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Astrochemistry: a new branch of astronomy
The Big Bang occurred roughly 13.8 billion years ago, a time so long it feels overwhelming to imagine. How exactly has the universe evolved from that moment to the present day, where we now live? Shota Notsu tackles this mystery through astrochemistry, a new branch of astronomy. He has two targets: how did the diverse array of planets form, and where did the complex organic molecules, the building blocks of life, come from?
“In the 1970s, observations of interstellar molecules using radio telescopes began in earnest. By the 1980s, Japan had built a 45-meter-diameter radio telescope at Nobeyama. Entering the 21st century, Hayabusa and Hayabusa2 spacecrafts were launched to explore asteroids. Meanwhile, the gigantic ALMA telescope, comprising 66 small radio telescopes arranged in a manner similar to the compound eyes of an insect, was constructed in Chile's Atacama Desert. Technological innovation has dramatically improved humanity’s “eyesight” for observing the universe. We can now observe in detail the formation of new planets far beyond our solar system. We are also increasingly able to analyze the water and organic molecules present there. It is against this backdrop that astrochemistry was born.”
Astrochemistry is a portmanteau of astronomy and chemistry. As such, it aims to explore and elucidate the origin, behavior, and evolution of matter existing in space through the lens of chemistry. The term itself has only recently become widely used.
“With the ALMA telescope beginning scientific observations in 2011, we gained the ability to observe in detail the protoplanetary disks, the gas and dust surrounding “baby” stars, and the organic molecules present within them. How do water and organic molecules chemically evolve in space? When did these molecules arrive on Earth, and how did that connect to the birth of life? These are the questions I want to answer. As a child, I loved humanities subjects, especially history. But before I knew it, I had skipped right over archaeology and ended up studying the incredibly ancient history of planetary formation (laughs).”
The mystery of "hot Jupiters"
In our solar system, terrestrial planets like Earth, which are primarily composed of rock, are closer to the Sun. Beyond them lie gas giants like Jupiter and Saturn. Further out still are the icy planets, Uranus and Neptune. Why are planets arranged in this way?
“Actually, Japan is quite prominent in this field. The “Kyoto Model,” developed by Professor Chushiro Hayashi and his group at Kyoto University in the 1980s, is now the standard. According to this model, planets form in the protoplanetary disk, which is composed of 99% gas and 1% dust and emerges right after a star is born. At this stage, the temperature of the inner part of the disk (closer to the star) is higher, so that water can only exist as a gas. Consequently, it is only rocky dust particles that “fuse” and grow into rocky bodies, leading to Earth-like planets. Meanwhile, in the outer regions, water freezes like rock, forming protoplanets consisting of large chunks of rock and ice. Then, gravity accretes the surrounding gas, inflating these protoplanets into gas giants like Jupiter. Icy planets like Uranus, located even further away from the Sun, took longer to grow as protoplanets and could not grow large enough before the gas in the disk dissipated. That is the birth of planets according to the Kyoto Model.”
The boundary between the forming region for Earth-like rocky planets and the forming region for gas giants like Jupiter is called the snowline. The Kyoto Model elucidated how the formation mechanisms differ inside and outside this snowline using mathematical equations. However, a major event challenged its validity.
“In 1995, the first planet around a main-sequence star outside our solar system (an exoplanet) was discovered. It was located remarkably close to its central star, even closer than Mercury in our solar system. Yet, it turned out to be a gas giant planet, which led to it being called a “hot Jupiter.” This, the traditional Kyoto Model could not explain, as according to the Kyoto Model, large gas planets must form in the outer regions of the disk.”
This discovery sparked theoretical research aimed at expanding the model into a more general planetary formation theory capable of explaining the diverse distribution and structure of exoplanets. New research actively pursued both theoretical and observational approaches, examining whether the model aligned with the characteristics of observed protoplanetary disks. Simultaneously, discoveries of exoplanets far beyond our solar system progressed through observations using visible light and near-infrared telescopes. Today, an astonishing number of approximately 6,000 planets have been found. But how is it possible to detect such tiny planets floating in space tens or hundreds of light-years away?
“One method is called the Doppler technique. Planets have a gravitational pull, so as a planet orbits its star, the star also moves ever so slightly. At this time, the wavelengths of the spectral lines (bright or dark lines emitted by atoms or molecules, whose intensity changes only at specific wavelengths) emitted by the star shift subtly due to the Doppler effect. This shift corresponds to a speed of just a few meters to tens of meters per second. By observing this shift, we can infer whether there is a planet there or not. Recently, the transit method is also often used. When an orbiting planet passes in front of its star, the star's light dims slightly. By observing this phenomenon, we can determine not only the presence of a planet but also its radius and atmospheric composition.”
Just how strong has astronomy's “eyesight” gotten?
“On-site” planet formation at V883 Orionis
Compared to earlier radio telescopes, the ALMA telescope offers approximately a hundred times higher spatial resolution. While earlier observations of protoplanetary disks could only reveal the difference in brightness between the center and outer regions, we can now discern structures like gaps in the protoplanetary disk created by massive gas planets and the circumplanetary disks forming around them. Notsu says it is almost like being on-site when planets form. Innovations in semiconductor technology have enabled the detailed analysis of even faint radio waves along their wavelength, allowing us to determine the temperature, quantity, rotational speed, and position relative to the central star of the dust and gas present at these planetary birthplaces.
"My primary studies involves submitting observation proposals for the ALMA telescope, developing theoretical models based on the acquired data, writing concept papers suggesting what could be learned from specific observations, and integrating theoretical models with observational data. Recently, I have also submitted observation proposals for projects like Japan's infrared astronomical satellite GREX-PLUS (launching in the 2030s) and NASA's far-infrared space telescope PRIMA (planned to launch in 2032), suggesting specific studies on the snowline."
