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Register a new userThe Super-Earth Opportunity - Search for Habitable Exoplanets in the 2020s
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The recent discovery of a staggering diversity of planets beyond the Solar System has brought with it a greatly expanded search space for habitable worlds. The Kepler exoplanet survey has revealed that most planets in our interstellar neighborhood are larger than Earth and smaller than Neptune. Collectively termed super-Earths and mini-Neptunes, some of these planets may have the conditions to support liquid water oceans, and thus Earth-like biology, despite differing in many ways from our own planet. In addition to their quantitative abundance, super-Earths are relatively large and are thus more easily detected than true Earth twins. As a result, super-Earths represent a uniquely powerful opportunity to discover and explore a panoply of fascinating and potentially habitable planets in 2020 - 2030 and beyond.
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We report the detection of GJ 832c, a super-Earth orbiting near the inner edge of the habitable zone of GJ 832, an M dwarf previously known to host a Jupiter analog in a nearly-circular 9.4-year orbit. The combination of precise radial-velocity measurements from three telescopes reveals the presence of a planet with a period of 35.68+/-0.03 days and minimum mass (m sin i) of 5.4+/-1.0 Earth masses. GJ 832c moves on a low-eccentricity orbit (e=0.18+/-0.13) towards the inner edge of the habitable zone. However, given the large mass of the planet, it seems likely that it would possess a massive atmosphere, which may well render the planet inhospitable. Indeed, it is perhaps more likely that GJ 832c is a super-Venus, featuring significant greenhouse forcing. With an outer giant planet and an interior, potentially rocky planet, the GJ 832 planetary system can be thought of as a miniature version of our own Solar system.
We present an analysis of the significantly expanded HARPS 2011 radial velocity data set for GJ 581 that was presented by Forveille et al. (2011). Our analysis reaches substantially different conclusions regarding the evidence for a Super-Earth-mass planet in the stars Habitable Zone. We were able to reproduce their reported chi_{ u}^2 and RMS values only after removing some outliers from their models and refitting the trimmed down RV set. A suite of 4000 N-body simulations of their Keplerian model all resulted in unstable systems and revealed that their reported 3.6sigma detection of e=0.32 for the eccentricity of GJ 581e is manifestly incompatible with the systems dynamical stability. Furthermore, their Keplerian model, when integrated only over the time baseline of the observations, significantly increases the chi_{ u}^2 and demonstrates the need for including non-Keplerian orbital precession when modeling this system. We find that a four-planet model with all of the planets on circular or nearly circular orbits provides both an excellent self-consistent fit to their RV data and also results in a very stable configuration. The periodogram of the residuals to a 4-planet all-circular-orbit model reveals significant peaks that suggest one or more additional planets in this system. We conclude that the present 240-point HARPS data set, when analyzed in its entirety, and modeled with fully self-consistent stable orbits, by and of itself does offer significant support for a fifth signal in the data with a period near 32 days. This signal has a False Alarm Probability of <4% and is consistent with a planet of minimum mass of 2.2 Earth-masses, orbiting squarely in the stars Habitable Zone at 0.13 AU, where liquid water on planetary surfaces is a distinct possibility
We report on radial velocity time series for two M0.0V stars, GJ338B and GJ338A, using the CARMENES spectrograph, complemented by ground-telescope photometry from Las Cumbres and Sierra Nevada observatories. We aim to explore the presence of small planets in tight orbits using the spectroscopic radial velocity technique. We obtained 159 and 70 radial velocity measurements of GJ338B and A, respectively, with the CARMENES visible channel. We also compiled additional relative radial velocity measurements from the literature and a collection of astrometric data that cover 200 a of observations to solve for the binary orbit. We found dynamical masses of 0.64$pm$0.07M$_odot$ for GJ338B and 0.69$pm$0.07M$_odot$ for GJ338A. The CARMENES radial velocity periodograms show significant peaks at 16.61$pm$0.04 d (GJ338B) and 16.3$^{+3.5}_{-1.3}$ d (GJ338A), which have counterparts at the same frequencies in CARMENES activity indicators and photometric light curves. We attribute these to stellar rotation. GJ338B shows two additional, significant signals at 8.27$pm$0.01 and 24.45$pm$0.02 d, with no obvious counterparts in the stellar activity indices. The former is likely the first harmonic of the stars rotation, while we ascribe the latter to the existence of a super-Earth planet with a minimum mass of 10.27$^{+1.47}_{-1.38}$$M_{oplus}$ orbiting GJ338B. GJ338B b lies inside the inner boundary of the habitable zone around its parent star. It is one of the least massive planets ever found around any member of stellar binaries. The masses, spectral types, brightnesses, and even the rotational periods are very similar for both stars, which are likely coeval and formed from the same molecular cloud, yet they differ in the architecture of their planetary systems.
