Identifying habitable (and possibly inhabited) planets around other stars is one of the greatest long-term goals of current exoplanet surveys. If one is to find Earth-like exoplanets in the habitable zones (HZs), one needs to know at what distance they should be found from their parent star. Most of the HZ limits that we see in the exoplanet literature were based on a 1-D climate model initially developed by Jim Kasting at Penn State. Jim and his collaborators published a seminal paper in 1993 about Habitable Zones around main-sequence stars (stars that fuse hydrogen into helium in their core, like our sun). According to that paper, the inner edge of the HZ is at 0.95 AU, and the outer edge is at 1.67 AU. (AU stands for “astronomical unit”, and 1 AU equals the mean Earth-sun distance.)
For two decades, the Kasting et al. (1993) results were the prime source for HZ estimates. In 2013, I (along with other collaborators including Jim) published revised HZ estimates, updating Jim’s old climate model. At the time Jim published his paper, no exoplanet was discovered around any Main-sequence star. People were not sure if Earth-size/mass planets even existed. Two decades later several observational surveys detected several super-Earth sized planets, indicating that small planets do exist and, in fact, are probably more common than larger planets. With this in mind, we set to re-calculate HZs. We updated several things in the climate model, but the most important one was how strong water and carbon dioxide absorb infra-red (IR) radiation. As you may know, infra-red radiation is extremely important for the greenhouse budget of a planet. The stronger the absorption of IR, the greater the greenhouse effect on a planet.
We considered an Earth-mass planet around different kinds of stars, and derived HZ limits. To our surprise, we found that the inner edge of the HZ (i.e, how close to a star one can push a planet before all the water on the surface is evaporated) is at 0.99 AU! (and the outer edge was almost the same as Jim’s old result: 1.70 AU). Remember that, by definition, Earth is at 1 AU. So this implies that we are just a step away from being un-inhabitable. There is a caveat: Our 1-D model can not model clouds, and water clouds can cool a planet by increasing the planet’s albedo (how much sunlight is reflected to space). A higher albedo means more sunlight is reflected, potentially cooling the surface of a planet. So, our 0.99 AU inner edge is a pessimistic limit. And that is ok. We want to be conservative in our HZ estimates, so that we don’t over count the number of potentially habitable planets in our Galaxy.
After we published this paper, several researchers in the exoplanet community asked us what are the HZ limits for super-Earth mass planets? or sub-Earth mass planets? That is the paper I will explain briefly here. To perform this calculation, we assumed that the background Nitrogen pressure scales with the size (or mass) of a planet. Meaning larger mass planets have more N2 (they acquire more volatiles during the formation). For the inner edge of the HZ, We found that the amount of N2 in the atmosphere just delays when the planet loses the surface water into the atmosphere, but does not affect the domination of water vapor to the greenhouse effect. In essence, more N2 acts as a light-scattering gas, increasing the albedo, to an extent. But as the planet warms more and more (as we push it closer to the star to find the inner edge), water-vapor completely dominates the atmosphere, and any scattering of light by N2 becomes negligible.
What actually matters is the gravity of the planet. A lower mass planet has less gravity. So compared to a larger mass planet, it has more water-vapor at a given height (it is puffy). When there is more water-vapor in the atmosphere, it absorbs more IR radiation, increasing the greenhouse effect. Hence, one can not push a lower mass planet as close to a star as a higher-mass planet (because as the Star’s radiation increases, more heat is absorbed and the low mass planet reaches it’s inner edge sooner than a high-mass planet). The outer edge of the HZ does not change much at all due to competing effects of albedo and the greenhouse effect of carbon-dioxide atmosphere. That is why the width of the HZ for a low mass planet is small compared to a high-mass planet. And that is the essence of our paper.