Cowboys & Aliens are Coming!
July 29, 2011 --
If aliens are going out of their way to kick up dust in the Wild West, as they do in the upcoming movie "Cowboys & Aliens," they must be coming from somewhere. Life could take root on a moon or a meteorite. But to nurture the kind of life that could destroy our saloons and harass our livestock, a planet might be the most suitable. So far, Kepler, a NASA orbiting telescope that searches for planets beyond our solar system, has detected over 1,200 exoplanets. Surely there must be a few candidates among this group that could meet some of the most basic requirements to host life? Explore some far-out worlds that could support aliens, be they cattle-rustling characters or a more peaceful people.
First, let's lay out some basic criteria. Kepler hasn't identified many rocky worlds and a solid surface is essential for life to take root. Size matters: The mass of the planet helps astrophysicists infer what it's made of. Some planets are Earth-sized. Others are several times the size of our planet. And then there are gas giants, which can range from "Neptune sized" to "super-Jupiters." Orbit: To support life, a planet must be in a stable orbit around its star -- no planets with wonky orbits that will eventually dump them into their star for a fiery death. Goldilocks Zone: This is a region not too hot or too cold that gives the planet enough distance from its parent star to have liquid water, key for life. Loner Stars: Single stars make better parents. In 2010, a pair of closely orbiting binary stars was spotted surrounded by what could be the debris of former planets. Unknowns: Some factors for life can't be confirmed one way or the other from the data available about extrasolar planets. These include: water, chemical compounds such as ammonia; a nitrogen-rich atmosphere; a magnetic field to repel solar and cosmic radiation; and more. BUT, some planets do have a head-start, beginning with Gliese 581D.
Located a mere 20 light-years away, practically our backyard in cosmic terms, Gliese 581d is situated on the "outer fringes" of the Goldilocks zone, orbiting a red dwarf star. The planet may be warm enough and wet enough to support life in much the same manner as Earth. It might also contain a thick carbon atmosphere. If we ever need a new Earth and have the means to get there, Gliese 581d may be our best bet for now.
When it was first detected and reported last year in Astrophysical Journal, Gliese 581g appeared to be the perfect candidate for a true "Earth-like" planet. Located in the same star system as Gliese 581d (and detected earlier), Gliese 581g seemed to be the right size and located within a habitable zone away from its parent star. Gliese 581g was said to have three times the mass of Earth, making it possible for the planet to hold an atmosphere. However, since its discovery, follow-up studies have alleged that Gliese 581g might have been a false alarm. In other words, the planet might not exist at all.
Dubbed a "waterworld" and located a mere 42 light-years from Earth, GJ 1214b orbits near a red dwarf star about one-fifth the size of our sun. What makes this planet unique is that it appears to be primarily composed of water, although GJ 1214b is 6.5 times the mass of Earth and 2.7 times wider, which classifies it as a "super-Earth." This planet also has a steamy atmosphere composed of thick, dense clouds of hydrogen, which, although it might not the case with this planet, could incubate life.
Situated 150 light-years from Earth, HD 209458b is a planet that holds traces of water vapor in its atmosphere, and also contains basic organic compounds that, on Earth, foster the development of life. But there are two factors working against HD 209458b as a suitable habitat. The planet is very hot due to its close proximity to its parents star, and it's a gas giant, so no solid surfaces.
If Kepler-10b were located further from its parent star, it might have had a chance of hosting life. Kepler-10b was the first "iron-clad proof of a rocky planet beyond our solar system" back in 2001. It was even dubbed the "missing link" of extrasolar planetary research. When it comes to the search for life, though, Kepler 10-b is missing a lot of other ingredients -- just minor things like water or an atmosphere.
When venturing to a new star system to explore the possibility of extraterrestrial life, trying a star that has already shown itself to nurture planets -- even if they're not the kind you're looking for -- could be a promising strategy. Project Icarus, an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination, has identified two stars located within 15 light-years that might fit the bill: "epsilon Eridani, a single K star 10.5 light-years away, and the red dwarf GJ 674, 14.8 light-years away." Indirect evidence has also shown that epsilon Eridani may already hold smaller worlds scientists simply haven't detected yet. Also, red dwarf star systems generally may be a safe haven for life.
