芝池 諭人

Giant planets are often regarded as primordial protoplanetary "architects". Their formation and interactions with planetesimals and other planets during and after the protoplanetary disc phase are key to setting the blueprint of the planetary systems we observe today. Catching nascent gas giants is our best opportunity to uncover these crucial steps in the planet formation process and shed light on the origins of our own solar system. Over the last decade, ALMA has revolutionised our understanding of planet formation; still, detecting signatures of Jupiter-like planets has remained out of reach. We argue that ALMA2040, a next-generation sub(mm) interferometer with improved sensitivity ($\times25$ in the continuum and $\times10$ for line emission) and a $\times3$ higher angular resolution compared to ALMA, will be sensitive to (sub)mm emission around a forming Jupiter-like planet orbiting a Sun-like star within $150\,{\rm pc}$ and allow kinematically characterising interactions of Jovian and sub-Jovian planets with their natal disc. ALMA2040 will provide unique and unprecedented insights into how gas giants carve gaps, accrete material, form satellites, and affect the architectures of their co-evolving planetary systems.

The outer planet region of the Solar System attracts scientists to explore fundamental questions of Solar System science, such as a comparative study with exoplanetary systems and the search for a second genesis in the ocean worlds. Until now, spacecraft exploration to the outer planet region has been realized only with large launch capabilities, multi-billion dollar budgets, multi-generational commitments, and RTG technology in many cases. Recent advances in telescopic observations of the outer planets from the Earth's vicinity such as JWST and dedicated planetary telescopes like Hisaki, as well as large-scale ground-based telescopes like ALMA, provide more frequent and shorter turnaround scientific outcomes than decade-long spacecraft missions. Thus, the traditional game of deep space exploration as described above needs to change to a more agile and sustainable way so that it can complement exciting discoveries from remote sensing of astronomical observatories and provide synergy with what in-situ exploration can only offer, such as observation at higher spatial and temporal resolutions, measurement of physical parameters and chemical compositions by direct contacts, and ultimate sample returns. As an innovative solution, JAXA has introduced a program concept called OPENS (Outer Planet Exploration by Novel Small Spacecraft) and has selected "OPENS-0" as the first candidate for its newly-created "Eco and Fast" small-class category in 2025. Aiming for the launch in the late 2020s, the OPENS-0 is an engineering demonstration mission of key enabling technologies for a 100-kg class spacecraft to independently explore the outer planet region. It is planned to perform multiple gravity assists at Venus and Earth, cruise the interplanetary space through the main asteroid belt, and eventually reach the Saturnian system as the final destination by the end of the 2030s. Due to its extremely constrained mass and power resources, no dedicated science payload resources could be allocated as of the proposal stage. However, complementary in-situ science that remote telescopic observations can never achieve is key to justify the OPENS program concept for the benefit of Solar System science. During its interplanetary cruising phase of OPENS-0 mission at heliocentric distances from 0.7 to 10 AU, the following scientific investigations are planned to conduct with multi-purpose engineering instruments onboard: (1) Earth photometry observation as a reference data for exoplanets, (2) Radio occultation observations of the Venusian atmosphere and the solar corona, (3) Optical observations of zodiacal light scattering and extragalactic background light, (4) Direct measurements of the spatial distribution of interplanetary dust, and (5) Flyby geological investigation of main belt asteroids. When the spacecraft reaches the Saturnian system as an extra success, integrated observations of the Saturnian ring structure are planned, targeting different physical quantities and spatial scales to advance our understanding of the formation and evolution of the planetary rings beyond Cassini's legacy. Candidate observations include measurements of fine ring structures and ring particles using optical cameras, characterization of the global ring structure through radio occultation, and direct measurements of the dust distribution from the ring region to the retrograde satellite dust torus. Once the OPENS-0 succeeds in demonstrating novel small spacecraft is capable of outer planet exploration as a complementary way from legacy flagship class missions, this expertise will be applied to subsequent missions (OPENS-1, 2, ... X) for other outer planet destinations through various flight opportunities, for examples as a solo rendezvous mission to an outer planet target with electric propulsion, a sample return mission combined with a larger orbital transfer vehicle, and mothership-daughtership collaboration with international flagship missions to enhance mission capabilities and mitigate risks of the mothership. Through this programmatic approach, the OPENS program aims to realize a higher frequency of outer planet exploration with less cost; thus, technological and scientific advancement of spacecraft exploration can become as fast as astronomical observations.

In 2025, JAXA/ISAS selected "OPENS-0" (Outer Planet Exploration by Novel and Small Spacecraft-Zero), which aims to demonstrate key enabling technologies for a single, small spacecraft to conduct outer planet exploration, as the first mission for its newly-created "Eco and Fast" small-class category.    While the 100-kg-class OPENS-0 spacecraft has severe limitations in both resources and operational capabilities, unlike legacy outer plane missions, its lighter mass and smaller size can be advantages for power and thermal management, as well as delta-V requirements.  Although the spacecraft design cannot enjoy abundant resources for dedicated scientific payloads, the OPENS-0 maximizes its scientific outputs, during 10+ years of the interplanetary cruising and multiple flyby occasions, with several "multi-purpose" instruments such as (1) the optical navigation telescopic and wide-field cameras to observe the "Pale Blue Dot" as a reference of Earth-like exoplanets, zodiacal light variation through the Main Asteroid  Belt, extragalactic background light in the outer planet region, main belt asteroid morphology; (2) the ultra-stable oscillator and deep space transponder for radio occultation of the solar flare, and the planetary atmospheres of Venus and Saturn; and (3) the PVDF-layered MLI for interplanetary micrometeoroid flux at 1-10 AU heliocentric distance.  Once the spacecraft eventually arrives in the vicinity of Saturn at the finale of the mission, all three instruments will be coordinated for integrated sciences focused on Saturn's ring structures, ranging from the retrograde satellite dust torus to the E-ring region, as well as the main rings and gaps, in complementary spatial resolutions among them.

It is generally accepted that the Galilean satellites formed out of the gas disk that accompanied Jupiter's formation. We construct a new scenario for the origin of the Galilean system, based on the capture of several planetesimal seeds and subsequent slow accretion of pebbles. Our “slow-pebble-accretion” scenario then simultaneously and consistently reproduces the following characteristics:(1) the mass of all the Galilean satellites;(2) the orbits of Io, Europa, and Ganymede captured in mutual 2: 1 mean motion resonances;(3) the ice mass fractions of all the Galilean satellites;(4) the unique ice-rock partially differentiated Callisto and the complete differentiation of the other satellites.