Are Planets Tidally Locked to Red Dwarfs Habitable? It’s Complicated

Astronomers are keenly interested in red dwarfs and the planets that orbit them. Up to 85% of the stars in the Milky Way could be red dwarfs, and 40% of them might host Earth-like exoplanets in their habitable zones, according to some research.

But there are some problems with their potential habitability. One of those problems is tidal locking.

Red dwarf stars are sometimes called “M-dwarfs,” but the terms can get a bit fuzzy.

The confusion between the terms red dwarf and M-dwarf stems from temperature and mass. One definition of a red dwarf is synonymous with an M-dwarf, based on a maximum temperature of 3,900 K and a maximum mass of 0.6 solar masses.

Another definition of a red dwarf includes hotter stars with a maximum temperature of 5,200 K and a maximum mass of 0.8 solar masses. This definition includes all K-type main-sequence stars, which are also called K-dwarfs.

Another red dwarf definition includes only part of the K-dwarf classification, and yet another includes some brown dwarfs.

The Hertzsprung-Russell diagram shows how the definitions overlap.

The definition of a red dwarf can be a bit fuzzy. Sometimes the definition included the brightest brown dwarfs, and sometimes it includes all or part of the K-dwarf classification. Image Credit: By User: Spacepotato – Modified version of Image: HR-diag-no-text.svg, written by Rursus and modified by Bhutajata, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2093830

In general, red dwarfs are the smallest and coolest main-sequence stars. Because they’re such low-mass objects, they develop slowly and live a long time. The least massive among them can maintain a constant luminosity for trillions of years, but there are no red dwarfs that old in the Universe yet.

However astronomers define them, there are an awful lot of them. If the estimate of 250 billion stars in the Milky Way is accurate, then 212 billion of them might be red dwarfs. Astronomers think that the red dwarf stellar population hosts a lot of rocky planets in their habitable zones. This is why the astronomy community makes such a pointed effort to study red dwarfs: they’re the Milky Way’s exoplanet cookie jar.

Red dwarfs are small and dim. Other stars can be so bright it’s near impossible to detect small planets when transiting in front of them. But red dwarfs are much less bright, so their light doesn’t create the same obstacle. The same dimness makes them difficult to observe from great distances, but astronomers have developed ways to work with these restrictions.

Our closest stellar neighbour is Proxima Centauri, a red dwarf shown in the small red circle. It’s too dim for the naked eye, but astronomers still detected three planets orbiting it. One is an Earth-sized planet in the red dwarf’s habitable zone. The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. Credit: Skatebiker at English Wikipedia (CC BY-SA 3.0)

The European Southern Observatory (ESO) runs a project called SPECULOOS, the Search for habitable Planets EClipsing ULtra-cOOl Stars. SPECULOOS is a system of four robotic cameras at the Paranal Observatory.

SPECULOOS’s mission is simple: to detect terrestrial planets as they transit across small, cool stars in our neighbourhood of the Milky Way. There are other attempts to find red dwarfs and characterize their population. The HARPS-N Red Dwarf Exoplanets Survey (HADES) analyzed the spectra of quiet, young M-dwarfs. The venerable Hubble Space Telescope has put in work, too, with its Habitable Zones and M dwarf Activity across Time (HAZMAT) observing program. Because red dwarfs are so numerous, Kepler, TESS, and others have all studied red dwarfs as part of their observations.

But SPECULOOS and the others don’t have the power to study red dwarfs in great detail. The planets they detect become targets for deeper observations, with the James Webb Space Telescope and powerful ground-based telescopes under construction right now. Those telescopes can study these planets’ atmospheres and reveal more clues to habitability.

This new paper is part of the effort to narrow down the list of red dwarfs for further study with the JWST and others. Observing time on these telescopes is in high demand, and identifying targets that can answer specific questions is essential. “In preparation for upcoming observations, it is increasingly
important that we understand the full range of possible M-dwarf planetary climates and their prospects for habitability,” the authors write.

