TOI-1685 b

Lore & Background

TOI-1685 b was discovered in 2021 via the transit method, with two discovery papers—Bluhm et al. 2021 and Hirano et al. 2021—reporting differing characteristics. Bluhm et al. gave a mass of about 3.78 Earths and radius of 1.70 Earths, while Hirano et al. reported 3.43 Earths and 2.0 Earths. Updated analysis in Burt et al. 2024 revised these values downward to a mass of 3.03 ±0.33 Earths and radius of 1.468 ±0.050 Earths, yielding a density of 5.3 ±0.8 g/cm³, similar to Earth's 5.51 g/cm³. This Earth-like density suggests a terrestrial composition. Initially considered possibly a water world (50% water, 50% enstatite), later observations from the James Webb Space Telescope (JWST) during its second cycle indicate it is likely a hot, barren world with negligible volatiles. Hydrogen-dominated atmospheres are ruled out; any primordial hydrogen or helium appears lost. Possible secondary atmospheres include pure heavy gases like CO₂, SO₂, H₂O, or CH₄, or a mixed atmosphere of CO, CO₂, and SO₂. Pure methane is unlikely, and CO₂- or water-dominated atmospheres would require high-altitude clouds or thin cloud-free conditions. Formation of such an ultra-short period planet in situ is deemed unlikely because dust particles would not sublimate so close to the star. TOI-1685 b likely migrated to its current position, possibly through dynamical interactions with other planets, though no additional planets are confirmed. Another scenario—that it is the remnant core of a hot Jupiter or hot Saturn—lacks supporting evidence, as the star shows no metallicity abnormalities.

Reader's Guide

TOI-1685 b holds significance as a benchmark ultra-short period super-Earth whose physical parameters were refined through multiple studies, resolving initial discrepancies between discovery papers. Its Earth-like density, despite extreme proximity to its host star, makes it a key example of a terrestrial planet that has retained a rocky composition rather than becoming a water world. The ruling out of a hydrogen-dominated atmosphere and the constraints on possible secondary atmospheres provide a foundation for understanding atmospheric escape and retention on close-in planets. The planet's formation history—likely involving migration rather than in-situ formation—adds to the growing evidence that many ultra-short period planets arrive at their orbits through dynamical processes. The lack of detected additional planets in the system and the absence of stellar metallicity anomalies challenge the hypothesis that such planets are stripped cores of giant planets. Observations from JWST further solidify its classification as a barren, volatile-poor world, offering a clear contrast to planets with thick atmospheres. TOI-1685 b thus serves as a reference point for studies of terrestrial planet evolution, atmospheric characterization, and orbital migration in compact systems.

Did You Know?

• TOI-1685 b orbits its star every 0.69 days, making it an ultra-short period planet.
• Its density of 5.3 g/cm³ is similar to Earth's density of 5.51 g/cm³.
• Initial mass estimates ranged from 3.43 to 3.78 Earths, later revised to 3.03 Earths.
• JWST observations during its second cycle helped rule out a water-world composition.