We perform a study of stellar flares for the 24,809 stars observed with 2 minute cadence during the first two months of the TESS mission. Flares may erode exoplanets' atmospheres and impact their ...habitability, but might also trigger the genesis of life around small stars. TESS provides a new sample of bright dwarf stars in our galactic neighborhood, collecting data for thousands of M dwarfs that might host habitable exoplanets. Here, we use an automated search for flares accompanied by visual inspection. Then, our public allesfitter code robustly selects the appropriate model for potentially complex flares via Bayesian evidence. We identify 1228 flaring stars, 673 of which are M dwarfs. Among 8695 flares in total, the largest superflare increased the stellar brightness by a factor of 16.1. Bolometric flare energies range from 1031.0 to 1036.9 erg, with a median of 1033.1 erg. Furthermore, we study the flare rate and energy as a function of stellar type and rotation period. We solidify past findings that fast rotating M dwarfs are the most likely to flare and that their flare amplitude is independent of the rotation period. Finally, we link our results to criteria for prebiotic chemistry, atmospheric loss through coronal mass ejections, and ozone sterilization. Four of our flaring M dwarfs host exoplanet candidates alerted on by TESS, for which we discuss how these effects can impact life. With upcoming TESS data releases, our flare analysis can be expanded to almost all bright small stars, aiding in defining criteria for exoplanet habitability.
Abstract
Waste gas products from technological civilizations may accumulate in an exoplanet atmosphere to detectable levels. We propose nitrogen trifluoride (NF
3
) and sulfur hexafluoride (SF
6
) as ...ideal technosignature gases. Earth life avoids producing or using any N–F or S–F bond-containing molecules and makes no fully fluorinated molecules with any element. NF
3
and SF
6
may be universal technosignatures owing to their special industrial properties, which unlike biosignature gases, are not species-dependent. Other key relevant qualities of NF
3
and SF
6
are: their extremely low water solubility, unique spectral features, and long atmospheric lifetimes. NF
3
has no non-human sources and was absent from Earth’s pre-industrial atmosphere. SF
6
is released in only tiny amounts from fluorine-containing minerals, and is likely produced in only trivial amounts by volcanic eruptions. We propose a strategy to rule out SF
6
’s abiotic source by simultaneous observations of SiF
4
, which is released by volcanoes in an order of magnitude higher abundance than SF
6
. Other fully fluorinated human-made molecules are of interest, but their chemical and spectral properties are unavailable. We summarize why life on Earth—and perhaps life elsewhere—avoids using F. We caution, however, that we cannot definitively disentangle an alien biochemistry byproduct from a technosignature gas.
WD 1145+017 is a unique white dwarf system that has a heavily polluted atmosphere, an infrared excess from a dust disk, numerous broad absorption lines from circumstellar gas, and changing transit ...features, likely from fragments of an actively disintegrating asteroid. Here, we present results from a large photometric and spectroscopic campaign with Hubble Space Telescope, Keck, Very Large Telescope (VLT), Spitzer, and many other smaller telescopes from 2015 to 2018. Somewhat surprisingly the ultraviolet (UV) transit depths are always shallower than those in the optical. We develop a model that can quantitatively explain the observed "bluing" and confirm the previous finding that: (1) the transiting objects, circumstellar gas, and white dwarf are all aligned along our line of sight; (2) the transiting object is blocking a larger fraction of the circumstellar gas than of the white dwarf itself. Because most circumstellar lines are concentrated in the UV, the UV flux appears to be less blocked compared to the optical during a transit, leading to a shallower UV transit. This scenario is further supported by the strong anticorrelation between optical transit depth and circumstellar line strength. We have yet to detect any wavelength-dependent transits caused by the transiting material around WD 1145+017.
The thermodynamic properties of a substance are key to predicting its behavior in physical and chemical systems. Specifically, the enthalpy of formation and entropy of a substance can be used to ...predict whether reactions involving that substance will proceed spontaneously under conditions of constant temperature and pressure, and if they do, what the heat and work yield of those reactions would be. Prediction of enthalpy and entropy of substances is therefore of value for substances for which those parameters have not been experimentally measured. We developed a database of 2869 experimental values of enthalpy of formation and 1403 values for entropy for substances composed of stable small molecules, derived from the literature. We developed a model for predicting enthalpy of formation and entropy from semiempirical quantum mechanical calculations of energy and atom counts, and applied the model to a comprehensive database of 16,417 small molecules. The database of small-molecule thermodynamic properties will be useful for predicting the outcome of any process that might involve the generation or destruction of volatile products, such as atmospheric chemistry, volcanism, or waste pyrolysis. Additionally, the collected experimental thermodynamic values will be of value to others developing models to predict enthalpy and entropy.
