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Water detected where giant planets like Earth may be forming

In a significant scientific breakthrough, the James Webb Space Telescope has detected water vapor in the rocky planet-forming zone of the PDS 70 star system

PDS 70 is an intriguing astronomical system that is situated approximately 370 light-years away. The fascinating aspect of this discovery is that it marks the first time that water has been detected in the terrestrial region of a disc, particularly one that is already recognized to host two or more protoplanets.

Search for extraterrestrial life on water planets

The quest for life outside of our planet is intrinsically linked to the search for water, the very element essential to life as we understand it. But how water actually arrived on Earth and whether the same processes could seed rocky exoplanets in distant star systems has been a topic of ongoing scientific debate.

The recent findings from the PDS 70 system may provide some compelling insights in this regard. This particular system comprises an inner disc and an outer disc, separated by a considerable gap of eight billion kilometers. Nestled within this gap are two known gas-giant planets. 

How the discovery was made

The water vapor discovery was made by the Mid-InfraRed Instrument (MIRI), a joint venture by NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA).

The MIRI has been successful in detecting water vapor within the inner disc of the PDS 70 system, at distances less than 160 million kilometers from the star. This area is of immense interest because it is the region where rocky, terrestrial planets like our Earth could potentially be forming.

Giulia Perotti, the lead author from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, stated, “We’ve seen water in other discs, but not so close in and in a system where planets are currently assembling. We couldn’t make this type of measurement before Webb.”

Her statement underscores the remarkable capabilities of the James Webb Space Telescope and its contributions to our understanding of the universe.

“This discovery is extremely exciting, as it probes the region where rocky planets similar to Earth typically form,” said study co-author Thomas Henning, director of MPIA. 

Henning also serves as the co-principal investigator of Webb’s MIRI and the principal investigator of the MINDS (MIRI Mid-Infrared Disk Survey) program that captured the data.

Planet-forming discs at PDS 70

PDS 70 is a K-type star, cooler and younger than our Sun, with an estimated age of about 5.4 million years. The presence of water vapor in such a relatively aged star, especially considering its planet-forming discs, comes as a surprise.

Planet-forming discs are comprised of gas and dust, the contents of which decline over time. This decline can be attributed to the central star’s radiation and winds eliminating the material, or the dust evolving into larger bodies that eventually form planets. 

In this light, the detection of water vapor in PDS 70 is remarkable since it implies a wet environment that could potentially allow the formation of rocky planets. This contradicts previous theories that suggested stellar radiation might lead to a dry environment in similarly aged discs, unsuitable for the formation of any rocky planets.

The researchers have yet to detect any planets forming within PDS 70’s inner disc, but they do see the essential building blocks for rocky worlds: silicates. The presence of water vapor indicates that if rocky planets are forming, they will have water available to them from the very start. 

“We find a relatively large amount of small dust grains. Combined with our detection of water vapour, the inner disc is a very exciting place,” said study co-author Rens Waters from Radboud University.

Two possible scenarios 

This extraordinary discovery poses further questions: Where did the water come from? The team behind the MINDS program has considered two scenarios. 

Water molecules could be forming directly in the place they were detected through the combination of hydrogen and oxygen atoms. 

Alternatively, ice-coated dust particles might be migrating from the cool outer disc to the hotter inner disc, where the water ice would sublimate and turn into vapor.

Moreover, how can water survive so close to the star, where the star’s ultraviolet light could theoretically break apart any water molecules? The most probable answer lies in the protection provided by surrounding materials, such as dust and other water molecules, that serve as a shield against destruction.

As a next step, the researchers plan to employ two of Webb’s other instruments, the Near-InfraRed Camera (NIRCam) and the Near-InfraRed Spectrograph (NIRSpec), to further study the PDS 70 system and potentially reveal even greater insights. 

The research has been published in the journal Nature.

More about PDS 70 and its potential water planets

PDS 70 is a star system approximately 370 light-years away from Earth in the constellation Centaurus. 

The PDS 70 system is particularly noteworthy due to the discovery of two planets in the process of formation, PDS 70b and PDS 70c.

These potential water planets were discovered in 2018 and 2019 respectively. Their formation is ongoing, providing astronomers with a rare opportunity to study planetary formation processes directly.

PDS 70b is a gas giant approximately 21 times the mass of Jupiter and orbits the star at a distance equivalent to about the distance between Uranus and the Sun in our solar system. PDS 70c, on the other hand, is around 10 times the mass of Jupiter and orbits at a greater distance from the star.

Also, it is noteworthy that PDS 70c was the first exoplanet directly observed to have a circumplanetary disc – a structure of dust and gas surrounding the planet, which is thought to be material left over from the planet’s formation. These discs are believed to be sites where moons can form.

More about gas giants

Gas giants, also known as Jovian planets, are a type of planet that is primarily composed of hydrogen and helium. The name “gas giant” was synonymous with “giant planet” until the latter term was clearly defined as a large planet that directly formed from the collapse of a molecular cloud (like a star) rather than through core accretion (like a terrestrial planet). This broad definition encompasses both traditional “gas giants” like Jupiter and Saturn and “ice giants” like Uranus and Neptune.

Defining characteristics

Size and mass

Gas giants are much larger and more massive than terrestrial planets. For example, Jupiter, the largest gas giant in our solar system, has a diameter more than 11 times that of Earth, and its mass is over 300 times greater.


Gas giants are primarily composed of hydrogen and helium, similar to the Sun. They may also contain heavier volatile substances, often called “ices.” For instance, in the cases of Uranus and Neptune, the term “ice giants” has been used to denote their different internal structure, which is thought to include a higher proportion of heavier volatile substances like water, ammonia, and methane.


Gas giants have thick atmospheres, extending many thousands of kilometers above their surfaces. These atmospheres are composed primarily of hydrogen and helium, with trace amounts of other gases that lend the planets their distinctive colors.

Rings and moons

All gas giants in our solar system have ring systems, although Saturn’s are the most prominent. They also have numerous moons.

Lack of solid surface

Unlike terrestrial planets, gas giants do not have a solid surface. The outer gaseous layer transitions gradually into the internal layers as pressure and temperature increase.


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