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.

Hypothesis: Does the Evolution of Complex Life Depend on the Star Type?

Earth is the only known example of a planet with life, so the history of life on Earth provides the only information regarding the timescale required for microscopic, single-celled life to evolve into bigger and more complex forms similar to plants, animals, or fungi. This process took about four billion years from the cooling of early Earth through today. But does this four billion year timescale apply when thinking about life on other planets?

It is certainly possible that complexity, on average, takes about four billion years to develop on any planet that already has life. If this were the case, then astronomers should search a wide range of stars (yellow dwarfs like our sun as well as cooler orange and red dwarf stars) because any of them might already have complex life. Although we really have no idea at all, this idea of “equal evolutionary time” is sometimes invoked by scientists as a default assumption: since we don’t know anything else, why not assume an average timescale of four billion years?

I suggest an alternative viewpoint to this assumption in a paper entitled “Does the evolution of complex life depend on the stellar spectral energy distribution?” and published in Astrobiology. In this paper, I present the hypothesis that the evolutionary timescale is constrained by the total energy that falls upon a planet and could actually be harnessed by life. Instead of assuming a fixed four billion year timescale, I calculate the amount of time it would take planets around different star types to accumulate the same amount of free energy that Earth received over its history. This assumption of “proportional evolutionary time” suggests that complex life on planets orbiting yellow dwarf stars like our sun might also take about four billion years to develop; however, planets around orange dwarf stars would take much longer, closer to five or ten billion years. And, following this hypothesis, planets orbiting red dwarfs would need a hundred billion years (longer than the present age of the universe) before they accumulated enough energy for complex life.

This idea remains a hypothesis until astronomers are able to search for signs of life around extrasolar planets. But this idea of proportional evolutionary time suggests that the coolest stars might not be the best places to look for complex life today.

Inferring the Climates of Red Dwarf Planets

Planets orbiting red dwarf stars are unique compared to other star systems because such planets are prone to falling into synchronous rotation, so that one side experiences perpetual day and the opposite side resides in permanent night. Such planets could still be habitable, sustaining liquid water and perhaps even life, so such systems continue to be targeted in the search for signs of life on exoplanets.

One starting point to looking for life on such worlds is to infer properties of an exoplanet climate from astronomical data. Eric Wolf, Ravi Kopparapu, and myself examine this problem in a paper titled “Simulated phase-dependent spectra of terrestrial aquaplanets in M dwarf systems” and published in The Astrophysical Journal. Infrared emission and reflected stellar light from a planet changes as it orbits its host star. We should that observations of these orbital changes in thermal energy could provide important information on the circulation state of the planet, the location of major cloud decks, and the abundance of water vapor. As the next generation of space telescope are designed and launched, methods such as these will become important tools for understanding the potential of M-dwarf systems to support life.

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.