Planets Orbiting M-dwarf Stars Research Articles

The Impact of Extended H2O Cross Sections on Temperate Anoxic Planet Atmospheres: Implications for Spectral Characterization of Habitable Worlds

JWST has created a new era of terrestrial exoplanet atmospheric characterization, and with it, the possibility to detect potential biosignature gases like CH4. Our interpretation of exoplanet atmospheric spectra, and the veracity of these interpretations, will be limited by our understanding of atmospheric processes and the accuracy of input modeling data. Molecular cross sections are essential inputs to these models. The photochemistry of temperate planets depends on photolysis reactions whose rates are governed by the dissociation cross sections of key molecules. H2O is one such molecule; the photolysis of H2O produces OH, a highly reactive and efficient sink for atmospheric trace gases. We investigate the photochemical effects of improved H2O cross sections on anoxic terrestrial planets as a function of host star spectral type and CH4 surface flux. Our results show that updated H2O cross sections, extended to wavelengths >200 nm, substantially impact the predicted abundances of trace gases destroyed by OH. The differences for anoxic terrestrial planets orbiting Sun-like host stars are greatest, showing changes of up to 3 orders of magnitude in surface CO levels, and over an order of magnitude in surface CH4 levels. These differences lead to observable changes in simulated planetary spectra, especially important in the context of future direct-imaging missions. In contrast, the atmospheres of planets orbiting M-dwarf stars are substantially less affected. Our results demonstrate a pressing need for refined dissociation cross-section data for H2O, where uncertainties remain, and other key molecules, especially at mid-UV wavelengths >200 nm.

No Evidence for More Earth-sized Planets in the Habitable Zone of Kepler's M versus FGK Stars

Reliable detections of Earth-sized planets in the habitable zone remain elusive in the Kepler sample, even for M dwarfs. The Kepler sample was once thought to contain a considerable number of M-dwarf stars (T eff < 4000 K), which hosted enough Earth-sized ([0.5, 1.5] R ⊕) planets to estimate their occurrence rate (η ⊕) in the habitable zone. However, updated stellar properties from Gaia have shifted many Kepler stars to earlier spectral type classifications, with most stars (and their planets) now measured to be larger and hotter than previously believed. Today, only one partially reliable Earth-sized candidate remains in the optimistic habitable zone, and zero in the conservative zone. Here we performed a new investigation of Kepler's Earth-sized planets orbiting M-dwarf stars, using occurrence rate models with considerations of updated parameters and candidate reliability. Extrapolating our models to low instellations, we found an occurrence rate of for the conservative habitable zone (and for the optimistic one), consistent with previous works when considering the large uncertainties. Comparing these estimates to those from similarly comprehensive studies of Sun-like stars, we found that the current Kepler sample does not offer evidence to support an increase in η ⊕ from FGK to M stars. While the Kepler sample is too sparse to resolve an occurrence trend between early and mid- to late M dwarfs for Earth-sized planets, studies including larger planets and/or data from the K2 and TESS missions are well suited to this task.

Extended Habitability of Exoplanets Due to Subglacial Water

Considering subglacial liquid water, a significant extension of the classical Habitable Zone is obtained. Elaborating on the model of Wandel it is shown how an atmosphere and liquid water could survive on tidally locked planets closely orbiting an M-dwarf host, extending the Habitable Zone boundary inwards. In addition, subglacial liquid water could extend the Habitable Zone beyond the outer boundary of the conservative Habitable Zone as well. These two results enhance the circumstellar region with a potential for liquid water well beyond the conservative boundaries of the classical Habitable Zone. It is argued that the probable recent JWST detection of atmospheric water vapor on the rocky Earth-sized exoplanet GJ 486 b, along with earlier detections of water on other planets orbiting M-dwarf stars, gives an empirical answer to the much-argued question of whether such planets can support liquid water, organic chemistry, and eventually life. It is shown how water on terrestrial planets closely orbiting M-dwarf stars may sustain in a subglacial melting layer. Finally, the model is applied to a few exoplanets demonstrating how water detection may constrain their atmospheric properties.

