Planet-hosting Stars Research Articles

Detailed chemical compositions of planet-hosting stars: II. Exploration of the interiors of terrestrial-type exoplanets

Abstract A major goal in the discovery and characterisation of exoplanets is to identify terrestrial-type worlds that are similar to (or otherwise distinct from) our Earth. Recent results have highlighted the importance of applying devolatilisation – i.e. depletion of volatiles – to the chemical composition of planet-hosting stars to constrain bulk composition and interiors of terrestrial-type exoplanets. In this work, we apply such an approach to a selected sample of 13 planet-hosting Sun-like stars, for which high-precision photospheric abundances have been determined in the first paper of the series. With the resultant devolatilised stellar composition (i.e. the model planetary bulk composition) as well as other constraints including mass and radius, we model the detailed mineralogy and interior structure of hypothetical, habitable-zone terrestrial planets (”exo-Earths”) around these stars. Model output shows that most of these exo-Earths are expected to have broadly Earth-like composition and interior structure, consistent with conclusions derived independently from analysis of polluted white dwarfs. The exceptions are the Kepler-10 and Kepler-37 exo-Earths, which we predict are strongly oxidised and thus would develop metallic cores much smaller than Earth. Investigating our devolatilisation model at its extremes as well as varying planetary mass and radius (within the terrestrial regime) reveals potential diversities in the interiors of terrestrial planets. By considering (i) high-precision stellar abundances, (ii) devolatilisation, and (iii) planetary mass and radius holistically, this work represents essential steps to explore the detailed mineralogy and interior structure of terrestrial-type exoplanets, which in turn are fundamental for our understanding of planetary dynamics and long-term evolution.

Revisiting BD-06 1339b: A Likely False Positive Caused by Stellar Activity

As long as astronomers have searched for exoplanets, the intrinsic variability of host stars has interfered with the ability to reliably detect and confirm exoplanets. One particular source of false positives is the presence of stellar magnetic or chromospheric activity that can mimic the radial velocity reflex motion of a planet. Here we present the results of a photometric data analysis for the known planet-hosting star BD –06°1339, observed by the Transiting Exoplanet Survey Satellite during Sector 6 at a cadence of 2 minutes. We discuss evidence that suggests that the observed 3.9-day periodic radial velocity signature may be caused by stellar activity rather than a planetary companion, since variability detected in the photometric data is consistent with the periodic signal. We conclude that the previously reported planetary signature is likely the result of a false-positive signal resulting from stellar activity, and we discuss the need for more data to confirm this conclusion.

The rotation of planet-hosting stars

ABSTRACT Understanding the distribution of angular momentum during the formation of planetary systems is a key topic in astrophysics. Data from the Kepler and Gaia missions allow to investigate whether stellar rotation is correlated with the presence of planets around Sun-like stars. Here, we perform a statistical analysis of the rotation period of 493 planet-hosting stars. These are matched to a control sample, without detected planets, with similar effective temperatures, masses, radii, metallicities, and ages. We find that planet-hosting stars rotate on average 1.63 ± 0.40 d slower. The difference in rotation is statistically significant both in samples including and not including planets confirmed by radial velocity follow-up observations. We also analyse the dependence of rotation distribution on various stellar and planetary properties. Our results could potentially be explained by planet detection biases depending on the rotation period of their host stars in both RV and transit methods. Alternatively, they could point to a physical link between the existence of planets and stellar rotation, emphasizing the need to understand the role of angular momentum in the formation and evolution planetary systems.

