habitability

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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.

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.

Planets in the habitable zone of low-mass, cool stars are expected to be in synchronous rotation, where one side of the planet always faces the host star (the substellar point) and the other side experiences perpetual night (the anti-stellar point). Previous studies using three-dimensional climate models have shown that slowly rotating plants orbiting these low-mass stars should develop thick water clouds form at substellar point, at the point at which the star is directly overhead, which should increase the reflectivity, and thus stabilize the planet against increased warming at the inner edge of the habitable zone.

However these studies did not use self-consistent orbital and rotational periods for synchronously rotating planets placed at different distances from the host star, which are a requirement from Kepler’s laws of motion. We address this issue in a new study led by Dr. Ravi Kopparapu, on which I am a co-author, titled “The inner edge of the habitable zone for synchronously rotating planets around low-mass stars using general circulation models.” In this study, we use correct relations between orbital and rotational periods to show that the inner edge of the habitable zone around low mass, cool stars is not as close as the estimates from previous studies. We also discuss how the stellar composition, or ‘metallicity,’ can affect the orbital distance of the habitable zone.

Small red stars, known by astronomers as “M-dwarfs”, are the most abundant type of star in the sky and are also the most long-lived of all stars. This means there are plenty nearby of M-dwarfs to search for possible habitable planets, and many current and planned exoplanet surveys emphasize the search for potential worlds orbiting within the habitable zone of these low-mass stars. Astrobiologists often use the term “habitability” to indicate a planet’s ability to sustain liquid water on its surface, thereby providing conditions where life might be able to develop and thrive. The corresponding “habitable zone” describes the range of orbital distances that can support these clement conditions and not lose the water to a rapid runaway greenhouse (from too close an orbit) or a cool condensing atmosphere (from too far an orbit).

The problem with planets orbiting M-dwarfs is that they are prone to fall into “synchronous rotation” so that one side of the planet always faces the star, while the other side remains in perpetual darkness. Synchronous rotation can occur as a result of tidal forces from gravitational interactions between two orbiting bodies (Earth’s moon is an example of an object in synchronous rotation, so that we only ever see one side from the ground). For a planet orbiting an M-dwarf, the “sub-stellar point” beneath a constant stream of starlight is ceaselessly warmed, while the opposing “anti-stellar point” receives no starlight at all and resides in total darkness. One potential problem is that the atmosphere may condense into large ice caps on the frigid night side of these planets, which could result in total atmosphere collapse and the loss of habitable conditions.

Fortunately, the large-scale motions of the atmosphere help to redistribute this energy and, in many studies with climate models, can help avoid this atmospheric freeze-out. In a paper published in the Monthly Notices of the Royal Astronomical Society, my co-author and I use a three-dimensional computer climate model to examine the role of geothermal heating on planets orbiting M-dwarfs. Geothermal heating is another consequence of tidal forces from a close orbit, and this additional surface warming can help to amplify the asymmetric distribution of energy transport toward the night side of the planet. This can help to induce the melting of ice near the anti-stellar point and create additional habitable area surrounding the night-time hemisphere.

We also examine the large-scale dynamical circulations on these synchronous rotating planets in comparison to the general circulation patterns on Earth. We demonstrate that the direction of of the meridional (i.e. north-south) circulation changes directions from one side of the sub-stellar point to the other. That is, a global average of the meridional circulation provides an incomplete picture of the large-scale dynamics because the eastern and western hemispheres each show strong motion but in opposite directions that cancel when summed together. Additionally, we examine the presence of a cross-polar circulation that transports energy and mass from the sub-stellar to anti-stellar point across the northern and southern poles. This also contributes toward maintaining climate stability and avoiding atmospheric freeze-out with a circulation pattern atypical of those observed on Earth.

Our study emphasizes the need for careful analysis when considering how the atmospheric dynamics of a synchronously rotating terrestrial planet may differ from our own. The study of Earth-like exoplanets must begin with analogies to observations on Earth, and studies like ours help to apply Earth system models toward more general planetary system. As research into planetary habitability continues, through theory as well as observations, we will indeed continue to observe how even basic physical principles can manifest in very different ways on these alien worlds.

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