That said, “being on-site” in Notsu’s studies means a distance spanning from about 100 light-years to several thousand light-years away from Earth. For example, the Orion molecular cloud complex is located approximately 1,300 light-years from Earth, and observations with the ALMA telescope have discovered numerous protoplanetary disks there. Molecular clouds are extremely cold, giant cloud-like structures of molecular gas and dust drifting through space.
“One astronomical object currently attracting attention is the protostar V883 Orionis. A freshly formed disk of gas and dust surrounds this young star, representing an active planet-forming site where gas and dust are pouring in from further outside the disk. By observing the light reaching us from V883 Orionis with the ALMA telescope, we have discovered that the snowline, which is typically located at a distance several times to ten times that between the Sun and Earth, is found here at a distance eighty times greater. The sudden brightening of the central protostar heated the surrounding disk, causing ice to sublimate and pushing the snowline outward. As a result, large amounts of water and organic molecules are now escaping from the ice surface into the gas.”
Where did the building blocks of life come from?
Observations of V883 Orionis using the ALMA telescope have detected numerous organic molecules there, including methanol, acetaldehyde, and methyl formate. Notsu proposes the following scenario leading to these complex organic molecules.
“Large amounts of carbon monoxide (CO) present within molecular clouds freeze onto the surfaces of dust particles, triggering reactions where hydrogen atoms attach, initially forming formaldehyde and methanol. As a protoplanetary disk forms from this molecular cloud, its temperature gradually rises. Within this environment, dust particles coagulate and grow. As the disk structure develops, further chemical reactions are triggered, likely leading to the formation of complex organic molecules such as acetaldehyde and methyl formate. Research into these chemical reaction processes has a history, led by groups including the research team in Hokkaido University. Collaborating to advance this research is critical.”
In 2014, the European probe Rosetta successfully landed on the nucleus of the comet 67P/Churyumov-Gerasimenko, which orbits within the solar system, and examined its material composition and structure. Comparing the organic molecular composition of this comet with that of the protostar V883 Orionis revealed striking similarities. Moreover, glycine, an amino acid essential to life on Earth, was also found on this comet.
“This means the chemical evolution of organic molecules at the site of planet formation was directly inherited by comets. I became convinced that research into the materials exploring the origin of the solar system and studies of protoplanetary disks are connected.”
Notsu believes that meticulously developing dynamic theoretical models of planet formation and making observations of the spectral lines of molecules present in various planet-forming environments are both indispensable to achieving the ultimate goal.
“How much of the material that forms the building blocks of life is created within the protoplanetary disk? And does it reach Earth via meteorites, or was it incorporated during Earth's formation? Fully clarifying all these questions is not easy, but I am confident that advances in research, including observations with the ALMA telescope, will gradually unravel this mystery step by step.”
Notsu emphasizes that solving these mysteries requires not only theoretical calculations and observations but also engaging in discussions and seeking guidance from scientists working in various fields of analysis, experimentation, and exploration.
“Astrochemistry is a field that absolutely requires interdisciplinary collaboration. There is still so much we do not understand and so much knowledge to acquire, which means we must keep studying every day. But as a researcher, I find that enjoyable. I really want to emphasize the unique appeal that comes from collaborating with people from various fields to conduct research.”
Postdoc life in the Netherlands: research and cultural experiences
One of the reasons Notsu decided to become a researcher was a joint science research program for Japanese and British high school students that he participated in during high school.
“For a week, I attended classes and conducted experiments alongside British high school students entirely in English. I was surprised that I managed to study and do experiments in English, since at that time I learned and used English only in English class. This also led me to think I could work overseas as a researcher using English and sparked an interest in the profession as well.”
The opportunity for research life abroad came soon after earning his doctorate. He secured a one-year position as an overseas research fellow at Leiden University, the oldest university in the Netherlands, founded in 1575.
“The Netherlands has become a major hub in Europe for planetary formation research and astrochemistry studies. Because of this, almost every week, outstanding researchers from around the world would come to discuss research ideas there. In such a privileged environment, I was able to meet many researchers and later collaborate with them on joint projects, which was great. I even got to experience something like a Dutch version of a lab retreat, which was also incredibly enjoyable. The nonhierarchical connections between researchers were also very refreshing.”
Notsu was particularly impressed by the public defense of the doctoral theses of graduate students in the research group. He says it was an elegant ceremony.
“Unlike in Japan, the review process is completed beforehand. The final public defense, held in a building on the university grounds originally constructed as a monastery in the 15th century, is closer to being a ceremony. The student being reviewed wears formal attire: a morning coat and bow tie. All the professors wear robes reminiscent of those seen in the Harry Potter films. After the thesis is defended, a ceremonial officer dressed in traditional attire, looking like a real-world wizard, enters and escorts the reviewing professors out. Then, the professors carrying scroll-like documents return to the room and recite archaic phrases like “I confer upon you the degree of Doctor of Philosophy.” For a history buff like me, it was an unforgettable experience (laughs). While postdoc life abroad has its challenges, I highly recommend it not only for broadening your research horizons but also for experiencing different cultures.”
Notsu, who competed in the 5,000 meters and 3,000 meters steeplechase for his high school track team, found sports a great icebreaker for connecting with colleagues in the Netherlands. He shares the following advice for young people aspiring to become researchers:
“You will have many experiences ahead. I hope you fully enjoy each one. That might mean reading books or participating in club activities. While these may not directly connect to your future path, give your all to what is in front of you. If something catches your interest, explore it deeply. These experiences will become valuable assets when you consider your future direction.”
※Year of interview:2025
Interview/Text: OTA Minoru
Photography: KAIZUKA Junichi
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