Tidal star–planet interaction and its observed impact on stellar activity in planet-hosting wide binary systems

ABSTRACT Tidal interaction between an exoplanet and its host star is a possible pathway to transfer angular momentum between the planetary orbit and the stellar spin. In cases where the planetary orbital period is shorter than the stellar rotation period, this may lead to angular momentum being transferred into the star’s rotation, possibly counteracting the intrinsic stellar spin-down induced by magnetic braking. Observationally, detecting altered rotational states of single, cool field stars is challenging, as precise ages for such stars are rarely available. Here we present an empirical investigation of the rotation and magnetic activity of a sample of planet-hosting stars that are accompanied by wide stellar companions. Without needing knowledge about the absolute ages of the stars, we test for relative differences in activity and rotation of the planet hosts and their co-eval companions, using X-ray observations to measure the stellar activity levels. Employing three different tidal interaction models, we find that host stars with planets that are expected to tidally interact display elevated activity levels compared to their companion stars. We also find that those activity levels agree with the observed rotational periods for the host stars along the usual rotation–activity relationships, implying that the effect is indeed caused by a tidal interaction and not a purely magnetic interaction that would be expected to affect the stellar activity, but not necessarily the rotation. We conclude that massive, close-in planets have an impact on the stellar rotational evolution, while the smaller, more distant planets do not have a significant influence.

Chemical Composition of Bright Stars in the Northern Hemisphere: Star–Planet Connection

Abstract In fulfilling the aims of the planetary and asteroseismic research missions, such as that of the NASA Transiting Exoplanet Survey Satellite (TESS) space telescope, accurate stellar atmospheric parameters and a detailed chemical composition are required as inputs. We have observed high-resolution spectra for all 848 bright (V < 8 mag) stars that are cooler than F5 spectral class in the area up to 12 deg surrounding the northern TESS continuous viewing zone and uniformly determined the main atmospheric parameters, ages, orbital parameters, velocity components, and precise abundances of up to 24 chemical species (C(C2), N(CN), [O i], Na i, Mg i, Al i, Si i, Si ii, Ca i, Ca ii, Sc i, Sc ii, Ti i, Ti ii, V i, Cr i, Cr ii, Mn i, Fe i, Fe ii, Co i, Ni i, Cu i, and Zn i) for 740 slowly rotating stars. The analysis of 25 planet-hosting stars in our sample drove us to the following conclusions: the dwarf stars hosting high-mass planets are more metal rich than those with low-mass planets. We find slightly negative C/O and Mg/Si slopes toward the stars with high-mass planets. All the low-mass planet hosts in our sample show positive Δ[El/Fe] versus condensation temperature slopes, in particular, the star with the largest number of various planets. The high-mass planet hosts have a diversity of slopes, but in more metal-rich, older, and cooler stars, the positive elemental abundance slopes are more common.

The TESS Faint-star Search: 1617 TOIs from the TESS Primary Mission

Abstract We present the detection of 1617 new transiting-planet candidates, identified in the Transiting Exoplanet Survey Satellite (TESS) full-frame images observed during the Primary Mission (Sectors 1–26). These candidates were initially detected by the Quick-Look Pipeline (QLP), which extracts full-frame image lightcurves for, and searches all stars brighter than, TESS magnitude T = 13.5 mag in each sector. However, QLP heavily relies on manual inspection for the identification of planet candidates, limiting vetting efforts to planet-hosting stars brighter than T = 10.5 mag and leaving millions of potential transit signals unvetted. We describe an independent vetting pipeline applied to QLP transit search results, incorporating both automated vetting tests and manual inspection to identify promising planet candidates around these fainter stars. The new candidates discovered by this ongoing project will allow TESS to significantly improve the statistical power of demographic studies of giant, close-in exoplanets.

New periodograms separating orbital radial velocities and spectral shape variation

We present new periodograms that are effective in distinguishing Doppler shift from spectral shape variability in astronomical spectra. These periodograms, building upon the concept of partial distance correlation, separate the periodic radial velocity modulation induced by orbital motion from that induced by stellar activity. These tools can be used to explore large spectroscopic databases in search of targets in which spectral shape variations obscure the orbital motion; such systems include active planet-hosting stars or binary systems with an intrinsically variable component. We provide a detailed prescription for calculating the periodograms, demonstrate their performance via simulations and real-life case studies, and provide a public Python implementation.

