Division of Science, NAOJ

How were our Solar System and exoplanets formed? Research to date has shown that planets are born within so-called protoplanetary disks – structures of gas and dust surrounding young stars. However, the detailed processes that lead to planet formation remain shrouded in mystery.

One feature that has long been thought to play an important role in planet formation is the spiral structure that can emerge within protoplanetary disks due to the self-gravity. Within these spirals, solid particles may efficiently collide and grow, eventually reaching planetary sizes. In some cases, the spirals themselves may directly fragment to planets. On the otherhand, it is also known that spiral structures can be created by massive planets that have already formed. This means that the presence of spirals alone does not allow researchers to determine whether planets are on the verge of forming or have already been born.

An international research team led by Tomohiro Yoshida, a graduate student at the National Astronomical Observatory of Japan (NAOJ) and the Graduate University for Advanced Studies (SOKENDAI), focused on a theoretical prediction that these two scenarios could be distinguished by the motion of the spirals. If the spirals are produced by the self-gravity of the disk prior to planet formation, they are expected to wind and eventually dissipate. In contrast, if the spirals are induced by an already-formed planet, they should retain their shape and co-rotate with the planet.

The team investigated the protoplanetary disk surrounding the young star IM Lupus, which exhibits prominent spiral arms. By combining images obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) over four observation periods in 2017, 2019, and 2024, spanning seven years, the researchers created a “movie” of the disk. The results revealed dynamic winding motions of the spirals. This finding indicates that the spirals are caused by the self-gravity of the disk. Such spirals are believed to promote planet formation, suggesting that this disk is caught in the very moment before planets are born.

These results were published as Tomohiro C. Yoshida et al., “Winding Motion of Spirals in a Gravitationally Unstable Protoplanetary Disk” in Nature Astronomy on September 24, 2025.
https://www.nature.com/articles/s41550-025-02639-y

Press release from NAOJ/ALMA project:
https://alma-telescope.jp/en/news/press/vimage-202509.html

Using high-resolution observations from ALMA (Atacama Large Millimeter/submillimeter Array) and JWST (James Webb Space Telescope), we have unveiled the intricate details of star formation in the nearby high-density cluster-forming region Oph A. The most striking discovery is seven planetary-mass objects. Three of them are associated with near-infrared point sources, suggesting they are extremely young free-floating planets or brown dwarfs (Fig1). The remaining four are dense cores without infrared emission, likely to evolve into free-floating planets or brown dwarfs in the future (Fig2). Remarkably, these dense cores appear connected to finger-like structures extending from the triple protostar system VLA1623-2417 (central-lower panel of Fig2), hinting that these planetary-mass objects may have been ejected from the protostellar system—offering new insights into their formation and ejection mechanisms. Additionally, a combined analysis of ALMA and JWST images revealed previously unseen features, including new protostellar outflows and jets, striped patterns on HII region shells likely generated by MHD waves, and warm gas flows from HII regions, shedding light on complex structures that had remained hidden until now.

Article Information

Fumitaka Nakamura , Ryohei Kawabe ,  Shuo Huang ,  Kazuya Saigo , Naomi Hirano ,  Shigehisa Takakuwa , Takeshi Kamazaki , Motohide Tamura , James Di Francesco , Rachel Friesen ,　Kazunari Iwasaki , and Chihomi Hara:
“Unveiling Stellar Feedback and Cloud Structure in the Rho Ophiuchi A Region with ALMA and JWST: Discovery of Substellar Cores, C18O Striations, and Protostellar Outflows”
(To be published in The Astrophysical Journal)
https://arxiv.org/abs/2509.01122

For decades, formation of high-mass stars exceeding 8 solar masses has remained one of astronomy’s greatest mysteries. In a recent study published in Astronomy & Astrophysics, a global team of astronomers, including researchers from Yunnan University, Shanghai Astronomical Observatory and Division of Science of NAOJ, has carried out cutting-edge observations toward a “hub-filament system” (HFS) molecular cloud—a cradle of high-mass star formation, and has uncovered stunning new evidence that challenges current theories and illuminates the multi-scale, dynamical  nature of high-mass stellar birth.

