Two new exoplanets highlight need for updated habitable zone definitions

A new paper reports two temperate planets orbiting fully convective mid-type M dwarfs and proposes a broader “temperate” zone defined by instellation flux between 0.1 and 5 times Earth’s solar constant (about 136–6,805 W/m²).

Astronomers are refining how they describe planets that receive moderate stellar energy as discoveries accrue. A new paper submitted to Monthly Notices of the Royal Astronomical Society introduces an expanded “temperate” zone and presents two new temperate planets around fully convective, mid-type M dwarfs. The work is presented alongside the TEMPOS survey, which links SPECULOOS ground-based follow-up with space data to measure precise radii for temperate planets around late M stars. The authors frame this as a step toward building a catalog of temperate M-dwarf planets for future atmospheric study. Key points: - The paper defines a temperate zone by instellation flux between 0.1 and 5 times Earth’s solar constant, which translates to roughly 136–6,805 W/m² at the top of a planet’s atmosphere. - The temperate zone is intentionally broader than the conservative habitable zone and is meant to capture planets that receive moderate levels of stellar energy rather than strictly those guaranteed to allow surface liquid water. - The study introduces two temperate planets: TOI-6716 b (about 0.91–1.05 Earth radii, most likely rocky) and TOI-7384 b (about 3.35–3.77 Earth radii, a sub-Neptune that may have a H/He envelope). - The planets orbit mid-type, fully convective M dwarfs and are described as occupying a sparsely populated region of temperate-planet parameter space; the authors note neither falls within even the optimistic habitable zones of their stars. - The paper presents TEMPOS (Temperate M Dwarf Planets With SPECULOOS) as a survey to produce precise radii for temperate planets transiting mid-to-late M dwarfs, using SPECULOOS ground-based observations alongside TESS. - The authors report that TOI-6716 b has a predicted transmission spectroscopy metric comparable to the TRAPPIST-1 planets and that TOI-7384 b’s size and predicted mass make it of interest for atmospheric characterization with JWST and other upcoming facilities. Summary: The paper expands the language used to describe planets receiving moderate stellar flux and adds two temperate planets around fully convective M dwarfs to the catalog of detected worlds. The authors position these discoveries within the TEMPOS program and note these planets as targets for future atmospheric observations with instruments such as JWST; detailed follow-up and catalog-building are described as the next steps in their research agenda.