The occurrence of small, short-period planets younger than 200 Myr with TESS

Abstract: Within the first few hundreds of millions of years, many physical processes sculpt the eventual properties of young planets. NASA's TESS mission has surveyed young stellar associations across the entire sky for transiting planets providing glimpses into the various stages of planetary evolution. Using our own detection pipeline, we search a magnitude-limited sample of 7219 young stars ($\leq$200 Myr) observed in the first four years of TESS for small (2-8 R$\oplus$), short period (1.6-20 days) transiting planets. The completeness of our survey is characterized by a series of injection and recovery simulations. Our analysis of TESS 2-minute cadence and Full Frame Image (FFI) light curves recover all known TOIs, as well as four new planet candidates not previously identified as TOIs. We derive an occurrence rate of $35^{+13}{-10}$% for mini-Neptunes and $27^{+10}_{-8}$% for super-Neptunes from the 2-minute cadence data, and $22^{+8.6}_{-6.8}$% for mini-Neptunes and $13^{+3.9}_{-4.9}$% for super-Neptunes from FFI data. To independently validate our results, we compare our survey yield with the predicted planet yield assuming Kepler planet statistics. We consistently find a mild increase in the occurrence of super-Neptunes and a significant increase in the occurrence of Neptune-sized planets with orbital periods of 6.2-12 days when compared to their mature counterparts. The young planet distribution from our study is most consistent with evolution models describing the early contraction of hydrogen-dominated atmospheres undergoing atmospheric escape and inconsistent with heavier atmosphere models offering only mild radial contraction early on.

• The paper presents a custom detection pipeline applied to 7,219 young stars, revealing the occurrence rates of mini- and super-Neptunes.
• Key results show occurrence rates of 35% (mini-Neptunes) and 27% (super-Neptunes) with varying cadence, highlighting distinct evolutionary trends.
• The findings support theories of rapid atmospheric contraction and photoevaporation, offering new insights into the early dynamical evolution of planetary systems.

The Occurrence of Young Exoplanets with TESS: Analysis and Implications

The investigated paper explores the occurrence rates of small, short-period exoplanets (ranging from 2 to 8 Earth radii, with orbital periods between 1.6 and 20 days) around stars younger than 200 million years (Myr) using data from the Transiting Exoplanet Survey Satellite (TESS). The study spans multiple disciplines within astrophysics, specifically focusing on the evolutionary processes affecting planetary systems in their formative stages.

Methods and Analysis

Researchers employed a bespoke detection pipeline on a magnitude-limited sample of 7,219 young stars identified by TESS over its first four years of operation. This pipeline effectively recaptured all known TESS Objects of Interest (TOIs) and identified four new planet candidates. The completeness of the survey was rigorously quantified through injection and recovery simulations, ensuring the robustness of their detection capabilities.

The occurrence rates derived from direct TESS observations were double-checked against prediction models that incorporated Kepler mission statistics, thereby bridging TESS's current observations with previous discoveries.

Results

The analysis revealed notable findings:

• Occurrence Rates: For mini-Neptunes, occurrence rates were calculated as 35% from 2-minute cadence data and 22% from full-frame images (FFIs). Similarly, super-Neptunes showed occurrence rates of 27% and 13%, respectively, indicating a mild uptick in young planetary populations compared to mature systems.
• Period Distribution: A significant discovery was the increased occurrence of Neptune-sized planets with orbital periods between 6.2 and 12 days among young stars. Such a distribution contrasts with mature counterparts, suggesting dynamically different initial conditions or evolutionary pathways.

Theoretical Implications

The elevated occurrence rates, particularly of Neptune-sized planets, provide empirical support for theoretical models that postulate faster atmospheric evolution or greater thermal contraction in young planetary systems. The results align with models of early evolution characterized by the rapid contraction of hydrogen-dominated atmospheres and atmospheric escape, challenging older models suggesting only mild radial contraction.

These findings could suggest that the early environment and dynamical interactions in young star clusters play a significant role in shaping planetary characteristics distinctly from their mature evolutionary states. The variety of mechanisms considered—such as photoevaporation, tidal interactions, and disk-driven migration—affect the observable characteristics of planets throughout their lifecycle.

Practical Implications and Future Directions

Practically, understanding these mechanisms can influence how we prioritize targets for atmospheric characterization, potentially optimizing observational resources to track atmospheric evolution. The study also lays groundwork for future missions that may test these initial conditions by observing atmospheric chemistry or structure directly.

Speculatively, as AI-driven analysis techniques improve, we might expect enhanced precision in identifying the subtle interactions at play within these young systems.

Conclusion

The paper represents a significant empirical contribution to our understanding of young exoplanet populations, highlighting distinct evolutionary trends. These insights contribute towards a broader comprehension of planet formation processes and the subsequent evolution pathways, which are crucial for developing accurate models of planetary systems within and beyond our solar vicinity. The work emphasizes the dynamic nature of planetary evolution and the richness of data still to be explored and understood with ongoing and future missions.