super-Earths Research Articles

HORuS transmission spectroscopy of 55 Cnc e

ABSTRACT The High Optical Resolution Spectrograph (HORuS) is a new high-resolution echelle spectrograph available on the 10.4-m Gran Telescopio Canarias (GTC). We report on the first HORuS observations of a transit of the super-Earth planet 55 Cnc e. We investigate the presence of Na i and Hα in its transmission spectrum and explore the capabilities of HORuS for planetary transmission spectroscopy. Our methodology leads to residuals in the difference spectrum between the in-transit and out-of-transit spectra for the Na i doublet lines of (3.4 ± 0.4) × 10−4, which sets an upper limit to the detection of line absorption from the planetary atmosphere that is one order of magnitude more stringent that those reported in the literature. We demonstrate that we are able to reach the photon-noise limit in the residual spectra using HORuS to a degree that we would be able to easily detect giant planets with larger atmospheres. In addition, we modelled the structure, chemistry, and transmission spectrum of 55 Cnc e using state-of-the-art open source tools.

How Flow Isolation May Set the Mass Scale for Super-Earth Planets

Abstract Much recent work on planet formation has focused on the growth of planets by accretion of grains whose aerodynamic properties make them marginally coupled to the nebular gas, a theory commonly referred to as “pebble accretion.” While the rapid growth rates of pebble accretion can ameliorate some problems in planet formation theory, they raise new concerns as well. A particular issue is the preponderance of observed planets that end their growth as “super-Earths” or “sub-Neptunes,” with masses in the range 2–10 M ⊕. Once planets reach this mass scale, growth by pebble accretion is so rapid that ubiquitously ending growth at super-Earth masses is difficult unless growth rates drop at this mass scale. In this work, we highlight this issue in detail using our previously published model of pebble accretion, and also propose a reason for this change in growth rate: feedback between the growing planet’s atmosphere and the gas disk inhibits accretion of smaller particle sizes by forcing them to flow around the growing planet instead of being accreted. For reasonable fiducial disk parameters, this “flow isolation” will inhibit accretion of all available particle sizes once the planet reaches super-Earth masses. We also demonstrate that the characteristics of this “flow isolation mass” agree with previously published trends identified in the Kepler planets.

Evidence for a high mutual inclination between the cold Jupiter and transiting super Earth orbiting π Men

ABSTRACT π Men hosts a transiting super Earth (P ≈ 6.27 d, m ≈ 4.82 M⊕, R ≈ 2.04 R⊕) discovered by TESS and a cold Jupiter (P ≈ 2093 d, msin I ≈ 10.02 MJup, e ≈ 0.64) discovered from radial velocity. We use Gaia DR2 and Hipparcos astrometry to derive the star’s velocity caused by the orbiting planets and constrain the cold Jupiter’s sky-projected inclination (Ib = 41°−65°). From this, we derive the mutual inclination (ΔI) between the two planets, and find that 49° < ΔI < 131° (1σ) and 28° < ΔI < 152° (2σ). We examine the dynamics of the system using N-body simulations, and find that potentially large oscillations in the super Earth’s eccentricity and inclination are suppressed by general relativistic precession. However, nodal precession of the inner orbit around the invariable plane causes the super Earth to only transit between 7 and 22 per cent of the time, and to usually be observed as misaligned with the stellar spin axis. We repeat our analysis for HAT-P-11, finding a large ΔI between its close-in Neptune and cold Jupiter and similar dynamics. π Men and HAT-P-11 are prime examples of systems where dynamically hot outer planets excite their inner planets, with the effects of increasing planet eccentricities, planet–star misalignments, and potentially reducing the transit multiplicity. Formation of such systems likely involves scattering between multiple giant planets or misaligned protoplanetary discs. Future imaging of the faint debris disc in π Men and precise constraints on its stellar spin orientation would provide strong tests for these formation scenarios.

Precise mass and radius of a transiting super-Earth planet orbiting the M dwarf TOI-1235: a planet in the radius gap?

