An article published in “The Astrophysical Journal” reports the results of a study on the habitable zone that goes beyond the so-called conservative zone because it’s based on rigid assumptions. Astrophysicist Amri Wandel of the Hebrew University of Jerusalem focused on examining the conditions existing in systems of low-mass stars: orange dwarfs (K-class stars) and red dwarfs (M-class stars). The study specifically considered planets tidally locked to their stars.
Amri Wandel conducted an analysis using climate models that account for global heat transport, the greenhouse effect, and albedo. This led him to conclude that these stars may host planets potentially habitable for Earth-like life forms orbiting outside the conservative habitable zone.
The image (Courtesy Amri Wandel, APJ, 2026. All rights reserved) contains a graph with the conservative habitable zone illustrated by the orange band and ellipses illustrating the extended habitable zone proposed by this study.
Astronomers and astrobiologists have sought to define the boundaries of a star system’s habitable zone with increasing precision. Within that zone, a planet with an Earth-like atmosphere may have liquid water on its surface. The width of the so-called conservative habitable zone is calculated according to rigid parameters, but there’s some debate on the subject. A further difficulty is posed by the possible environmental conditions on a planet’s surface: perhaps, both Venus and Mars had habitable potential early in their existence, but conditions have subsequently changed significantly on both planets.
In systems of low-mass stars, the habitable zone is close to them because a planet must be close to it to receive enough energy to make it habitable. However, the consequence is that these planets are tidally locked to their star and always show the same face to it, just like the Moon does to Earth. According to Amri Wandel, these conditions require specific testing to assess their habitability.
Astronomers and astrobiologists previously thought that planets tidally locked to a low-mass star would have a dayside too hot and a nightside too cold to develop Earth-like life forms. Amri Wandel has applied climate models that indicate that an atmosphere not very efficient at transporting heat could allow the nightside to maintain temperatures sufficient for liquid water to exist on the surface.
One of the parameters used to estimate the habitable zone of a star system is the wet greenhouse effect limit. In estimating the internal boundary, it’s the limit beyond which a planet’s surface temperatures would be so high as to cause a quick and irreversible loss of water into space through evaporation. According to simulations conducted by Amri Wandel, if around 30% of the heat received from the star is transported to the night side of a planet, water can remain liquid on the surface or, in some cases, underground. This means that the habitable zone’s inner boundary is closer to its star than the conservative one.
Amri Wandel mentioned observations conducted with the James Webb Space Telescope of the exoplanets GJ 486 b and TOI 270 d, which showed traces of water vapor. These two exoplanets are closer to their stars than the inner boundary of their systems’ conservative habitable zone and orbit red dwarfs. These observations still require confirmation that water vapor is indeed present on these exoplanets, but they certainly deserve follow-up observations.
In estimating the outer boundary of the habitable zone, Amri Wandel believes that heating processes, which can be geological, generated by the decay of radioactive elements, or tidal forces exerted by other celestial bodies in the system, could be sufficient to maintain surface temperatures high enough to have liquid water.
In the solar system, there are several cases of moons and even dwarf planets where there are subsurface oceans of liquid water, or at least they existed in the past. Sometimes, such a situation may only be temporary, but they are still interesting cases, such as that of Ceres, where it was theorized years ago that liquid water might have existed in this dwarf planet’s subsurface.
Amri Wandel’s study shows that the search for extraterrestrial life shouldn’t be too limiting, even when the goal is Earth-like life. The ongoing discovery and study of new exoplanets provides new information that will allow to refine the models underlying this type of study. These are complex models because they must take into account many factors, including red dwarf activity, which can be destructive to planet atmospheres.
