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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.

Red dwarf stars outnumber yellow dwarf stars like our sun by over a factor of ten. Observations of exoplanets have also shown that rocky, and potentially habitable, planets are just as common around red dwarfs as yellow dwarfs. But if these much smaller stars are more commonplace, then why do we find ourselves around a yellow star like the sun, instead of a red dwarf?

My co-authors and I attempt to address this question of selection bias in a recent paper titled “Why do we find ourselves around a yellow star instead of a red star?” and published in International Journal of Astrobiology. We take a statistical approach to thinking about the region around all stars where life is most likely to develop. The liquid water habitable zone provides the best observational constraint on where we would expect to find planets that could support conscious observers like us, and this study examines the probability of finding oneself on a planet in the habitable zone of a yellow dwarf star, compared to a red dwarf. The results show that even though red dwarfs are much more numerous, they have a narrower habitable zone than yellow dwarfs, so our existence around a star like the sun is actually to be expected.

This study also considers that red dwarf stars will be even more numerous in the distant future of the universe, due to their much longer lifetimes than other stars. If these red dwarf stars will eventually become the predominant place for conscious observers to develop, then why do we not instead find ourselves around a red dwarf star billions or trillions of years into the future? The statistics for this aspect of the problem suggest that our existence around a yellow dwarf star today, compared to a red dwarf star in the future, might be a slight statistical anomaly—perhaps on the order of finding oneself born ambidextrous or with perfect pitch. But this statistical unlikelihood might also suggest that life is wholly impossibly around red dwarf stars, or else any type of conscious observers that do develop around such stars will be drastically different from our type of conscious life.

The outer edge of the habitable zone is traditionally defined as the outermost orbital distance at which a planet could sustain liquid water on its surface. At this distance orbit, Earth-like planets with plate tectonics (or a similar process for recycling volatiles) should build up dense carbon dioxide atmospheres that help offset the reduction in starlight. Carbon dioxide released from volcanoes provides additional greenhouse warming, although rainwater dissolves some of this. The amount of carbonic acid that dissolves in rainwater and reaches the ground depends upon the temperature: the colder it gets, the less carbon dioxide gets rained out of the atmosphere. This feedback is part of the carbonate-silicate cycle, which regulates an Earth-like planet’s carbon dioxide over geologic (million year) time scales.

In a recent paper published in The Astrophysical Journal, titled “Limit cycles can reduce the width of the habitable zone,” my co-authors and I examine the propensity of this carbonate-silicate cycle to cause a planet to oscillate between completely frozen and completely ice-free climate states. We update a simplified climate model to account for the increase in weathering that occurs as a planet builds up a dense carbon dioxide atmosphere. Beginning with a planet in completely ice-covered conditions, we allow volcanic outgassing of carbon dioxide to continue until the planet melts from the enhanced greenhouse effect. However, under certain conditions, the planet will then start to rain out and weather the atmospheric carbon dioxide at such a fast rate that the greenhouse effect decreases and the planet again plummets into global glaciation.

This type of climate cycle between glacial and ice-free states is not likely to occur on Earth today, but such cycles might have been possible on early Earth during the Hadean eon. Extrasolar planets may also be prone to this type of climate cycling, although predicting whether or not this should occur depends upon knowing a planet’s volcanic outgassing rate. Our climate calculations place boundaries on the conditions under which we should expect such climate cycles to occur for Earth-like planets orbiting a range of different stars.

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