TRAPPIST-1 is probably the most well-known ultra-cool, or red dwarf, star. It is host to several rocky, roughly Earth-sized planets. Astronomers think it’s no accident that ultra-cool stars and red dwarfs are hosts to many smaller, rocky planets, and they hope that SPECULOOS will find them. Credit: NASA/JPL-Caltech

Observations of M-dwarfs have revealed a lot. But astronomers still don’t have definitive answers to some important questions regarding these stars and the habitability of their planets. Do they flare too violently? Do they emit too much powerful UV and X-ray radiation? Do they strip away the atmospheres of stars in their habitable zones?

There’s another big question surrounding red dwarf habitability: tidal locking also called synchronous rotation.

Because M-dwarfs are not highly luminous, their habitable zones are closer than stars like our Sun. Planets must be close to M-dwarfs to be in the smaller habitable zones. But that proximity puts them in their stars’ gravitational grip, preventing them from rotating. So planets in M-dwarf habitable zones are likely tidally locked to their stars.

A new study examined tidally locked M-dwarf planets to understand what conditions could make their terminator regions habitable. The study is “Terminator Habitability: The Case for Limited Water Availability on M-dwarf Planets,” and it’s been accepted for publication in The Astronomical Journal. The lead author is Ana Lobo, a Ph.D. Candidate at the Caltech Division of Geological and Planetary Sciences.

When a planet is tidally locked to its star, it creates what planetary scientists sometimes call a stellar eyeball region. The part of the planet directly facing the star is warmed, but beyond the terminator line, it’s not. This can create a planet with liquid water in the stellar eyeball but frozen water everywhere else.

This is an artist’s impression of the exoplanet TRAPPIST-1f. It’s likely an example of a “cold” eyeball planet with an ice shell pierced by an ocean on the side facing the star. Image Credit: By NASA/JPL-Caltech – Catalog page · Full-res (MOV), Public Domain, https://commons.wikimedia.org/w/index.php?curid=56552366

The paper’s authors explain the focus of their research in their introduction. “Previous studies have focused on scenarios where fractional habitability is confined to the substellar or “eye” region, but in this paper, we explore the possibility of planets with terminator habitability, defined by the existence of a habitable band at the transition between a scorching dayside and a glacial nightside.”

Scientists have wondered about tidally locked exoplanets and how they might be habitable since the early days of exoplanet discoveries. The temperature extremes between the day and night might not be extreme if a planet’s atmosphere circulates enough. On exoplanets with significant surface water, ocean heat transport could affect day and night side temperatures, potentially moderating temperatures.

But what water-to-land ratios can create a habitable terminator zone?

This artist’s illustration shows the terminator on an exoplanet. The region would experience perpetual twilight. Image Credit: Image Science and Analysis Laboratory, NASA Johnson Space Center.

In this paper, the authors modelled exoplanets with different land and water coverage ratios. They wanted to determine how that ratio affected the planet-encircling band of habitability centred on the terminator.

Without atmospheric or oceanic circulation, the nightside of these tidally-locked planets is likely frozen solid. Conversely, the dayside could see a concentration of water vapour that never dissipates, creating a runaway greenhouse effect. But depending on how much heat the planet can circulate, the band of habitability around the terminator could be broader or skinnier.

“In this paper,” the authors write, “we explore climate at the inner edge of the M-dwarf habitable zone to determine how fractional habitability changes as dayside temperatures start to exceed habitable limits.” Though no strictly defined temperature determines habitability, the authors work with a 0 to 50 degrees Celsius range.

The paper focuses on a specific star named AD Leonis. They chose AD Leonis not because exoplanets are orbiting it but because it’s a well-understood star that’s representative of brighter red dwarf stars, where astronomers have found most habitable-zone exoplanets. It’s also near the Sun—only 16 light-years away— so it’s relatively easy to observe. (AD Leonis is a known flare star, but its flaring activity wasn’t part of this study.)

