Expanding Horizons in Exoplanetary Studies: Beyond Habitable Zones

Expanding the Concept of Habitable Zones

Scientists from the University of Birmingham and the University of Oxford have proposed expanding the astronomical concept of the ‘habitable zone’ by introducing a broader category-the ‘temperate zone’. Their work suggests that traditional criteria for habitability are insufficient for describing the growing diversity of exoplanets and are ill-suited for selecting objects for atmospheric studies. In this new classification, the ‘temperate zone’ is defined as the range of stellar radiation at which the surface energy flow on a planet ranges from 0.1 to 5 Earth values. For comparison, Earth’s solar constant-the amount of energy received per square meter at the top of the atmosphere-is about 1361 W/m², translating to approximately 124.7 BTUs/hr. Thus, the proposed range covers radiation levels from approximately 136 to 6805 W/m². This interval significantly broadens the classical ‘habitable zone’, limited by conditions for the existence of liquid water on the surface, allowing the inclusion of planets with moderate illumination, potentially suitable for studying their atmospheres.

Application in the TEMPOS Project

This approach was applied to data from the TEMPOS project (Temperate M Dwarf Planets With SPECULOOS), implemented as part of the SPECULOOS program aimed at searching for planets around ultracool stars. Observations led to the discovery of two exoplanets-TOI-6716 b and TOI-7384 b, both orbiting M-class red dwarfs. TOI-6716 b has a radius from 0.91 to 1.05 times Earth’s radius and is estimated to be a rocky planet. TOI-7384 b belongs to the sub-Neptune class; its radius ranges from 3.35 to 3.77 Earth radii, and it is believed to have a rocky core and a massive hydrogen-helium envelope. Neither of these planets falls even within the expanded version of the classical ‘habitable zone’ of their stars. However, the authors emphasize their value. TOI-6716 b is considered a promising target for observations using the James Webb Space Telescope (JWST) if it retains an atmosphere. The size and estimated mass of TOI-7384 b also make it a suitable object for atmospheric studies using JWST and future instruments.

Future Prospects with the James Webb Space Telescope

In the latest updates, the James Webb Space Telescope has continued to provide groundbreaking data on a wide array of celestial bodies, accentuating findings related to the atmospheric composition of distant planets such as these. The flexibility in classification brought by the introduction of the ‘temperate zone’ allows for a more nuanced approach to exoplanet classification and prioritization of observations, particularly in systems with low-mass stars. This paradigm shift aims to enhance the search for planetary atmospheres capable of supporting diverse forms of life, even in environments previously deemed unsuitable.