Fluctuations for Planets Around Binaries

Planets orbiting a binary pair of stars continue to be discovered by astronomers. Earth-like planets that could host liquid water, and perhaps life, may be just as likely to occur in a binary system as a single star system.

The two stars in the binary pair orbit around each other, while the planet orbits them both. This leads to a situation where the amount of radiation from each star changes by a small amount as the planet moves, causing an increase and then decrease in the starlight received with time. If this effect is too extreme, then it could potentially prevent such planets from maintaining liquid water on their surfaces.

My co-authors and I address this problem in a paper entitled “Constraining the magnitude of climate extremes from time-varying instellation on a circumbinary planet” and published in Journal of Geophysical Research – Planets. We use a simple climate model to calculate the maximum temperature that could be expected for the most extreme, but physically possible, case of a planet orbiting a binary pair. Even in the most extreme cases, we find that such a planet would be able to support liquid water in at least some parts of its surface.

Rather than sterilize the planet, the temperature variation from a binary pair acts more like a driver of seasons. Planets orbiting a binary pair may therefore experience unique seasons and weather patterns, but these would not be strong enough to make life impossible.

Habitable Zones for Binary Star Systems

Although our sun is the only star in our Solar System, about half of stars are in binary systems, with two central stars orbiting their center of mass. Astronomers have recently started to detect planets in binary systems, which suggests that binary systems could conceivably host planets with just as much diversity as single star systems. Could planets orbiting binary stars be good places to search for signs of life?

My co-authors and I explore this question in a paper entitled “Habitable zone boundaries for circumbinary planets” and published in Publications of the Astronomical Society of the Pacific. We calculate the liquid water habitable zone for a planet orbiting a binary pair, which depends upon the particular combination of stars in the system. Dimmer red dwarf stars emit more infrared radiation than brighter yellow dwarf stars like our sun, for example; varying this combination of star types in the system can have a noticeable effect on the planet’s climate. But in general, planets orbiting a binary pair of stars should be about as likely to have habitable conditions as a similar planet orbiting a single star.

Is Climate Change Ever Good?

The cumulative effects of human actions over past centuries such as widespread deforestation and the abundant use of fossil fuels has caused changes in the present climate state that would otherwise not have occurred. This trend of an increase in the global average temperature shows no evidence of slowing into the future, which suggests that uncomfortable climate change will persist in the centuries to come. Most of the problematic aspects of climate change involve its negative impact on humans. This includes shifts in farmable regions of the world, destabilization of parts of the world due to fluctuating food prices, changes in flooding and drought patterns, and forced migration due to sea level rise.

A warming planet might pose problems for humans, but how would climate change affect the welfare of other species on Earth? In a recent paper titled “Is climate change morally good from non-anthropocentric perspectives?” and published in Ethics, Policy and Environment, Toby Svoboda and I examine the impact of climate change on non-human organisms. If we temporarily set aside the interests of humans, might it be possible that climate change provides net benefits to other organisms?

The context of this study is the belief by some people in “nonanthropocentrism” or “anti-humanism” as a philosophy. Such beliefs tend to place human interests beneath those of other organisms in Earth’s community of life. If such philosophical positions are accepted at face value, then this might suggest that climate change is in fact a good thing. Climate change might cause the decline of civilization and even a reduction in biodiversity, but it might allow other organisms to flourish as a result. The net effect could be a much more thriving planet, albeit one in which humans are worse off.

This is not to say that we should ignore the effects of climate change on humans. Instead, this analysis demonstrates that nonanthropocentrism and anti-humanism may be in conflict with modern attempts at mitigating climate change. Our analysis therefore challenges adherents of nonanthropocentric ethics to examine the extent to which non-human interests should take priority over the collective interests of human civilization.

Exomoon Atmospheres

As astronomers continue to search for potentially habitable planets orbiting other stars, some have also started to consider the possibility of habitable moons orbiting giant planets in such systems. Such exomoons could be the size of Mars, Earth, or even a few times larger, based on observations of similar large moons orbiting Jupiter and Saturn. A few scientific studies have demonstrated that exomoons should be expected in other planetary systems and could even be observable with the next generation of space telescopes.

I recently published a paper with René Heller in Monthly Notices of the Royal Astronomical Society titled “Exploring exomoon atmospheres with an idealized general circulation model.” This study presents the first three-dimensional climate modeling of exomoon atmospheres. Exomoon atmospheres receive daily instellation from the host star of the planetary system, which would make them similar in climate to an Earth-like planet. But exomoons are also in synchronous rotation with the host giant planet (similar to the synchronous rotation of our moon around Earth). Exomoons therefore receive additional thermal energy at the top of their atmospheres from their host planet, in addition to the star. This configuration leads to a climate with warmer poles (a phenomenon known as “polar amplification”) and stronger dynamical energy transport.

Some exomoons atmospheres could enter a runaway greenhouse from the additional thermal energy of the host planet, but others should be able to maintain stable and potentially habitable atmospheres. Exomoons remain viable prospects in the search for life, and future astronomical surveys will gradually reveal the frequency of such worlds.