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Scientific Rationale
Scientific Rationale
Planetary systems are now known to be diverse, thanks to the discovery of over 6,000 exoplanets. How was this diversity formed? To fully understand the origin of the diversity, it is essential to study evolution of planetary systems from formation to maturity across a wide range of ages. Young planetary systems offer a unique window into the critical evolutionary processes that shape mature planetary architectures. As illustrated in Fig. 1 (left), after disk dispersal, planets undergo profound structural, dynamical, and atmospheric transformations that shape the observed diversity of mature planetary systems. Studying young systems enables researchers to observe these processes while they are still ongoing, providing unique insights into planet formation and evolutionary pathways that are otherwise obscured in mature systems.
Fig. 1. Left: Timeline of planet formation and evolution and the rough numbers, N, of such systems currently known. Young (a few My<age<1Gyr) systems are still underexplored and that is the focus of this proposal. They are key to understanding critical evolutionary processes that shape mature planetary systems. Right: Distribution of planetary radius and orbital period of young systems (markers) and mature systems (contours). The youngest systems (red) appear inflated relative to their older counterparts (blue) indicating substantial shrinkage within ~100 My timescale consistent with XUV photoevaporation theory.
However, most known planetary systems to date are several billion years old, while young systems (a few Myr–1 Gyr) remain rare due to observational challenges. High stellar activity in young stars causes large photometric variability and radial velocity (RV) jitter, often masking planetary signals. Nonetheless, discoveries of young systems are steadily increasing thanks to large transit surveys like K2 and TESS targeting stars in newly identified young clusters and associations from Gaia (Perren et al. 2023). Advances in instrumentation, data analysis, and empirical models—such as lithium depletion (Jeffries et al. 2025) and gyrochronology (Bouma et al. 2023)—have improved detection and stellar age estimates, yielding more transiting planets around young field stars (e.g., de Leon et al. 2023).
As shown in Fig. 1 (right), the emerging population of very young transiting planets (<100 Myr) tends to exhibit significantly larger, or inflated, radii compared to older counterparts at similar orbital separations (e.g., Vach et al. 2024). This trend is attributed to the extended H-He envelopes of very young planets, which remain distended by residual heat from formation. Over time, these envelopes contract as a result of XUV-driven photoevaporation and secular cooling (Owen et al. 2023), leading to the observed convergence toward the mature sub-Neptune radius distribution by ~1 Gyr (David et al. 2020; Petigura et al. 2022). The youngest planets, often orbiting pre-main-sequence G, K and M-type stars, thus provide rare snapshots of planets during the early stages of planetary evolution.
Topics and Goals of this Splinter Session
Topics and Goals of this Splinter Session
Under the science theme “Cool Stars with Protoplanetary Disks and Exoplanetary Systems,” we propose a splinter session titled “Young Stars with Evolving Planetary Systems” to explore the critical evolutionary phase that bridges the gap between protoplanetary disk dispersal and the emergence of mature planetary systems. Key open questions include identifying the dominant mechanisms driving planetary radius evolution in the first 100 Myr, quantifying how stellar magnetic activity modulates atmospheric loss, and determining when and how planetary orbits evolve. Addressing these gaps requires coordinated efforts across theory, stellar characterization, and exoplanet surveys and follow-up characterization.
This session will focus on topics including, but not limited to:
Detections and properties of young exoplanets: New discoveries of young exoplanets with ages less than 1 Gyr, and measurements of their fundamental properties, leveraging on TESS, Gaia, and ground-based facilities
Evolving planetary atmospheres: Atmospheric characterization of young exoplanets in evolution, including their compositions, extent, and escape, as observed by JWST, HST, and ground-based facilities
Evolving orbital architectures: Characterization of orbital obliquities, eccentricities, and resonances in young planets
Theory of planetary system evolution: Theoretical studies of early evolution of planetary systems and planets, including planet formation, orbital migration, atmospheric loss, internal cooling, giant impacts etc, shaping planetary system structure
Stellar age-dating: Advances in techniques such as gyrochronology, lithium depletion, and chemical tagging, applied to linking stellar and planetary evolution
Comparative planetary populations: Studies across different ages and environments to bridge protoplanetary disks, young planetary systems, and mature exoplanet systems
Relevance and Timeliness
Relevance and Timeliness
This splinter session is closely relevant to one of the main CS23 themes “Cool Stars with Protoplanetary Disks and Exoplanetary Systems.” This session builds on the legacy of previous Cool Stars meetings by linking stellar evolution to the early stages of planetary system development, bridging protoplanetary disks to mature planets, and formation to long-term evolution. The topic is especially timely given the rapid expansion of the young exoplanet sample thanks to TESS and Gaia, the first atmospheric and structural characterizations from HST and JWST (e.g. Barat et al. 2025), and the imminent arrival of PLATO, Ariel, and the ELTs, which will enable systematic studies of planetary evolution across stellar ages and environments. Recent advances in stellar age-dating techniques, high-precision time-domain surveys, and multiwavelength observations now allow unprecedented constraints on early planet formation, atmospheric loss, orbital dynamics, and star-planet interactions. By bringing together observers and theorists across these communities, the session will consolidate current knowledge, highlight critical open questions, and coordinate strategies for interpreting the next generation of young planet discoveries, including comparative population studies, planetary structure evolution, and the role of magnetic activity in shaping emerging planetary systems.
Splinter Session Format
Splinter Session Format
We plan to structure each 90-min session to include one invited talk (18 minutes total: 15-min presentation and 3-min Q&A) and six contributed talks (12 minutes total: 10-min presentation and 2-min Q&A). With two 90-min sessions, our splinter session will thus feature a total of two invited talks and twelve contributed talks. We may reserve the final slot for a dedicated discussion to foster possible future collaborations.
Splinter Session Organizers
Splinter Session Organizers
Norio Narita (Chair: The University of Tokyo, Japan)
John Livingston (Astrobiology Center, Japan)
Kiyoe Kawauchi (Ritsumeikan, Japan)
Enric Palle (Instituto de Astrofisica de Canarias, Spain)
Jaume Orell-Miquel (The University of Texas at Austin, USA)
Eric Gaidos (University of Hawaii, USA)
Fei Dai (University of Hawaii, USA)
Splinter Session Date
Splinter Session Date
Afternoon on June 15, 2026 (Mon)
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