We report the confirmation of a transiting planet around the bright weakly active M0.5 V star TOI-1235 (TYC 4384–1735–1, V ≈ 11.5 mag), whose transit signal was detected in the photometric time series of sectors 14, 20, and 21 of the TESS space mission. We confirm the planetary nature of the transit signal, which has a period of 3.44 d, by using precise RV measurements with the CARMENES, HARPS-N, and iSHELL spectrographs, supplemented by high-resolution imaging and ground-based photometry. A comparison of the properties derived for TOI-1235 b with theoretical models reveals that the planet has a rocky composition, with a bulk density slightly higher than that of Earth. In particular, we measure a mass of Mp = 5.9 ± 0.6 M⊕ and a radius of Rp = 1.69 ± 0.08 R⊕, which together result in a density of ρp = 6.7− 1.1+ 1.3 g cm−3. When compared with other well-characterized exoplanetary systems, the particular combination of planetary radius and mass places our discovery in the radius gap, which is a transition region between rocky planets and planets with significant atmospheric envelopes. A few examples of planets occupying the radius gap are known to date. While the exact location of the radius gap for M dwarfs is still a matter of debate, our results constrain it to be located at around 1.7 R⊕ or larger at the insolation levels received by TOI-1235 b (~60 S⊕). This makes it an extremely interesting object for further studies of planet formation and atmospheric evolution.

Influence of learner motivational dispositions on MOOC completion

This study examined the role motivational dispositions had on completing a massive open online course (MOOC) using identifiable data from 10,726 students who enrolled in an iteration of the HarvardX MOOC, Super Earths and Life. As part of the course registration process, learners had the option to complete a pre-course survey and self-report information including their level of education, gender and registration motivations. Using these pre-course survey responses, latent profiles linked to learners’ course performance were created. Results showed education background, gender, and motivation were all significantly related to students’ performance. Furthermore, students with intrinsic motivational dispositions performed better than students with extrinsic dispositions, and females performed better than males.

Solid accretion onto planetary cores in radiative disks

The solid accretion rate, which is necessary to grow gas giant planetary cores within the disk lifetime, has been a major constraint for theories of planet formation. We tested the solid accretion rate efficiency on planetary cores of different masses embedded in their birth disk by means of 3D radiation-hydrodynamics, where we followed the evolution of a swarm of embedded solids of different sizes. We found that by using a realistic equation of state and radiative cooling, the disk at 5 au is able to efficiently cool and reduce its aspect ratio. As a result, the pebble isolation mass is reached before the core grows to 10 M⊕, thus fully stopping the pebble flux and creating a transition disk. Moreover, the reduced isolation mass halts the solid accretion before the core reaches the critical mass, leading to a barrier to giant planet formation, and this explains the large abundance of super-Earth planets in the observed population.

The CARMENES search for exoplanets around M dwarfs

Aims. We report on radial velocity time series for two M0.0 V stars, GJ 338 B and GJ 338 A, 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. Methods. We obtained 159 and 70 radial velocity measurements of GJ 338 B and A, respectively, with the CARMENES visible channel between 2016 January and 2018 October. 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. Results. We found dynamical masses of 0.64 ± 0.07 M⊙ for GJ 338 B and 0.69 ± 0.07 M⊙ for GJ 338 A. The CARMENES radial velocity periodograms show significant peaks at 16.61 ± 0.04 d (GJ 338 B) and 16.3−1.3+3.5 d (GJ 338 A), which have counterparts at the same frequencies in CARMENES activity indicators and photometric light curves. We attribute these to stellar rotation. GJ 338 B shows two additional, significant signals at 8.27 ± 0.01 and 24.45 ± 0.02 d, with no obvious counterparts in the stellar activity indices. The former is likely the first harmonic of the star’s rotation, while we ascribe the latter to the existence of a super-Earth planet with a minimum mass of 10.27−1.38+1.47 M⊕ orbiting GJ 338 B. We have not detected signals of likely planetary origin around GJ 338 A. Conclusions. GJ 338 Bb 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.

