Lars A. Buchhave

We characterize the radius-dependent observed transiting eccentricity distribution of 219 warm ( P = 8–50 days) systems with only one transiting planetary candidate identified during Sectors 1–69 of the Transiting Exoplanet Survey Satellite (TESS) mission. Using the “photoeccentric effect” in a hierarchical Bayesian framework, we first model the population using discrete planetary size bins (sub-Neptunes, sub-Saturns, and Jovians). We then develop a continuous mixture model with weights governed by a logistic sigmoid function of radius. We find that the warm-single population is best described by two components: a dominant low-eccentricity mode (〈 e _low 〉 = 0.03 9 − 0.038 + 0.018 ) and a secondary dynamically excited mode (〈 e _high 〉 = 0.46 6 − 0.068 + 0.067 ). The fraction of planets belonging to this high-eccentricity component increases strongly with planet radius, characterized by a transition at a break radius of R _br = 9 . 2 − 1.1 + 1.9 R _⊕ . This trend places warm sub-Saturns predominantly on the same low-eccentricity track as sub-Neptunes. In contrast, warm Jovians (8–16 R _⊕ ) are frequently eccentric, with 6 5 − 12 + 13 % of the population in the high-eccentricity mode. Under the assumption of a two-component model, we see tentative evidence for a bimodal Jovian distribution at ∼2.7 σ . Finally, we identify a nonnegligible tail of highly eccentric sub-Neptunes (1–4 R _⊕ ), which comprise 16 . 2 − 6.4 + 5.2 % of the population, consistent with excitation by nontransiting external companions.

We present the discovery of the planetary system orbiting the bright ( V = 7.2), nearby (35 pc), Sun-like ( M _⋆ = 1.050 ± 0.044 M _⊙ , R _⋆ = 1.129 ± 0.013 R _⊙ ) star, HD 60779. We report two Transiting Exoplanet Survey Satellite transits and a subsequent Characterising Exoplanet Satellite transit of HD 60779 b, a sub-Neptune ( 3.24 6 − 0.097 + 0.100 R ⊕ ) planet on a 29.98617 5 − 0.000033 + 0.000030 day orbit. Additionally, 313 High Accuracy Radial velocity Planet Searcher for the Northern hemisphere radial velocity (RV) measurements reveal the mass of planet b ( 14.1 9 − 0.96 + 0.97 M ⊕ ) and the presence of an outer planet, HD 60779 c ( P _orb = 104.79 ± 0.23 days, M p sin i = 28.3 ± 1.5 M ⊕ ). Both of these planets’ orbits are consistent with being circular, suggesting that they have a dynamically quiet history. The data are not sufficient to determine whether planet c transits. HD 60779’s uniquely high systemic RV (129.75 ± 0.12 km s ⁻¹ ) allows its Ly α emission to avoid absorption by the interstellar medium, making it a prime candidate for probing atmospheric escape from HD 60779 b. HD 60779 is also the third brightest host of a P _orb > 25 day sub-Neptune with a measured mass and radius, distinguishing it in terms of accessibility to spectroscopic characterization.

The radii of small exoplanets form two populations, super-Earths and sub-Neptunes, separated by a gap known as the radius valley. This feature could be produced by the removal of atmospheres by stellar or internal heating or by the lack of an initial envelope. We used transit photometry and radial velocity measurements to detect and characterize four exoplanets orbiting LHS 1903, a red dwarf star in the Milky Way’s thick disk. These four planets have orbital periods ranging from 2.2 to 29.3 days and span the radius valley within a single planetary system. The derived densities indicate that LHS 1903 b is rocky, whereas LHS 1903 c and LHS 1903 d have extended atmospheres. The most distant planet from the host star, LHS 1903 e, has no gaseous envelope, indicating that it formed from gas-depleted material.

