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Register a new userDetectability of biosignatures on LHS 1140 b
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Terrestrial extrasolar planets around low-mass stars are prime targets when searching for atmospheric biosignatures with current and near-future telescopes. The habitable-zone Super-Earth LHS 1140 b could hold a hydrogen-dominated atmosphere and is an excellent candidate for detecting atmospheric features. In this study, we investigate how the instellation and planetary parameters influence the atmospheric climate, chemistry, and spectral appearance of LHS 1140 b. We study the detectability of selected molecules, in particular potential biosignatures, with the upcoming James Webb Space Telescope (JWST) and Extremely Large Telescope (ELT). In a first step we use the coupled climate-chemistry model, 1D-TERRA, to simulate a range of assumed atmospheric chemical compositions dominated by H$_2$ and CO$_2$. Further, we vary the concentrations of CH$_4$ by several orders of magnitude. In a second step we calculate transmission spectra of the simulated atmospheres and compare them to recent transit observations. Finally, we determine the observation time required to detect spectral bands with low resolution spectroscopy using JWST and the cross-correlation technique using ELT. In H$_2$-dominated and CH$_4$-rich atmospheres O$_2$ has strong chemical sinks, leading to low concentrations of O$_2$ and O$_3$. The potential biosignatures NH$_3$, PH$_3$, CH$_3$Cl and N$_2$O are less sensitive to the concentration of H$_2$, CO$_2$ and CH$_4$ in the atmosphere. In the simulated H$_2$-dominated atmosphere the detection of these gases might be feasible within 20 to 100 observation hours with ELT or JWST, when assuming weak extinction by hazes. If further observations of LHS 1140 b suggest a thin, clear, hydrogen-dominated atmosphere, the planet would be one of the best known targets to detect biosignature gases in the atmosphere of a habitable-zone rocky exoplanet with upcoming telescopes.
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Atmospheric characterisation of temperate, rocky planets is the holy grail of exoplanet studies. These worlds are at the limits of our capabilities with current instrumentation in transmission spectroscopy and challenge our state-of-the-art statistical techniques. Here we present the transmission spectrum of the temperate Super-Earth LHS 1140b using the Hubble Space Telescope (HST). The Wide Field Camera 3 (WFC3) G141 grism data of this habitable zone (T$_{rm{eq}}$ = 235 K) Super-Earth (R = 1.7 $R_oplus$), shows tentative evidence of water. However, the signal-to-noise ratio, and thus the significance of the detection, is low and stellar contamination models can cause modulation over the spectral band probed. We attempt to correct for contamination using these models and find that, while many still lead to evidence for water, some could provide reasonable fits to the data without the need for molecular absorption although most of these cause also features in the visible ground-based data which are nonphysical. Future observations with the James Webb Space Telescope (JWST) would be capable of confirming, or refuting, this atmospheric detection.
LHS 1140 is an M dwarf known to host two known transiting planets at orbital periods of 3.77 and 24.7 days. The external planet (LHS 1140 b) is a rocky super-Earth that is located in the middle of the habitable zone of this low-mass star, placing this system at the forefront of the habitable exoplanet exploration. We further characterize this system by improving the physical and orbital properties and search for additional planetary-mass components in the system, also exploring the possibility of co-orbitals. We collected 113 new radial velocity observations with ESPRESSO over a 1.5-year time span with an average photon-noise precision of 1.07 m/s. We determine new masses with a precision of 6% for LHS 1140 b ($6.48 pm 0.46~M_{oplus}$) and 9% for LHS 1140 c ($m_c=1.78 pm 0.17~M_{oplus}$), reducing by half the previously published uncertainties. Although both planets have Earth-like bulk compositions, the internal structure analysis suggests that LHS 1140 b might be iron-enriched. In both cases, the water content is compatible to a maximum fraction of 10-12% in mass, which is equivalent to a deep ocean layer of $779 pm 650$ km for the habitable-zone planet LHS 1140 b. Our results also provide evidence for a new planet candidate in the system ($m_d= 4.8pm1.1~M_{oplus}$) on a ~78.9-day orbital period, which is detected through three independent methods. The analysis also allows us to discard other planets above 0.5 $M_{oplus}$ for periods shorter than 10 days and above 2 $M_{oplus}$ for periods up to one year. Finally, our analysis discards co-orbital planets of LHS 1140 b down to 1 $M_{oplus}$. Indications for a possible co-orbital signal in LHS 1140 c are detected in both radial velocity and photometric data, however. The new characterization of the system make it a key target for atmospheric studies of rocky worlds at different stellar irradiations