Using the world’s most advanced (sub)millimeter interferometer, ALMA, the research team conducted ~3000AU resolution observations at the 1.3mm wavelength toward the HFS I18308 cloud, a high star-forming region with a textbook example of HFS morphologies (left panel, Figure 1).  The team revealed dual fragmentation modes. Two hub-composing filaments (F1 and F2) exhibit a cylinder-like fragmentation mode, with the quasi-periodic core spacings regulated by the turbulence-dominated fragmentation mechanism. In contrast, the central hub clump shows a spherical-like fragmentation mode, with the core spacings regulated by gravity-dominated Jeans fragmentation mechanism. These findings challenge models predicting a single fragmentation mode across all density scales within molecular clouds (e.g., the global gravitational collapse model).

Moreover, the team did not find high-mass prestellar cores surpassing 30 solar masses; and instead all relatively low-mass cores show a systematic increase in mass and density with evolution. These observed facts support a multi-scale accretion scenario: low-mass prestellar cores form via Jeans fragmentation in the hub, collapse into intermediate-mass protostars, and grow into high-mass stars through hierarchical mass accretion from the filaments, hub clump, and cores (right panel, Figure 1).

Article Information

L. M. Zhen, H-L. Liu, X. Lu, Y. Cheng, R. Galván-Madrid, H. B. Liu, P. Sanhueza, T. Liu, D. T. Yang, F. Nakamura, S. H. Jiao, L. Chen, Y. Q. Guo, S. Y. Feng, Q. Zhang, X. C. Liu, K. Wang, Q. L. Gu, Q. Y. Luo, Y. Lin, P. S. Li, S. H. Li, K. Tanaka , A. E. Guzmán, “Hierarchical fragmentation in HFS I18308 observed as part of the INFANT survey”, Astronomy & Astrophysics, 2025, 70, A47.
https://doi.org/10.1051/0004-6361/202554634

Corresponding Authors: Y. Cheng, H-L. Liu, X. Lu

Authors: Takashi Hamana (NAOJ), Chiaki Hikage (Kavil IPMU), Masamune Oguri (Chiba University), Masato Shirasaki (NAOJ & The Institute of Statistical Mathematics), Surhud More (The Inter-University Center for Astronomy and Astrophysics)

Overview:
Cosmic shear is the coherent distortion of the shapes of distant galaxies caused by gravitational lensing of intervening large-scale structures, and is one of the most powerful tools for cosmology.
We perform a cosmic shear analysis of Hyper Supreme-Cam Subaru Strategic Program first-year data (HSC-Y1) to derive cosmological constraints. For an estimator of cosmic shear signals, we adopt orthogonal sets of E/B-integrals (COSEBIs) which allows us to separate the cosmic shear signals into independent E/B-modes.
We measure cosmic shear E/B-mode COSEBIs from shapes of ~10 million distant galaxies with good signal-to-noise ratios (Figure-1). We find that the B-mode signals are consistent with zero, which is consistent with the prediction of the standard gravity theory.
We perform a standard Bayesian likelihood analysis for cosmological inference from the measured E-mode COSEBIs, in which we adopt a covariance matrix derived from realistic mock catalogs constructed from full-sky gravitational lensing simulations done using the super-computer system XC30 at CfCA, NAOJ. We obtain constraints on cosmological parameters, the total matter density parameter and the amplitude parameter of the density fluctuation at present universe (Figure-2). It is found that our results are consistent with results of the CMB experiment by Planck mission, supporting the standard cold-dark matter universe model with the cosmological constant.

Figure-1: Comparison of the HSC-Y1 tomographic cosmic shear COSEBIs with the best-fitting theoretical model for the cold-dark matter model with the cosmological constant. Left and right triangular-tiled panels show E and B-mode cosmic shear COSEBIs⁠, respectively. Error bars represent the square-root of the diagonal elements of the covariance matrix. The solid lines in the top panels correspond to the best-fitting (maximum likelihood) model.

Figure-2: Marginalized posterior contours (68% and 95% confidence levels) with a blue scale for a visual overview of the distribution (darker for higher posterior) in the plane of the total matter density parameter (horizontal axis) and the amplitude parameter of the density fluctuation at present universe (vertical axis).

See the URLs below for the details.