A Possible Transit of a Disintegrating Exoplanet in the Nearby Multiplanet System DMPP-1

Abstract We analyze TESS photometry of DMPP-1 (HD 38677; TIC 66560666), a nearby F8V star hosting hot super-Earth planets and a warm Neptune. Using the Transit Least Squares algorithm and other methods we find a transit signal at days with depth ppm and false alarm probability 1.6%. This is shallower than hitherto published TESS discoveries. The 3.285 day signal is recovered for several, but not all, methods for detrending stellar astrophysical variability. Further observations are needed to improve the significance of the detection. If this transit were due to an Earth-like rocky planet it would have been detected in the RV data, but it is not. The TESS data cover seven individual transits, one of which is consistent with zero depth. The insolation of the putative planet is 990 S ⊕, typical of fluxes experienced by the three known catastrophically disintegrating exoplanets (CDEs). The transits can be self-consistently attributed to a CDE with a mass below the RV detection threshold. We searched for transits of the known RV planets, finding null results and detection thresholds of <100 ppm, which we quantify for each. The DMPP-1 planetary system was discovered as a consequence of circumstellar gas attributed to ablation of hot planets. The RV planets may have been ablated to near-pure iron cores. We place limits on the orbital inclinations of the RV planets where the expected transit depth exceeds the detection threshold. If the 3.2854 day transit detection is confirmed, e.g., with CHEOPS photometry, DMPP-1 would be a first-rate target for James Webb Space Telescope spectroscopy.

MCMCI: A code to fully characterise an exoplanetary system

Context. Useful information can be retrieved by analysing the transit light curve of a planet-hosting star or induced radial velocity oscillations. However, inferring the physical parameters of the planet, such as mass, size, and semi-major axis, requires preliminary knowledge of some parameters of the host star, especially its mass or radius, which are generally inferred through theoretical evolutionary models. Aims. We seek to present and test a whole algorithm devoted to the complete characterisation of an exoplanetary system thanks to the global analysis of photometric or radial velocity time series combined with observational stellar parameters derived either from spectroscopy or photometry. Methods. We developed an integrated tool called MCMCI. This tool combines the Markov chain Monte Carlo (MCMC) approach of analysing photometric or radial velocity time series with a proper interpolation within stellar evolutionary isochrones and tracks, known as isochrone placement, to be performed at each chain step, to retrieve stellar theoretical parameters such as age, mass, and radius. Results. We tested the MCMCI on the HD 219134 multi-planetary system hosting two transiting rocky super Earths and on WASP-4, which hosts a bloated hot Jupiter. Even considering different input approaches, a final convergence was reached within the code, we found good agreement with the results already stated in the literature and we obtained more precise output parameters, especially concerning planetary masses. Conclusions. The MCMCI tool offers the opportunity to perform an integrated analysis of an exoplanetary system without splitting it into the preliminary stellar characterisation through theoretical models. Rather this approach favours a close interaction between light curve analysis and isochrones, so that the parameters recovered at each step of the MCMC enter as inputs for purposes of isochrone placement.

Formation of planetary populations − II. Effects of initial disc size and radial dust drift

ABSTRACT Recent ALMA observations indicate that while a range of disc sizes exist, typical disc radii are small, and that radial dust drift affects the distribution of solids in discs. Here, we explore the consequences of these features in planet population synthesis models. A key feature of our model is planet traps – barriers to otherwise rapid type-I migration of forming planets – for which we include the ice line, heat transition, and outer edge of the dead zone. We find that the ice line plays a fundamental role in the formation of warm Jupiters. In particular, the ratio of super Earths to warm Jupiters formed at the ice line depends sensitively on the initial disc radius. Initial gas disc radii of ∼50 au results in the largest super Earth populations, while both larger and smaller disc sizes result in the ice line producing more gas giants near 1 au. This transition between typical planet class formed at the ice line at various disc radii confirms that planet formation is fundamentally linked to disc properties (in this case, disc size), and is a result that is only seen when dust evolution effects are included in our models. Additionally, we find that including radial dust drift results in the formation of more super Earths between 0.1 and 1 au, having shorter orbital radii than those produced in models where dust evolution effects are not included.

