Periodic Reporting for period 4 - EXOCONDENSE (Climate Dynamics of Exoplanets with Condensible Atmospheres)
Reporting period: 2022-04-01 to 2023-09-30
EXOCONDENSE is aimed at understanding the atmospheres of planets around stars other than our own, stimulated by the revolution in ability of astronomers to detect exoplanets and to characterize their climates and atmospheres. With the next generation of ground and space-based telescopes, smaller (potentially Earth-like) planets are coming into view. All planets deepen our understanding of the fundamental nature of planetary climate and its evolution, including planets that are not likely to be habitable for any form of life.
Condensible substances play a ubiquitous role in planetary atmospheres. Depending on the temperature of a planet, a range of substances other than water can condense. Phase change effects -- loosely speaking, condensation -- profoundly affect the dynamics of planetary climate, and the way we interpret astronomical observations. Condensation transports heat from one place to another , forms clouds which feed back on climate, and provides feedback on the energy balance of the planet even when in a gaseous state. Condensation and later precipitation affect the chemical evolution of an atmosphere.
It is the overall objective of this project to develop a comprehensive understanding of the effects of condensation in exoplanet atmospheres, through developing and carrying out a new suite of simulations of atmospheres involving condensible substances. There is an emphasis on developing predictions that can be tested against observations.
It is important for the same reason that understanding the Big Bang is important. It satisfies a deeply and widely held desire to understand our origins, and our place in the Universe. Up until twenty years ago, for all we knew planets are a very rare event in the Universe. Now, astronomy has opened a window into one of the deepest questions of all: Are We Alone?
Condensible substances play a ubiquitous role in planetary atmospheres. Depending on the temperature of a planet, a range of substances other than water can condense. Phase change effects -- loosely speaking, condensation -- profoundly affect the dynamics of planetary climate, and the way we interpret astronomical observations. Condensation transports heat from one place to another , forms clouds which feed back on climate, and provides feedback on the energy balance of the planet even when in a gaseous state. Condensation and later precipitation affect the chemical evolution of an atmosphere.
It is the overall objective of this project to develop a comprehensive understanding of the effects of condensation in exoplanet atmospheres, through developing and carrying out a new suite of simulations of atmospheres involving condensible substances. There is an emphasis on developing predictions that can be tested against observations.
It is important for the same reason that understanding the Big Bang is important. It satisfies a deeply and widely held desire to understand our origins, and our place in the Universe. Up until twenty years ago, for all we knew planets are a very rare event in the Universe. Now, astronomy has opened a window into one of the deepest questions of all: Are We Alone?
The project has substantially advanced the understanding of the behaviour of condensible substances in exoplanet atmospheres, and laid the groundwork for interpretation of data from observations with the James Webb Space telescope. Accomplishments have been in two general areas: global circulation models of exoplanet atmospheres, and regional scale simulations of three dimensional convection involving condensation and clouds. Dissemination has been in the form of journal articles, doctoral dissertations, presentations at domestic and international meetings, and publicly available model code. A few highlights of these results are presented below:
- A series of journal articles concerning basic aspects of exoplanet dynamics which provide the underpinning for understanding condensation processes, drawing on concepts in linear wave dynamics that were originally developed for understanding the Earth's tropical circulation, especially phenomena such as El Nino.
- The first simulation of the lava planet 55 Cancri-e which took into account the possibility of a non-condensible background atmosphere, which appears to be necessary to explain the relatively warm night-side temperatures of the planet.
- The first simulation of the global circulation of a brown dwarf star in orbit around and irradiated by a host star. Brown dwarfs are an outstanding and highly observable laboratory for probing the effects of clouds and condensation. The EXOCONDENSE team has published additional work probing condensation behaviour on such objects, employing our generalized version of the CM1 three dimensional convection model. The same model has been used for the first simulation of convection in pure steam atmospheres.
- Produced the first model of the outer edge of the habitable zone to incorporate modern advances in the understanding of geochemical weathering processes. This work has considerable bearing on the interpretation of observations which seek the signature of a chemical-weathering thermostat in action. .
-Developed simulations of subNeptune exoplanets, with particular application to the exoplanet K2-18b. subNeptunes are low-density planets bigger than Earth and smaller than Neptune, having an extensive fluid envelope compared to Earth's atmosphere and ocean. These results include the first incorporation of stabilization of the atmosphere against convection by the presence of a water-rich layer in a Hydrogen atmosphere, and delineate the circumstances under which a liquid water ocean can exist. An additional article on subNeptunes described how the runaway greenhouse phenomenon affects the interior structure of the planet.
-Developed one dimensional and three dimensional models of post-runaway pure steam atmospheres, applying them to elucidate the fate of planets that have undergone a runaway greenhouse process.
-Used convection-resolving simulations to simulate 3D convection in planetary atmospheres in which the buoyancy is affected strongly by gradients in composition (e.g the hydrogen to water vapour ratio).
- A series of journal articles concerning basic aspects of exoplanet dynamics which provide the underpinning for understanding condensation processes, drawing on concepts in linear wave dynamics that were originally developed for understanding the Earth's tropical circulation, especially phenomena such as El Nino.
