Water in star formation: Rho Ophiuchi
​Star-forming cloud Rho Ophiuchi, imaged by the Herschel Space Observatory, with illustration of water molecules. ​​​
​ ESA/Herschel/NASA/JPL-Caltech​

From interstellar clouds to habitable planets

​An international team of astronomers, among them scientists from Chalmers, has published a comprehensive survey of water’s journey through space. Using data from the Herschel Space Observatory, they have shown that life's most important molecule can thrive in all new-born solar systems - not just ours.
Only ten years ago, it was not known how and where water is formed in space, and how it eventually ends up on a planet like Earth. 

Now, an international research team has put together everything scientists know about water in interstellar clouds, and the origin of water on newborn, potentially habitable, planets. The article, published in the journal Astronomy & Astrophysics, is based on observations with the Herschel Space Observatory. 

Space telescopes

Seeing water in space is a challenge for astronomers. Even the best ground-based telescopes are affected by water vapour in our own atmosphere.

Following an early pioneering effort by the Swedish science satellite Odin​, the Herschel Space Observatory was launched in 2009 by the European Space Agency, ESA.

During its four-year mission, Herschel had as one of its main objectives to investigate water in space. Of particular importance was the instrument HIFI, which was built under Dutch leadership with important contributions from Sweden, and in particular from Chalmers.

In the new study, Ewine van Dishoeck and her colleagues have been able to study how water molecules follow each part of the process that leads to the birth of new stars and planets.

Starts with Ice

The new study shows that most of the water is formed as ice on tiny dust particles in cold and tenuous interstellar clouds.

When a cloud collapses into new stars and planets, this water is largely preserved and quickly anchored into pebble-sized dust particles. In the rotating disk around the young star, these pebbles then form the building blocks for new planets.

"Water is mostly transported as ice from large interstellar clouds to these disks. The ice seems not to melt or break up on the way in. We can't say yet exactly how much water there is in these disks, but it's enough to form oceans on Earth-like planets", says Per Bjerkeli, astronomer at Chalmers.

Earth's water has also migrated here in this way, the researchers believe. Furthermore, they have calculated that most new solar systems are born with enough water to fill several thousand oceans. 

"It's fascinating to realise that when you drink a glass of water, most of those molecules were made more than 4.5 billion years ago in the cloud from which our sun and the planets formed", says Ewine Van Dishoeck.

For water molecules, the road from interstellar clouds to the drinking glass is complex, the scientists show. Previous studies with the Herschel Space Observatory showed how hot water vapour seen and copiously produced near forming stars is lost to space in powerful outflows. Now, the researchers have also been able to trace both cold water vapor and ice deposits in these star systems, among other things by examining weak signals from heavy water (where the molecule H20 contains one or two atoms of heavy hydrogen, or deuterium).

Many mysteries remain concerning water's path to the planets which new and future telescopes will have to address. NASA/ESA's James Webb telescope, which will be launched later this year, as well as the radio telescope ALMA in Chile and the future radio telescope SKA all have roles to play. The instrument MIRI on board the James Webb Telescope will be able to detect warm water vapour in the innermost zones of dust disks.

– Herschel has already shown that planet-forming disks are rich in water ice. With MIRI we can now follow that trail into the regions where Earth-like planets are formed, says Michiel Hogerheijde, astronomer at Leiden University and the University of Amsterdam.

More about the research and the Herschel Space Observatory 

Herschel was a space telescope of the European Space Agency (ESA) built in cooperation with NASA. Its HIFI and PACS instruments were used for the water research. HIFI was designed and built by a consortium of institutes and university departments across Europe, Canada, and the United States under the leadership of SRON Netherlands Institute for Space Research, the Netherlands, with major contributions from Germany, France, and the USA. The PACS instrument was developed by a consortium of institutes and universities across Europe led by the Max Planck Institute for Extraterrestrial Physics in Germany. Chalmers scientists played an active role in the scientific planning for Herschel, and were involved in several projects using data from the telescope.

The results are published in a paper by Ewine F. van Dishoeck et al., Water in star-forming regions: Physics and chemistry from clouds to disks as probed by Herschel spectroscopy, in the journal Astronomy & Astrophysics. Link to the paper: https://www.aanda.org/10.1051/0004-6361/202039084​ (see also https://arxiv.org/abs/2102.02225).

Ewine van Dishoeck led the water research programme WISH (Water in Star-forming regions with Herschel). ​The team consists of 50 astronomers, among them Chalmers scientists Per Bjerkeli, René Liseau och Magnus Persson, and Bengt Larsson (Stockholm University).


A (top) - The path of water molecules from vast interstellar clouds to potentially habitable planets has been traced in the star-forming region Rho Ophiuchi, 440 light years distant in the constellation Ophiuchus. This wide-angle image from Herschel, taken in light with a wavelength between 70 and 250 micrometers with the telescope's camera PACS, is 4 degrees wide (equivalent to eight full moons). In the brightest part of the image (above right) lies the young star VLA 1623, subject of detailed observations of water with the instrument HIFI.
Image: ESA / Herschel / NASA / JPL-Caltech, CC BY-SA 3.0 IGO; Acknowledgment: R. Hurt (JPL-Caltech)

B - The star-forming region Rho Ophiuchi, 440 light years distant in the constellation Ophiuchus. This wide-angle image from Herschel, taken in light with a wavelength between 70 and 250 micrometers with the telescope's camera PACS, is 4 degrees wide (equivalent to eight full moons). In the brightest part of the image (above right) lies the young star VLA 1623, subject of detailed observations of water with the instrument HIFI.
Image: ESA / Herschel / NASA / JPL-Caltech, CC BY-SA 3.0 IGO; Acknowledgment: R. Hurt (JPL-Caltech)

C - This illustration shows the Journey of water from interstellar clouds to habitable worlds. From top left to bottom right: water in a cold interstellar cloud, near a young, forming star with an outflow, in a protoplanetary disc, in a comet and in the oceans of an exoplanet. The first three stages show the spectrum of water vapour measured by the HIFI instrument on the Herschel space observatory. The signals from the cold interstellar cloud and from the protoplanetary disk have been exaggerated in this image by a factor of 100 compared to those from the young, forming star in the centre.​
Image: ESA / ALMA / NASA / L. E. Kristensen


Robert Cumming, communications officer, Onsala Space Observatory, +46 70 493 31 14, robert.cumming@chalmers.se

Per Bjerkeli, astronomer, Department of Space, Earth and Environment, +46 31 772 64 30, per.bjerkeli@chalmers.se

Page manager Published: Tue 20 Apr 2021.