Link to the paper (PASJ web page): https://academic.oup.com/pasj/article-abstract/74/4/923/6610012
Hyper Suprime-Cam Subaru Strategic Program: https://hsc.mtk.nao.ac.jp/ssp/
Center for Computational Astrophysics (CfCA): https://www.cfca.nao.ac.jp/

The research field of exoplanets has entered an era in which we not only detect new planets but also investigate (characterize) in detail the properties of already detected planets. An international research team that Prof. Masahiro Ikoma of the Division of Science, NAOJ, joins has developed and applied a new analysis method (atmospheric retrieval method) to infrared spectroscopic data for 25 hot Jupiters obtained by the secondary eclipse observation method with the Hubble Space Telescope and Spitzer Space Telescope. The team has succeeded in extracting some correlations between their atmospheric properties including the temperature structure and chemical compositions, which had not been known before. This research has led to a new phase from the characterization of “individual” planets to that of planetary “populations.” The findings obtained in this study will be further investigated and verified by the James Web Space Telescope (JWST) which was launched last year and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (Ariel), which is scheduled for launch in 2029. The characterization of planetary populations developed in this study will also be useful and crucial to understanding the characteristics, formation, and evolution of various types of planets.

So far, about 5,000 exoplanets have been discovered. However, no planetary system with a structure like that of our Solar System has been found among them, indicating that planetary systems are indeed diverse. In particular, gas giant planets called “hot Jupiters,” which orbit close to their host stars, do not exist in our Solar System, and their characteristics and formation remain a mystery. To characterize the atmospheres of the 25 hot Jupiters, the research team reanalyzed a vast amount of archived data consisting of approximately 600 hours of Hubble Space Telescope observations and more than 400 hours of Spitzer Space Telescope observations. They found that secondary eclipses (see Figure 2) were observed for all the 25 hot Jupiters, which occur when an exoplanet passes behind the central star as seen from the Earth, and the apparent decrease in the central star’s luminosity is called a “secondary eclipse. By spectroscopically observing the apparent decrease in the luminosity of the central star, we can estimate the vertical distribution of the atmospheric composition and temperature of the planet.

As a result of these systematic characterizations of many hot Jupiters, the research team found several clear trends and correlations among the hot Jupiter properties. For example, they found that in more than half of the atmospheres in their sample, the temperature increases with altitude (so-called temperature inversion). And those atmospheres were found to contain titanium oxide (TiO), vanadium oxide (VO), iron hydride (FeH), or hydrogen negative ions (H-) at temperatures higher than 2000 K. From this fact, one can infer that planetary atmospheres being hot enough to contain such metallic species absorb large amounts of central stellar light and heat the upper atmosphere, resulting in a temperature inversion. For the relatively cooler atmospheres, the team found that there are two populations, one in which H2O is detected and the other in which no H2O is detected. For the latter population, it was suggested that H2O is not chemically produced because of the relatively high carbon content of the atmosphere. In addition, such population study has led to validating or disproving what had previously been suggested by the characterization of individual planets.

This research is presented in the following paper published on April 25, 2022, in the Astrophysical Journal Supplement Series: Changeat et al. “Five key exoplanet questions answered via an analysis of 25 hot Jupiter atmospheres in eclipse”

The Astrophysical Journal Supplement Series : https://iopscience.iop.org/article/10.3847/1538-4365/ac5cc2

YouTube : Hubble Helps Answer Key Exoplanet Questions

A research group led by National Astronomical Observatory of Japan, Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA), and Osaka Prefecture University has developed a new ultra-wideband, high-sensitivity eQ receiver which is capable of observing from 30 to 50 GHz (a wavelength of about 7mm). It was successfully installed on the NRO 45m radio telescope and detected SiO maser line from the late-type star, RR Aql (see Fig. 1). Fumitaka Nakamura, Ryohei Kawabe, and Kotomi Taniguchi from our Division of Science participated in the installation campaign. Chau-Ching Chiong, who is in charge of the receiver development at ASIAA, also participated via zoom from Taiwan. Recently, ALMA started installation of Band-1 receivers with similar band (33-50 GHz). The eQ receiver has the widest bandwidth and highest sensitivity in the similar frequency range. In addition, this is the first time that a receiver developed overseas has been mounted on the 45m telescope.

We will mainly promote the following three sciences: (1) Zeeman measurements with SO/CCS lines, (2) detection of line emission from high-z galaxies, and (3) astrochemistry in 30-50GHz, and will prepare for upcoming ALMA Band-1 observations.

Figure 1: the eQ frist light with a SiO maser line from RR Aql.  

Figure 2: the eQ receiver and our installation team

An international research team including Dr. Hidetoshi Sano has succeeded for the first time in quantifying the proton and electron components of cosmic rays in a supernova remnant. At least 70% of the very-high-energy gamma rays emitted from cosmic rays are due to relativistic protons, according to the novel imaging analysis of radio, X-ray, and gamma-ray radiation. The acceleration site of protons, the main components of cosmic rays, has been a 100-year mystery in modern astrophysics, this is the first time that the amount of cosmic rays being produced in a supernova remnant has been quantitatively shown and is an epoch-making step in the elucidation of the origin of cosmic rays.

