宇宙科学談話会
ISAS Space Science Colloquium & Space Science Seminar
FY2026
Emerging boundaries in the planet formation process
Dr. Erik Petigura
University of California, Los Angeles
The eight planets in the solar system fall neatly into three main categories: rocky terrestrials, hydrogen-dominated Jovians, and ice giants. These planet classes reflect different formation environments, processes, and timescales. Extrasolar planets, in contrast, span a continuum of sizes, masses, and orbit. They demand a richer taxonomy. Recently, large, multivariate studies of transiting exoplanets have revealed new dividing lines in the planet formation process. Transitions in the occurrence, eccentricity, and host star metallicity distributions have shown that close-in planets larger than Neptune form according to very different pathways compared to their smaller counterparts. At the same time, new synergies involving RVs, direct imaging, and astrometry are beginning to illuminate the boundaries between the most massive planets (formed by core accretion) and the least massive brown dwarfs (formed by direct collapse). Surprisingly, this empirical boundary occurs well below the deuterium burning limit. Studies like this offer a preview of the types of insights we can look forward to with the upcoming release of Gaia DR4 in December of this year, which will dramatically expand our census of giant planets and brown dwarfs.
Conference Hall (2nd floor/ Research and Administration Building A), Via Zoom
The centre of the Milky Way
Dr. Mattia Sormani
University of Insubria (Como, Italy)
I will give an introduction to the structure and dynamics of the central 3 kpc of the Milky Way. The Galactic bar efficiently drives gas from the Galactic disc towards the centre at a rate of ~0.8 Msun/yr, creating a ring-like accumulation of molecular gas at a radius of R=120pc known as the Central Molecular Zone (CMZ). The CMZ is the Milky Way analogue of the star-forming nuclear rings commonly observed at the centre of external barred galaxies, and its formation is governed by a dynamical process similar to that responsible for creating gaps in Saturn's rings. Once in the CMZ ring, approximately 10% of the gas is converted into stars by intense star formation activity. Over Gyr timescales, this activity builds up a flattened stellar system known as the nuclear stellar disc (NSD), which currently has a mass of 10^9 Msun and dominates the gravitational potential of the Milky Way at 30pc<R<300pc. Most of the remaining gas is ejected perpendicularly to the plane by a Galactic outflow powered either by stellar feedback and/or AGN activity. A small fraction continues moving radially inwards, feeding the circum-nuclear disc (CND) at radii of few pc and contributing to the in-situ growth of the nuclear star cluster (NSC). Only a tiny fraction eventually enters the sphere of influence of the central black hole SgrA* at R<1pc.
Conference Hall (2nd floor/ Research and Administration Building A), Via Zoom