In the last few years many exoplanets in the habitable zone (HZ) of M-dwarfs have been discovered, but the X-ray/UV activity of cool stars is very different from that of our Sun. The high-energy radiation environment influences the habitability, plays a crucial role for abiogenesis, and impacts planetary atmospheres. LHS 1140b is a super-Earth-size planet orbiting in the HZ of LHS 1140, an M4.5 dwarf at ~15 parsecs. We present the results of a Swift X-ray/UV observing campaign. We characterize for the first time the X-ray/UV radiation environment of LHS 1140b. We measure the variability of the near ultraviolet (NUV) flux and estimate the far ultraviolet (FUV) flux with a correlation between FUV and NUV flux of a sample of low-mass stars in the GALEX archive. We highlight the presence of a dominating X-ray source close to the J2000 coordinates of LHS 1140, characterize its spectrum, and derive an X-ray flux upper limit for LHS 1140. We find that this contaminant source could have influenced the previously estimated spectral energy distribution. No significant variation of the NUV flux of LHS 1140 is found over 3 months, and we do not observe any flare during the 38 ks on the target. LHS 1140 is in the 25th percentile of least variable M4-M5 dwarfs of the GALEX sample. Analyzing the UV flux experienced by the HZ planet LHS 1140b, we find that outside the atmosphere it receives a NUV flux <2% with respect to that of the present-day Earth, while the FUV/NUV ratio is ~100-200 times higher. This represents a lower limit to the true FUV/NUV ratio since the GALEX FUV band does not include Lyman-alpha, which dominates the FUV output of low-mass stars. This is a warning for future searches for biomarkers, which must take into account this high ratio. The relatively low level and stability of UV flux experienced by LHS 1140b should be favorable for its present-day habitability.
We report the first ground-based transit observation of K2-3d, a 1.5 R_Earth planet supposedly within the habitable zone around a bright M-dwarf host star, using the Okayama 188 cm telescope and the multi(grz)-band imager MuSCAT. Although the depth of the transit (0.7 mmag) is smaller than the photometric precisions (1.2, 0.9, and 1.2 mmag per 60 s for the g, r, and z bands, respectively), we marginally but consistently identify the transit signal in all three bands, by taking advantage of the transit parameters from K2, and by introducing a novel technique that leverages multi-band information to reduce the systematics caused by second-order extinction. We also revisit previously analyzed Spitzer transit observations of K2-3d to investigate the possibility of systematic offsets in transit timing, and find that all the timing data can be explained well by a linear ephemeris. We revise the orbital period of K2-3d to be 44.55612 pm 0.00021 days, which corrects the predicted transit times for 2019, i.e., the era of the James Webb Space Telescope, by sim80 minutes. Our observation demonstrates that (1) even ground-based, 2 m class telescopes can play an important role in refining the transit ephemeris of small-sized, long-period planets, and that (2) a multi-band imager is useful to reduce the systematics of atmospheric origin, in particular for bluer bands and for observations conducted at low-altitude observatories.
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