We pretty much know how gravity works. A star system [usually] revolves around its galactic center, a planet around its parent star, a moon around its planet, a human around its fridge.
But apparently, if a human can revolve around two love interests, so can a planet.
Kepler 16b is not your average floozy, but a full sized, very well-documented exoplanet. Gas giant to be exact. And the very first to be discovered orbiting two stars.
Some time ago I wrote an article (“Is our Solar System the odd one out in a universe full of chaos?”) where I talked about the peculiarity of our Sun being a lone star. When we look around our galactic neighborhood, we discover that most stars with a mass similar to our Sun, down to the size of Red dwarfs, come in pairs or in groups of three. The long lived cosmic dance of these objects is part of what makes the universe so creative, in terms of events, energy and radiation. Yet, when it comes to planets orbiting these stars, their systems follow a very familiar rule of gravity we also witness and experience during our orbit around the Sun.
Before we dig into the physics of Kepler 16b, let’s see what makes the binary system it orbits so prone for this phenomenon to happen inside the Milky Way, only 254 light years away from Earth.
The Kepler 16 binary system is composed of two stars: Kepler 16A and Kepler 16B. Why the name? Because they were discovered by the – now defunct – Kepler Space Telescope, in commission between March 2009 and October 2018, observing no less than 530,506 stars and 2662 planets throughout its long run. Kepler 16A is a main-sequence Orange dwarf with approximately 68% the mass of our Sun and approximately 65% its radius. Its companion, Kepler 16B, is a Red dwarf with a fifth of the mass of the Sun and a fifth of its radius. We already know about Red dwarfs, their dim luminosity and impressively long life span. Kepler 16A, however more massive than its smaller partner is also more than six times dimmer than the Sun, therefore it will outlive our parent star by billions of years. Tens of billions!
As a perpetual courtesy towards my readers, I never avoid giving details, as to entice the curiosity of all of you out there, who’s minds wonder towards the possibilities our universe has to offer. Orange dwarfs, such as Kepler 16A, also denominated as K-type main-sequence stars, are three to four times more numerous than G-type main-sequence stars (Yellow dwarfs), such as our Sun. They are the perfect candidates to host exoplanets with extraterrestrial life. Orange dwarfs emit much less ultraviolet radiation than Yellow dwarfs, making them more life-friendly. Furthermore, if Sun-like stars are stable for a debatable 10 billion years, Orange dwarfs remain in their very stable main-sequence life span for 40 to 80 billion years, making it possible for life to develop and flourish to degrees we might not comprehend, but hope to discover. To put it in simpler terms, no K-type main-sequence star (/Orange dwarf) that has ever ignited in the entire history of the Universe has ever burned out through its hydrogen gas fuel yet. However old such a star might be, considering the overall age of the Universe, it’s still in its teenage years. However, Kepler 16A is definitely not a good candidate for life hosting exoplanets. And it’s because of its Red dwarf companion, Kepler 16B, very low in luminosity but fully capable of violent solar storms, ejecting flares that will sterilize all its surrounding planets.
The Kepler 16 binary system is very compressed, with the two stars orbiting around a gravitational center, with an average distance of only 0,22 AU, much closer than Mercury ever gets to the Sun (Mercury’s perihelion is 0,30 AU). It takes the two stars just 41 days to complete an orbit around each other. From what it appears, their cosmic dance seems to be stable, without the Red dwarf vacuuming its larger companion’s outer layers of hydrogen gas. At least for the next two billion years, when the system might become unstable.
The very close proximity between the two Kepler 16s has made it possible for the remaining debris from the early stages of the system to coalesce into a Saturn-sized gas giant. At an average distance of 0,71 AU, Kepler 16b orbits the binary system right outside the habitable zone. The exoplanet is part gas, part rock and ice, with an estimated surface temperature of -83°C and a full orbit in 228 days. Habitability on the exoplanet is highly impossible due to the radioactive nature of Kepler 16b. However, there is an ongoing debate to whether or not Kepler 16b might be orbited by a rocky moon. If we look in our own backyard, we know that radioactive worlds such as Jupiter or Saturn create huge magnetic fields around them and are surrounded by potentially habitable moons, such as Titan for Saturn or Ganymede for Jupiter (further space missions will prove whether or not these moons are hosting uni- or multicellular forms of life. I believe they are! Both of them!). For a system as far as Kepler 16, we are still unable to see smaller details, as in smaller and more distant exoplanets orbiting the two stars, and the moons that might orbit these exoplanets, including Gas Giant Kepler 16b. The gravitational effect on a rocky moon ranging from the size of Earth to the size of Mars and the Gas Giant’s magnetic field, could open up the possibility for a fully habitable world orbiting a very unfriendly exoplanet.
And yes, if you reached this part of this article and are still wondering what’s so familiar about this exoplanet orbiting two stars, know that the Smithsonian Center has indeed called it Tatooine as a legacy to the legendary fictional world where the Star Wars saga ignited in the 70’s.
Kepler 16b might not be the best candidate for extraterrestrial life, but it fuels our drive to further explore exoplanets orbiting Orange dwarfs and better our technology so that we can peak with more detail into what might really be the very best candidates for hosting life in the universe: the rocky moons of gas giants. – Roman Alexander
ASTROPEEPS.COM 
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