- The first simulation of the lava planet 55 Cancri-e which took into account the possibility of a non-condensible background atmosphere, which appears to be necessary to explain the relatively warm night-side temperatures of the planet.
- The first simulation of the global circulation of a brown dwarf star in orbit around and irradiated by a host star. Brown dwarfs are an outstanding and highly observable laboratory for probing the effects of clouds and condensation. The EXOCONDENSE team has published additional work probing condensation behaviour on such objects, employing our generalized version of the CM1 three dimensional convection model. The same model has been used for the first simulation of convection in pure steam atmospheres.
- Produced the first model of the outer edge of the habitable zone to incorporate modern advances in the understanding of geochemical weathering processes. This work has considerable bearing on the interpretation of observations which seek the signature of a chemical-weathering thermostat in action. .
-Developed simulations of subNeptune exoplanets, with particular application to the exoplanet K2-18b. subNeptunes are low-density planets bigger than Earth and smaller than Neptune, having an extensive fluid envelope compared to Earth's atmosphere and ocean. These results include the first incorporation of stabilization of the atmosphere against convection by the presence of a water-rich layer in a Hydrogen atmosphere, and delineate the circumstances under which a liquid water ocean can exist. An additional article on subNeptunes described how the runaway greenhouse phenomenon affects the interior structure of the planet.
-Developed one dimensional and three dimensional models of post-runaway pure steam atmospheres, applying them to elucidate the fate of planets that have undergone a runaway greenhouse process.
-Used convection-resolving simulations to simulate 3D convection in planetary atmospheres in which the buoyancy is affected strongly by gradients in composition (e.g the hydrogen to water vapour ratio).
The project has achieved the following advances beyond the state of the art.
- Pioneered the use of the novel cube-sphere representation of atmospheres in exploration of the general circulation of tide-locked exoplanets.
- Developed a novel stochastic scheme for computing the electromagnetic radiation exiting from a planet's atmosphere, which has substantial advantages in the interpretation of astronomical data.
- Developed a new approach to computing transfer of energy by electromagnetic radiation in pure-steam (runaway greenhouse) atmospheres, which captures the essential aspects of the process, but is much more computationally efficient than full real-gas radiative transfer calculations. We have pioneered the use of advanced geochemical representations of the processes that control the amount of carbon dioxide in a planetary atmosphere.
- Developed one of the world’s few flexible three dimensional convection models suitable for a wide range of planetary atmospheres, and applied it to a range of atmospheres, including those of brown dwarfs.
* Developed a new theory of the way the hydrological cycle on a planet translates into a sink of carbon dioxide. This class of new models provides the basis for new estimates of the outer edge of the habitable zone, since turning potential habitability into actual habitability requires that the atmosphere contain carbon dioxide with an appropriate range of concentrations.
In the course of the project we have developed a new understanding of convection including the effects of composition on buoyancy and the effects of condensible species that make up a major part of the mass of the convecting atmospheres; we also have used these insights to develop new representations of convection for use in global circulation models.
To that end, we have:
- Carried out three dimensional simulations of the climate and atmospheric evolution of planets which are undergoing a runaway greenhouse.
- Developed and applied a new class of models of atmospheres whose source is evaporation from the hottest portions of a planet's surface.
- Completed a study of the hydrological cycle on the temperate subNeptune exoplanet K2-18b, with emphasis on understanding the effects of latent heat release and compositional buoyancy on global circulation patterns, which was the first such study to incorporate the effects of compositional buoyancy.
- Pioneered the use of the novel cube-sphere representation of atmospheres in exploration of the general circulation of tide-locked exoplanets.
- Developed a novel stochastic scheme for computing the electromagnetic radiation exiting from a planet's atmosphere, which has substantial advantages in the interpretation of astronomical data.
- Developed a new approach to computing transfer of energy by electromagnetic radiation in pure-steam (runaway greenhouse) atmospheres, which captures the essential aspects of the process, but is much more computationally efficient than full real-gas radiative transfer calculations. We have pioneered the use of advanced geochemical representations of the processes that control the amount of carbon dioxide in a planetary atmosphere.
- Developed one of the world’s few flexible three dimensional convection models suitable for a wide range of planetary atmospheres, and applied it to a range of atmospheres, including those of brown dwarfs.
* Developed a new theory of the way the hydrological cycle on a planet translates into a sink of carbon dioxide. This class of new models provides the basis for new estimates of the outer edge of the habitable zone, since turning potential habitability into actual habitability requires that the atmosphere contain carbon dioxide with an appropriate range of concentrations.
In the course of the project we have developed a new understanding of convection including the effects of composition on buoyancy and the effects of condensible species that make up a major part of the mass of the convecting atmospheres; we also have used these insights to develop new representations of convection for use in global circulation models.
To that end, we have:
- Carried out three dimensional simulations of the climate and atmospheric evolution of planets which are undergoing a runaway greenhouse.
- Developed and applied a new class of models of atmospheres whose source is evaporation from the hottest portions of a planet's surface.
- Completed a study of the hydrological cycle on the temperate subNeptune exoplanet K2-18b, with emphasis on understanding the effects of latent heat release and compositional buoyancy on global circulation patterns, which was the first such study to incorporate the effects of compositional buoyancy.