Survey of Surveys

Context. In the present-day panorama of large spectroscopic surveys, the amount, diversity, and complexity of the available data continuously increase. The overarching goal of studying the formation and evolution of our Galaxy is hampered by the heterogeneity of instruments, selection functions, analysis methods, and measured quantities. Aims. We present a comprehensive catalogue, the Survey of Surveys (SoS), built by homogeneously merging the radial velocity (RV) determinations of the largest ground-based spectroscopic surveys to date, such as APOGEE, GALAH, Gaia-ESO, RAVE, and LAMOST, using Gaia as a reference. This pilot study serves to prove the concept and to test the methodology that we plan to apply in the future to the stellar parameters and abundance ratios as well. Methods. We have devised a multi-staged procedure that includes: (i) the cross match between Gaia and the spectroscopic surveys using the official Gaia cross-match algorithm, (ii) the normalisation of uncertainties using repeated measurements or the three-cornered hat method, (iii) the cross calibration of the RVs as a function of the main parameters on which depend (magnitude, effective temperature, surface gravity, metallicity, and signal-to-noise ratio) to remove trends and zero point offsets, and (iv) the comparison with external high-resolution samples, such as the Gaia RV standards and the Geneva-Copenhagen survey, to validate the homogenisation procedure and to calibrate the RV zero-point of the SoS catalogue. Results. We provide the largest homogenised RV catalogue to date, containing almost 11 million stars, of which about half come exclusively from Gaia and half in combination with the ground-based surveys. We estimate the accuracy of the RV zero-point to be about 0.16−0.31 km s−1 and the RV precision to be in the range 0.05−1.50 km s−1 depending on the type of star and on its survey provenance. We validate the SoS RVs with open clusters from a high resolution homogeneous samples and provide the systemic velocity of 55 individual open clusters. Additionally, we provide median RVs for 532 clusters recently discovered by Gaia data. Conclusions. The SoS is publicly available and ready to be applied to various research projects, such as the study of star clusters, Galactic archaeology, stellar streams, or the characterisation of planet-hosting stars, to name a few. We also plan to include survey updates and more data sources in future versions of the SoS.

The Influence of 10 Unique Chemical Elements in Shaping the Distribution of Kepler Planets

Abstract The chemical abundances of planet-hosting stars offer a glimpse into the composition of planet-forming environments. To further understand this connection, we make the first ever measurement of the correlation between planet occurrence and chemical abundances for ten different elements (C, Mg, Al, Si, S, K, Ca, Mn, Fe, and Ni). Leveraging data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and Gaia to derive precise stellar parameters ( σ R ⋆ ≈ 2.3 % , σ M ⋆ ≈ 4.5 % ) for a sample of 1018 Kepler Objects of Interest, we construct a sample of well-vetted Kepler planets with precisely measured radii ( σ R p ≈ 3.4 % ). After controlling for biases in the Kepler detection pipeline and the selection function of the APOGEE survey, we characterize the relationship between planet occurrence and chemical abundance as the number density of nuclei of each element in a star’s photosphere raised to a power, β. varies by planet type, but is consistent within our uncertainties across all ten elements. For hot planets (P = 1–10 days), an enhancement in any element of 0.1 dex corresponds to an increased occurrence of ≈20% for super-Earths (R p = 1–1.9 R ⊕) and ≈60% for sub-Neptunes (R p = 1.9–4 R ⊕). Trends are weaker for warm (P = 10–100 days) planets of all sizes and for all elements, with the potential exception of sub-Saturns (R p = 4–8 R ⊕). Finally, we conclude this work with a caution to interpreting trends between planet occurrence and stellar age due to degeneracies caused by Galactic chemical evolution and make predictions for planet occurrence rates in nearby open clusters to facilitate demographics studies of young planetary systems.

