16 research outputs found
BICEPS: An improved characterization model for low- and intermediate-mass exoplanets
Context. The number of exoplanets with precise mass and radius measurements is constantly increasing thanks to novel ground- and space-based facilities such as HARPS, ESPRESSO, CHEOPS, and TESS. The accuracy and robustness of the planetary characterization largely depends on the quality of the data, but also requires a planetary structure model, capable of accurately modeling the interior and atmospheres of exoplanets over a large range of boundary conditions.
Aims. Our goal is to provide an improved characterization model for planets with masses between 0.5 and 30 Earth masses, equilibrium temperatures below <2000 K, and a wide range of planetary compositions and physical phases.
Methods. In this work, we present the Bayesian Interior Characterization of ExoPlanetS (BICEPS) model, which combines an adaptive Markov chain Monte Carlo sampling method with a state-of-the-art planetary structure model. BICEPS incorporates many recently developed equations of state suited for large ranges of pressures and temperatures, a description for solid and molten planetary cores and mantles, a gaseous envelope composed of hydrogen, helium, and water (with compositional gradients), and a non-gray atmospheric model.
Results. We find that the usage of updated equations of state has a significant impact on the interior structure prediction. The impact varies, depending on the planetary composition. For dense rocky planets, BICEPS predicts radii a few percent different to prior internal structure models. For volatile rich planets, we find differences of 10% or even larger. When applying BICEPS to a particular exoplanet, TOI-130 b, we inferred a 25% larger water mass fraction and a 15% smaller core than previous models.
Conclusions. The presented exoplanet characterization model is a robust method applicable over a large range of planetary masses, compositions, and thermal boundary conditions. We show the importance of implementing state-of-the-art equations of state for the encountered thermodynamic conditions of exoplanets. Hence, using BICEPS improves the predictive strength of the characterization process compared to previous methods.ISSN:0004-6361ISSN:1432-074
AQUA: A Collection of HO Equations of State for Planetary Models
Water is one of the key chemical elements in planetary structure modelling.
Due to its complex phase diagram, equations of state cover often only parts of
the pressure - temperature space needed in planetary modelling. We construct an
equation of state of HO spanning a very wide range from 0.1 Pa to 400 TPa
and 150 K to K, which can be used to model the interior of planets. We
combine equations of state valid in localised regions to form a continuous
equation of state spanning over said pressure and temperature range. We provide
tabulated values for the most important thermodynamic quantities, i.e.,
density, adiabatic temperature gradient, entropy, internal energy and bulk
speed of sound of water over this pressure and temperature range. For better
usability we also calculated density - temperature and density - internal
energy grids. We discuss further the impact of this equation of state on the
mass radius relation of planets compared to other popular equation of states
like ANEOS and QEOS. AQUA is a combination of existing equation of state useful
for planetary models. We show that AQUA is in most regions a thermodynamic
consistent description of water. At pressures above 10 GPa AQUA predicts
systematic larger densities than ANEOS or QEOS. A feature which was already
present in a previously proposed equation of state, which is the main
underlying equation of this work. We show that the choice of the equation of
state can have a large impact on the mass-radius relation, which highlights the
importance of future developments in the field of equation of states and
regarding experimental data of water at high pressures.Comment: 20 pages, 12 Figures, Accepted for publication in A&
Exoplanet characterization using conditional invertible neural networks
The characterization of an exoplanet's interior is an inverse problem, which
requires statistical methods such as Bayesian inference in order to be solved.
Current methods employ Markov Chain Monte Carlo (MCMC) sampling to infer the
posterior probability of planetary structure parameters for a given exoplanet.
These methods are time consuming since they require the calculation of a large
number of planetary structure models. To speed up the inference process when
characterizing an exoplanet, we propose to use conditional invertible neural
networks (cINNs) to calculate the posterior probability of the internal
structure parameters. cINNs are a special type of neural network which excel in
solving inverse problems. We constructed a cINN using FrEIA, which was then
trained on a database of internal structure models to recover
the inverse mapping between internal structure parameters and observable
features (i.e., planetary mass, planetary radius and composition of the host
star). The cINN method was compared to a Metropolis-Hastings MCMC. For that we
repeated the characterization of the exoplanet K2-111 b, using both the MCMC
method and the trained cINN. We show that the inferred posterior probability of
the internal structure parameters from both methods are very similar, with the
biggest differences seen in the exoplanet's water content. Thus cINNs are a
possible alternative to the standard time-consuming sampling methods. Indeed,
using cINNs allows for orders of magnitude faster inference of an exoplanet's
composition than what is possible using an MCMC method, however, it still
requires the computation of a large database of internal structures to train
the cINN. Since this database is only computed once, we found that using a cINN
is more efficient than an MCMC, when more than 10 exoplanets are characterized
using the same cINN.Comment: 15 pages, 13 figures, submitted to Astronomy & Astrophysic
The Nominal Range of Rocky Planet Masses, Radii, Surface Gravities and Bulk Densities
The two primary observable quantities of an exoplanet--its mass and
radius--alone are not sufficient to probe a rocky exoplanet's interior
composition and mineralogy. To overcome this, host-star abundances of the
primary planet-building elements (Mg, Si, Fe) are typically used as a proxy for
the planet's bulk composition. The majority of small exoplanet hosts, however,
do not have available abundance data. Here we present the open-source ExoPlex
mass-radius-composition solver. Unlike previous open-source mass-radius
solvers, ExoPlex calculates the core chemistry and equilibrium mantle
mineralogy for a bulk composition, including effects of mantle FeO content,
core light elements and surface water/ice. We utilize ExoPlex to calculate the
planetary radii, surface gravities and bulk densities for 10 model planets
up to 2 R across these geochemistries, adopting the distribution of
FGK stellar abundances to estimate of the range of bulk exoplanet compositions.
