Red Dwarfs: Their Impact on Biosignatures

We’re in the midst of a significant period defining the biosignatures life can produce and determining how we might identify them. Centauri Dreams regular Alex Tolley today looks at a paper offering a unique contribution to this effort. The work of Sarah Rugheimer and Lisa Kaltenegger, the paper looks at how exoplanet spectra change for different types of host star and different epochs of planetary evolution. As Alex points out, the effects are profound, especially given the fact that red dwarfs will be our testbed for biosignature detection as we probe planetary atmospheres during transits around nearby stars. How stellar class affects our analysis will affect our strategies especially as we probe early Earth atmosphere equivalents. What will we find, for example, at TRAPPIST-1?

By Alex Tolley

As the search for life on exoplanets ramps up, the question arises as to which types of stars represent the best targets. Based on distribution, M-Dwarfs are very attractive as they represent 3/4 of all stars in our galaxy. Their long lifetimes offer abundant opportunities for life to evolve, and to resist extinction as their stars increase in luminosity. On Earth, terrestrial life might last another one billion years before the level of CO2 in the atmosphere is forced to be reduced below photosynthetic requirements for plants to survive. All but lithophilic life might be extinct within 1 ½ billion years. An additional advantage for astronomers is that spectra of exoplanet atmospheres will be easier to distinguish around low luminosity stars. [6, 7]

From a purely numbers game, M-Dwarfs are most attractive targets:

“Temperate terrestrial planets transiting M-dwarf stars are often touted as the poor-astronomer’s Earth analog, since they are easier to detect and characterize than a true Earth twin. Based on what we currently know, however, M-Dwarf planets are the most common habitable worlds“ [1]

Image: Gliese 581 from a planet in its HZ. Credit: David Hardy.

That M-Dwarf rocky worlds may be the most common habitable world is due to:

“1. rocky planets are much more common in the temperate zones of M-Dwarfs (…) than in the temperate zones of Sun-like Stars (…) 2. small stars are more common than big stars (…) 3. the tidally-locked nature of these planets is not a challenge to climate and may double the width of the habitable zone (…) 4. the red stellar radiation results in a weaker ice-albedo feedback

and hence stabler climate (…), and (…) 5. the slow main sequence evolution of M-Dwarfs means that a geological thermostat is not strictly necessary to maintain habitable conditions for billions of years (…). Studying temperate terrestrial planets around M-Dwarfs is our best shot at understanding habitability writ large.” [1]

There are negatives for life around M-Dwarfs too. The closeness of the habitable zone (HZ) to the star results in tidal locking that may impact the stability of the atmosphere, as well as the intense flares that may strip the atmospheres from these worlds. However, these negatives for habitability and hence life may be compensated by the ubiquity of such worlds and the relative ease of studying them remotely. For lithophilic life, surface conditions largely can be ignored.

After the lifeless Hadean, the Archean and Proterozoic eons had life that was purely prokaryotic. During this time photosynthesis evolved that eventually resulted in an atmosphere with O2 and very little CO2 and CH4. This phase of life’s history covers the long period when Earth’s atmosphere changed from a largely reducing one of N2, CO2, and some CH4, to one that becomes oxidizing. The Phanerozoic, starting around 500 mya encompasses the period when O2 pressures increased to the level they are today and terrestrial, multicellular life blossomed in diversity.

If Earth’s history is any guide, life in our galaxy will be mostly unicellular bacteria, living in a reducing atmosphere. If that is a correct hypothesis, then most life in the galaxy will be non-photosynthetic, perhaps with biologies similar to the Archaea. A biosignature of such microbial life will still require looking for a disequilibrium in gases, mainly CO2 and CH4, rather than O2 and CH4 [2, 3]. Archaea include the extremophiles living in a diverse array of environments, including the lithosphere. Such organisms may well survive the harsher conditions of a tidally locked world, especially regarding the impact of flares.