A model for spin–orbit commensurability and synchronous starspot activity in stars with close-by planets

Context. The rotation period of some planet-hosting stars appears to be in close commensurability with the orbital period of their close-by planets. In some cases, starspots rotating with a commensurable period have been detected, while the star displays latitudinal differential rotation. Aims. A model is proposed to interpret such a phenomenon based on the excitation of resonant oscillations in the interior magnetic field of the star by a component of the tidal potential with a very low frequency in the reference frame rotating with the star. Methods. A magnetic flux tube located in the overshoot layer of the star is assumed in order to study the excitation of the resonant oscillations in the magnetostrophic regime. The model considers a planet on a circular oblique orbit, and the growth timescale of the oscillations is estimated. To keep the system in resonance with the exciting potential despite the variations in the magnetic field or tidal frequency, a self-regulating mechanism is proposed. Results. The model is applied to ten systems and proves capable of accounting for the observed close commensurability in eight of them by assuming a magnetic field between 102 and 104 G. Systems with distant low-mass planets, such as AU Mic and HAT-P-11, cannot be interpreted by the proposed model. Conclusions. Consequences for the spin–orbit evolution of the systems, including the dynamical tides and gyrochronology of planet-hosting stars, are discussed together with the effects on the chromospheric features produced by star–planet magnetic interactions.

Unbound Close Stellar Encounters in the Solar Neighborhood

Abstract We present a catalog of unbound stellar pairs, within 100 pc of the Sun, that are undergoing close, hyperbolic, encounters. The data are drawn from the GAIA EDR3 catalog, and the limiting factors are errors in the radial distance and unknown velocities along the line of sight. Such stellar pairs have been suggested to be possible events associated with the migration of technological civilizations between stars. As such, this sample may represent a finite set of targets for a SETI search based on this hypothesis. Our catalog contains a total of 132 close passage events, featuring stars from across the entire main sequence, with 16 pairs featuring at least one main-sequence star of spectral type between K1 and F3. Many of these stars are also in binaries, so that we isolate eight single stars as the most likely candidates to search for an ongoing migration event—HD 87978, HD 92577, HD 50669, HD 44006, HD 80790, LSPM J2126+5338, LSPM J0646+1829 and HD 192486. Among host stars of known planets, the stars GJ 433 and HR 858 are the best candidates.

Orbital Stability of Planet-hosting Triple-star Systems according to Hill: Applications to Alpha Centauri and 16 Cygni

Abstract In this study we investigate aspects of orbital stability for the Alpha Centauri and 16 Cygni systems. They are planet-hosting triple star systems of highly hierarchic nature. For each system, orbital stability of the outlying stellar component and the observed exoplanet(s) are explored through assessing Hill stability. Orbital stability is identified for all components, including the observed system planets.

SWEET-Cat 2.0: The Cat just got SWEETer

Aims. The catalog of Stars With ExoplanETs (SWEET-Cat) was originally introduced in 2013. Since then many more exoplanets have been confirmed, increasing significantly the number of host stars listed there. A crucial step toward a comprehensive understanding of these new worlds is the precise and homogeneous characterization of their host stars. Better spectroscopic stellar parameters along with new results from Gaia eDR3 provide updated and precise parameters for the discovered planets. A new version of the catalog, whose homogeneity in the derivation of the parameters is key to unraveling star–planet connections, is available to the community. Methods. We made use of high-resolution spectra for planet-host stars, either observed by our team or collected through public archives. The spectroscopic stellar parameters were derived for the spectra following the same homogeneous process using ARES and MOOG (ARES+MOOG) as for the previous SWEET-Cat releases. We re-derived parameters for the stars in the catalog using better quality spectra and/or using the most recent versions of the codes. Moreover, the new SWEET-Cat table can now be more easily combined with the planet properties listed both at the Extrasolar Planets Encyclopedia and at the NASA exoplanet archive to perform statistical analyses of exoplanets. We also made use of the recent Gaia eDR3 parallaxes and respective photometry to derive consistent and accurate surface gravity values for the host stars. Results. We increased the number of stars with homogeneous parameters by more than 40% (from 645 to 928). We reviewed and updated the metallicity distributions of stars hosting planets with different mass regimes comparing the low-mass planets (< 30 M⊕) with the high-mass planets. The new data strengthen previous results showing the possible trend in the metallicity-period-mass diagram for low-mass planets.