The origin of cosmic rays, the particles with the highest energy in the universe, has been a great mystery since their discovery in 1912. Because cosmic rays promote the chemical evolution of interstellar matter, understanding their origin is critical in understanding the evolution of our Galaxy. The cosmic rays are thought to be accelerated by supernova remnants (the after-effects of supernova explostions) in our Galaxy and traveled to the Earth at almost the speed of light. Recent progress in gamma-ray observations has revealed that many supernova remnants emit gamma-rays at teraelectronvolts (TeV) energies. If gamma rays are produced by protons, which are the main component of cosmic rays, then the supernova remnant origin of cosmic rays can be verified. However, gamma rays are also produced by electrons, it is necessary to determine whether the proton or electron origin is dominant, and to measure the ratio of the two contributions. The results of this study provide compelling evidence of gamma rays originating from the proton component, which is the main component of cosmic rays, and clarify that Galactic cosmic rays are produced by supernova remnants.

The originality of this research is that gamma-ray radiation is represented by a linear combination of proton and electron components. Astronomers knew a relation that the intensity of gamma-ray from protons is proportional to the interstellar gas density obtained by radio-line imaging observations. On the other hand, gamma-rays from electrons are also expected to be proportional to X-ray intensity from electrons. Therefore, they expressed the total gamma-ray intensity as the sum of two gamma-ray components, one from the proton origin and the other from the electron origin. This led to a unified understanding of three independent observables. This method was first proposed in this study. As a result, it was shown that gamma rays from protons and electrons account for 70% and 30% of the total gamma-rays, respectively. This is the first time that the two origins have been quantified. The results also demonstrate that gamma rays from protons are dominated in interstellar gas-rich regions, whereas gamma rays from electrons are enhanced in the gas-poor region. This confirms that the two mechanisms work together and supporting the predictions of previous theoretical studies.

This research was presented in the following paper on July 9, 2021, in the Astrophysical Journal.

Fukui, Sano, Yamane et al. “Pursuing the Origin of the Gamma Rays in RX J1713.7−3946 Quantifying the Hadronic and Leptonic Components”, ApJ, 915, 84 (2021)
DOI: https://doi.org/10.3847/1538-4357/abff4a

See the URL below for the details.
Nagoya University: https://en.nagoya-u.ac.jp/research/activities/news/2021/08/post-2.html

Dr Yuko Matsushita released her research results.

See the URL below for the details.

ALMA – NAOJ: https://alma-telescope.jp/en/news/jetrotation-202108

Kyushu University: https://www.kyushu-u.ac.jp/ja/researches/view/625

The Astrophysical Journal: https://iopscience.iop.org/article/10.3847/1538-4357/ac069f/meta

Authors: Kangrou Guo, Eiichiro Kokubo

Standard models of planet formation explains how planets form in axisymmetric, unperturbed disks in single star systems. However, it is possible that giant planets could have already formed when other planetary embryos start to grow. We investigate the dynamics of planetesimals under the perturbation of a giant planet in a gaseous disk. Our aim is to understand the effect of the planet’s perturbation on the formation of giant planet cores outside the orbit of the planet. We calculate the orbital evolution of planetesimals ranging from 10^13 to 10^20 g, with a Jupiter-mass planet located at 5.2 au. We find orbital alignment of planetesimals distributed in about 9-15 au, except for the mean motion resonance (MMR) locations. The degree of alignment increases with increasing distance from the planet and decreasing planetesimal mass. When the orbits of two objects are aligned, they encounter on tangential trajectories with low relative velocities, which lead to a higher chance of accumulation. The typical velocity dispersion for identical-mass planetesimals is of the order 10 m s^−1, except for the MMR locations. The relative velocity decreases with increasing distance from the planet and decreasing mass ratio of planetesimals. When the eccentricity vectors of planetesimals reach equilibrium under the gas drag and secular perturbation, the relative velocity becomes lower when the masses of two planetesimals are both on the larger end of the mass spectrum. Our results show that with a giant planet embedded in the disk, the growth of another planetary core outside the planet orbit might be accelerated in certain locations.