The question then arises, if we look for a biosignature around stars of different spectral types, will the star’s type have an impact on the planet’s atmosphere, detectable spectral markers, and any potential biosignatures?

This question is examined in a paper by Rugheimer and Kaltenegger [5]. The authors modeled the spectra of atmospheres to simulate Earth-like worlds – rocky worlds large enough to hold an atmosphere and presumably with a mix of ocean and continents, rather than water worlds – orbiting in the HZ of different star types F, G, K and M. Their simulations cover the state of evolution of those worlds as if they were an Earth relocated to other stars, so that the spectra for different gas mixtures could be modeled.

The light of an M-Dwarf is shifted so that the UV component is much diminished. This affects the reactions of the gases in the atmosphere. Photolysis is reduced, reducing the loss of H20, which in turn, as a greenhouse gas, warms the surface more than with a hotter star. CH4 in particular is not lost and may even result in a runaway accumulation in some cases. The increase in H2O increases the cloud cover in the troposphere, which in turn increases the planet’s albedo. The increased IR component of the M-Dwarf’s output increases the surface temperature as well and may well further increase cloud formation.

The photolysis of water and the oxidation of CH4 is shown below. UV is required which results in the reduced loss of H2O and CH4 on exoplanets around M-Dwarfs.

H2O + hv (ƛ < 200 nm) -> H + OH

CH4 + OH -> CH3 + H2O

Similarly, UV is required to split O2 allowing O3 formation.

O2 + hv (ƛ < 240 nm) -> O + O

O + O2 -> O3

Previously, Kaltenegger [4] had modeled the atmospheres of Earth-like worlds around different stars and constructed synthetic spectra to determine the visibility of different biosignature gases in the visible and near-infrared.

Following on, Rugenheimer et al modeled gases for 4 different periods – 3.9, 2.0, 0.8 and 0 Ga for the 4 star types. The initial gas mixes are shown in Table 1.

Table1. Gas mixing ratios for 4 eons. N2 not shown.

Because the stars age at different rates, the periods are standardized to Earth. As M-Dwarfs age far more slowly than our sun, the different luminosities are modeled as if their planets are further out from their star earlier in its history to simulate the lower luminosity.

The result of the simulations shows that some markers will be difficult to observe under different spectral types of stars.

The impact of the star type is shown in Figure 1. Temperature and 5 gases are profiled with altitude, The M-Dwarfs show clear differences from the hotter star types. Of particular note are the higher H2O and CH4 atmosphere ratios, particularly at higher altitudes.

Figure 1. Planetary temperature vs. altitude profiles and mixing ratio profiles for H2O, O3, CH4, OH, and N2O (left to right) for a planet orbiting the grid of FGKM stellar models with a prebiotic atmosphere corresponding to 3.9 Ga (first row), the early rise of oxygen at 2.0 Ga (second row), the start of multicellular life on Earth at 0.8 Ga (third row), and the modern atmosphere (fourth row). Source: Rugheimer & Kaltenegger 2017 [5]

Figure 2 shows the simulated spectra for the star types. Because of the loss of shorter wavelengths with M-Dwarf stars, the O2 signatures are largely lost. This means that even should a planet around an M-Dwarf evolve photosynthesis and create an oxidizing atmosphere, this may not be detectable around such a world.

Figure 2. Disk-integrated VIS/NIR spectra at a resolution of 800 at the TOA for an Earth-like planet for the grid of stellar and geological epoch models assuming 60% Earth-analogue cloud coverage. For individual features highlighting the O2, O3, and H2O/CH4 bands in the VIS spectrum. Source: Rugheimer & Kaltenegger 2017 [5] [TOA = Top of the atmosphere – AMT]

In contrast, the strong markers for CO2 and CH4 are well represented in the spectrum for M type stars. This creates a complication for a biosignature for early life comparable to the Archean and early Proterozoic periods on Earth. An atmosphere of CO2 and CH4 assumes that the CH4 is due to methanogens being the dominant source of CH4, far outstripping geologic sources. On the Hadean Earth, CH4 outgassing should be rapidly eliminated by UV. During the Archean, the biogenic production of CH4 maintains the CH4 and therefore the disequilibrium biosignature. But on an M-Dwarf world, this CH4 photolysis is largely absent, resulting in a CO2/CH4 biosignature that is a false positive.

