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.

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.

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.