The Galactic Be star binary MWC 656 was long considered the only known Be star+black hole (BH) system, making it a critical benchmark for models of massive binary evolution and for the expected X-ray emission of Be+BH binaries. However, recent dynamical measurements cast doubt on the presence of a BH companion. We present new multi-epoch ultraviolet spectroscopy from the Hubble Space Telescope, combined with high-resolution optical spectra, to reassess the nature of the companion. The far-ultraviolet spectra reveal high-ionisation features – including prominent N V and He II lines – which are absent in the spectra of normal Be stars and are indicative of a hot, luminous companion. Spectral modelling shows that these features cannot originate from the Be star or from an accretion disc around a compact object. Instead, we find that the data are best explained by a hot ( T _eff ≈ 85 kK), compact, hydrogen-deficient star with strong wind signatures, consistent with an intermediate-mass stripped star. Our revised orbital solution and composite spectroscopic modelling yield a companion mass of M ₂ = 1.48 ^+0.55 _−0.46 M _⊙ , definitively ruling out a BH and disfavouring a white dwarf. MWC 656 thus joins the growing class of Be + stripped star binaries. The system’s unusual properties – including a high companion temperature and wind strength – extend the known parameter space of such binaries. The continued absence of confirmed OBe+BH binaries in the Galaxy highlights a growing tension with population synthesis models.

JWST is transforming our ability to characterize small exoplanets, from sub-Neptunes to rocky worlds. A key open question is whether highly irradiated rocky planets can retain atmospheres or are stripped bare by stellar irradiation—a boundary that remains to be mapped observationally. Here we present the first JWST secondary eclipse observations of the rocky exoplanet GJ 3473 b, obtained with MIRI F1500W photometry. Using four visits, we confidently detect the eclipse at an average depth of 186 ± 45 ppm, somewhat lower than expected for a blackbody. We test a wide range of data reduction and analysis assumptions and provide new insights into MIRI detector settling behavior that will benefit future observations. We model a suite of airless surfaces with varied compositions, textures, and degrees of space weathering, as well as idealized atmospheric scenarios including the possibility of atmospheric collapse. Both atmospheric and bare-rock interpretations remain consistent with the data, but we exclude thick CO ₂ atmospheres, placing a 95% credible upper limit of 1.2–6.5 bar on the surface pressure. We also find tentative evidence for visit-to-visit variability in eclipse depth (33–371 ppm), though additional data are required to confirm this. Our results highlight the challenges and intrinsic degeneracies in interpreting MIRI F1500W eclipse measurements of rocky exoplanets, indicating that such observations alone may not uniquely distinguish between bare-rock and atmospheric scenarios. Future spectroscopic or phase-curve observations will be required to determine whether or not GJ 3473 b hosts a substantial atmosphere.

The measurement of exoplanet masses using the radial velocity (RV) technique is currently limited by stellar activity, which introduces quasiperiodic variability signals that must be modeled and removed to enhance the sensitivity of the RV measurements to exoplanet signals. Neural networks have previously been demonstrated effective in modeling stellar activity signals in HARPS-N solar data using white light cross correlation functions (CCFs). Building on this work, we train a neural network on 6 yr of HARPS-N solar data with additional parameters commonly associated to stellar activity, including chromatic CCFs, line shape metrics, spectral activity indicators, total solar irradiance (TSI) light curves from SORCE and TSIS-1, and TSI time derivatives. Our results show that parameters such as the bisector inverse slope and Na D equivalent widths (EWs) do not significantly improve the neural network’s ability to predict activity-induced RV variations compared to using the white light CCFs alone. However, parameters such as unsigned magnetic flux, the TSI and its time derivative, S-index, H α EW, chromatic CCFs, contrast, and FWHM do improve the neural network's ability to predict RV scatter. Our new model reduces the RV scatter in a held-out test set from 147.1 cm s ⁻¹ to 93.3 cm s ⁻¹ , consistent with supergranulation noise levels reported in previous studies. These results suggest that finding effective tracers for (super)granulation will be critical to train models capable of further mitigating RV jitter, and necessary for characterizing Earth analogs.

We characterize the radius-dependent eccentricity distribution of 347 warm (P = 8-200 days) systems with only one transiting planetary candidate identified during Sectors 1-69 of the TESS mission. Using the ``photoeccentric effect'' in a hierarchical Bayesian framework, we first model the population using discrete planetary size bins (sub-Neptunes, sub-Saturns, and Jovians). We then develop a continuous mixture model with weights governed by a logistic sigmoid function of radius. We find that the warm-single population is best described by two components: a dominant low-eccentricity mode ( = 0.070-0.068+0.026) and a secondary dynamically excited mode ( = 0.616-0.075+0.091). The fraction of planets belonging to this high-eccentricity component increases strongly with planet radius, characterized by a transition at a break radius of R_br = 9.8-1.1+1.4 R_e. This trend places warm sub-Saturns predominantly on the same low-eccentricity track as sub-Neptunes. In contrast, warm Jovians (8--16 R_e) are frequently eccentric, with 59+-13% of the population in the high eccentricity mode. We detect this bimodality at >4sigma, providing statistically significant evidence that warm gas giants are sculpted by two distinct pathways, or a single mechanism with subsequent eccentricity excitation. Finally, we identify a non-negligible tail of highly eccentric sub-Neptunes (1--4 R_e), which comprise 14.9-6.5+5.1% of the population, consistent with excitation by non-transiting external companions.