A long-term goal of exoplanet studies is the identification and detection of biosignature gases. Beyond the most discussed biosignature gas O
, only a handful of gases have been considered in detail. ...In this study, we evaluate phosphine (PH
). On Earth, PH
is associated with anaerobic ecosystems, and as such, it is a potential biosignature gas in anoxic exoplanets. We simulate the atmospheres of habitable terrestrial planets with CO
- and H
-dominated atmospheres and find that PH
can accumulate to detectable concentrations on planets with surface production fluxes of 10
to 10
cm
s
(corresponding to surface concentrations of 10s of ppb to 100s of ppm), depending on atmospheric composition and ultraviolet (UV) irradiation. While high, the surface flux values are comparable to the global terrestrial production rate of methane or CH
(10
cm
s
) and below the maximum local terrestrial PH
production rate (10
cm
s
). As with other gases, PH
can more readily accumulate on low-UV planets, for example, planets orbiting quiet M dwarfs or with a photochemically generated UV shield. PH
has three strong spectral features such that in any atmosphere scenario one of the three will be unique compared with other dominant spectroscopic molecules. Phosphine's weakness as a biosignature gas is its high reactivity, requiring high outgassing rates for detectability. We calculate that tens of hours of JWST (James Webb Space Telescope) time are required for a potential detection of PH
. Yet, because PH
is spectrally active in the same wavelength regions as other atmospherically important molecules (such as H
O and CH
), searches for PH
can be carried out at no additional observational cost to searches for other molecular species relevant to characterizing exoplanet habitability. Phosphine is a promising biosignature gas, as it has no known abiotic false positives on terrestrial planets from any source that could generate the high fluxes required for detection.
Planets residing in circumstellar habitable zones offer us the best opportunities to test hypotheses of life's potential pervasiveness and complexity. Constraining the precise boundaries of ...habitability and its observational discriminants is critical to maximizing our chances at remote life detection with future instruments. Conventionally, calculations of the inner edge of the habitable zone (IHZ) have been performed using both 1D radiative-convective and 3D general circulation models. However, these models lack interactive 3D chemistry and do not resolve the mesosphere and lower thermosphere region of the upper atmosphere. Here, we employ a 3D high-top chemistry-climate model (CCM) to simulate the atmospheres of synchronously rotating planets orbiting at the inner edge of habitable zones of K- and M-dwarf stars (between Teff = 2600 and 4000 K). While our IHZ climate predictions are in good agreement with general circulation model studies, we find noteworthy departures in simulated ozone and HOx photochemistry. For instance, climates around inactive stars do not typically enter the classical moist greenhouse regime even with high ( 10−3 mol mol−1) stratospheric water vapor mixing ratios, which suggests that planets around inactive M-stars may only experience minor water-loss over geologically significant timescales. In addition, we find much thinner ozone layers on potentially habitable moist greenhouse atmospheres, as ozone experiences rapid destruction via reaction with hydrogen oxide radicals. Using our CCM results as inputs, our simulated transmission spectra show that both water vapor and ozone features could be detectable by instruments NIRSpec and MIRI LRS on board the James Webb Space Telescope.
Abstract
The search for signs of life beyond Earth is a crucial driving motivation of exoplanet science, fueling new work on biosignature gases in habitable exoplanet atmospheres. We study carbonyls, ...a category of molecules containing the C=O double bond, following our proposal to systematically identify plausible biosignature gas candidates from a list of all small volatile molecules. We rule out carbonyls as biosignature gases due to both their high water solubility and their high photolysis rate, despite their ubiquitous production by life on Earth, their critical importance in Earth’s life biochemistry, and their uniquely identifiable molecular spectral features in mid- to low-resolution spectroscopy. Even in scenarios where life is a large net source of carbonyls, we demonstrate that detection of carbonyls is not possible on even the most ideal habitable exoplanet, because only 10 ppb of carbonyls can accumulate under our most optimistic assumptions. Moreover, high biological fluxes of organic carbon gases, while mathematically possible, are likely biologically unattainable due to the resulting huge waste of carbon—a main building block for life. Our simulations show that photochemical processing of carbonyls leads to generation of CO in quantities that can reengineer the atmosphere, affirming the ambiguity of CO as an antibiosignature. Overall, we find that the expression of a carbonyl-producing biosphere by CO, though potentially detectable by the James Webb Space Telescope, is unable to be uniquely traced back to carbonyls. While carbonyls fail as a bioindicator, by investigating them we have made a significant step toward systematically assessing the biosignature gas potential of all small volatile molecules.
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