The role of magma oceans in maintaining surface water on rocky planets orbiting M-dwarfs

ABSTRACT Earth-like planets orbiting M-dwarf stars, M-Earths, are currently the best targets to search for signatures of life. Life as we know it requires water. The habitability of M-Earths is jeopardized by water loss to space: high flux from young M-dwarf stars can drive the loss of 3–20 Earth oceans from otherwise habitable planets. We develop a 0-D box model for Earth-mass terrestrial exoplanets, orbiting within the habitable zone, which tracks water loss to space and exchange between reservoirs during an early surface magma ocean phase and the longer deep-water cycling phase. A key feature is the duration of the surface magma ocean, assumed concurrent with the runaway greenhouse. This time-scale can discriminate between desiccated planets, planets with desiccated mantles but substantial surface water, and planets with significant water sequestered in the mantle. A longer-lived surface magma ocean helps M-Earths retain water: dissolution of water in the magma provides a barrier against significant loss to space during the earliest, most active stage of the host M-dwarf, depending on the water saturation limit of the magma. Although a short-lived basal magma ocean can be beneficial to surface habitability, a long-lived basal magma ocean may sequester significant water in the mantle at the detriment of surface habitability. We find that magma oceans and deep-water cycling can maintain or recover habitable surface conditions on Earth-like planets at the inner edge of the habitable zone around late M-dwarf stars – these planets would otherwise be desiccated if they form with less than ∼10 terrestrial oceans of water.

High Tide or Riptide on the Cosmic Shoreline? A Water-rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ 486b from JWST Observations

Planets orbiting M-dwarf stars are prime targets in the search for rocky exoplanet atmospheres. The small size of M dwarfs renders their planets exceptional targets for transmission spectroscopy, facilitating atmospheric characterization. However, it remains unknown whether their host stars’ highly variable extreme-UV radiation environments allow atmospheres to persist. With JWST, we have begun to determine whether or not the most favorable rocky worlds orbiting M dwarfs have detectable atmospheres. Here, we present a 2.8–5.2 μm JWST NIRSpec/G395H transmission spectrum of the warm (700 K, 40.3× Earth’s insolation) super-Earth GJ 486b (1.3 R ⊕ and 3.0 M ⊕). The measured spectrum from our two transits of GJ 486b deviates from a flat line at 2.2σ − 3.3σ, based on three independent reductions. Through a combination of forward and retrieval models, we determine that GJ 486b either has a water-rich atmosphere (with the most stringent constraint on the retrieved water abundance of H2O > 10% to 2σ) or the transmission spectrum is contaminated by water present in cool unocculted starspots. We also find that the measured stellar spectrum is best fit by a stellar model with cool starspots and hot faculae. While both retrieval scenarios provide equal quality fits () to our NIRSpec/G395H observations, shorter wavelength observations can break this degeneracy and reveal if GJ 486b sustains a water-rich atmosphere.

High-resolution Transmission Spectroscopy of the Terrestrial Exoplanet GJ 486b

Terrestrial exoplanets orbiting M-dwarf stars are promising targets for transmission spectroscopy with existing or near-future instrumentation. The atmospheric composition of such rocky planets remains an open question, especially given the high X-ray and ultraviolet flux from their host M dwarfs that can drive atmospheric escape. The 1.3 R ⊕ exoplanet GJ 486b (T eq ∼ 700 K), orbiting an M3.5 star, is expected to have one of the strongest transmission spectroscopy signals among known terrestrial exoplanets. We observed three transits of GJ 486b using three different high-resolution spectrographs: IRD on Subaru, IGRINS on Gemini-South, and SPIRou on the Canada–France–Hawai’i Telescope. We searched for atmospheric absorption from a wide variety of molecular species via the cross-correlation method, but did not detect any robust atmospheric signals. Nevertheless, our observations are sufficiently sensitive to rule out several clear atmospheric scenarios via injection and recovery tests, and extend comparative exoplanetology into the terrestrial regime. Our results suggest that GJ 486b does not possess a clear H2/He-dominated atmosphere, nor a clear 100% water-vapor atmosphere. Other secondary atmospheres with high mean molecular weights or H2/He-dominated atmospheres with clouds remain possible. Our findings provide further evidence suggesting that terrestrial planets orbiting M-dwarf stars may experience significant atmospheric loss.