A Spectroscopic Analysis of the California-Kepler Survey Sample. II. Correlations of Stellar Metallicities with Planetary Architectures

Abstract We present independent and self-consistent metallicities for a sample of 807 planet-hosting stars from the California-Kepler Survey from an LTE spectroscopic analysis using a selected sample of Fe i and Fe ii lines. Correlations between host-star metallicities, planet radii, and planetary architecture (orbital periods—warm or hot—and multiplicity—single or multiple) were investigated using nonparametric statistical tests. In addition to confirming previous results from the literature, e.g., that overall host-star metallicity distributions differ between hot and warm planetary systems of all types, we report on a new finding: when comparing the median metallicities of hot versus warm systems, the difference for multiple super-Earths is considerably larger when compared to that difference in single super-Earths. The metallicity cumulative distribution functions of hot single super-Earths versus warm single super-Earths indicate different parent stellar populations, while for sub-Neptunes this is not the case. The transition radius between sub-Neptunes and sub-Saturns was examined by comparing the APOGEE metallicity distribution for the Milky Way thin disk in the solar neighborhood with metallicity distributions of host stars segregated based upon the largest known planet in their system. These comparisons reveal increasingly different metallicity distributions as the radius of the largest planet in the systems increases, with the parent stellar metallicities becoming significantly different for R p > 2.7 R ⊕. The behavior of the p-values as a function of planet radius undergoes a large slope change at R p = 4.4 ± 0.5 R ⊕, indicating the radius boundary between small and large planets.

Predictions of Astrometric Jitter for Sun-like Stars. II. Dependence on Inclination, Metallicity, and Active-region Nesting

Abstract Ultra-precise astrometry from the Gaia mission is expected to lead to astrometric detections of more than 20,000 exoplanets in our Galaxy. One of the factors that could hamper such detections is the astrometric jitter caused by the magnetic activity of the planet host stars. In our previous study, we modeled astrometric jitter for the Sun observed equator-on. In this work, we generalize our model and calculate the photocenter jitter as it would be measured by the Gaia and Small-JASMINE missions for stars with solar rotation rate and effective temperature, but with various values of the inclination angle of the stellar rotation axis. In addition, we consider the effect of metallicity and of nesting of active regions (i.e., the tendency of active regions to emerge in the vicinity of each other). We find that, while the jitter of stars observed equator-on does not have any long-term trends and can be easily filtered out, the photocenters of stars observed out of their equatorial planes experience systematic shifts over the course of the activity cycle. Such trends allow the jitter to be detected with continuous measurements, in which case it can interfere with planet detectability. An increase in the metallicity is found to increase the jitter caused by stellar activity. Active-region nesting can further enhance the peak-to-peak amplitude of the photocenter jitter to a level that could be detected by Gaia.

Solar Neighborhood Exoplanet Host Stars within a Galactic Context

Abstract Ages, metallicities, and Galactic orbit parameters are considered for those stars with Henry Draper catalog designations that have been discovered to host planetary systems. There are quite a few such stars with above-solar metallicity and ages up to 4 Gyr older than the Sun. Some planet-hosting stars have Galactic orbits that take them within 4 kpc of the Galactic Center, while a substantial fraction attain displacements from the Galactic mid-plane consistent with the thick disk.

