Are We Redefining Habitability? Two New Exoplanets Challenge Everything We Thought We Knew
At the dawn of the exoplanet era, astronomical goals were relatively straightforward. Researchers first aimed to discover as many planets beyond our Solar System as possible, in order to build a statistical understanding of planetary populations. The second priority was equally clear: to determine whether any of these worlds resided within the so-called habitable zone of their host stars.
At that time, the definition of a habitable zone was intentionally simple. Any planet orbiting at a distance where liquid water could exist on its surface was considered potentially habitable. Yet as thousands of exoplanets have since been discovered, this once-useful simplicity has begun to show its limitations.
Can habitability really be reduced to distance alone?
From Classical Habitable Zones to Optimistic and Conservative Definitions
As exoplanet diversity expanded, scientists were compelled to refine their assumptions. Habitability is now commonly discussed in terms of optimistic and conservative habitable zones, each accounting for different planetary and stellar complexities.
The optimistic habitable zone is broader by design. It incorporates factors such as geothermal heating, which could extend the outer edge of habitability, and planetary rotation or cloud feedback mechanisms that might prevent runaway greenhouse effects closer to the star.
In contrast, the conservative habitable zone is more restrictive. Its inner boundary is defined by the onset of extreme greenhouse conditions, while its outer edge is limited by the condensation of carbon dioxide from the atmosphere, which could deprive a planet of sufficient warming to sustain liquid surface water.
But even these refined categories may no longer be enough.
Introducing the Temperate Zone: A New Framework for Exoplanet Habitability
New research published in Monthly Notices of the Royal Astronomical Society adds another layer of nuance by introducing a formally defined temperate zone. The study, titled “Two temperate Earth- and Neptune-sized planets orbiting fully convective M dwarfs,” argues that habitability should be discussed along a broader physical continuum.
“As the diversity of exoplanets continues to grow, it is important to revisit assumptions about habitability and classical HZ definitions,” the authors write.
While the term temperate has appeared in earlier studies, it was often left undefined or used interchangeably with habitable. In this work, however, the concept is explicitly quantified.
Instellation Flux and the Physical Meaning of the Temperate Zone
The researchers define the temperate zone using instellation flux, also known as insolation flux—the amount of stellar energy reaching a planet’s surface. Specifically, the temperate zone is defined as:
0.1 < S/S⊕ < 5
This range corresponds to stellar energy levels between approximately 136 watts per square meter and 6,805 watts per square meter, relative to Earth’s solar constant of about 1,361 watts per square meter at the top of the atmosphere.
Rather than focusing exclusively on surface liquid water, the temperate zone categorizes planets that receive moderate levels of stellar energy. This deliberately broader framework captures worlds that may fall outside classical habitable zones, yet still possess atmospheres or thermal conditions worthy of study.
Is it possible that some of the most scientifically valuable worlds lie just beyond traditional definitions?
The TEMPOS Survey and the Role of M Dwarf Stars in Exoplanet Discovery
The paper also serves as an introduction to TEMPOS—Temperate M Dwarf Planets With SPECULOOS. SPECULOOS, short for Search for habitable Planets EClipsing ULtra-cOOl Stars, is a ground-based survey designed to find Earth-sized planets orbiting small, cool stars.
TEMPOS builds on this effort by producing a catalog of precise planetary radii for temperate exoplanets transiting mid- to late-type M dwarfs, also known as red dwarfs.
These stars are particularly valuable targets. Due to their low effective temperatures—often below three thousand four hundred kelvin—temperate planets orbit closer in, making transits more frequent and easier to detect.
“Planet equilibrium temperature scales with both semimajor axis and stellar effective temperature,” the authors explain, highlighting why M dwarfs dominate current temperate planet discoveries.
TOI-6716 b and TOI-7384 b: Two Temperate Planets in a Sparse Parameter Space
Within this refined framework, the study introduces two newly confirmed temperate exoplanets: TOI-6716 b and TOI-7384 b.
TOI-6716 b is an Earth-sized planet, measuring between approximately zero point nine one and one point zero five Earth radii, making it likely rocky in composition. TOI-7384 b, by contrast, is a sub-Neptune, with a radius between three point three five and three point seven seven Earth radii, possibly consisting of a rocky core enveloped by a thick hydrogen-helium atmosphere.
Notably, neither planet resides within even the optimistic habitable zone of its star. Yet their placement near the inner edge of the temperate zone makes them scientifically compelling.
“These planets populate an otherwise sparse region of temperate planet parameter space,” the authors note, opening new opportunities as definitions of habitability continue to evolve.
Why Temperate Exoplanets Matter for Future Atmospheric Studies
The ultimate objective of identifying temperate exoplanets is not classification alone, but atmospheric characterization.
TOI-6716 b, in particular, exhibits a predicted Transmission Spectroscopy Metric comparable to the planets of the TRAPPIST-1 system. This makes it a promising target for the James Webb Space Telescope, provided it has retained an atmosphere.
Similarly, TOI-7384 b’s size and estimated mass render it suitable for atmospheric analysis with JWST and next-generation observatories.
What chemical signatures might these atmospheres hold—and what could they reveal about planetary evolution beyond our Solar System?
Redefining Habitability in an Expanding Exoplanet Universe
Taken together, these discoveries underscore the power of combining TESS observations with coordinated ground-based surveys. More importantly, they highlight a shift in thinking: habitability is no longer a binary condition, but a spectrum.
“Together these discoveries show the power of building a catalog of temperate planets around fully convective M dwarfs for atmospheric studies in the coming decade,” the researchers conclude.
As observational capabilities advance and definitions broaden, one question remains open—and increasingly urgent:
Are we ready to recognize life-friendly worlds that do not look like Earth at all?
Source: Are We Redefining Habitability? Two New Exoplanets Challenge Everything We Thought We Knew
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Are We Redefining Habitability? Two New Exoplanets Challenge Everything We Thought We Knew