Nov. 8, 2011 --
Despite the occasional report of an extraterrestrial sighting, be it through a microscope revealing curious shapes in a meteorite or a photo of wispy lights taken at the blurry end of a camera lens, aliens have yet to make contact with humans. Even the White House yesterday put out a statement declaring that the federal government "has no evidence that any life exists outside our planet, or that an extraterrestrial presence has contacted or engaged any member of the human race." Humans may not yet have encountered life outside of our planet, but many scientists see it as an inevitability. In 1960, astronomer Frank Drake came up with the now eponymous equation which provided an estimate of the number of civilizations in our galaxy. Although scientists continue to debate the application of his formula as well as alternatives, Drake's own solution to the equation is 10,000 civilizations, suggesting intelligent, technologically advanced life outside our planet is common. How these different civilizations, including our own, find each other is an important question for anyone here on Earth looking for extraterrestrials. Explore how aliens might stumble upon our planet -- and how we might actually spot them first.
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Before we can began to search the skies, we have to start by narrowing down our options. Sticking within our own galaxy is a good start, since we're more likely to spot a neighbor closer to us than one further away. Astronomers may also elect to focus their attention on stars closer to the center of the Milky Way, where 90 percent of its stars are clustered. Furthermore, the stars here are a billion times older than the sun, giving life more time to develop biologically and technologically. Many stars are unsuitable for nurturing life, and even stars that do have the appropriate "spectral type" may host exoplanets inhospitable to life due to their location relative to their parent star, size or composition. These criteria would not only help us find aliens, but also help them find us. After all, Earth would stand out as a hospitable planet, according to a paper published in 2007 in Astrophysical Journal.
If aliens are looking for us, they're scanning the same, vast, dark and mostly empty expanse of space that we are. It's a good thing then that we're leaving the lights on to make it easier to find us. According to Abraham Loeb, of Harvard University and Edwin Turner, from Princeton University, by scanning the skies for artificial illumination as opposed to naturally occurring light sources, both human and extraterrestrial astronomers might be able to find signs of life. Existing telescopes would be able to see a city the size of Tokyo as far as the edges of our solar system.
BLOG: CITY LIGHTS COULD REVEAL ET
For more than 25 years, the SETI Institute has been scouring the skies for signs of alien life. However, long before the institute was established, scientists have tried to catch a communication signal from another world. Scientists looking for alien signals use a combination of optical and radio telescopes, such as the one seen here. Dropping in on a signal without knowing the source of the communication is the tricky part, however, and researchers narrow down their search by targeting specific kinds of stars. With their citizen science program, SETI@home, the institute has enlisted three million additional observers analyzing data for traces of an alien signal.
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Have aliens already stopped by for a visit, even though we weren't at the door to meet them? If they have, shouldn't they have left something behind? An artificial object of alien origin could be lurking in our solar system without our knowledge. As Discovery News' Ray Villard explains: "In a paper published in the 1960s, Carl Sagan, using the Drake Equation, statistically estimated that Earth might be visited every few tens of thousands of years by an extraterrestrial civilization." Further out beyond our solar system, aliens may have left what essentially amount to interstellar billboards large enough to be seen by, say, a planet-hunting telescope like Kepler. These last two scenarios, of course, envision an extremely technologically advanced civilization well beyond the engineering capabilities of humankind. At the same time, humans have sent spacecraft beyond the solar system, including Pioneer 10 and 11 as well as Voyager 1 and 2. All of these spacecraft are equipped with what are essentially calling cards for the human race -- small plaques in the case of the Pioneer spacecraft and golden records for the Voyager spacecraft (seen here).
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Humans may rely primarily on fossil fuels as their primary means of energy, but that doesn't mean extraterrestrials in a far off civilization have the same power source. Solar power could be one option, though not quite with the same black panels we use on Earth. A super civilization could even tap into a black hole to meet its energy needs. If aliens are tapping to these cosmic bodies, that should make them all the more detectable from Earth. How would we know whether an alien race was relying on a black hole as a source of energy? As Discovery News' Ray Villard explains: "Tell-tale evidence would come from measurements that showed the black hole weighed less than 3.5 solar masses. That's the minimum mass for crushing matter into a black hole via a supernova core-collapse."
In one of the most unusual -- and highly unlikely -- first-contact scenarios, aliens would be able to recognize us by the level of greenhouse gas emissions we pump into our atmosphere. Not only that, according to a hypothesis put forward by researchers affiliated with NASA and Pennsylvania State University (though not directly tied with either institution), but aliens may use that as cause to wipe out the human race. In this bizarre set of circumstance, aliens view human advancement as a destructive force spiraling out of control. To avoid the threat of a future adversary, extraterrestrials clear out the competition.
BLOG: TO SAVE THE GALAXY, DESTROY HUMANITY
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.
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.
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.
“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.
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.