If photosynthesis evolves, the O2 signal can be detected at the longer wavelength of 760 nm, but only if there is no cloud cover, as shown in figure 3. For an M-Dwarf planet, clouds mask the O2 signal, and we expect more cloud cover due to the increased H2O on such worlds.

Figure 3. Disk-integrated spectra (R = 800) of the O2 feature at 0.76 m for clear sky in relative reflectivity (left) and the detectable reflected emergent flux for clear sky (middle) and 60% cloud cover (right). Source: Rugheimer & Kaltenegger 2017 [5]. Note the loss of detectable O2 feature for M-type stars – AMT

Fortunately, ozone (O3) can be detected strongly in the IR around 9500 nm, so we can hope to detect photosynthetic life when the O2 partial pressure increases. Figure 4 shows that the O3 signature can be detected in the Archean in the Phanerozoic, but not the Proterozoic.

Figure 4. Smoothed, disk-integrated IR spectra at the TOA for an Earth-like planet for the grid of stellar and geological epoch models assuming 60% Earth-analogue cloud coverage. For individual features highlighting the O3, H2O/CH4, and CO2 bands in the IR spectrum see Figs. 9, 10, and 11, respectively. Source: Rugheimer & Kaltenegger 2017 [5]

While current instruments cannot resolve spectra in sufficient detail to detect the needed signatures of gases, the authors conclude

“These spectra can be a useful input to design instruments and to optimize the observation strategy for direct imaging or secondary eclipse observations with EELT or JWST as well as other future mission design concepts such as LUVOIR/HDST.”

To conclude, the type of star complicates biosignature detection, especially the co-presence of CO2 and CH4 in Archean and early Proterozoic eons that dominate the history of life on Earth. Not only is the star’s light shifted, hiding shorter wavelength signals, but the light itself impacts the equilibrium composition of atmospheric gases which can lead to biosignature ambiguity.

While the ubiquity of M-Dwarf stars and the longevity of low O2 atmospheres due to the time to evolve photosynthesis on Earth and the delay before the atmosphere builds up its O2 partial pressure, favors M-Dwarf stars as targets for looking for early life, the potential of false positives for the Archaean and early Proterozoic equivalent eons complicates the search for life on these worlds using expected biosignatures for worlds around sol-like stars. There is still work to be done to resolve these issues.

References

1. N. B. Cowan et al “Characterizing Transiting Planet Atmospheres through 2025”

2015 PASP 127 311. DOI: https://doi.org/10.1086/680855

2. Tolley, A, “Detecting Early Life on Exoplanets”, 02/23/2018. https://www.centauri-dreams.org/2018/02/23/detecting-early-life-on-exoplanets/

3. Krissansen-Totton et al “Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life” 2018 Science Advances Vol. 4, no. 1. DOI: 10.1126/sciadv.aao5747

4. Kaltenegger et al “Spectral Evolution of an Earth-like Planet”, The Astrophysical Journal, 658:598Y616, 2007 March 20 (abstract).

5. Rugheimer, Kaltenegger “Spectra of Earth-like Planets Through Geological Evolution Around FGKM Stars”, The Astrophysical Journal 854(1). DOI: 10.3847/1538-4357/aaa47a

6. Burrows, A. S ”Spectra as windows into exoplanet atmospheres,” 2014, PNAS, 111, 12601 (abstract)

7. Ehrenreich D “Transmission spectra of exoplanet atmospheres” 2011 http://www-astro.physik.tu-berlin.de/plato-2011/talks/PLATO_SC2011_S03T06_Ehrenreich.pdf