Planetary Habitability

Within the past decade, we've discovered a lot of new planets around stars other than our Sun (for an up-to-date catalogue, visit www.exoplanets.org)  These extrasolar planets, or exoplanets for short, can exist under conditions very different than for planets in our own Solar system. From Jupiter-sized giants orbiting their host star more tightly than Mercury to tidally locked rocky planets with permanent day and night hemispheres, the diversity of observed worlds is impressive and suggests that habitable worlds may differ substantially from Earth.

The most broadly accepted definition for a 'habitable' planet is one which can sustain liquid water at its surface. While it is theoretically possible that life could arise by other means, life as we know it depends on the presence of liquid water. In our search for exoplanets that could sustain life, we are mainly interested in identifying planets within a range of orbital distances referred to as the 'habitable zone.'  

While the stellar flux (ie the radiative energy received by a given surface area over a certain amount of time, a quantity related to temperature) depends only on the orbit's radius, surface temperatures (and by extension the habitability of the planet) may be governed by the climate system.  Venus, despite being at the inner edge of our habitable zone, has a thick carbon dioxide atmosphere which leaves the surface too warm for life to develop. At the other extreme, planets with no atmosphere cannot be habitable, as liquid water cannot exist at zero pressure.  

Over a planet's lifetime, the stellar flux may vary due to several processes: [1] Stars getting brighter with age [2] Planets migrating inward or outward due to dynamical interactions [3] Orbits may not always be circular. With a changing energy input, the habitability of a planet over time depends on the stability of the climate system and its feedback processes. Positive feedback processes amplify temperature change.  One example is related to the reflectivity of ice vs oceans. Increasing global temperatures would melt ice, thereby reducing the planet's overall reflectivity and allowing for more absorption of light.  This would increase temperatures even further.  One primary negative feedback which maintains equilibrium here on Earth is related to weathering processes between the atmosphere's carbon dioxide and minerals at the surface. A positive perturbation to global temperatures would melt ice, thereby exposing more land and increasing average precipitation rates. As a result, weathering rates would increase, thereby pulling more carbon dioxide out of the atmosphere, weakening our greenhouse effect, and reducing temperatures back to equilibrium.  The strength of these feedback mechanisms can influence a planet's resilience to changes in thermal forcing.

On Earth, our orbit is nearly circular, allowing for a more stable climate system over time. This may not be the case, however, for many planets within binary star systems. With the presence of a second star beyond the orbital radius of the planet, gravitational dynamics may warp the shape of the orbit to be more elliptical, thereby varying energy input over the course of the orbit.

At sufficiently high angles between the orbit of the planet and the secondary star, the shape of the planet's orbit would oscillate. Over short enough timescales, these oscillations may create potentially extreme  climate cycles. Depending on the strength of the climate's feedbacks and how elliptical the orbit becomes, a planet like Earth could periodically freeze or boil over if it were to temporarily exit the habitable zone. Whether or not such events would render the planet permanently uninhabitable would have to be determined via climate modeling.

With the gravitational influence of a second star (upper right), the shape of a planet's orbit can oscillate between circular and elliptical, causing potentially extreme climate cycles. Note: The timescale over which the orbit changes shape is much longer than that over which the planet orbits the primary star.

Given the growing number of planets detected within binary systems, it is important to understand the limits of habitability under the physical conditions that would be expected with the presence of a second star.  If such worlds could indeed maintain stable climates with liquid water, the extreme climate cycles would present developing life with many challenges that were never present on Earth. I'll leave those topics to the biologists.

I'll end this post with some imaginative thinking about what a double sunset would look like on such a planet. Perhaps two stars may be better than one.

Public Domain Image Modified by Nathan Baskin