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Register a new userSuper-Earths, M Dwarfs, and Photosynthetic Organisms: Habitability in the Lab
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In a few years, space telescopes will investigate our Galaxy to detect evidence of life, mainly by observing rocky planets. In the last decade, the observation of exoplanet atmospheres and the theoretical works on biosignature gasses have experienced a considerable acceleration. The~most attractive feature of the realm of exoplanets is that 40% of M dwarfs host super-Earths with a minimum mass between 1 and 30 Earth masses, orbital periods shorter than 50 days, and radii between those of the Earth and Neptune (1--3.8 R$_oplus$). Moreover, the recent finding of cyanobacteria able to use far-red (FR) light for oxygenic photosynthesis due to the synthesis of chlorophylls $d$ and $f$, extending in vivo light absorption up to 750 nm, suggests the possibility of exotic photosynthesis in planets around M dwarfs. Using innovative laboratory instrumentation, we exposed different cyanobacteria to an M dwarf star simulated irradiation, comparing their responses to those under solar and FR simulated lights.~As expected, in FR light, only the cyanobacteria able to synthesize chlorophyll $d$ and $f$ could grow. Surprisingly, all strains, both able or unable to use FR light, grew and photosynthesized under the M dwarf generated spectrum in a similar way to the solar light and much more efficiently than under the FR one. Our findings highlight the importance of simulating both the visible and FR light components of an M dwarf spectrum to correctly evaluate the photosynthetic performances of oxygenic organisms exposed under such an exotic light~condition.
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We announce the discovery of two planets orbiting the M dwarfs GJ 251 ($0.360pm0.015$ M$_odot$) and HD 238090 ($0.578pm0.021$ M$_odot$) based on CARMENES radial velocity (RV) data. In addition, we independently confirm with CARMENES data the existence of Lalande 21185 b, a planet that has recently been discovered with the SOPHIE spectrograph. All three planets belong to the class of warm or temperate super-Earths and share similar properties. The orbital periods are 14.24 d, 13.67 d, and 12.95 d and the minimum masses are $4.0pm0.4$ $M_oplus$, $6.9pm0.9$ $M_oplus$, and $2.7pm0.3$ $M_oplus$ for GJ 251 b, HD 238090 b, and Lalande 21185 b, respectively. Based on the orbital and stellar properties, we estimate equilibrium temperatures of $351.0pm1.4$ K for GJ 251 b, $469.6pm2.6$ K for HD 238090 b, and $370.1pm6.8$ K for Lalande 21185 b. For the latter we resolve the daily aliases that were present in the SOPHIE data and that hindered an unambiguous determination of the orbital period. We find no significant signals in any of our spectral activity indicators at the planetary periods. The RV observations were accompanied by contemporaneous photometric observations. We derive stellar rotation periods of $122.1pm2.2$ d and $96.7pm3.7$ d for GJ 251 and HD 238090, respectively. The RV data of all three stars exhibit significant signals at the rotational period or its first harmonic. For GJ 251 and Lalande 21185, we also find long-period signals around 600 d, and 2900 d, respectively, which we tentatively attribute to long-term magnetic cycles. We apply a Bayesian approach to carefully model the Keplerian signals simultaneously with the stellar activity using Gaussian process regression models and extensively search for additional significant planetary signals hidden behind the stellar activity.
