Climate Cycles Complicate Habitability

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.

Colonizing Mars

National space agencies and private corporations have declared plans to send humans to the red planet, with longer-term planets of settlement and resource extraction likely to follow. Such actions may conflict with the Outer Space Treaty of 1967, which currently prohibits any sovereign claims in space.

In a recent Space Policy paper written by Sara Bruhns and myself, titled “A pragmatic approach to sovereignty on Mars,” we develop a practical approach toward allowing the settlement of space and use of its resources through a “bounded first possession” model with a required planetary park system. We suggest that exclusive economic claims could be made without establishing sovereignty on Mars, and we propose a model for management and conflict resolution on Mars that build upon lessons from history. We also recommend revisions to the Outer Space Treaty to resolve the current ambiguity of how nations, corporations, and individuals can use the resources of space.

How Habitable are Planets Orbiting Red Dwarfs?

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.

Short Story Collection: Tales From Spaceship Earth

I am pleased to announce that the first volume of the Blue Marble Space Short Story Collection is now available! This volume, titled Tales From Spaceship Earth, includes stories from six different scientists at the Blue Marble Space Institute of Science and is the first in an ongoing series of science-informed fiction.

This collection of stories reflects an intersection of each author’s knowledge of science and vision of the future. These unique perspectives range from the near-term evolution of the space station program, to the beginnings of martian colonies, to the ultimate end of life on Earth–all of which grapple with critical issues of our transformation into a spacefaring society.

This book is available from Amazon as in both paperback and Kindle formats. Proceeds from this collection directly support the research of BMSIS scientists.