The composition of rocky exoplanets in the context of stars’ composition provides important constraints to formation theories. In this study, we select a sample of exoplanets with mass and radius measurements with an uncertainty $<25\%$ and obtain their interior structure. We calculate compositional markers, ratios of iron to magnesium and silicon, as well as core-mass fractions (cmf) that fit the planetary parameters, and compare them to the stars’. We find four key results that successful planet formation theories need to predict: (1) In a population sense, the composition of rocky planets spans a wider range than stars. The stars’ Fe/Si distribution is close to a Gaussian distribution $1.63^{+0.91}_{-0.85}$, while the planets’ distribution peaks at lower values and has a longer tail, $1.15^{+1.43}_{-0.76}$. It is easier to see the discrepancy in cmf space, where primordial stellar composition is $0.32^{+0.14}_{-0.12}$, while rocky planets’ follow a broader distribution $0.24^{+0.33}_{-0.18}$. (2) We introduce uncompressed density ($\overline{\rho _0}$ at reference pressure/temperature) as a metric to compare compositions. With this, we find what seems to be the maximum iron enrichment that rocky planets attain during formation ($\overline{\rho _0}\sim 6$ and cmf ∼0.8). (3) Highly irradiated planets exhibit a large range of compositions. If these planets are the result of atmospheric evaporation, iron enrichment and perhaps depletion must happen before gas dispersal. And (4), we identify a group of highly-irradiated planets that, if rocky, would be 2-fold depleted in Fe/Si with respect to the stars. Without a reliable theory for forming iron-depleted planets, these are interesting targets for follow up.

Kepler revealed that roughly one-third of Sunlike stars host planets orbiting within 100 days and between the size of Earth and Neptune. How do these planets form, what are they made of, and do they represent a continuous population or multiple populations? To help address these questions, we began the Magellan-TESS Survey (MTS), which uses Magellan II/PFS to obtain radial velocity (RV) masses of 30 TESS-detected exoplanets and develops an analysis framework that connects observed planet distributions to underlying populations. In the past, small-planet RV measurements have been challenging to obtain due to host star faintness and low RV semiamplitudes and challenging to interpret due to the potential biases in target selection and observation planning decisions. The MTS attempts to minimize these biases by focusing on bright TESS targets and employing a quantitative selection function and observing strategy. In this paper, we (1) describe our motivation and survey strategy, (2) present our first catalog of planet density constraints for 27 TESS Objects of Interest (TOIs; 22 in our population analysis sample, 12 that are members of the same systems), and (3) employ a hierarchical Bayesian model to produce preliminary constraints on the mass–radius (M-R) relation. We find that the biases causing previous M-R relations to predict fairly high masses at 1 R ⊕ have been reduced. This work can inform more detailed studies of individual systems and offer a framework that can be applied to future RV surveys with the goal of population inferences.

Exploring the properties of exoplanets near or inside the radius valley provides insight on the transition from the rocky super-Earths to the larger, hydrogen-rich atmosphere mini-Neptunes. Here, we report the discovery of TOI-1452b, a transiting super-Earth (R p = 1.67 ± 0.07 R ⊕) in an 11.1 day temperate orbit (T eq = 326 ± 7 K) around the primary member (H = 10.0, T eff = 3185 ± 50 K) of a nearby visual-binary M dwarf. The transits were first detected by the Transiting Exoplanet Survey Satellite, then successfully isolated between the two 3.″2 companions with ground-based photometry from the Observatoire du Mont-Mégantic and MuSCAT3. The planetary nature of TOI-1452b was established through high-precision velocimetry with the near-infrared SPIRou spectropolarimeter as part of the ongoing SPIRou Legacy Survey. The measured planetary mass (4.8 ± 1.3 M ⊕) and inferred bulk density ( 5.6 − 1.6 + 1.8 g cm−3) is suggestive of a rocky core surrounded by a volatile-rich envelope. More quantitatively, the mass and radius of TOI-1452b, combined with the stellar abundance of refractory elements (Fe, Mg, and Si) measured by SPIRou, is consistent with a core-mass fraction of 18% ± 6% and a water-mass fraction of 22 − 13 + 21 %. The water world candidate TOI-1452b is a prime target for future atmospheric characterization with JWST, featuring a transmission spectroscopy metric similar to other well-known temperate small planets such as LHS 1140b and K2-18 b. The system is located near Webb’s northern continuous viewing zone, implying that is can be followed at almost any moment of the year.

TOI-561 is a galactic thick-disk star hosting an ultra-short-period (0.45-day-orbit) planet with a radius of 1.37 R ⊕, making it one of the most metal-poor ([Fe/H] = −0.41) and oldest (≈10 Gyr) sites where an Earth-sized planet has been found. We present new simultaneous radial velocity (RV) measurements from Gemini-N/MAROON-X and Keck/HIRES, which we combined with literature RVs to derive a mass of M b = 2.24 ± 0.20 M ⊕. We also used two new sectors of TESS photometry to improve the radius determination, finding R b = 1.37 ± 0.04 R ⊕ and confirming that TOI-561 b is one of the lowest-density super-Earths measured to date (ρ b = 4.8 ± 0.5 g cm−3). This density is consistent with an iron-poor rocky composition reflective of the host star’s iron and rock-building element abundances; however, it is also consistent with a low-density planet with a volatile envelope. The equilibrium temperature of the planet (∼2300 K) suggests that this envelope would likely be composed of high mean molecular weight species, such as water vapor, carbon dioxide, or silicate vapor, and is likely not primordial. We also demonstrate that the composition determination is sensitive to the choice of stellar parameters and that further measurements are needed to determine whether TOI-561 b is a bare rocky planet, a rocky planet with an optically thin atmosphere, or a rare example of a nonprimordial envelope on a planet with a radius smaller than 1.5 R ⊕.