As we seek out planets orbiting stars inside their habitable zones, astronomical techniques are becoming so sophisticated that, one day, we may be able to probe the atmosphere of a distant exo-Earth - i.e. a rocky exoplanet possessing liquid water on its surface with potential biosignatures in its atmosphere.
But let's take this idea one step further.
If there's one thing we are beginning to realize with exoplanetary studies, it's that there is a huge variety of alien worlds out there and, of the billions of stars in our galaxy, just about every conceivable configuration of exoplanet size and orbit should be possible.
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In a new study presented at the American Astronomical Society's Division for Planetary Sciences meeting in Tucson, Ariz., earlier this month, researchers from the California Institute of Technology (Caltech) discussed the possibility of habitable binary planets; a configuration that, if the conditions are right, life could take root on both bodies orbiting inside the habitable zone of their star.
Probably the most familiar example of what could be considered to be a binary planet is that of the Pluto-Charon system. Although Charon is officially recognized as the biggest moon of Pluto and not a binary partner, in a recent Discovery News article I argued the case for making dwarf planet Pluto and satellite Charon a binary planet. Although this possibility was discussed in 2006 when the International Astronomical Union debated Pluto's planetary status, eventually, the "dwarf planet" designation was (controversially) settled on.
In the case of Pluto and Charon, their barycenter (the point in space at which both masses orbit) is well above Pluto's surface. The gravitational tugging of Charon is substantial, shifting their orbital focal point into the space between the two worlds. As observed by the fast-approaching NASA News Horizons spacecraft (that will flyby Pluto and its system of moons in July 2015), the two masses have a very distinctive wobble. In comparison, Earth's moon does tug on Earth, but the Earth-moon system's wobble creates a barycenter deep inside our planet near the Earth's core.
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But say if, somewhere in the Milky Way, there are two worlds of approximately equal mass, in a binary dance like Pluto and Charon, orbiting their star at a distance where the temperature conditions are right for liquid water to persist on their surfaces?
As discussed by Caltech undergraduate student Keegan Ryan, graduate student Miki Nakajima and planetary scientist David Stevenson, this scenario isn't that far fetched. During the formation of rocky planetary bodies around a star, it's possible that two large masses may drift close enough to begin to gravitationally interact. This interaction can result in the merging of both masses to form a larger planet. Alternatively, the two masses may collide energetically, kicking up vast quantities of debris.
Scientists believe the latter scenario led to the formation of Earth's only natural satellite; when another planetary body smashed into our young Earth, the debris produced coalesced to form the moon. The colliding body careened away from the Earth and was likely ejected from the solar system.
But say if there's another collision scenario where two like-mass worlds interacted with one another, but did not merge or collide, instead becoming locked in a stable orbit around one another for billions of years.
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"There is a good reason to believe terrestrial binary planetary systems may be possible," writes a Caltech press release. "In a grazing collision the angular momentum is too high to be contained within a single rotating body (it would fission) and if the bodies barely touch then they could retain their identity. However, it requires an encounter where the bodies are initially approaching each other at low enough velocity."
Through the use of a computer model utilizing a method called Smoothed Particle Hydrodynamics (SPH), a collection of tens of thousands of interacting particles could be simulated. The method can be used to simulate the agglomeration of protoplanetary bodies and the formation and evolution of moons. But the researchers ran the simulation for a huge variety of initial conditions. Sure enough, though both masses undergo huge tidal distortions, planetary binaries of approximate Earth-like masses are possible. From this model, the possible exoplanetary binary configuration can be characterized and astronomers can begin hunting for observational evidence of their existence.
Due to their close proximity, these planetary binaries will be tidally locked, where one side of each world will continuously face one another. We are familiar with this scenario with Earth's moon - the moon is tidally locked with Earth, showing only one hemisphere, the moon's near-side.
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Though only imagined in science fiction to date, this Caltech simulation proves that, in exoplanetary studies where any configuration seems possible, a scenario where two bona fide Earth-like exoplanets could be locked in a stable binary system, potentially within their star's habitable zone.
One can imagine looking up from one of those worlds where the second planet is constantly high in the sky, and as the binary system rotates, orbiting their star, the planet above progresses through its phases, much like the moon does around the Earth. Depending on how the binary is aligned relative to the star, there may even be periodic eclipse events where one world is cast into darkness by its binary twin blocking star light.
And what if alien life evolved on one or both of these worlds? And what if they became technologically advanced enough to travel between both planets? Though these musings are purely hypothetical, it does make you think that having a second habitable (or, at least, potentially habitable) world in a binary orbit with your own planet could motivate a fevered space race that would dwarf our space race of the 1960s. That could be the genesis of a powerful and sustained space-faring alien race.
via Physorg.com, h/t Franck Marchis