An ultra-short period rocky super-Earth orbiting the G2-star HD 80653

Ultra-short period (USP) planets are a class of exoplanets with periods shorter than one day. The origin of this sub-population of planets is still unclear, with different formation scenarios highly dependent on the composition of the USP planets. A better understanding of this class of exoplanets will, therefore, require an increase in the sample of such planets that have accurate and precise masses and radii, which also includes estimates of the level of irradiation and information about possible companions. Here we report a detailed characterization of a USP planet around the solar-type star HD 80653 ≡EP 251279430 using the K2 light curve and 108 precise radial velocities obtained with the HARPS-N spectrograph, installed on the Telescopio Nazionale Galileo. From the K2 C16 data, we found one super-Earth planet (Rb = 1.613 ± 0.071 R⊕) transiting the star on a short-period orbit (Pb = 0.719573 ± 0.000021 d). From our radial velocity measurements, we constrained the mass of HD 80653 b to Mb = 5.60 ± 0.43 M⊕. We also detected a clear long-term trend in the radial velocity data. We derived the fundamental stellar parameters and determined a radius of R⋆ = 1.22 ± 0.01 R⊙ and mass of M⋆ = 1.18 ± 0.04 M⊙, suggesting that HD 80653 has an age of 2.7 ± 1.2 Gyr. The bulk density (ρb = 7.4 ± 1.1 g cm−3) of the planet is consistent with an Earth-like composition of rock and iron with no thick atmosphere. Our analysis of the K2 photometry also suggests hints of a shallow secondary eclipse with a depth of 8.1 ± 3.7 ppm. Flux variations along the orbital phase are consistent with zero. The most important contribution might come from the day-side thermal emission from the surface of the planet at T ~ 3480 K.

An ablating 2.6 M⊕ planet in an eccentric binary from the Dispersed Matter Planet Project

Earth mass exoplanets are difficult to detect. The Dispersed Matter Planet Project (DMPP) identifies stars which are likely to host the most detectable low mass exoplanets. The star DMPP-3 (HD 42936) shows signs of circumstellar absorption, indicative of mass loss from ablating planets. Here we report the radial velocity (RV) discovery of a highly eccentric 507 d binary companion and a hot super-Earth planet in a 6.67 d orbit around the primary star. DMPP-3A is a solar type star while DMPP-3B is just massive enough to fuse hydrogen. The binary, with semi-major axis 1.22 $\pm$ 0.02 AU, is considerably tighter than others known to host planets orbiting only one of the component stars. The configuration of the DMPP-3 planetary system is rare and indicates dynamical interactions, though the evolutionary history is not entirely clear. DMPP-3Ab is possibly the residual core of a giant planet precursor, consistent with the inferred circumstellar gas shroud.

Super-Earth masses sculpted by pebble isolation around stars of different masses

We developed a pebble-driven core accretion model to study the formation and evolution of planets around stars in the stellar mass range of 0.08 M⊙–1 M⊙. By Monte Carlo sampling of the initial conditions, the growth and migration of a large number of individual protoplanetary embryos were simulated in a population synthesis manner. We tested two hypotheses for the birth locations of embryos: at the water ice line or log-uniformly distributed over entire protoplanetary disks. Two types of disks with different turbulent viscous parametersαtof 10−3and 10−4are also investigated to shed light on the role of outward migration of protoplanets. The forming planets are compared with the observed exoplanets in terms of mass, semimajor axis, metallicity, and water content. We find that gas giant planets are likely to form when the characteristic disk sizes are larger, the disk accretion rates are higher, the disks are more metal rich, and/or their stellar hosts are more massive. Our model shows that first, the characteristic mass of super-Earth is set by the pebble isolation mass. Super-Earth masses increase linearly with the mass of its stellar host, which corresponds to one Earth mass around a late M-dwarf star and 20 Earth masses around a solar-mass star. Second, the low-mass planets, up to 20 M⊕, can form around stars with a wide range of metallicities, while massive gas giant planets are preferred to grow around metal rich stars. Third, super-Earth planets that are mainly composed of silicates, with relatively low water fractions, can form from protoplanetary embryos at the water ice line in weakly turbulent disks where outward migration is suppressed. However, if the embryos are formed over a wide range of radial distances, the super-Earths would end up having a distinctive, bimodal composition in water mass. Altogether, our model succeeds in quantitatively reproducing several important observed properties of exoplanets and correlations with their stellar hosts.