LHS 1140 is a nearby mid-M dwarf known to host a temperate rocky super-Earth (LHS 1140 b) on a 24.737-day orbit. Based on photometric observations by MEarth and Spitzer as well as Doppler spectroscopy from HARPS, we report the discovery of an additional transiting rocky companion (LHS 1140 c) with a mass of $1.81pm0.39~{rm M_{Earth}}$ and a radius of $1.282pm0.024~{rm R_{Earth}}$ on a tighter, 3.77795-day orbit. We also obtain more precise estimates of the mass and radius of LHS 1140 b to be $6.98pm0.89~{rm M_{Earth}}$ and $1.727pm0.032~{rm R_{Earth}}$. The mean densities of planets b and c are $7.5pm1.0~rm{g/cm^3}$ and $4.7pm1.1~rm{g/cm^3}$, respectively, both consistent with the Earths ratio of iron to magnesium silicate. The orbital eccentricities of LHS 1140 b and c are consistent with circular orbits and constrained to be below 0.06 and 0.31, respectively, with 90% confidence. Because the orbits of the two planets are co-planar and because we know from previous analyses of Kepler data that compact systems of small planets orbiting M dwarfs are commonplace, a search for more transiting planets in the LHS 1140 system could be fruitful. LHS 1140 c is one of the few known nearby terrestrial planets whose atmosphere could be studied with the upcoming James Webb Space Telescope.
We investigate atmospheric responses of modeled hypothetical Earth-like planets in the habitable zone of the M-dwarf AD Leonis to reduced oxygen (O2), removed biomass (dead Earth), varying carbon dioxide (CO2) and surface relative humidity (sRH). Results suggest large O2 differences between the reduced O2 and dead scenarios in the lower but not the upper atmosphere. Ozone (O3) and nitrous oxide (N2O) also show this behavior. Methane depends on hydroxyl (OH), its main sink. Abiotic production of N2O occurs in the upper layers. Chloromethane (CH3Cl) decreases everywhere on decreasing biomass. Changing CO2 (from x1 to x100 present atmospheric level (PAL)) and surface relative humidity (sRH) (from 0.1 percent to 100 percent) does not influence CH3Cl as much as lowering biomass. Therefore, CH3Cl can be considered a good biosignature. Changing sRH and CO2 has a greater influence on temperature than O2 and biomass alone. Changing the biomass produces ~6 kilometer (km) in effective height (H) in transmission compared with changing CO2 and sRH ( about 25km). In transmission O2 is discernible at 0.76 microns for greater than 0.1 PAL. The O3 9.6 micron band was weak for the low O2 runs and difficult to discern from dead Earth, however O3 at 0.3 microns could serve as an indicator to distinguish between reduced O2 and dead Earth. Spectral features of N2O and CH3Cl corresponded to some km H. CH4 could be detectable tens of parsecs away with ELT except for the 10-4 and 10-6 PAL O2 scenarios. O2 is barely detectable for the 1 PAL O2 case and unfeasible at lower abundances.
The tectonic regime of rocky planets fundamentally influences their long-term evolution and cycling of volatiles between interior and atmosphere. Earth is the only known planet with active plate tectonics, but observations of exoplanets may deliver insights into the diversity of tectonic regimes beyond the solar system. Observations of the thermal phase curve of super-Earth LHS 3844b reveal a solid surface and lack of a substantial atmosphere, with a temperature contrast between the substellar and antistellar point of around 1000 K. Here, we use these constraints on the planets surface to constrain the interior dynamics and tectonic regimes of LHS 3844b using numerical models of interior flow. We investigate the style of interior convection by assessing how upwellings and downwellings are organized and how tectonic regimes manifest. We discover three viable convective regimes with a mobile surface: (1) spatially uniform distribution of upwellings and downwellings, (2) prominent downwelling on the dayside and upwellings on the nightside, and (3) prominent downwelling on the nightside and upwellings on the dayside. Hemispheric tectonics is observed for regimes (2) and (3) as a direct consequence of the day-to-night temperature contrast. Such a tectonic mode is absent in the present-day solar system and has never been inferred from astrophysical observations of exoplanets. Our models offer distinct predictions for volcanism and outgassing linked to the tectonic regime, which may explain secondary features in phase curves and allow future observations to constrain the diversity of super-Earth interiors.
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