We outline the distribution of radii, surface gravity and bulk
densities that define planets as "nominally rocky." Planets outside this range
require compositions outside those expected from stellar abundance data, likely
making them either Fe-enriched super-Mercuries, or volatile-enriched
mini-Neptunes. We apply our classification scheme to a sample of 85
well-resolved exoplanets without available host-star abundances. We estimate
only 9 planets are within the "nominally rocky planet zone" at
confidence, while and of this sample can be reasonably
classified as super-Mercuries or volatile-rich, respectively. Our results
provide observers with a self-consistent way to broadly classify a planet as
likely rocky, Mercury-like or volatile-enriched, using mass and radius
measurements alone.Comment: 41 pages, 21 figures, 2 tables. Accepted to Ap
A pair of Sub-Neptunes transiting the bright K-dwarf TOI-1064 characterised with CHEOPS
Funding: TGW, ACC, and KH acknowledge support from STFC consolidated grant numbers ST/R000824/1 and ST/V000861/1, and UKSA grant ST/R003203/1.We report the discovery and characterization of a pair of sub-Neptunes transiting the bright K-dwarf TOI-1064 (TIC 79748331), initially detected in the Transiting Exoplanet Survey Satellite (TESS) photometry. To characterize the system, we performed and retrieved the CHaracterising ExOPlanets Satellite (CHEOPS), TESS, and ground-based photometry, the High Accuracy Radial velocity Planet Searcher (HARPS) high-resolution spectroscopy, and Gemini speckle imaging. We characterize the host star and determine Teff,⋆=4734±67K, R⋆=0.726±0.007R⊙, and M⋆=0.748±0.032M⊙. We present a novel detrending method based on point spread function shape-change modelling and demonstrate its suitability to correct flux variations in CHEOPS data. We confirm the planetary nature of both bodies and find that TOI-1064 b has an orbital period of Pb = 6.44387 ± 0.00003 d, a radius of Rb = 2.59 ± 0.04 R⊕, and a mass of Mb=13.5+1.7−1.8 M⊕, whilst TOI-1064 c has an orbital period of Pc=12.22657+0.00005−0.00004 d, a radius of Rc = 2.65 ± 0.04 R⊕, and a 3σ upper mass limit of 8.5 M⊕. From the high-precision photometry we obtain radius uncertainties of ∼1.6 per cent, allowing us to conduct internal structure and atmospheric escape modelling. TOI-1064 b is one of the densest, well-characterized sub-Neptunes, with a tenuous atmosphere that can be explained by the loss of a primordial envelope following migration through the protoplanetary disc. It is likely that TOI-1064 c has an extended atmosphere due to the tentative low density, however further radial velocities are needed to confirm this scenario and the similar radii, different masses nature of this system. The high-precision data and modelling of TOI-1064 b are important for planets in this region of mass–radius space, and it allow us to identify a trend in bulk density–stellar metallicity for massive sub-Neptunes that may hint at the formation of this population of planets.Publisher PDFPeer reviewe
The nature of the radius valley
The existence of a radius valley in the Kepler size distribution stands as one of the most important observational constraints to understand the origin and composition of exoplanets with radii between those of Earth and Neptune. In this work we provide insights into the existence of the radius valley, first from a pure formation point of view and then from a combined formation-evolution model. We run global planet formation simulations including the evolution of dust by coagulation, drift, and fragmentation, and the evolution of the gaseous disc by viscous accretion and photoevaporation. A planet grows from a moon-mass embryo by either silicate or icy pebble accretion, depending on its position with respect to the water ice line. We include gas accretion, type I–II migration, and photoevaporation driven mass-loss after formation. We perform an extensive parameter study evaluating a wide range of disc properties and initial locations of the embryo. We find that due to the change in dust properties at the water ice line, rocky cores form typically with ∼3 M⊕ and have a maximum mass of ∼5 M⊕, while icy cores peak at ∼10 M⊕, with masses lower than 5 M⊕ being scarce. When neglecting the gaseous envelope, the formed rocky and icy cores account naturally for the two peaks of the Kepler size distribution. The presence of massive envelopes yields planets more massive than ∼10 M⊕ with radii above 4 R⊕. While the first peak of the Kepler size distribution is undoubtedly populated by bare rocky cores, as shown extensively in the past, the second peak can host half-rock–half-water planets with thin or non-existent H-He atmospheres, as suggested by a few previous studies. Some additional mechanisms inhibiting gas accretion or promoting envelope mass-loss should operate at short orbital periods to explain the presence of ∼10–40 M⊕ planets falling in the second peak of the size distribution
TESS Reveals a Short-period Sub-Neptune Sibling (HD 86226c) to a Known Long-period Giant Planet
The Transiting Exoplanet Survey Satellite mission was designed to find transiting planets around bright, nearby stars. Here, we present the detection and mass measurement of a small, short-period (≈4 days) transiting planet around the bright (V = 7.9), solar-type star HD 86226 (TOI-652, TIC 22221375), previously known to host a long-period (~1600 days) giant planet. HD 86226c (TOI-652.01) has a radius of 2.16 ± 0.08 R ⊕ and a mass of M ⊕, based on archival and new radial velocity data. We also update the parameters of the longer-period, not-known-to-transit planet, and find it to be less eccentric and less massive than previously reported. The density of the transiting planet is 3.97 g cm−3, which is low enough to suggest that the planet has at least a small volatile envelope, but the mass fractions of rock, iron, and water are not well-constrained. Given the host star brightness, planet period, and location of the planet near both the "radius gap" and the "hot Neptune desert," HD 86226c is an interesting candidate for transmission spectroscopy to further refine its composition