Tidally Induced Radius Inflation of Sub-Neptunes

Abstract Recent work suggests that many short-period super-Earth and sub-Neptune planets may have significant spin axis tilts (“obliquities”). When planets are locked in high-obliquity states, the tidal dissipation rate may increase by several orders of magnitude. This intensified heat deposition within the planets’ interiors should generate significant structural consequences, including atmospheric inflation leading to larger transit radii. Using up-to-date radius estimates from Gaia Data Release 2, we show evidence for ∼50% larger average radii of planets wide of first-order mean-motion resonances, a population of planets with a theorized frequent occurrence of high obliquities. We investigate whether this radius trend could be a signature of obliquity tides. Using an adaptation of the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution toolkit, we model the atmospheric evolution of sub-Neptune-mass planets in response to additional internal heat from obliquity tides. The degree of radius inflation predicted by the models is ∼10%–100% for tidal luminosities ≳10−5 of the incident stellar power; this degree of inflation is broadly consistent with the observations and can approximately be described by power-law relationships. We present a few case studies of very low density “super-puff” planets—Kepler-79 d, Kepler-31 c, and Kepler-27 b—and show that they are strong candidates for potentially having undergone tidally induced radius inflation. We also discuss how the discrepancy between the two populations of planets with masses derived from radial velocities and transit timing variations is connected to the radius distribution features we have identified. Altogether, the calculations in this work confirm that tidal dissipation has nonnegligible consequences for the structural properties of short-period sub-Neptunes.

Super-Earth ingestion can explain the anomalously high metal abundances of M67 Y2235

Abstract We investigate the hypothesis that ingestion of a terrestrial or super-Earth planet could cause the anomalously high metal abundances seen in a turn-off star in the open cluster M67, when compared to other turn-off stars in the same cluster. We show that the mass of the convective envelope of the star is likely only 3.45 × 10−3 M⊙, and hence 5.2 M⊕ of rock is required to obtain the observed 0.128 dex metal enhancement. Rocky planets dissolve entirely in the convective envelope if they enter it with sufficiently tangential orbits: we find that the critical condition for dissolution is that the planet’s radial velocity must be less than 40% of its total velocity at the stellar surface; or, equivalently, the impact parameter must be greater than about 0.9. We model the delivery of rocky planets to the stellar surface both by planet-planet scattering in a realistic multi-planet system, and by Lidov–Kozai cycles driven by a more massive planetary or stellar companion. In both cases almost all planets that are ingested arrive at the star on grazing orbits and hence will dissolve in the surface convection zone. We conclude that super-Earth ingestion is a good explanation for the metal enhancement in M67 Y2235, and that a high-resolution spectroscopic survey of stellar abundances around the turn-off and main sequence of M67 has the potential to constrain the frequency of late-time dynamical instability in planetary systems.

GJ 357: a low-mass planetary system uncovered by precision radial velocities and dynamical simulations

ABSTRACT We report the detection of a new planetary system orbiting the nearby M2.5V star GJ 357, using precision radial velocities from three separate echelle spectrographs, High Accuracy Radial velocity Planet Searcher (HARPS), High Resolution Echelle Spectrograph (HiRES), and Ultraviolet and Visible Echelle Spectrograph (UVES). Three small planets have been confirmed in the system, with periods of 9.125 ± 0.001, 3.9306 ± 0.0003, and 55.70 ± 0.05 d, and minimum masses of 3.33 ± 0.48, 2.09 ± 0.32, and 6.72 ± 0.94 M⊕, respectively. The second planet in our system, GJ 357 c, was recently shown to transit by the Transiting Exoplanet Survey Satellite (TESS), but we could find no transit signatures for the other two planets. Dynamical analysis reveals the system is likely to be close to coplanar, is stable on Myr time-scales, and places strong upper limits on the masses of the two non-transiting planets GJ 357 b and GJ 357 d of 4.25 and 11.20 M⊕, respectively. Therefore, we confirm the system contains at least two super-Earths, and either a third super-Earth or mini-Neptune planet. GJ 357 b and GJ 357 c are found to be close to a 7:3 mean motion resonance, however no libration of the orbital parameters was found in our simulations. Analysis of the photometric light curve of the star from the TESS, when combined with our radial velocities, reveals GJ 357 c has an absolute mass, radius, and density of $2.248^{+0.117}_{-0.120}$ M⊕, $1.167^{+0.037}_{-0.036}$ R⊕, and $7.757^{+0.889}_{-0.789}$ g cm−3, respectively. Comparison to super-Earth structure models reveals the planet is likely an iron-dominated world. The GJ 357 system adds to the small sample of low-mass planetary systems with well constrained masses, and further observational and dynamical follow-up is warranted to better understand the overall population of small multiplanet systems in the solar neighbourhood.

