In the last 15 years, two major international projects—the Herschel Space Observatory and the Atacama Large Millimeter Array (ALMA)—have transformed researchers’ ability to investigate interstellar space at the farthest reaches of the universe. The 2018 recipient of the James Craig Watson Medal of the National Academy of Sciences, Ewine F. van Dishoeck, has been involved in both projects from their inception. van Dishoeck’s research has revealed the chemistry of the cosmos on both macroscopic and microscopic levels, detailing the formation of stars and planets as well as the molecular composition of interstellar clouds, dust, and disks. Her work holds tantalizing clues about the development of water and organic matter—the building blocks of life as we know it—on Earth as well as on exoplanets. A professor of molecular astrophysics at Leiden University in The Netherlands, as well as the former director of Leiden’s Raymond and Beverly Sackler Laboratory for Astrophysics, van Dishoeck was elected as a foreign associate to the National Academy of Sciences in 2001. PNAS recently spoke to van Dishoeck about her research.
Ewine F. van Dishoeck. Image courtesy of Elodie Burrillon (photographer).
PNAS: You started working on the European Space Agency’s Herschel Space Observatory in 1982, and it was finally launched in 2009 with a 4-year lifespan. What was it like developing a project with such a long lead time?
van Dishoeck: I was a PhD student at Leiden University when we began planning for Herschel. On Herschel or similar endeavors like ALMA, from the project’s inception researchers push for technical specifications that will supersede previous devices, which guarantees a long time for research and development. At some point, physics dictates what the technology can do. Only when you’ve reached that stage can you start thinking about a launch date. We’re also fortunate that we could adapt some of the instrumentation from Herschel for use in ALMA.
We used Herschel primarily to track water, developing the WISH (Water in Star-forming regions with Herschel) program to ask: Where is water formed in a galaxy, where does it end up, and what does it look like near stars of different sizes—low mass ones, like our Sun, and larger ones like those found in Orion?
Water is one of the keys to our existence and our survival. But it’s very hard to detect water in space using ground-based telescopes because of the interference from water in our atmosphere. A space telescope like Herschel is ideal as it’s free from this interference.
We found water in all regions associated with star and planet formation, including any young solar systems. Water mostly takes the form of ice in interstellar clouds before the formation of the star itself. Ice builds on the surfaces of dust grains in the clouds, which seed protostars. We had a wonderful synergy of these results with work from the Sackler Laboratory for Astrophysics, which simulated the formation of water under similar conditions.
As the interstellar cloud collapses into a forming star, more water forms through high-temperature chemistry. Here, it’s associated with outflows that are like massive waterfalls: billions of Iguazu Falls! However, this water isn’t entering a new Solar System and is essentially lost to space. The next stage of star formation is when the cloud condenses into an accretion disk, and Herschel gave us a big surprise: the water signals were much weaker than we had expected, about an order-of-magnitude weaker. We suspected that the water that entered these disks as ice became locked up very early on in bodies that form planetesimals.
PNAS: ALMA came online just as Herschel’s lifespan concluded. What have you been able to learn from the northern Chile observatory?
van Dishoeck: ALMA amazed us from the beginning. Before the first official scientific cycle, we identified a complex organic molecule, glycolaldehyde, which is a sugar, in the gas near a binary solar mass star system called IRAS 16293-2422. But our biggest surprise came from an investigation of an accretion disk around the young star Oph-IRS 48. We knew from previous observations that this disk had a hole in the center, which is indicative that a giant planet may have formed there. So we were expecting to see a nice round donut shape with a hole in it. Instead, because of ALMA’s improved spatial resolution we saw something that looked like a very asymmetric cashew.
It turns out that we had the first observational evidence of a dust trap, which had been theorized for decades. A giant planet, formed in part by ice from an accretion disk, creates a pressure bump in the gas of the disk, which leads to dust collecting in that bump. This research confirmed evidence from Herschel that water was being locked into these larger bodies early on.
The combination of these instruments, Herschel and ALMA, allowed us to put the puzzle together; we couldn’t have done it with just one instrument or one wavelength. We’re interested in using ALMA, the upcoming James Webb Space Telescope, which should be launched in a few years, and eventually the European Southern Observatory’s Extremely Large Telescope, which is being constructed in Chile, to follow the story of planet formation.
PNAS: In addition to water and sugar, you have also observed prebiotic, complex organic molecules like methyl isocyanate in celestial bodies. What does this imply?
van Dishoeck: What I can definitively say is that the ingredients needed to make biogenic molecules like DNA and RNA are found around every forming protostar. They are there at an early stage, incorporating into bodies at least as large as comets, which we know are the building blocks of terrestrial planets. Whether these molecules survive or are delivered at the late stage of planet formation, that’s the part of it we don’t know very well.