Are We Alone?
Taking into account the number of exoplanets that have been detected, as well as the vastly greater number that are estimated to be out there, some astrophysicists are convinced that extraterrestrial life is inevitable. After all, the Milky Way may be loaded with as many as 50 billion alien worlds. Some even think we'll find alien life by 2020. Others, however, say it may not exist at all. Recently, astrophysicists David Spiegel of Princeton University and Edwin Turner from the University of Tokyo suggested we might be alone in the universe, based on their interpretation of the Drake equation, a formula meant to determine loosely the probability of the existence of life beyond Earth. According to their analysis, just because life on Earth took shape early, endured and prospered doesn't mean the same process would naturally and inevitably occur elsewhere in the universe. Discovering life elsewhere, however, would be the only means of settling this debate. Unless the aliens find us first, of course.
In an effort to search for intelligent extraterrestrials, astronomers have completed their first “directed” SETI search. Unfortunately, it turned up no evidence of transmitting aliens. But that’s hardly surprising.
By focusing the Green Bank radio telescope, located in West Virginia, on stars hosting (candidate) exoplanets, it is hoped that one of those star systems may also play host to a sufficiently evolved alien race capable of transmitting radio signals into space. But in a study headed by Andrew Siemion, of the University of California, Berkeley, the conclusion of this first attempt is blunt: “No signals of extraterrestrial origin were found.”
With the help of the Kepler space telescope, Siemion’s team was able to identify which stars in Kepler’s field of view host exoplanets with certain characteristics. By selecting star systems hosting worlds in their habitable zones, systems containing 5 or more exoplanets and super-Earths with an orbital period of over 50 days, the astronomers hope that the evolution of intelligent life may have been possible, and thus may contain a transmitting alien race.
But it’s a bit like trying to find a needle in a haystack, when you have no clue where the haystack is.
“In particular, we can offer no argument that an advanced, intelligent civilization necessarily produces narrow-band radio emission, either intentional or otherwise,” the astronomers caution in a paper submitted to the arXiv pre-print service today. “Thus we are probing only a potential subset of such civilizations, where the size of the subset is difficult to estimate.
“The search for extraterrestrial intelligence is still in its infancy, and there is much parameter space left to explore.”
86 stars were selected for this directed search, and over a period of three months in 2011 (February to April), the Green Bank antennae detected 52 candidate narrow-band radio signals (less than 5 Hz) between 1-2 GHz. There are no known natural mechanisms for generating radio signals at such narrow frequencies. “Emission no more than a few Hz in spectral width is, as far as we know, an unmistakable indicator of engineering by an intelligent civilization,” they added.
After careful analysis, all signals were ruled out as being anything extraterrestrial; they were identified as interference from artificial terrestrial sources.
Though this search didn’t turn up evidence for ET’s phonecall, it represents a critical step forward in the detection of intelligent civilizations beyond our solar system. Whereas previous searches for transmitting aliens depended on educated guesswork, Kepler can be used as a guide for radio telescopes to hone-in on systems known to contain small worlds with some orbital characteristics like Earth.
This non-discovery is a result in itself — the researchers can now place important limits on the likelihood of finding a sufficiently advanced alien race in the Milky Way. Generating a powerful radio signal requires a lot of energy, so the team point out that they will most likely detect a civilization capable of generating an isotropic signal (i.e. a radio transmission that is emitted in all directions). This would require the civilization to harness the total power output of their host star, making them a Kardashev type II civilization. The results from this study give a statistical likelihood of no more than on-in-a-million chance of sun-like stars playing host to such advanced civilizations.
Refining SETI searches are key to us one day detecting hypothetical extraterrestrial intelligences. The next logical step would be to probe exoplanetary atmospheres for spectroscopic traces of a biosphere, a path Kepler has started to lead us down.
Source: arXiv blog
Publication: “A 1.1 to 1.9 GHz SETI Survey of the Kepler Field: I. A Search for Narrow-band Emission from Select Targets,” arXiv:1302.0845v1 [astro-ph.GA]
Image: The Greenbank Radio Telescope. Credit: NRAO
Note: The original version of this article stated that ex-SETI chief Jill Tarter headed this research. This was not the case, Tarter was a contributing researcher. Corrections have been made to reflect this.