normal water. Image Credit: ALMA (ESO/NAOJ/NRAO), J. Tobin, B. Saxton (NRAO/AUI/NSF), CC BY

Completion of the Water Trail

In 2021, the Atacama Large Millimeter/submillimeter Array measured V883 Ori for six hours. The data revealed a strong signature of semi-heavy and normal water coming from the proto-planetary disk of V883 Ori. We measured the ratio of semi-heavy to normal water and found that the ratio was very similar to the ratios found in comets as well as the ratios found in younger protostar systems.

These results fill the water trail gap that makes a direct link between water in the interstellar medium, protostars, proto-planetary disks, and Earth-like planets through a process of inheritance, not chemical reset. .

The new results definitively show that a large portion of Earth’s water likely formed billions of years ago, before the sun even rose. Confirming this missing part of the universe’s waterways offers clues to the origin of water on Earth. Scientists have previously suggested that most of the water on Earth come from comets impacting the planet. The fact that Earth has less semi-heavy water than comets and V883 Ori, but more than the chemical reset theory would, means that water on Earth probably came from more than one source.The conversation

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Photo Credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO), CC BY