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Title: Climate Regimes Across the Habitable Zone: A Comparison of Synchronous Rocky M and K Dwarf Planets
Authors: Ana H. Lobo and Aomawa L. Shields
First Author’s Institution: University of California, Irvine
Status: Published in ApJ
When it comes to planet hunting, the hot star on the block is the humble M dwarf: the coolest and dimmest type of star in the universe. They’re also the most abundant type of star, which combined with their small size makes them pretty good targets for rocky Earth-like planet hunting.
However, the jury’s still out on whether M dwarfs are likely to host habitable rocky Earth-like planets, as habitability — as we know it — requires a vast set of conditions on top of those needed to form a planet that simply resembles Earth in size and composition. What we do know is that one requirement for Earth-like life is the maintained and continued presence of water.
We have the luxury of living within our Sun’s “Goldilocks zone,” i.e., a “just right” distance from the Sun where it isn’t too hot or too cold for water to either boil off or freeze up permanently. An exoplanet’s atmosphere must also survive the onslaught of stellar storms brought on by its host star, which, in the case of an M-dwarf host, can be extreme from the very beginning.
This, unfortunately, considerably pares down the chances of finding a habitable planet orbiting an M dwarf, although there are scenarios in which it could be possible.
Today’s authors propose an alternate target: K dwarfs.
Choose Your Host Star!
K dwarfs are essentially M dwarfs’ slightly more massive and more stable siblings, albeit overshadowed. The gentler stellar activity that K dwarfs offer gives them an edge over M dwarfs, as fewer violent storms enhance volatile delivery and biosignature detection, and overall provide more of a fighting chance for life. However, we’re uncertain how common small rocky planets are around K dwarfs, and there hasn’t been much study on the kinds of climates K-dwarf planets could have. The authors thus set out to simulate two potentially habitable scenarios on both K-dwarf and M-dwarf planets with the goal of understanding how dayside climate and water availability vary between host stellar types.
For both scenarios, they assume tidally locked planets with Earth-like radii and surface gravities, and simplified atmospheres and orbits. They model the M-dwarf host star after AD Leonis and the K-dwarf host star after Epsilon Eridani. For both stellar classes, today’s authors play with water worlds and sand worlds (planets with surfaces made entirely of sand) by varying their orbital periods and instellation fluxes — the amount of stellar flux received by a planetary surface. An overview of how these planets’ temperatures vary across their surfaces can be seen in Figure 1.
Figure 1: A diagram of the surfaces of different K-dwarf and M-dwarf hosted planets simulated by the authors projected onto a 2D map, and how their temperature varies across the surface. The center of each projection corresponds to a latitude of 0 degrees and a longitude of 0 degrees. The top two rows correspond to planets hosted by an M dwarf, where the top row contains water worlds, and the bottom row contains sand worlds. The bottom two rows contain planets hosted by a K dwarf, where the top contains water worlds and the bottom contains sand worlds. Instellation flux values increase from left to right. [Lobo & Shields 2024]
It’s an Eyeball Summer
The K-dwarf water world planets today’s authors simulate are home to an “eyeball” regime, where the dayside contains a habitable region with moderate temperatures on surface areas where the star is nearly directly overhead, somewhat resembling an eyeball, and the nightside is dominated by freezing cold temperatures. These make up the third row in Figure 1.
The authors find that with increasing instellation, these planets inch closer to the moist greenhouse limit, the point at which surface temperatures become too high to support liquid water, leading to the evaporation of a planet’s atmosphere. We might expect this to happen with increased instellation flux or decreased orbital distance. One simulated K-dwarf planet surpasses this limit once instellation flux is ramped up over 50%. But surprisingly, the nightside is still frozen! Meanwhile, a similar M-dwarf planet with 50% increased instellation flux enters a runaway greenhouse state, where evaporating oceans become trapped in the atmosphere, preventing heat escape. This leads to an uninhabitable rise in temperature, much like a pressure cooker.
The K-dwarf planets’ resistance to increased instellation seems to support the notion of them being more stable than their M-dwarf-hosted cousins, but there’s more to the tale than this.
Life in The Terminator Zone
The second scenario is characterized by a “terminator band.” This is a world of extremes where the dayside is home to hellish temperatures and the nightside is uninhabitably cold, yet a temperate band persists at the boundary where day and night meet. As seen in Figure 1, temperate terminator zones can exist even on “eyeball” planets. It turns out that, depending on whether a planet has reached the moist greenhouse limit, the terminator might be easier to analyze on actual observed planets, as it tends to be cloud-free at lower instellations.
Unfortunately, the terminator zone never reaches habitable temperatures on water worlds around K dwarfs or M dwarfs. Terminator habitability does occur for some M-dwarf and K-dwarf sand worlds, and one K-dwarf planet even becomes a borderline case, where the eyeball center is uninhabitably hot but most of the dayside is habitable.
Simulated M-dwarf and K-dwarf sand planets have been found to fall into terminator habitability zones in the inner habitable zone, and one might be tempted to conclude that cooler dayside temperatures make K-dwarf planets less likely to contain habitable terminators when placed in the outer habitable zone. However, the authors state this may vary on planetary surface composition and orbital distance, as darker materials such as basalt could absorb more stellar energy, keeping the terminator warmer for longer at greater orbital distances.
Which is Better for Habitability?
While planets hosted by M dwarf and K dwarfs experience similar climate trends, it takes different instellations to incur different states, which in themselves can look different depending on the host stellar type. While it took much more instellation flux for a K-dwarf planet to reach the moist greenhouse limit, its nightside was still frozen. This might seem like straightforward evidence for planets around K dwarfs being more stable in general, but the authors state that dayside water retention might be more difficult than expected, as much of the water would be packed into the nightside.
While K-dwarf systems might be more stable for habitability, water worlds may be easier to find in the mid to outer Goldilocks zone. Rocky worlds might be better found in the inner zone, although we don’t know how common they are around K dwarfs yet.
Where Would You Live?
With future expected advances in telescope power, such as the upcoming Large UV/Optical/IR Surveyor (LUVOIR) mission, it should eventually become easier to observe K-dwarf-hosted planets and learn more about their compositions and atmospheres. Then we can find out more about their ability to retain water.
With this in mind, where would you want to live? A planet hosted by an M dwarf or a K dwarf?
Original astrobite edited by Kat Lee.
About the author, Diana Solano-Oropeza:
I’m a first-year astronomy PhD student at Cornell University, where I study exoplanets, stars, and habitability using Gaia data. I earned my BS in physics at Drexel University before entering the Bridge to the PhD in STEM program at Columbia University. There, I researched TESS-detected exoplanets for two years. My hobbies include practicing Muay Thai, fictionwriting, and playing video games. You can check out my website at https://dianasolano-oropeza.com/.