We have exactly one data point at which exoplanets can be measured for their habitability: the earth. As far as we know, life evolved only on that one light blue point orbiting a single star in the center of a spiral arm of an otherwise inconspicuous galaxy.
However, most of the stars in the Milky Way are not like the sun and hang around in space all alone. Instead, up to 85 percent of the stars There may be at least one companion in orbit of each other (so it’s nice that the sun makes us keep them company).
This of course complicates the search for life, as the potential habitability of individual stars is easier to assess. Binary companions bring additional gravitational interactions and stellar radiation with them to confuse the microbes trying to wriggle out of the primeval mud.
A few years ago, astrophysicist Siegfried Eggl, now at the University of Illinois Urbana-Champaign and the University of Washington, developed an analytical framework for determining the habitable zones for binary stars in light of these additional complications.
Now he and his colleagues – Nikolaos Georgakarakos and Ian Dobbs-Dixon of New York University Abu Dhabi in the United Arab Emirates – have applied this framework to well-known binaries that host giant exoplanets to look for potential habitability.
“We used data collected from the Kepler spaceship, such as star mass, star brightness, the position of a giant planet, and other parameters to create a method of identifying two-sun systems that can host habitable Earth-like planets” Eggl explained.
The nine systems the team examined were all identified by the Kepler mission: Kepler-16Kepler-34, Kepler-35, Kepler-38, Kepler-64, Kepler-413, Kepler-453, Kepler-1647 and Kepler-1661. These systems were all analyzed by the team using equations rather than simulations, which are much more time consuming.
“It is an analysis method that requires almost no computational effort.” Said Eggl.
“There are some parts that use numerical models to input information, such as the way the atmosphere interacts with different amounts and spectra of sunlight. This is really hard to figure out analytically, so we have precomputed atmospheric models for that used.
“The advantage of our approach is that anyone can apply our equations to other systems to determine where best to look for Earth-like worlds.”
Of the nine systems, two were identified as particularly bad. Kepler-16 and Kepler-1647 host giant planets that are too poorly positioned to create a stable habitable zone – a region where exoplanets are not so close to the star that the surface water evaporates and not so far that it freezes completely .
Kepler-16 already has a smaller habitable zone due to gravitational disturbances from the binary companion. In both systems, the giant planet makes the entire habitable zone dynamically unstable.
However, five of the systems could actually have habitable worlds: Kepler-34, Kepler-35, Kepler-38, Kepler-64, and Kepler-413, with Kepler-38 showing particular promise.
Even so, the habitability conditions on any planet with two suns require a complicated balancing act.
“If a planet gets too close to its suns, its oceans can boil away. If the planet is too far away, or even ejected from a system, the water on its surface will eventually freeze, as will the atmosphere itself, as does CO2, which is seasonal forms polar ice caps Mars, ” Eggl explained.
“Once we confirm that a potentially habitable planet is in a stable orbit, we can study how much radiation it will receive from the two stars over time. By modeling the evolution of the stars and planetary orbits, we can see the actual amount or radiation estimate the planet is receiving. “
We knowThanks to the retired exoplanet hunting telescope Kepler, these exoplanets can actually form in binary star systems despite the additional gravitational disturbances. The work of the team shows that these exoplanets could possibly also be habitable.
A wide web is desirable when looking for exoplanets that could harbor life – but not if that wide web catches systems that we know are inhospitable. This new finding could help define the parameters for future work in the search for life outside of our own little pocket of space.
The research was published in Limits in astronomy and space science.