The TESS-Keck Survey II: An Ultra-Short Period Rocky Planet and its Siblings Transiting the Galactic Thick-Disk Star TOI-561 (2009.03071v2)

Abstract: We report the discovery of TOI-561, a multi-planet system in the galactic thick disk that contains a rocky, ultra-short period planet (USP). This bright ($V=10.2$) star hosts three small transiting planets identified in photometry from the NASA TESS mission: TOI-561 b (TOI-561.02, P=0.44 days, $R_b = 1.45\pm0.11\,R_\oplus$), c (TOI-561.01, P=10.8 days, $R_c=2.90\pm0.13\,R_\oplus$), and d (TOI-561.03, P=16.3 days, $R_d=2.32\pm0.16\,R_\oplus$). The star is chemically ([Fe/H]$=-0.41\pm0.05$, [$\alpha$/H]$=+0.23\pm0.05$) and kinematically consistent with the galactic thick disk population, making TOI-561 one of the oldest ($10\pm3\,$Gyr) and most metal-poor planetary systems discovered yet. We dynamically confirm planets b and c with radial velocities from the W. M. Keck Observatory High Resolution Echelle Spectrometer. Planet b has a mass and density of $3.2\pm0.8\,M_\oplus$ and $5.5^{+2.0}_{-1.6}\,$g$\,$cm$^{-3}$, consistent with a rocky composition. Its lower-than-average density is consistent with an iron-poor composition, although an Earth-like iron-to-silicates ratio is not ruled out. Planet c is $7.0\pm2.3\,M_\oplus$ and $1.6\pm0.6\,$g$\,$cm$^{-3}$, consistent with an interior rocky core overlaid with a low-mass volatile envelope. Several attributes of the photometry for planet d (which we did not detect dynamically) complicate the analysis, but we vet the planet with high-contrast imaging, ground-based photometric follow-up and radial velocities. TOI-561 b is the first rocky world around a galactic thick-disk star confirmed with radial velocities and one of the best rocky planets for thermal emission studies.

• The paper confirms the discovery of a rocky ultra-short period planet, TOI-561 b, with precise measurements of its radius, mass, and density.
• The study employs combined TESS observations and high-resolution Keck HIRES spectroscopy to characterize the planetary system and its ancient thick-disk host star.
• The results challenge existing formation models by showing that tightly orbiting rocky planets can endure around a metal-poor, 10-billion-year-old star.

Analysis of the {\it TESS}-Keck Survey II: An Ultra-Short Period Rocky Planet and its Siblings

The paper presents the discovery and analysis of the TOI-561 exoplanetary system, identifying it as a unique object within our galaxy's thick-disk. This system hosts a remarkable rocky ultra-short period (USP) planet, TOI-561 b, and two additional planets, TOI-561 c and TOI-561 d. The discovery primarily involved observations from the TESS mission and subsequent follow-up with ground-based facilities including Keck Observatory's HIRES and others for high-resolution spectroscopy and imaging.

Key Findings

1. TOI-561 b Characteristics: The shortest-period planet, TOI-561 b, was confirmed to have a rocky composition, with a radius of approximately 1.45 R_⊕ and a mass of 3.2 M_⊕. Its density was measured at 5.5 g/cm³, supporting a rocky composition hypothesis. This numerical result refines the understanding of USP planets, supporting the presence of rocky bodies in these extreme orbital environments.
2. Chemical and Age Properties of TOI-561: The host star is distinguished by its kinematic and chemical association with the galactic thick disk, with low metallicity and enhanced alpha elements ([Fe/H]= -0.41, [α/Fe] = +0.23). This suggests a notably ancient age of approximately 10 billion years for the system, making it one of the oldest exoplanet systems with known rocky planets, challenging prevailing models of planetary formation in metal-poor environments.
3. System Dynamics and Stability: Radial velocity (RV) measurements confirm the presence of TOI-561 b and TOI-561 c, with the mass of the former determined as having a dense rocky structure, whereas TOI-561 c appears to possess a volatile envelope. TOI-561 d's status remains less certain due to aliasing in its transit period, although a possible orbital period of 16.3 days was considered.
4. Prospects for Atmospheric Studies: Their TSM and ESM calculations position TOI-561 b as a prime candidate for thermal emission studies, potentially illuminating atmospheric loss or composition. The system's properties make it an enticing target for JWST and other advanced spectroscopic missions aimed at probing exoplanet atmospheres.
5. Theoretical Implications: The characterization of TOI-561 b and its siblings enriches the discussion on the formation and evolutionary pathways of ancient planetary systems. Particularly, the persistence of such small, dense, highly irradiated planets over billions of years informs both theoretical models of planet migration and survival in harsh proximities to their stars.
6. Challenges and Limitations: The robustness of the TOI-561 d planet signature remains uncertain, manifesting complexities in data interpretation and underscoring the necessity for ongoing observational campaigns to refine planetary parameters and dynamics, especially within the context of USP planets and their broader system architectures.

Conclusion

TOI-561 exemplifies the diversity and complexity of exoplanetary systems, expanding the boundaries of known planetary formation environments to include ancient, metal-poor regions of the galaxy. The results underscore the efficacy of the TESS mission, complemented by ground-based facilities like Keck Observatory's HIRES, in uncovering the intricate dynamics and compositions of distant worlds. Future investigations, particularly those exploring atmospheric composition and structure, will be crucial in elucidating the evolutionary pathways of exoplanets around different types of stellar environments. This paper lays a foundation for theoretical advancements in understanding the longevity and chemical diversity of planetary systems.