We present observations of two bright M dwarfs (TOI-1634 and TOI-1685: $J=9.5-9.6$) hosting ultra-short period (USP) planets, identified by the TESS mission. The two stars are similar in temperature, mass, and radius ($T_mathrm{eff},approx,3500$ K, $M_star,approx,0.45-0.46,M_odot$, and $R_starapprox 0.45-0.46,R_odot$), and the planets are both super-Earth-sized ($1.25,R_oplus<R_p<2.0,R_oplus$). For both systems, light curves from the ground-based photometry exhibit planetary transits, whose depths are consistent with those by the TESS photometry. We also refine the transit ephemerides based on the ground-based photometry, finding the orbital periods of $P=0.9893436pm0.0000020$ day and $P=0.6691416pm0.0000019$ day for TOI-1634b and TOI-1685b, respectively. Through intensive radial velocity (RV) observations using IRD on the Subaru 8.2m telescope, we confirm the planetary nature of the TOIs, and measure their masses: $10.14pm0.95,M_oplus$ and $3.43pm0.93,M_oplus$ for TOI-1634b and TOI-1685b, respectively, when the observed RVs are fitted with a single-planet circular-orbit model. Combining those with the planet radii of $R_p=1.749pm 0.079,R_oplus$ (TOI-1634b) and $1.459pm0.065,R_oplus$ (TOI-1685b), we find that both USP planets have mean densities consistent with an Earth-like internal composition, which is typical for small USP planets. TOI-1634b is currently the most massive USP planet in this category, and it resides near the radius valley, which makes it a benchmark planet in the context of discussing the size limit of rocky planet cores as well as testing the formation scenarios for USP planets. Excess scatter in the RV residuals for TOI-1685 suggests the presence of a possible secondary planet or unknown activity/instrumental noise in the RV data, but further observations are required to check those possibilities.
We present estimations of dipolar magnetic moments for terrestrial exoplanets using the Olson & Christiansen (2006) scaling law and assuming their interior structure is similar to Earth. We find that the dipolar moment of fast rotating planets (where the Coriolis force dominates convection in the core), may amount up to ~80 times the magnetic moment of Earth, M_Earth, for at least part of the planets lifetime. For very slow rotating planets (where the force of inertia dominates), the dipolar magnetic moment only reaches up to ~1.5 M_Earth. Applying our calculations to currently confirmed rocky exoplanets, we find that CoRoT-7b, Kepler-10b and 55 Cnc e can sustain dynamos up to ~ 18, 15 and 13 M_Earth, respectively. Our results also indicate that the magnetic moment of rocky exoplanets not only depends on their rotation rate, but also on their formation history, thermal state, age and composition, as well as the geometry of the field. These results apply to all rocky planets, but have important implications for the particular case of exoplanets in the Habitable Zone of M-dwarfs.
We announce the discovery of two planetary companions orbiting around the low mass stars Ross 1020 (GJ 3779, M4.0V) and LP 819-052 (GJ 1265, M4.5V). The discovery is based on the analysis of CARMENES radial velocity observations in the visual channel as part of its survey for exoplanets around M dwarfs. In the case of GJ 1265, CARMENES observations were complemented with publicly available Doppler measurements from HARPS. The datasets reveal one planetary companion for each star that share very similar properties: minimum masses of $8.0pm0.5$ M$_{oplus}$ and $7.4pm0.5$ M$_{oplus}$ in low-eccentricity orbits with periods of $3.023pm0.001$ d and $3.651pm0.001$ d for GJ 3779 b and GJ 1265 b, respectively. The periodic signals around three days found in the radial velocity data have no counterpart in any spectral activity indicator. Besides, we collected available photometric data for the two host stars, which confirm that the additional Doppler variations found at periods around 95 d can be attributed to the rotation of the stars. The addition of these planets in a mass-period diagram of known planets around M dwarfs suggests a bimodal distribution with a lack of short-period low-mass planets in the range of 2-5 M$_{oplus}$. It also indicates that super-Earths (> 5 M$_{oplus}$) currently detected by radial velocity and transit techniques around M stars are usually found in systems dominated by a single planet.
Habitability has been generally defined as the capability of an environment to support life. Ecologists have been using Habitat Suitability Models (HSMs) for more than four decades to study the habitability of Earth from local to global scales. Astrobiologists have been proposing different habitability models for some time, with little integration and consistency among them, being different in function to those used by ecologists. Habitability models are not only used to determine if environments are habitable or not, but they also are used to characterize what key factors are responsible for the gradual transition from low to high habitability states. Here we review and compare some of the different models used by ecologists and astrobiologists and suggest how they could be integrated into new habitability standards. Such standards will help to improve the comparison and characterization of potentially habitable environments, prioritize target selections, and study correlations between habitability and biosignatures. Habitability models are the foundation of planetary habitability science and the synergy between ecologists and astrobiologists is necessary to expand our understanding of the habitability of Earth, the Solar System, and extrasolar planets.
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