Inner rocky super-Earth formation: distinguishing the formation pathways in viscously heated and passive discs

Observations have revealed that super-Earths (planets up to 10 Earth masses) are the most abundant type of planets in the inner systems. Their formation is strongly linked to the structure of the protoplanetary disc, which determines growth and migration. In the pebble accretion scenario, planets grow to the pebble isolation mass, at which the planet carves a small gap in the gas disc halting the pebble flux and thus its growth. The pebble isolation mass scales with the disc’s aspect ratio, which directly depends on the heating source of the protoplanetary disc. I compare the growth of super-Earths in viscously heated discs, where viscous heating dissipates within the first million years, and discs purely heated by the central star with super-Earth observations from the Kepler mission. This allows two formation pathways of super-Earths to be distinguished in the inner systems within this framework. Planets growing within 1 Myr in the viscously heated inner disc reach pebble isolation masses that correspond directly to the inferred masses of the Kepler observations for systems that feature planets in resonance or not in resonance. However, to explain the period ratio distribution of Kepler planets – where most Kepler planet pairs are not in mean motion resonance configurations – a fraction of these resonant chains has to be broken. In case the planets are born early in a viscously heated disc, these resonant chains thus have to be broken without planetary mergers, for example through the magnetic rebound effect, and the final system architecture should feature low mutual inclinations. If super-Earths form either late or in purely passive discs, the pebble isolation mass is too small (around 2–3 Earth masses) to explain the Kepler observations, implying that planetary mergers have to play a significant role in determining the final system architecture. Resonant planetary systems thus have to experience mergers already during the gas disc phase, so the planets can get trapped in resonance after reaching 5–10 Earth masses. In case instabilities are dominating the system architecture, the systems should not be flat, but feature mutually inclined orbits. This implies that future observations of planetary systems with radial velocities (RV) and transits (for example through the Transiting Exoplanet Survey Satellite (TESS) and its follow up RV surveys) could distinguish between these two formation channels of super-Earth and thus constrain planet formation theories.

Effects of magnetic fields on the location of the evaporation valley for low-mass exoplanets

ABSTRACT The observed distribution of radii for exoplanets shows a bimodal form that can be explained by mass-loss from planetary atmospheres due to high-energy radiation emitted by their host stars. The location of the minimum of this radius distribution depends on the mass–radius relation, which in turn depends on the composition of planetary cores. Current studies suggest that super-Earth and mini-Neptune planets have iron-rich and hence largely Earth-like composition cores. This paper explores how non-zero planetary magnetic fields can decrease the expected mass-loss rates from these planets. These lower mass-loss rates, in turn, affect the location of the minimum of the radius distribution and the inferred chemical composition of the planetary cores.

Publisher Correction: A super-Earth and two sub-Neptunes transiting the nearby and quiet M dwarf TOI-270

Publisher Correction: A super-Earth and two sub-Neptunes transiting the nearby and quiet M dwarf TOI-270

Same masses, different sizes

Exoplanets Exoplanets with masses between those of Earth and Neptune come in two types that display a bimodal distribution of densities: “super Earths,” with rocky bodies and small (or no) atmospheres, and “mini Neptunes,” with extended atmospheres of hydrogen and helium surrounding rocky cores. That difference might reflect the removal of light gasses from some of their atmospheres by stellar activity. Gandolfi et al. have discovered two exoplanets orbiting the star HD15337. Planet c is a mini Neptune with a 50% larger radius than planet b, a super Earth. As both planets have identical masses (within the uncertainties) and orbit the same star, studying them will help explain the atmospheric loss processes that lead to these planet types. Astrophys. J. Lett. 876 , L24 (2019).