Circulation States of Synchronous Rotators

Some planets around low mass stars are expected to be in synchronous rotation, so that the star is continually fixed upon one side. This not only causes one hemisphere to experience permanent day and the other to reside in permanent night (with perpetual twilight along the “terminator” at the edges), but this also drives the climate into a regime fundamentally unlike any seen in the atmosphere of Earth.

In a recent paper titled, “Demarcating circulation regimes of synchronously rotating terrestrial planets within the habitable zone,” my co-authors and I analyze a set of climate model calculations to examine the dependence upon stellar effective temperature of the atmospheric dynamics of planets as they move closer to the inner edge of the
habitable zone. These results show that the surface temperature contrast between day and night hemispheres decreases with an increase in incident stellar flux. This trend is opposite that seen on gas giants, where the same forcing shows an increase in the day-night atmosphere temperature contrast.

We define three dynamical regimes in terms of the dynamical quantities known as the Rossby deformation radius (the ratio of buoyancy to rotation) and the Rhines length (the maximum extent of turbulent structures). The slow rotation regime is characterized by a mean zonal circulation that spans from the day to night side. Slow rotation requires that both the Rossby deformation radius and the Rhines length exceed planetary radius, which occurs for planets with rotation rate > 20 days. Rapid rotators show a mean zonal circulation that only partially spans a hemisphere, with banded cloud formation beneath the substellar point. The rapid rotation regime is defined by the Rossby deformation radius being less than planetary radius, which occurs for planets with rotation rate < 5 days. In between is the Rhines rotation regime, which retains a mean zonal circulation from day to night side but also features midlatitude turbulence-driven zonal jets. Rhines rotators are expected for planets with rotation rate between 5 to 20 days, where the Rhines length is less than planetary radius but the Rossby deformation radius is greater than planetary radius. The dynamical state can be inferred from observations of orbital period and spectral type of the host star as well as from comparing the morphology of the thermal emission phase curves of synchronously rotating planets. Such phase curves will be potentially useful tools for characterizing planets with the next generation of space telescopes.

Does Intelligence Depend on Star Color?

Our civilization on Earth resides around a yellow dwarf star, but orange and red dwarf stars are much more numerous in the galaxy. Does this suggest that we should orient the search for extraterrestrial intelligence (SETI) toward these smaller and redder star systems? Or are certain star types better abodes for intelligent life than others?

Myself and Ravi Kopparapu address these questions in a chapter titled, “The Drake equation as a function of spectral type and time,” which appears in the book Habitability of the Universe Before Earth. Our approach invokes the Drake equation, which is a probabilistic device for estimating the number of communicative civilizations in the galaxy. The conventional terms of the Drake equation are the rate of star formation, the fraction of stars with planets, the number of habitable planets per system, the probability of life, the probability of intelligence, the probability of communicative technology, and the average lifetime of such civilizations. The product of all these terms yields the number of civilizations in the galaxy that SETI could conceivably discover.

Our book chapter analyzes calculations of the liquid water habitable zone for plants orbiting yellow, orange, and red dwarf stars. These habitable zone calculations provide constraint on the terms of the Drake equation. In particular we consider the dependence of each term of the Drake equation on both the stellar type (color) of the star, as well as the change in each parameter over time since the galaxy formed. We suggest that the habitability of red dwarf systems may peak in the far future, while the present time is optimal for habitability around yellow and orange dwarf stars.

Water Loss on Planets Orbiting Low-Mass Stars

An Earth-like planet tends to increase its water vapor content as its mean temperature increases. The inner edge of the habitable zone is defined by the point at which such a planet begins to lose its water, thus rendering it uninhabitable. A “moist greenhouse” occurs when the (usually dry) upper atmosphere becomes wet, which results in the destruction of water molecules by incoming sunlight. Another process knows as a “runaway greenhouse” occurs due to the increased greenhouse effect of water vapor in the lower atmosphere, which further drives evaporation and more warming. Either of these processes could cause a planet at the inner edge of the habitable zone to lose its oceans entirely.

In a recent paper published in The Astrophysical Journal, titled “Habitable Moist Atmospheres On Terrestrial Planets Near the Inner Edge Of the Habitable Zone Around M-dwarfs,” my co-authors and I conduct three-dimensional climate simulations of planets orbiting low-mass stars. We show that planets near the inner edge of the habitable zone should generally first enter a moist greenhouse state, although planets around the coolest stars we analyzed should directly transition into a runaway greenhouse state instead. Some of these planets orbiting low-mass stars could experience very slow water loss that could last up to the lifetime of the star, which could allow habitable conditions to persist even during a moist or runaway greenhouse.

Snowball and Greenhouse Atmospheres Around Other Stars

Earth’s climate has been shown by a wide range of climate models to be bistable, which means that it can exist in both a frozen state or a warm, ice-free state with the same amount of solar energy. Earth today resides in a warm state with small ice caps, but geologic evidence suggests that the Neoproterozoic Earth, about 500 Myr ago, may have been frozen all the way down to the equator.

Other terrestrial planets orbiting other stars should behave similarly, although the transition point between climate states may differ for stars that are brighter and dimmer than the sun. In a recent paper published in The Astrophysical Journal, titled “Constraints on climate and habitability for Earth-like exoplanets determined from a general circulation model,” my co-authors and I analyze three-dimensional climate simulations of planets orbiting a range of stars. We show that planets can exist around such stars as a frozen snowball, partial melt with an equatorial waterbelt, temperature conditions everywhere, and a hothouse with gradual water loss. Stars slightly cooler than the sun could maintain habitable conditions for longer periods of time, due to the slower rate of water loss.