TOI-2257 b

Lore & Background

TOI-2257 b was discovered via the transit method by TESS in November 2021, and it has one of the longest orbital periods of any TESS planet. Its existence is supported by photometry and high-resolution observations from ground-based telescopes, and it is considered unlikely to be a false positive. The planet's mass and density are not directly measured, but mass-radius relationships predict a mass roughly 3.4–10 times that of Earth, consistent with a Neptune-like world. Its average equilibrium temperature is 256 K, similar to the average temperature of Alert, Canada, but varies from about 193 K at aphelion to 373 K at perihelion due to its eccentric orbit. The planet's eccentricity is the highest recorded around an M-type star and the third highest of any known mini-Neptune as of 2021. The host star, TOI-2257, is an M3V star with a temperature of 3,430 K, a metallicity of -0.27, an age of about 8 billion years, and about 0.3 times the mass and 0.33 times the radius of the Sun.

Reader's Guide

TOI-2257 b's significance lies in its combination of a habitable-zone location and extreme orbital eccentricity, which challenges traditional models of planetary habitability. The planet receives 37/50ths of the sunlight Earth gets, placing it well within the habitable zone, but its eccentric orbit takes it through both the 'too hot' and 'too cold' zones briefly. Its equilibrium temperature could range from 317 K to 239 K, both within Earth's thermal amplitude, and with a greenhouse effect similar to Earth's, the temperature would be around 289 K. The planet is likely tidally locked if it had a circular orbit, but its high eccentricity suggests it may be in a 3:2 spin-orbit resonance instead, where the year is 1.5 times as long as the day. Models of such a planet, if oceanic, predict open water in lower and middle latitudes and sea ice above 60° latitude, a configuration known as the 'striped-ball planet.' The eccentricity could compromise habitability by triggering a runaway greenhouse or glaciation if the planet spends too long at extreme temperatures, but it may also work against tidal locking. Future observations, including from the JWST, aim to detect water vapor in its atmosphere and search for additional planets in the system.

Did You Know?

• TOI-2257 b has the highest orbital eccentricity ever recorded around an M-type star.
• The planet is likely in a 3:2 spin-orbit resonance rather than being tidally locked.
• It receives 37/50ths of the sunlight that Earth gets from the Sun.