This is an artist’s illustration of the nearby red dwarf AD Leonis, also known as Gliese 388. Like other red dwarfs, it’s known to flare violently, but flaring wasn’t part of this study. Image Credit: National Astronomical Observatory of Japan.

The team of researchers performed two sets of simulations to explore terminator habitability. One set involved water-abundant aquaplanets, and the other involved water-limited land planets. The team compared the results to examine how these planets might be habitable.

They used a simulated planet named Aq34 as a starting point because, in this simulated scenario, it has an Earth-like solar constant and a mostly temperate dayside climate.

This image from the study shows the simulated planets. The researchers simulated six aquaplanets and three land planets. Image Credit: Lobo et al. 2022

The research showed that some of the variables produced competing effects. For example, a higher mean planetary temperature can produce more water vapour which can act as a greenhouse gas. But increased water vapour also means more cloud cover. That can raise the planet’s albedo, reflecting more stellar energy away from it and helping it stay cooler.

This figure from the study shows how variables can have competing effects. (a) shows the surface temperature and winds. (b) shows the mid-troposphere height and wind patterns. (c) shows top-of-atmosphere albedo. (d) shows the surface evaporation rates and precipitation reaching the
surface in yellow contours. (c) shows that regions with abundant clouds reflect a larger fraction of incoming SW (shortwave flux or stellar radiation), reducing warming in the stellar eye but also increasing the greenhouse effect. Image Credit: Lobo et al. 2022

The authors point out that for a planet to have a habitable terminator zone, it must have a large swing between its dayside and nightside temperatures. “By definition, in order for terminator habitability to occur, a planet must sustain large day-night temperature gradients,” they write. Only that dynamic can produce a wide enough terminator region to create habitable temperatures.

The research shows that simulated ocean planets can’t produce a habitable terminator region. The closer one of these planets is to the red dwarf, the higher its stellar flux, reducing the difference between day and night side temperatures. Those planets would produce a homogenous climate before the dayside reached a runaway greenhouse effect. They never passed through a state where the terminator was habitable.

Water-limited planets fared differently. As stellar flux increases, “… large day-night temperature gradients are easily achievable without entering a runaway greenhouse state,” the authors explain. That helps create a habitable terminator zone. “We also find that the water-limited land planet configurations may be favourable in terms of long-term climate stability,” meaning the terminator could be habitable for extended periods.

“We find that a temperate terminator is not achievable with aquaplanet simulations that seek to reproduce ocean-covered planets but can easily occur on water-limited land planets,” they conclude.

Astronomers have difficulty determining the water content of red dwarfs. Radial velocity studies can measure how much a planet tugs on its star and can provide some understanding of the planet’s density when combined with size measurements. A lower-density planet likely has more water. But those measurements aren’t certain.

Astronomers think that water-limited exoplanets may be more abundant than water-abundant planets, but more research is needed to solidify that understanding. If it’s true, it bodes well for habitability, according to this research. “Therefore, terminator habitability may represent a significant fraction of habitable M-dwarf planets,” the authors write.

But if a habitable terminator is more likely on water-limited exoplanets, that may affect the possibility of life. Life needs water, after all. “Overall, the lack of abundant surface water in these simulations could
pose a challenge for life to arise under these conditions,” the team writes.

There are some confounding variables on these planets. What if the available water is locked away in glaciers on the planet’s nightside? What if the atmosphere is so thick and mixed so effectively that the entire world is too hot? Those questions can be answered in degrees, but only by more research.

This artist’s illustration shows an eyeball world with all the water frozen into glaciers on the nightside and the dayside totally barren. Image Credit: By Ittiz – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=114028109

We need more research into red dwarf planets and their land-water configurations. This study is a good starting point and can help astronomers choose good targets for follow-up observations with the James Webb. The authors acknowledge their work’s limitations in their final comment.

“We expect that future studies exploring a broader range of land planet configurations, particularly those using future generations of surface and ice models, will find a wide range of habitable terminator scenarios in regimes intermediate to the water-limited and aquaplanet cases considered here.”


Evan Gough

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