The measurement of exoplanet masses using the radial velocity (RV) technique is currently limited by stellar activity, which introduces quasiperiodic variability signals that must be modeled and removed to enhance the sensitivity of the RV measurements to exoplanet signals. Neural networks have previously been demonstrated effective in modeling stellar activity signals in HARPS-N solar data using white light cross correlation functions (CCFs). Building on this work, we train a neural network on six years of HARPS-N solar data with additional parameters commonly associated to stellar activity, including chromatic CCFs, line shape metrics, spectral activity indicators, total solar irradiance (TSI) light curves from SORCE and TSIS-1, and TSI time derivatives. Our results show that parameters such as the bisector inverse slope and Na D equivalent widths do not significantly improve the neural network's ability to predict activity-induced RV variations compared to using the white light CCFs alone. However, parameters such as unsigned magnetic flux, the TSI and its time derivative, S-index, H-alpha equivalent width, chromatic CCFs, contrast, and full width at half maximum do improve the neural network's ability to predict RV scatter. Our new model reduces the RV scatter in a held-out test set from 147.1 cm/s to 93.3 cm/s, consistent with supergranulation noise levels reported in previous studies. These results suggest that finding effective tracers for (super)granulation will be critical to train models capable of further mitigating RV jitter, and necessary for characterizing Earth analogues.

Context. The HARPS-N solar telescope has been observing the Sun every possible day since the summer of 2015. We have recently released 10 years of these data, which are available online. Aims. The goal of this paper is to present the different optimisations made to the ESPRESSO data reduction software used to extract the published HARPS-N solar spectra, describe the data curation, and perform some analyses that demonstrate the extreme radial velocity (RV) precision of those data. Methods. By analysing all of the HARPS-N wavelength solutions over 13 years, we brought to light instrumental systematics at the 1 m s ⁻¹ level. We mitigated those systematics by curating the thorium line list used to derive the wavelength solution and applying a correction to the drift of thorium lines induced by the aging of thorium-argon hollow cathode lamps. After optimisation, we demonstrated a peak-to-peak precision on the HARPS-N wavelength solution better than 0.75 m s ⁻¹ over 13 years. We then carefully curated the decade of HARPS-N re-reduced solar observations by rejecting 30% of the data affected either by clouds, bad atmospheric conditions, or well-understood instrumental systematics. Finally, we corrected the curated data for spurious sub-meter-per-second RV effects caused by erroneous instrumental drift measurements and by changes in the spectral blaze function over time. Results. After curation and correction, a total of 109,466 HARPS-N solar spectra and respective RVs over a decade were made available. The median photon-noise precision of the RV data is 0.28 m s ⁻¹ , and on daily timescales, the median RV rms is 0.49 m s ⁻¹ , which is similar to the level imposed by stellar granulation signals. On 10 year timescales, the large RV rms of 2.95 m s ⁻¹ results from the RV signature of the Sun’s magnetic cycle. Through modelling of this long-term effect using the Bremen composite magnesium II activity index, we demonstrate a long-term RV precision of 0.41 m s ⁻¹ . We also analysed contemporaneous HARPS-N and NEID solar RVs and found the data from both instruments to be of similar quality and precision. However, an analysis of the RV difference between these two RV datasets over the three available years gave a surprisingly large RV rms of 1.3 m s ⁻¹ . This variation is dominated by an unexplained trend that could be caused by a different sensitivity to stellar activity of the two datasets. Once this trend was modelled, the overall RV rms for three years reached 0.79 m s ⁻¹ , and the RV rms during the low-activity phase decreased to 0.6 m s ⁻¹ , compatible with what is expected from supergranulation. Conclusions. This decade of high-cadence HARPS-N solar observations with short- and long-term precision below one m s ⁻¹ represents a crucial dataset in the pursuit of further understanding the stellar activity signals in solar-type stars and advancing other science cases requiring such extreme precision.