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.
BLOG: ARE UFOS ALIEN? NO!
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.
BLOG: MAN LOOKS FOR ALIENS, LOSES JOB
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
In our quest to discover strange new life on strange new worlds, a group of astronomers has modeled potential alien worlds using Earth’s biological history as a framework. From this they have determined that if we are to detect extraterrestrial biology, we should fine-tune our search to the color purple.
As we discover more and more worlds orbiting other stars in ever more biologically-pleasing orbits, the question “are we alone?” becomes increasingly acute. It’s inevitable that we will soon discover an alien world with Earth-like dimensions, orbiting a sun-like star within its habitable zone. But until we develop the means to remotely probe that world’s atmosphere, we can never be sure if it is truly habitable.
Looking for a “true” Earth analog is fraught with challenges. Are we looking for a planet with the same characteristics as modern Earth, or do we try to model our planets during different epochs and work out when Earth life would have been at its most detectable? Life on Earth has been around for the best part of 4 billion years, when would have been best for an alien civilization to detect terrestrial life and what would they have needed to look for?
It’s exactly this question that an international team of researchers is trying to answer.
“Clearly what we know about our planet will be our guideline for the characterization of (small rocky worlds in the habitable zones of their stars),” writes the team, headed by Esther Sanroma of the Instituto de Astrofísica de Canarias (IAC), Spain, in a paper accepted for publication in the Astrophysical Journal. “But the Earth has been inhabited for at least 3.8 (billion years), and its appearance has changed with time.”
3 billion years ago, during the Archean eon, the Earth was likely dominated by purple bacteria, a photosynthetic microorganism that inhabited the land and ancient seas. These organisms would have had a very distinctive spectroscopic fingerprint and a tell-tail sign that Earth was covered in a basic form of life.
By modeling different distributions of this microbe throughout the planet — in the oceans, on the land, around coastlines and during different atmospheric conditions — Sanroma’s team used a radiative transfer model “to simulate the visible and near-(infrared) radiation reflected by our planet.” By doing so, they were able to determine that by using multi-color photometric observations, distant observers would be able to “distinguish between an Archean Earth in which purple bacteria inhabit vast extensions of the planet, and a present-day Earth with continents covered by deserts, vegetation or microbial mats.”
When looking for Earth-like worlds, the researchers emphasize the need for exoplanet hunters to be aware that they may not discover a modern-looking Earth-like world, they may stumble across a purple bacteria-dominated world with a very distinctive photometric signature more fitting with an ancient Archean eon Earth-like world.
“Earth is the only planet where life is known to exist; thus observations of our planet will be a key instrument for characterization and the search for life elsewhere. However, even if we discovered a second Earth, it is very unlikely that it would present a stage of evolution similar to the present-day Earth.”
Purple plants may thrive under binary stars.University of St. Andrews
This isn’t the first time that purple alien worlds have been discussed as a possibility. In 2011, researchers examined the exotic energy-generating regimes hypothetical alien plant life would need to develop under sunlight from binary stars.
Over 25 percent of sun-like stars and 50 percent of red dwarf stars exist in binary pairs. Should there be any planets in orbit around binary systems, any life — be it flora or fauna, or some alien form of life that we can’t comprehend, let alone categorize — would be exposed to a broad spectra of light, stretching far into ultraviolet wavelengths. The upshot of this would be purple hued (or even black) plant life that has evolved to optimize photosynthesis.
It seems that in the hunt for extraterrestrial life, all roads lead to purple.
Publication: Characterizing the purple Earth: Modelling the globally-integrated spectral variability of the Archean Earth, E. Sanromá, E. Pallé, M. N. Parenteau, N. Y. Kiang, A. M. Gutiérrez-Navarro, R. López, P. Montañés-Rodríguez, 2013. arXiv:1311.1145 [astro-ph.EP]
Image credit: NASA (purple added)