Link to the paper (arXiv): https://arxiv.org/abs/2106.06240

Figure 1: Schematic illustration of alignment of orbits. The dashed curves indicate parts of the orbits that are below the reference plane (yellow). (a): non-aligned orbits, m1 and m2 encounter with high relative velocity. (b): aligned orbits, m1 and m2 encounter on almost tangential orbits and with low relative velocity. This figure shows that even when two particles are on eccentric and inclined orbits, they can encounter with a low relative velocity when their orbits are well-aligned.

Figure 9 (b): Color maps of relative velocities at about 0.5 million years near 12 au. The relative velocity increases with increasing mass ratio. The typical relative velocity of identical-mass particles is on the order of 10 m/s.

Figure 10: Comparison of growth timescales in the aligned case (yellow) and non-aligned case (red) at different orbital distances. When the orbits are aligned, the growth timescale is significantly shorter – the growth of another planetary core outside the giant planet perturber can be accelerated under the coupling effect of secular perturbation and nebula gas drag.

Contact: Kangrou Guo (carol.kwok@grad.nao.ac.jp)

A worldwide team of astronomers has discovered evidence for a new type of stellar explosion — an electron-capture supernova. While this type has been theorized for over 40 years, this is the first observed example. These findings provide clues to the precursor of the Crab Nebula, a supernova observed by cultures all over the world in 1054.

Stars are governed by a balance of gravity causing them to contract and pressure preventing them from contracting. If this balance is disturbed, the results can be a runaway reaction, such as the star exploding in a supernova. Astronomers know of two main supernova types. One is a thermonuclear supernova — the explosion of a white dwarf star (less than about 8 times the mass of the Sun) that steals matter from its partner in a binary star system. The other is a core-collapse supernova where a massive star, one more than about 10 times the mass of the Sun, runs out of nuclear fuel and has its iron core collapse, creating a black hole or neutron star. The electron capture supernovae fit between these two groups, a type of core-collapse that happens to less massive stars, down to about 8 solar masses.

In an electron capture supernova, the pressure become great enough to force electrons into atomic nuclei (Fig. 1). The theory, including the observable characteristics the supernova should have, was formulated in 1980 by Ken’ichi Nomoto of the University of Tokyo and others. The progenitor star should have a lot of mass, lose much of it before exploding, and this material near the dying star should be of an unusual chemical composition. Then the electron capture supernova should be weak, have little radioactive fallout, and have neutron-rich elements in the core.

In this study by the Global Supernova Project, the team found that the supernova SN 2018zd (Fig. 2) had many unusual characteristics, and that these characteristics matched the predictions for an electron capture supernova. During the analysis, members of the team at the National Astronomical Observatory of Japan, Takashi Moriya (Assistant Professor) and Nozomu Tominaga (Professor) assisted with theoretical models of the progenitor star and interpretation of predictions of the changes in brightness expected from electron capture supernovae.

This is the first confirmed example, but it has long been suspected that the Crab Nebula was created by this type of explosion. In 1054 AD, a supernova was observed and according to Chinese records and Japanese records was so bright that it could be seen in the daytime for 23 days, and at night for nearly two years. The resulting remnant — the Crab Nebula — has been studied in detail. It was previously the best candidate for an electron capture supernova, but there was uncertainty partly because the explosion happened nearly a thousand years ago. The new results increase the confidence that SN 1054 was an electron capture supernova. It also explains why that supernova was relatively bright compared to the models — its luminosity was probably enhanced by the supernova ejecta colliding with material cast off by the progenitor star as was seen in SN 2018zd.

These results appeared as Daichi Hiramatsu et al. “The electron-capture origin of supernova 2018zd” in Nature Astronomy on June 28, 2021.

Fig. 1

Artist impressions of a super-asymptotic giant branch star (left) and its core (right) made up of oxygen (O), neon (Ne), and magnesium (Mg).  A super-asymptotic giant branch star is the end state of stars in a mass range of around 8-10 solar masses, whose core is pressure supported by electrons (e-).  When the core becomes dense enough, neon and magnesium start to eat up electrons (so called electron-capture reactions), reducing the core pressure and inducing a core-collapse supernova explosion.

Credit: S. Wilkinson; Las Cumbres Observatory

Fig. 2

Las Cumbres Observatory and Hubble Space Telescope color composite of the electron-capture supernova 2018zd (the large white dot on the right) and the host starburst galaxy NGC 2146 (towards the left).

Credit: NASA/STScI/J. DePasquale; Las Cumbres Observatory