Paper Title: The time evolution of the ultraviolet habitable zone
Authors: R. Spinelli, F. Borsa, Ghirlanda, G. Ghisellini, F. Haardt, and F. Rigamonti
First-author institution: Palermo Astronomical Observatory, Palermo, Italy
Status: Published in MNRAS Letters, September 2024
The search for extrasolar life is a multidisciplinary endeavor, pulling together fields like astrophysics, chemistry, and biology. This is especially true when analyzing the impact a central star will have on a potentially life-hosting exoplanet. One such impact is the possibility for liquid water to exist on the surface of the planet. In the case of life-as-we-know-it, liquid water is non-negotiable when it comes to habitability, so much so that when we describe the region around a star where it is possible for liquid water to exist, we call it the “circumstellar habitable zone” (CHZ), or sometimes just “habitable zone”. Unfortunately, this nickname is something of a misnomer, as liquid water is not the only requirement to sustain life as-we-know-it. Today’s authors take a deeper look into another habitability issue: UV radiation.
Previously, the authors of today’s paper have outlined the role high-energy radiation around an exoplanet has on the possible formation of life. In particular, they highlight the delicate balance between too much and too little UV radiation. On the one hand, UV is necessary to create the building blocks for life-as-we-know-it. On the other, an excess of UV radiation can evaporate atmospheres, destroy biomolecules, and induce apoptosis in cellular life (this is what happens when you get a sunburn!) The region around a star where abiogenesis can occur without destroying a possible life-hosting environment is called the ultraviolet habitable zone (UHZ). A planet capable of hosting life as-we-know-it, therefore, must sit (or have once sat) in both the CHZ and UHZ simultaneously.
The catch is that stellar evolution causes the locations of the CHZ and UHZ to change over time. This is the primary focus of today’s paper. The authors pull a small sample of well-known M-dwarf and K-type stars which have been observed to host planetary systems containing rocky planets. M-dwarfs are especially important for this, as they are the most common type of star we observe, and the ones around which we observe the most rocky CHZ exoplanets. In particular, they prioritize sources which have age estimates (or lower bounds) in the literature, masses less than 0.9 times the mass of the sun, and have been observed in the near ultraviolet (NUV) by the Swift telescope. They compile a final list of 14 sources.
For each M-dwarf in the sample, the authors use the MESA Isochrones and Stellar Tracks (MIST) code to obtain luminosity and effective temperature estimates at different times in the star’s evolution. From these, they are able to obtain estimates for the locations of each star’s CHZ boundaries as a function of stellar age.
To determine the time-evolution of the UHZ, the authors make use of a previous study, which outlines the high-energy evolution of a large number of M- and K-type stars in young moving groups and clusters, as well as a number of very nearby field stars. These sources were divided into groups based on star type (primarily indicated by mass) and age, and, using GALEX observations, were able to obtain median NUV fluxes for each group. From this work, the authors of today’s paper were able to scale the GALEX fluxes from the large study and the Swift fluxes from their 14-source sample for direct comparison. From here, they are able to estimate the time-evolution of the NUV emission and, from their previous work, calculated the extent of the UHZ with time, for three different values of theoretical atmospheric transmission. The results of the CHZ and UHZ calculations are visualized in Fig. 1.
They find that for many of the relatively cool M-dwarf stars, the present-day CHZ and UHZ are not overlapping. In comparison, for the two hotter K-stars in the sample (Kepler-62 and HD 40307), the CHZ and UHZ almost entirely overlap for atmospheric transmissions of 100% and 50%.
Current understandings of abiogenesis, however, tell us that prebiotic molecules take a long time to form in sufficient amounts. Therefore, the duration of overlap of the habitable zones is a more significant factor than the extent. Today’s study finds that for typical late M-dwarfs, the habitable zones will have a 25% overlap for 1.3, 0.5, and 0.03 billion years for 100, 50, and 10% atmospheric NUV transmission, respectively. The durations are 2.2, 1.2, and 0.2 billion years for early M-dwarfs, and 28, 20, and 0.5 billion years for K-type stars. They note, however, that this calculation neglects the possibility that the atmospheric transmission may change over time.
Comparing with our population of observed rocky exoplanets, today’s authors find that none of the existing CHZ planets around M-dwarfs are presently receiving enough NUV radiation to create prebiotic molecules. They conclude, based on the relatively short duration of CHZ-UHZ overlap, that M-dwarf stars may not emit sufficient high-energy radiation to trigger the onset of life-as-we-know-it. This represents a strong departure from CHZ-exclusive studies of planet habitability, which have previously primarily focused on these cool, common stars.
Astrobite edited by: Maria Vincent
Featured image credit: NASA/JPL-Caltech
Discover more from astrobites
Subscribe to get the latest posts to your email.