Radius and Mass Distribution of Ultra-short-period Planets

Abstract Ultra-short-period (USP) planets are an enigmatic subset of exoplanets defined by having orbital periods <1 day. It is still not understood how USP planets form, or to what degree they differ from planets with longer orbital periods. Most USP planets have radii <2 R ⊕, while planets that orbit further from their star extend to Jupiter size (>10 R ⊕). Several theories attempt to explain the formation and composition of USP planets: they could be remnant cores of larger gas giants that lost their atmospheres due to photoevaporation or Roche-lobe overflow, or they could have developed through mass accretion in the innermost part of the protoplanetary disk. The radius and mass distribution of USP planets could provide important clues to distinguish between potential formation mechanisms. In this study, we first verify and update the Kepler catalog of USP planet host star properties, incorporating new data collected by the Gaia mission where applicable. We then use the transit depths measured by Kepler to derive a radius distribution and present occurrence rates for USP planets. Using spherical and tidally distorted planet models, we then derive a mass distribution for USP planets. Comparisons between the updated USP planet mass distribution and simulated planetary systems offer further insights into the formation and evolutionary processes shaping USP planet populations.

The TESS Mission Target Selection Procedure

We describe the target selection procedure by which stars are selected for 2 minute and 20 s observations by TESS. We first list the technical requirements of the TESS instrument and ground systems processing that limit the total number of target slots. We then describe algorithms used by the TESS Payload Operation Center (POC) to merge candidate targets requested by the various TESS mission elements (the Target Selection Working Group, TESS Asteroseismic Science Consortium, and Guest Investigator office). Lastly, we summarize the properties of the observed TESS targets over the two-year primary TESS mission. We find that the POC target selection algorithm results in 2.1–3.4 times as many observed targets as target slots allocated for each mission element. We also find that the sky distribution of observed targets is different from the sky distributions of candidate targets due to technical constraints that require a relatively even distribution of targets across the TESS fields of view. We caution researchers exploring statistical analyses of TESS planet-host stars that the population of observed targets cannot be characterized by any simple set of criteria applied to the properties of the input Candidate Target Lists.

Why do more massive stars host larger planets?

Aims. It has been suggested that planetary radii increase with stellar mass for planet sizes smaller than 6 R⊕ and host masses lower than 1 M⊙. In this study, we explore whether this inferred relation of planetary size and host star mass can be explained by a higher planetary mass of planets orbiting higher-mass stars, inflation of the planetary radius due to the difference in stellar irradiation, or different planetary compositions and structures. Methods. Using exoplanetary data of planets with measured masses and radii, we investigated the relations between stellar mass and various planetary properties for G and K stars. We confirm that more massive stars host larger and more massive planets. Results. We find that the differences in the planetary masses and temperatures are insufficient to explain the measured differences in radii for planets surrounding different stellar types. We show that the larger planetary radii can be explained by a larger fraction of volatile material (H-He atmospheres) in planets surrounding more massive stars. Conclusions. We conclude that planets around more massive stars are most probably larger as a result of larger H-He atmospheres. Our findings imply that planets forming around more massive stars tend to accrete H-He atmospheres more efficiently.

OGLE-2018-BLG-1185b: A Low-mass Microlensing Planet Orbiting a Low-mass Dwarf

Abstract We report an analysis of the planetary microlensing event OGLE-2018-BLG-1185, which was observed by a large number of ground-based telescopes and by the Spitzer Space Telescope. The ground-based light curve indicates a low planet–host star mass ratio of q = (6.9 ± 0.2) × 10−5, which is near the peak of the wide-orbit exoplanet mass-ratio distribution. We estimate the host star and planet masses with a Bayesian analysis using the measured angular Einstein radius under the assumption that stars of all masses have an equal probability of hosting the planet. The flux variation observed by Spitzer is marginal, but still places a constraint on the microlens parallax. Imposing a conservative constraint that this flux variation should be Δf Spz < 4 instrumental flux units yields a host mass of M host = 0.37 − 0.21 + 0.35 M ⊙ and a planet mass of m p = 8.4 − 4.7 + 7.9 M ⊕ . A Bayesian analysis including the full parallax constraint from Spitzer suggests smaller host star and planet masses of M host = 0.091 − 0.018 + 0.064 M ⊙ and m p = 2.1 − 0.4 + 1.5 M ⊕ , respectively. Future high-resolution imaging observations with the Hubble Space Telescope or Extremely Large Telescope could distinguish between these two scenarios and help reveal the planetary system properties in more detail.