The First Habitable Zone Earth-sized Planet from TESS. I: Validation of the TOI-700 System

Abstract: We present the discovery and validation of a three-planet system orbiting the nearby (31.1 pc) M2 dwarf star TOI-700 (TIC 150428135). TOI-700 lies in the TESS continuous viewing zone in the Southern Ecliptic Hemisphere; observations spanning 11 sectors reveal three planets with radii ranging from 1 R$\oplus$ to 2.6 R$\oplus$ and orbital periods ranging from 9.98 to 37.43 days. Ground-based follow-up combined with diagnostic vetting and validation tests enable us to rule out common astrophysical false-positive scenarios and validate the system of planets. The outermost planet, TOI-700 d, has a radius of $1.19\pm0.11$ R$\oplus$ and resides in the conservative habitable zone of its host star, where it receives a flux from its star that is approximately 86% of the Earth's insolation. In contrast to some other low-mass stars that host Earth-sized planets in their habitable zones, TOI-700 exhibits low levels of stellar activity, presenting a valuable opportunity to study potentially-rocky planets over a wide range of conditions affecting atmospheric escape. While atmospheric characterization of TOI-700 d with the James Webb Space Telescope (JWST) will be challenging, the larger sub-Neptune, TOI-700 c (R = 2.63 R$\oplus$), will be an excellent target for JWST and beyond. TESS is scheduled to return to the Southern Hemisphere and observe TOI-700 for an additional 11 sectors in its extended mission, which should provide further constraints on the known planet parameters and searches for additional planets and transit timing variations in the system.

• The paper validates the TOI-700 system by confirming three planets, with the outer Earth-sized planet located in the habitable zone.
• It employs TESS data and ground-based follow-up to determine orbital periods (9.98–37.43 days) and rule out false positives.
• The findings provide a basis for future research on planetary formation, atmospheric dynamics, and habitability around low-activity M dwarfs.

An Overview of the Validation of the TOI-700 Planetary System

The study titled "The First Habitable Zone Earth-sized Planet from TESS. I: Validation of the TOI-700 System" presents a detailed analysis and validation of a three-planet system identified around the M2 dwarf star TOI-700. This discovery, based on observations from the Transiting Exoplanet Survey Satellite (TESS), highlights an Earth-sized planet residing within the star's habitable zone—an area where conditions could potentially support liquid water.

Key Findings

The study identified three planets orbiting TOI-700, located at a distance of 31.1 parsecs from Earth. These planets exhibit radii ranging from 1 to 2.6 times that of Earth and possess orbital periods spanning 9.98 to 37.43 days. Ground-based observations paired with diagnostic vetting established the legitimacy of these planetary bodies, effectively negating common false-positive scenarios.

The outermost planet, TOI-700 d, is of particular interest. With a size approximately 1.19 times Earth's radius, it inhabits the star's conservative habitable zone—a region receiving approximately 86% of Earth's insolation. Remarkably, TOI-700 displays low stellar activity, making it a particularly intriguing candidate for analyzing potentially rocky planets, especially in terms of atmospheric escape mechanisms. While the James Webb Space Telescope (JWST) may face challenges in characterizing TOI-700 d's atmosphere, the system's sub-Neptune planet, TOI-700 c, presents a superior target for such studies.

Implications and Potential for Further Research

The systematic validation of the TOI-700 system has meaningful implications for our understanding of planetary formation and atmospheric dynamics, particularly in systems orbiting cool stars. The TOI-700 system's architectural uniqueness, featuring a variance in planet size and type, enriches ongoing discussions around planetary evolution, migration patterns, and atmospheric retention influenced by proximity to the parent star.

The paper outlines the system's potential for future observational campaigns, with TESS scheduled to re-visit the Southern Hemisphere, thus providing additional observation opportunities and refinement of planetary parameters. The findings could guide future studies focusing on transit timing variations (TTVs) and additional possible planet detections.

Conclusion

The validated TOI-700 planetary system, particularly the Earth-sized TOI-700 d in the habitable zone, expands our catalog of exoplanets suitable for habitability studies. As TESS and potentially the JWST continue to probe these planetary environments, the study underscores the scientific potential of low-active M dwarfs as prime subjects for evaluating conditions akin to those necessary for life's proliferation. Further investigations might elucidate intricate atmospheric characteristics and enrich models of planetary system evolution.