Terminator Habitability: The Case for Limited Water Availability on M-dwarf Planets

Rocky planets orbiting M-dwarf stars are among the most promising and abundant astronomical targets for detecting habitable climates. Planets in the M-dwarf habitable zone are likely synchronously rotating, such that we expect significant day–night temperature differences and potentially limited fractional habitability. 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. Using a global climate model, we show that for water-limited planets it is possible to have scorching temperatures in the “eye” and freezing temperatures on the nightside, while maintaining a temperate climate in the terminator region, due to reduced atmospheric energy transport. On water-rich planets, however, increasing the stellar flux leads to increased atmospheric energy transport and a reduction in day–night temperature differences, such that the terminator does not remain habitable once the dayside temperatures approach runaway or moist greenhouse limits. We also show that while water-abundant simulations may result in larger fractional habitability, they are vulnerable to water loss through cold trapping on the nightside surface or atmospheric water vapor escape, suggesting that even if planets were formed with abundant water, their climates could become water-limited and subject to terminator habitability.

Origins space telescope: from first light to life

The Origins Space Telescope (Origins) is one of four science and technology definition studies selected by the National Aeronautics and Space Administration (NASA) in preparation of the 2020 Astronomy and Astrophysics Decadal survey in the US. Origins will trace the history of our origins from the time dust and heavy elements permanently altered the cosmic landscape to present-day life. It is designed to answer three major science questions: How do galaxies form stars, make metals, and grow their central supermassive black holes from reionization? How do the conditions for habitability develop during the process of planet formation? Do planets orbiting M-dwarf stars support life? Origins operates at mid- to far-infrared wavelengths from ~ 2.8 μm to 588 μm, and is more than 1000 times more sensitive than prior far-IR missions due to its cold (~ 4.5 K) aperture and state-of-the-art instruments.

The Effect of Substellar Continent Size on Ocean Dynamics of Proxima Centauri b

Abstract The potential habitability of tidally locked planets orbiting M-dwarf stars has been widely investigated in recent work, typically with a nondynamic ocean and without continents. On Earth, ocean dynamics are a primary means of heat and nutrient distribution. Continents are a critical source of nutrients, strongly influence ocean dynamics, and participate in climate regulation. In this work, we investigate how the size of a substellar land mass affects the oceans ability to transport heat and upwell nutrients on the tidally locked planet Proxima Centauri b using the ROCKE-3D coupled ocean-atmosphere general circulation model (GCM). We find that dayside ice-free ocean and nutrient delivery to the mixed layer via upwelling are maintained across all continent sizes. We also find that Proxima Centauri b’s climate is more sensitive to differences among atmospheric GCMs than to the inclusion of ocean dynamics in ROCKE-3D. Finally, we find that Proxima Centauri b transitions from a “lobster” state where ocean heat transport distributes heat away from the substellar point to an “eyeball” state where heat transport is restricted and surface temperature decreases symmetrically from the substellar point when the continent size exceeds ∼20% of the surface area. Our work suggests that both a dynamic ocean and continents are unlikely to decrease the habitability prospects of nearby tidally locked targets like Proxima Centauri b that could be investigated with future observations by the James Webb Space Telescope.

Habitability and Water Loss Limits on Eccentric Planets Orbiting Main-sequence Stars

Abstract A planet’s climate can be strongly affected by its orbital eccentricity and obliquity. Here we use a one-dimensional energy balance model modified to include a simple runaway greenhouse (RGH) parameterization to explore the effects of these two parameters on the climate of Earth-like aqua planets—completely ocean-covered planets—orbiting F-, G-, K-, and M-dwarf stars. We find that the range of instellations for which planets exhibit habitable surface conditions throughout an orbit decreases with increasing eccentricity. However, the appearance of temporarily habitable conditions during an orbit creates an eccentric habitable zone (EHZ) that is sensitive to orbital eccentricity and obliquity, planetary latitude, and the spectral type of the host star. We find that the fraction of a planet’s orbit over which it exhibits habitable surface conditions is larger on eccentric planets orbiting M-dwarf stars, due to the lower broadband planetary albedos of these planets. Planets with larger obliquities have smaller EHZs, but exhibit warmer climates if they do not enter a snowball state during their orbits. We also find no transient RGH state on planets at all eccentricities. Rather, planets spend their entire orbits either in an RGH or not. For G-dwarf planets receiving 100% of the modern solar constant and with eccentricities above 0.55, an entire Earth ocean inventory can be lost in 3.6 Gyr. M-dwarf planets, due to their larger incident X-ray and extreme ultraviolet flux, can become desiccated in only 690 Myr with eccentricities above 0.38. This work has important implications for eccentric planets that may exhibit surface habitability despite technically departing from the traditional habitable zone as they orbit their host stars.

