Story

The earth is filled with beautiful nature. Her wish is to study the Universe that gave birth to the Earth.

　Nami Sakai focuses on the epic evolution that connects the far end of the Universe to the Earth. Why did the Earth become such a planet of miracles? Her research aims to elucidate the mysteries of the origin and evolution of the Solar System. This article follows the challenges of Sakai, who cultivated a “new astronomy” from the perspectives of physics and chemistry.

　My research began with an interest in the origins of the Solar System, which is familiar to us, humankind, and the unique natural environment of the Earth. I want to understand how the Solar System and the Earth came to the way they are in the vast Universe. I currently focus on the evolution of molecules in the Universe through observation of complex molecules found around baby stars (protostars).

　When I was a graduate course student and a rookie researcher at the Department of Physics, Graduate School of Science, the University of Tokyo, I worked on research related to carbon atoms. I initially planned to conduct observations using the Mt. Fuji submillimeter wave telescope, but just as I began my research, the Mt. Fuji Weather Station closed due to changes in the observation methods by the Meteorological Agency, and the telescope, which was powered by electricity from the station, could no longer be used.

　Thinking that I could not study what I wanted to, I considered transferring to another laboratory, even one overseas. However, my supervisor told me about a paper published by a French team who reported that they had detected spectral lines of methyl formate around a protostar in Ophiuchus in 2003, when I was still an undergraduate student. It was an incredible discovery at the time, as their report indicated the possibility that organic molecules such as methyl formate had already formed when the Solar System was formed. While wondering if there were similar sources other than the one in Ophiuchus, I also wanted to know if it was common. That is why I started conducting observations using the 45m telescope at the Nobeyama Radio Observatory in Nagano Prefecture by targeting NGC1333IRAS4A, a protostar in Perseus region, with the aim of finding organic molecules around the protostar, “trying to catch the second lightning in a bottle.”

A new discovery thanks to high-sensitivity observation at an untried level, instead of focusing on cost performance

　Sakai began conducting observations at the Nobeyama Radio Observatory. However, this was an observation of the same target source at the same frequency for an unusually long time, unlike the conventional approach of other researchers. She was even cautioned that the cost performance was too low. However, she patiently continued the long hours of observation and consequently achieved a high-sensitivity observation that no one else had ever tried.

　I considered the possibility that the spectral line of methyl formate may be so weak that it could be detected only by observing one source over a long time and reducing the noise in the data, and I conducted the observation for 100 hours, which was unprecedented at Nobeyama. Naturally, such a long time of integration (equivalent to long-time exposure with a camera) was not normal, and some called it ridiculous. I still stuck to the observation method I believed in as I was born with a rebellious spirit. I conducted approximately 6 hours of tracking observation per day, and the spectral line started to appear after about a week. I was so excited and started looking for the “third lightning,” spurred on by the joy of being the first to know something that no one else in the world knew.

　When I was observing protostar L1527 in Taurus, I noticed that only one of the two spectral lines emitted by methyl formate was detected. This did not seem right to me because these two spectral lines should be detected at nearly the same intensity. I referred to the database and found that it corresponded to the spectral line for the carbon chain molecule. This was really epoch-making. Somehow, the area around this protostar was rich in carbon chain molecules instead of methyl formate. In other words, this was an unexpected observational result suggesting that the chemical composition varies for each source. This was the driving force for me to obtain observation time with telescopes around the world, conduct observations together with many collaborators, and write a considerable number of papers in two and a half years. My doctoral thesis at the University of Tokyo, which proved the existence of “chemical diversity,” became the starting point for my subsequent research.

　After completing my phD course, and as an assistant professor, I decided to study what kind of formation process and physical environment leads to what kind of chemical composition of a source. By finding the answer, we may have an idea of how the Solar System was formed in the history of the Universe. Just as we learned that the ancestors of humans were apes, I wanted to trace the route that our Solar System took. By figuring this out, I thought I could elucidate why the Solar System gave birth to the miracle planet called Earth in the Universe.

Anguish during the process of pursuing astronomy from the perspective both of physics and chemistry

　However, Sakai’s research was an anomaly in the field of physics. To justify why a researcher belonging to the Physics Department should conduct astronomical observation research from chemical point of view, she had to explain how studying chemical composition changes in astronomical sources helps us understand their physical evolution. That is, an excuse was needed.

　For researchers in physics, the methyl formate and carbon chain molecules that I study belong to the research field of chemistry, which is complex and difficult to understand. Looking back on my days there, when asked “Why are you studying chemistry in the Physics Department?” I had to explain the purpose and significance of my research every time, such as “I’m trying to understand the relationship between chemical evolution and physical evolution through the changes in chemical composition.”

