Displays

We explore the application of machine-learning, based on mixture density neural networks (MDNs), to the interior characterization of low-mass exoplanets up to 25 Earth masses constrained by mass, radius, and fluid Love number k₂. MDNs are a special subset of neural networks, able to predict the parameters of a Gaussian mixture distribution instead of single output values, which enables them to learn and approximate probability distributions. With a dataset of 900,000 synthetic planets, consisting of an iron-rich core, a silicate mantle, a high-pressure ice shell, and a gaseous H/He envelope, we train an MDN using planetary mass and radius as inputs to the network. We show that the MDN is able to infer the distribution of possible thicknesses of each planetary layer from mass and radius of the planet. This approach obviates the time-consuming task of calculating such distributions with a dedicated set of forward models for each individual planet.

The fluid Love number k₂ bears constraints on the mass distribution in the planets' interior and will be measured for an increasing number of exoplanets in the future. Adding k₂ as an input to the MDN significantly decreases the degeneracy of possible interior structures.

How to cite: Baumeister, P., Padovan, S., Tosi, N., Montavon, G., Nettelmann, N., MacKenzie, J., and Godolt, M.: Machine-learning inference of the interior structure of low-mass exoplanets, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7520, https://doi.org/10.5194/egusphere-egu2020-7520, 2020.

Until about 600 million years ago, our planet experienced temporary snowball conditions, with continental and sea ices covering a large fraction of its surface. This points to a potential bistability of Earth’s climate, that can have at least two different (statistical) equilibrium states for the same external forcing (i.e., solar radiation). Here we explore the probability of finding bistable climates in rocky exoplanets, and consider the properties of planetary climates obtained by varying the semi-major orbital axis (thus, received stellar radiation), eccentricity and obliquity, and atmospheric pressure. To this goal, we use the Earth-like planet surface temperature model (ESTM), an extension of 1D Energy Balance Models developed to provide a numerically efficient climate estimator for parameter sensitivity studies and long climatic simulations. After verifying that the ESTM is able to reproduce Earth climate bistability, we identify the range of parameter space where climate bistability is detected. An intriguing result of the present work is that the planetary conditions that support climate bistability are remarkably similar to those required for the sustainance of complex, multicellular life on the planetary surface. The exploration of potential climate bistability proceeds with the case of a Earth-like planet partially covered by vegetation that generates a positive vegetation-albedo feedback, in the spirit of the Charney conceptual model. In this case, it is shown that the presence of this vegetation feedback can induce relevant changes in climate dynamics and alter the range of habitable conditions for the planet.

How to cite: Provenzale, A., Murante, G., Vladilo, G., Silva, L., Bisesi, E., Palazzi, E., and von Hardenberg, J.: Climate bistability of rocky exoplanets, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2471, https://doi.org/10.5194/egusphere-egu2020-2471, 2020.

Superflares of energies up to 10³⁸ ergs have been studied from Kepler and Gaia observations, and estimates of their energy and frequency on different types of stars is improving rapidly. Flares with energies up to 10³⁵ ergs occur about once every 2000-3000 years on slow rotating stars like the Sun, but the occurrence rate is ∼ 100 times higher for younger, faster rotating stars of the same class. More than a dozen potentially habitable planets, like Proxima Centauri b and TRAPPIST-1 e, are in close-in configurations and their proximity to the host star makes them highly sensitive to stellar activity. Episodic events such as flares have the potential to cause severe damage to close-in planets, adversely impacting their habitability. Stellar Energetic Particles (SEPs) emanating from Stellar Proton Events (SPEs) cause atmospheric damage (erosion and photochemical changes), and produce secondary particles, which in turn results in enhanced radiation dosage on planetary surfaces. Taking particle spectra from 70 major solar events (observed between 1956 and 2012) as proxy, we use the GEANT4 Monte Carlo model to simulate SPE interactions with exoplanetary atmospheres. We have demonstrated that radiation dose varies significantly with charged particle spectra and an event of a given fluence can have a drastically different effect depending on the spectrum. Our results show that radiation dose can vary by about five orders of magnitude for a given fluence. In terms of shielding, we found that atmospheric depth is a major factor in determining radiation dose on the planetary surface. Radiation dose is reduced by three orders of magnitude corresponding to an increase in the atmospheric depth by an order of magnitude. We found that the planetary magnetic field is an important but a less significant factor compared to atmospheric depth. The dose is reduced by a factor of about thirty corresponding to an increase in the magnetospheric strength by an order of magnitude.

How to cite: Atri, D.: Stellar Proton Events and Exoplanetary Habitability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12792, https://doi.org/10.5194/egusphere-egu2020-12792, 2020.

In the last two decades, the field of exoplanets has witnessed a tremendous creative surge. Research in exoplanets now encompasses a wide range of fields ranging from astrophysics to heliophysics and climate science. One of the primary objectives of studying exoplanets is to determine the criteria for habitability, and whether certain exoplanets meet these requirements. The classical definition of the Habitable Zone (HZ) is the region around a star where liquid water can exist on the planetary surface given sufficient atmospheric pressure. However, this definition largely ignores the impact of the stellar wind and stellar magnetic activity on the erosion of an exoplanet's atmosphere. Amongst the many factors that determine habitability, understanding the mechanisms of atmospheric loss is of paramount importance.

We will discuss the impact of exoplanetary space weather on the long-term climate evolution and habitability, which offers fresh insights concerning the habitability of exoplanets, especially those orbiting M-dwarfs, such as Proxima b and the TRAPPIST-1 planets. We will focus on a wide range of atmospheric compositions, ranging from exo-Venus candidates to Earth twins, as many factors remain unresolved at this stage. Future missions such as the James Webb Space Telescope (JWST) will play a crucial role in constraining the atmospheres of those exoplanets. For each of these cases, we will demonstrate the importance of the exoplanetary space weather on atmospheric ion loss and habitability.

How to cite: Dong, C.: Atmospheric escape from rocky M-dwarf planets orbiting within the habitable zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13528, https://doi.org/10.5194/egusphere-egu2020-13528, 2020.

As the number of confirmed exoplanets has increased, so too has the diversity in their physical parameters, namely their mass and radius. A common practice is to place these planets on a Mass-Radius diagram with various calculated density curves corresponding to some bulk composition. However, these lines don’t necessarily correspond to the structure of the planet found using interior models, particularly for low mass planets with masses less than 20 M_⊕ and 4 R_⊕, which we call “sub-Neptunes.” Planets in this range can have highly degenerate solutions with no solar system analog, from so-called “ocean worlds” to small dense cores with extended primary composition atmospheres. We have created a model that is able to cover the range of solutions possible for sub-Neptunes, with various levels of complexity for both the interior and atmosphere. This includes both an isothermal and semi-grey atmosphere, along with a high-pressure solar composition envelope when atmospheric pressures exceed approximately 1000 bar. We then apply this model to known sub-Neptunes located in the extended habitable zone of their star using a hydrogen-helium dominated atmosphere. An atmospheric escape model is used to investigate the longevity of the atmosphere and its effect on the overall habitability of the planet.

How to cite: MacKenzie, J., Baumeister, P., Godolt, M., Tosi, N., Kubyshkina, D., and Fossati, L.: Modeling the Atmospheric Contribution to the Interior Characterization of sub-Neptunes and its Effect on Habitability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16746, https://doi.org/10.5194/egusphere-egu2020-16746, 2020.