Gliese 12 b, A Temperate Earth-sized Planet at 12 Parsecs Discovered with TESS and CHEOPS (2405.13118v1)

Abstract: We report on the discovery of Gliese 12 b, the nearest transiting temperate, Earth-sized planet found to date. Gliese 12 is a bright ($V=12.6$ mag, $K=7.8$ mag) metal-poor M4V star only $12.162\pm0.005$ pc away from the Solar System with one of the lowest stellar activity levels known for an M-dwarf. A planet candidate was detected by TESS based on only 3 transits in sectors 42, 43, and 57, with an ambiguity in the orbital period due to observational gaps. We performed follow-up transit observations with CHEOPS and ground-based photometry with MINERVA-Australis, SPECULOOS, and Purple Mountain Observatory, as well as further TESS observations in sector 70. We statistically validate Gliese 12 b as a planet with an orbital period of $12.76144\pm0.00006$ days and a radius of $1.0\pm{0.1}$ R$_\oplus$, resulting in an equilibrium temperature of $\sim$315K. Gliese 12 b has excellent future prospects for precise mass measurement, which may inform how planetary internal structure is affected by the stellar compositional environment. Gliese 12 b also represents one of the best targets to study whether Earth-like planets orbiting cool stars can retain their atmospheres, a crucial step to advance our understanding of habitability on Earth and across the Galaxy.

• The paper reports the discovery of Gliese 12 b, a temperate Earth-sized exoplanet with a radius of 1.0 Earth radii, an equilibrium temperature of 315 K, and a 12.76-day orbital period.
• Its proximity at 12 parsecs and inactive M4V host star make Gliese 12 b an exceptional target for atmospheric characterization using telescopes like JWST.
• Gliese 12 b offers insights into planet formation in low-metallicity environments and atmospheric retention near the inner edge of the habitable zone around M dwarfs.

Overview of the Discovery and Implications of Gliese 12 b

The paper documents the discovery and validation of Gliese 12 b, a temperate Earth-sized exoplanet orbiting the nearby M dwarf star Gliese 12. Utilizing data from several observational campaigns, including NASA's TESS mission and the ESA's CHEOPS telescope, the authors present a compelling case for the authenticity and importance of Gliese 12 b within the field of exoplanetary science.

Discovery and Characterization

Gliese 12 b was identified based on transit signals initially discovered by TESS. However, initial observations presented ambiguity concerning the planet's orbital period, suggesting either a 12.76-day or 25.52-day period due to gaps in the TESS data. Subsequent observations by CHEOPS and additional ground-based observations with MINERVA, SPECULOOS, and PMO photometers resolved this period ambiguity, confirming an orbital period of precisely 12.76144 days. With a radius measured at $1.0 \pm 0.1$ Earth radii and an equilibrium temperature of approximately 315 K, Gliese 12 b is classified as a terrestrial planet. Notably, the host star, Gliese 12, is a relatively inactive M4V star, which enhances the prospects for atmospheric characterization by reducing noise due to stellar variability.

Significance in Exoplanetary Science

From an observational standpoint, Gliese 12 b is exceptionally well-positioned for detailed follow-up investigations. Its proximity at just 12 parsecs from Earth and the low stellar activity of its host star make it an ideal candidate for atmospheric analysis using spectroscopy, including the capabilities provided by the James Webb Space Telescope (JWST). The low activity levels significantly mitigate challenges associated with stellar contamination that have obscured atmospheric features on other exoplanets orbiting active M dwarfs.

Theoretical Implications

The presence of Gliese 12 b invites significant theoretical investigations regarding planet formation and evolution under a low-metallicity environment, a subject that remains sparsely explored due to a scarcity of suitable targets. The potential to precisely measure its mass with high-precision spectroscopic instruments like HARPS-N could allow for inferences about its internal structure, thus contributing data to address the hypothesized correlation between the composition of host stars and the density of terrestrial planets.

Furthermore, Gliese 12 b's position near its star's inner habitable zone boundary offers invaluable insights into the atmospheric retention capabilities of Earth-sized exoplanets around M dwarfs. The outcomes of such studies would enhance understanding of atmospheric erosion processes and their implications for habitability, as habitable conditions critically depend on the possession of a stable atmosphere over geological timescales.

Prospects for Further Exploration

In light of its favorable characteristics, Gliese 12 b is a prime target for future missions aiming to understand the atmospheric compositions of planets in similar stellar environments. It provides an invaluable testbed to paper the effects of a stellar environment on small, rocky exoplanets and to validate climate models predicting atmospheric dynamics and potential habitability on such bodies.

Ongoing and future observations focusing on mass measurements and atmospheric characterization could make Gliese 12 b a cornerstone in the paper of terrestrial planets around low-mass stars, offering tangible contributions to the broader endeavor of assessing the habitability of worlds beyond our Solar System.