The Effect of Land Fraction and Host Star Spectral Energy Distribution on the Planetary Albedo of Terrestrial Worlds

AbstractThe energy balance and climate of planets can be affected by the reflective properties of their land, ocean, and frozen surfaces. Here we investigate the effect of host star spectral energy distribution (SED) on the albedo of these surfaces using a one-dimensional energy balance model. Incorporating spectra of M-, K-, G-, and F-dwarf stars, we determined the effect of varying fractional and latitudinal distribution of land and ocean surfaces as a function of host star SED on the overall planetary albedo, climate, and ice-albedo feedback response. While noting that the spatial distribution of land masses on a given planet will have an effect on the overall planetary energy balance, we find that terrestrial planets with higher average land/ocean fractions are relatively cooler and have higher albedo regardless of star type. For Earth-like planets orbiting M-dwarf stars, the increased absorption of water ice in the near-infrared, where M-dwarf stars emit much of their energy, resulted in warmer global mean surface temperatures, ice lines at higher latitudes, and increased climate stability as the ice-albedo feedback became negative at high land fractions. Conversely, planets covered largely by ocean, and especially those orbiting bright stars, had a considerably different energy balance due to the contrast between the reflective land and the absorptive ocean surface, which in turn resulted in warmer average surface temperatures than land-covered planets and a stronger potential ice-albedo feedback. While dependent on the properties of individual planetary systems, our results place some constraints on a range of climate states of terrestrial exoplanets based on albedo and incident flux.

Triaxial deformation and asynchronous rotation of rocky planets in the habitable zone of low-mass stars

Rocky planets orbiting M-dwarf stars in the habitable zone tend to be driven to synchronous rotation by tidal dissipation, potentially causing difficulties for maintaining a habitable climate on the planet. However, the planet may be captured into asynchronous spin-orbit resonances, and this capture may be more likely if the planet has a sufficiently large intrinsic triaxial deformation. We derive the analytic expression for the maximum triaxiality of a rocky planet, with and without a liquid envelope, as a function of the planet's radius, density, rigidity and critical strain of fracture. The derived maximum triaxiality is consistent with the observed triaxialities for terrestrial planets in the solar system, and indicates that rocky planets in the habitable zone of M-dwarfs can in principle be in a state of asynchronous spin-orbit resonances.

MAGNETOSPHERIC STRUCTURE AND ATMOSPHERIC JOULE HEATING OF HABITABLE PLANETS ORBITING M-DWARF STARS

We study the magnetospheric structure and the ionospheric Joule Heating of planets orbiting M-dwarf stars in the habitable zone using a set of magnetohydrodynamic (MHD) models. The stellar wind solution is used to drive a model for the planetary magnetosphere, which is coupled with a model for the planetary ionosphere. Our simulations reveal that the space environment around close-in habitable planets is extreme, and the stellar wind plasma conditions change from sub- to super-Alfvenic along the planetary orbit. As a result, the magnetospheric structure changes dramatically with a bow shock forming in the super-Alfvenic sectors, while no bow shock forms in the sub-Alfvenic sectors. The planets reside most of the time in the sub-Alfvenic sectors with poor atmospheric protection. A significant amount of Joule Heating is provided at the top of the atmosphere as a result of the planetary interaction with the stellar wind. For the steady-state solution, the heating is about 0.1-3\% of the total incoming stellar irradiation, and it is enhanced by 50\% for the time-dependent case. The significant Joule Heating obtained here should be considered in models for the atmospheres of habitable planets in terms of the thickness of the atmosphere, the top-side temperature and density, the boundary conditions for the atmospheric pressure, and particle radiation and transport.

An Astrobiological Experiment to Explore the Habitability of Tidally Locked M-Dwarf Planets

AbstractWe present a summary of a three-year academic research proposal drafted during the Sao Paulo Advanced School of Astrobiology (SPASA) to prepare for upcoming observations of tidally locked planets orbiting M-dwarf stars. The primary experimental goal of the suggested research is to expose extremophiles from analogue environments to a modified space simulation chamber reproducing the environmental parameters of a tidally locked planet in the habitable zone of a late-type star. Here we focus on a description of the astronomical analysis used to define the parameters for this climate simulation.