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.

We have a full festival schedule ahead of us. Really looking forward to Pinnacle Jam this year, where we have the amazing opportunity to open for Dark Star Orchestra!

April 8 – Willie’s Midnight Crazy Train Campout IV (Lehighton, PA)

May 14 – Celebration of Life 3 (Lehighton, PA)

May 20 – Strawberry Jam (Laurens, NY)

June 4 – Gypsy Wind Festival (Northumberland, PA)

June 17 – Stonehenge Music Festival (Jermyn, PA)

July 2 – Liberty Fest 6 (Roaring Branch, PA)

July 4 – Central PA 4th Fest (State College, PA)

July 29 – Grateful Get Down (Hamburg, PA)

July 30 – Pinnacle Jam (Bethel, PA)

August 7 – A Bear’s Picnic (Milmont, PA)

August 27 – Route 22 Fest 4 (Huntingdom, PA)

September 4 – Peace of Mind 6 (Halifax, PA)

September 10 – Still Grateful Fest 5 (Jermyn, PA)

September 17 – Hippie’s 6th Annual Fest (White Haven, PA)

If you’re in the State College area, we’ll be playing right before the fireworks begin on the 4th, so come join us to celebrate our independence. We’re also looking forward to more regional travel this year into New York state and West Virginia. See you around this summer!

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.

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