Which planets are most likely to have surface water?
An Astrobiology Thought Experiment
This thought experiment is presented by Distinguished Professor of Astronomy Chris Impey in his University of Arizona Coursera course, Astrobiology: Exploring Other Worlds. In this exercise, a team of astrobiologists is investigating three exoplanets. They are trying to determine which is most likely to have liquid surface water.
All three exoplanets have been categorized as “Earth-like” and orbit their host stars at a distance of 1 AU, the same as Earth. However, each planet orbits a different type of star. Exoplanet-1 orbits an A-class star, exoplanet-2 orbits a G-class star, and exoplanet-3 orbits an M-class star. To determine if liquid water may be found on the surface of any of these worlds, we must consider where their respective habitable zones are located.
The habitable zone is an oversimplified term that identifies the range of distance a planet can be from its host star where it will retain liquid water on its surface. In other words, the habitable zone is the range of distance suitable for life as we know it. Within the habitable zone, the surface temperature of a planet is between 0 ℃, the freezing point of water, and 100 ℃, the boiling point of water.
The range of the habitable zone has a proportional relationship to the mass of the star. As the star’s mass increases, consequently increasing its luminosity and temperature due to higher exerted gravitational forces, the distance a planet must be from that star to hold liquid water on its surface also increases. The opposite is true as well; lower-mass stars have closer and more narrow habitable zones.
Stars are categorized by their spectral type, which indicates their mass. At one end of the spectrum lie M-class stars, which have the lowest mass, lowest luminosity, and lowest temperatures. At the other end are O-class stars, being the most massive and luminous, while also holding the highest temperatures. The full spectrum is O, B, A, F, G, K, and M.
In this thought experiment, we are provided with data about three Earth-like exoplanets that orbit their host stars at 1 AU: exoplanet-1, which orbits an A-class star, exoplanet-2, which orbits a G-class star, and exoplanet-3, which orbits an M-class star.
One researcher claims that because the three planets orbit at 1 AU, they will all retain liquid water on their surfaces. However, there are many factors to consider when defining the range of the habitable zone. The claim that the planets will all have liquid water is understandable when just looking at planetary data, but it is important to also consider stellar data, as the type of star affects the range of the habitable zone.
Another researcher claims that only exoplanet-2 and exoplanet-3 will have liquid water on their surfaces, but not exoplanet-1. This researcher believes exoplanet-1 isn’t within the habitable zone because it is orbiting too close to an A-class star. However, once again, this researcher isn’t considering enough data about both the exoplanets and their host stars.
Four main factors come into play when determining an exoplanet’s habitability: the spectral type of its host star, its orbital distance, the surface temperature of the planet, and the atmosphere of the planet. However, in this scenario, the given information includes that the three exoplanets are Earth-like and orbit at 1 AU. It can be inferred that the only relevant information is stellar data, which is provided in Reference Table a: Lifetime and habitable zone range by spectral type. Knowing that each planet orbits a different type of star at the same distance, it can be automatically ruled out that all of the three planets are habitable.
To start, exoplanet-1 orbits an A-class, which according to the table, has a habitable zone range of 3.5–7.1 AU. Exoplanet-1 orbits at a distance of 1 AU and can be identified as uninhabitable. It orbits far too close and would therefore not be able to hold liquid surface water.
Next, exoplanet-2 orbits a G-class star. The table tells us that G-class stars have a habitable zone range of 0.9–1.8 AU. It can be inferred that exoplanet-2, orbiting at a distance of 1 AU, could have liquid surface water, as it falls in the given range.
Lastly, we have exoplanet-3 which orbits an M-class star. Once again, the table provided tells us that M-class stars have a habitable zone range of 0.3–0.4 AU. This range is significantly closer to the host star than the distance exoplanet-3 is orbiting, being 1 AU. Any surface water would likely be in a frozen state (not taking into account geological activity and other internal heat sources). This leads to the determination that exoplanet-3 is not habitable, as it is too far away from its star.