　To begin with, physics is an academic field that tries to elucidate the fundamental mechanisms of phenomena, and it focuses on aspects such as changes in energy and laws of motion. On the other hand, the field of chemistry differs in that it focuses on the diversity of substances and chemical reactions, and it pursues the structures and properties of molecules and changes caused by interactions and reactions. I was not entirely comfortable about pursuing research that integrates both aspects in the Physics Department, such as how the chemical composition of existing substances changes in concurrence with the birth of a planetary system, and how and why the chemical composition differs depending on the system.

　When questioned by others, I used to reply, “Chemical composition is an important parameter in the field of observational astronomy, and it can reveal matters that cannot be found from general measurements such as temperature and density.” Every time I repeated this explanation, I felt uncomfortable and thought I should not have to make excuses, since the elucidation of changes in chemical composition or the history of chemical evolution that follows the birth of a star is an important research theme in itself.

Uniquely creating a turning point toward a new field of research

　Going deeper into astronomy from the fusion of physics and chemistry, the uncomfortable feeling of having to make excuses lingered, even though all she wanted was to learn about chemical evolution. While hesitating about how to continue her research, she came across an open recruitment notice for the RIKEN.

　I learned about the RIKEN when Dr. Tahei Tahara, a Chief Scientist at RIKEN, gave a presentation at a workshop held at the University of Tokyo. Because of Dr. Tahara’s attitude and enthusiasm, I became interested in the RIKEN research environment. I looked into the history of RIKEN and found that there were many researchers who studied in interdisciplinary areas with extremely active exchanges between researchers from different fields. Its liberal research environment was very attractive to me. I felt it would be easier to continue my research, which straddles the boundaries of physics and chemistry because many of the researchers at RIKEN considered it was crucial to cooperate with different fields and because they were very interested in new areas in each research field. So I applied for a position and moved to RIKEN.

　The advertisement for an Associate Chief Scientist at RIKEN did not specify the field. They also offered startup funding for five years. They allowed me to continue interdisciplinary research that exceeded the boundaries of physics and chemistry, and it has become possible to conduct fulfilling research. It is also easy to discuss and collaborate with people from different research fields, which has accelerated my work. Furthermore, we have been able to expand the scope of activities by creating a community for discussing astrochemistry, and make connections that had not been possible through prior research activities.

　Many of the researchers at RIKEN are very interested in what researchers in other fields are studying. Since we have ample opportunities for communication on a daily basis, we often help each other solve research questions and problems. I consider this one of the assets I did not have access to before I came to RIKEN.

Searching for the origin of the Earth with colleagues by continuing interdisciplinary research

　Sakai has accelerated her research since moving to RIKEN. At present, she is creating a unique “system = miniature universe” by applying the mechanism of radio telescopes and is conducting observations.

　The nature of my research makes it impossible to directly collect data at the observation target because it is 500 light years away—I could never reach it during my lifetime. We will therefore try to elucidate what spectral line patterns can be obtained by experimentally creating a model environment, a “miniature universe,” inside my laboratory at RIKEN using molecules and their isotopoologues that likely exist in the target source, and conducting observations. Using this information as a type of dictionary, we will analyze the astronomical observation data. Right now, We are mapping out the basics.

　This system uses the same mechanism that we use to observe the sky with radio telescopes. It comes from the idea that if there are molecules whose spectral patterns we have not identified, why don’t we place the intended molecule inside a gas cell in a room and observe it? The advantage of my group is that we have members with experience in developing and operating submillimeter wave receivers at the Mt. Fuji telescope, including myself. We were able to manufacture the superconductor receiver, which requires excellent skills, and construct the exact same reception system as a radio telescope within the laboratory.

　We started using this model environment around 2020. After three or four years, we finally achieved accurate measurement of not only the wavelength but also the intensity of the spectral lines of targeted molecules in astronomical interest. We can now conduct research by direct comparison with the spectral lines from the observation data collected at the Atacama Large Millimeter/Submillimeter Array in Chile.

　Elucidating the process of molecule formation in the Universe―With the research theme to elucidate the origin of the Solar System from chemical evolution while working on astronomical research using interdisciplinary methods, the name of the laboratory I established at RIKEN is the “Star and Planet Formation Laboratory.” We are working on the formation of stars and planets by collaborating with researchers from various fields and institutions, including astronomy, surface science, and quantum science. By advancing research toward the goal of understanding how rare Earth (the Solar System as well) is in the vast universe, our group has the potential to become a starting point for creating new interdisciplinary research areas. During my time as an undergraduate, graduate student, and early-career researcher, there were moments of hesitation at the crossroads of different fields. However, it was precisely because I did not limit myself to a single field that I am where I am today. I hope to keep demonstrating that there is a path for such research.

(Titles omitted in the article)