Authors: Monica R. Vidaurri, Sandra T. Bastelberger, Eric T. Wolf, Shawn Domagal-Goldman, Ravi Kumar Kopparapu
First Author’s Institution: Stanford University
Status: Accepted for publication in the Planetary Science Journal (Open Access), also available on arXiv (https://arxiv.org/abs/2204.10919)
Planetary Habitability: Water, Water, Everywhere?
Classically, the benchmark for planetary habitability is liquid water: if a planet’s surface temperature is above freezing and below boiling, then it lies within its star’s habitable zone. The Earth, obviously, is our best example of an inhabited, habitable planet in the habitable zone. Mars, now too cold for liquid water, is thought to have historically been much warmer, habitable, and possibly even inhabited! The evidence for this is strong enough that searching for past Martian life is a main science goal of the Perseverance rover! However, there’s one more planet in our Solar System on the other end of the habitability spectrum.
Last month, as part of Earth Week ⨉ Astrobites, we heard from Dr. David Grinspoon about comparative planetology, looking at the differences in the environments on Earth, Mars, and Venus. If Mars is the end state of a planet unable to sustain a greenhouse effect (not just a result of carbon emissions, but necessary to maintain liquid water), then Venus is the end state of a planet with a runaway greenhouse effect. Venus absorbed more radiation from the sun than it could emit, which heated the planet to the point where all liquid water (if it had any) evaporated, and then was lost to space through photodissociation. Historically, the orbital radius at which a planet would be subject to this effect from its host star has defined the inner edge of the habitable zone. Any closer, and the planet would lose its water. However, this isn’t a hard limit, and the actual orbital radii where Venus analogs can be found varies significantly with both stellar and planetary parameters. In this paper, the authors run a grid of simulations to determine exactly where the “Venus zone,” which overlaps with the classical habitable zone, is for stars at varying masses and temperatures.
How to Build an Uninhabitable Planet
In order for a planet to have a runaway greenhouse, there must be sufficient greenhouse gases in its atmosphere. The specific amount is different for every planet, but for any particular planet, this primarily depends on how much radiation it receives from its host star. As the planet is irradiated by the star, different atmospheric species absorb and emit different amounts of starlight at different pressures/altitudes, heating and cooling the atmosphere. By increasing the amount of carbon dioxide relative to nitrogen and water (starting with present-day Earth values for CO2 and N2, and maintaining a certain humidity level), the authors can eventually induce a runaway greenhouse on the planet, producing a “Venus analog” planet. The authors then decrease the stellar flux, effectively moving the planet further away from its host star, until the runaway greenhouse can no longer be sustained. This yields the outer edge of the “Venus zone”, past which no increase in the amount of carbon dioxide will be able to run away.
This process is repeated for each stellar type in the grid: an F star (7200K), a Sun-like G star (5780K), a K star (4400K), and two M stars (M3 (3400K) and M5 (3000K) dwarfs). Habitable-zone planets around M stars are particularly tempting targets for further study, as they have significant observational benefits, such as short orbital periods due to their close-in habitable zones, as well as high transit depths compared to planets around larger Sun-like stars.
Including a variety of stellar types in the model is important, not only because of their different brightnesses, but also their different spectra. As seen in Figure 1, larger and hotter stars are less efficient heaters of planetary atmospheres than smaller, cooler stars. An F star emits much more of its light as shorter-wavelength radiation compared to an M star, which is brighter in the infrared. The higher IR flux from an M star is better absorbed by greenhouse gases in planetary atmospheres, heating the planet more than an F star.
Is Every Habitable Planet a Potential Venus?
Luckily, the authors found that while the Venus zone has significant overlap with the classical habitable zone, there are still places where habitable planets can exist with zero risk of runaway (see Figure 2). However, since there is so much overlap between the habitable zone and the Venus zone, future investigators must be careful to take the possibility of runaway greenhouses into consideration when studying potentially-habitable planets. In addition, there is significant uncertainty in Venus’s atmospheric and geological history, though new in-situ measurements by the upcoming fleet of Venus missions (NASA’s DAVINCI+ and VERITAS, as well as ESA’s EnVision) will be able to uncover much of this unclear history. Just like the atmospheres of these simulated planets, the search for habitable planets is heating up. Work like this will hopefully ensure that the future Earth 2.0 isn’t actually a Venus 2.0!
Astrobite edited by Catherine Clark
Featured image credit: Paramount Pictures, NASA, JAXA/ISAS