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.
The Search for Extraterrestrial Intelligence, or SETI, is a speculative venture. There is currently zero evidence for the existence of extraterrestrial life, let alone intelligent extraterrestrial life — why the heck are we bothering to search for life forms advanced enough to blast radio waves into space? As speculative as this hunt may be, it is based on sound science methodology and, should the slightest hint of alien intelligence be detected, it would transform our understanding about our place in this grand cosmic sandbox.
Listening to random stars in the hope that one may play host to a transmitting alien civilization is beyond speculative, however — given enough time and computational power, yes, it only ever has a low probability of success. But say if we know that a given star plays host to a planet, not too dissimilar to Earth, orbiting within a magical distance from its host star that is just the right temperature for liquid water to exist on its rocky surface? Wouldn’t it make sense to aim our radio telescopes at those star systems?
Wouldn’t it make even more sense to aim our radio antennae at star systems containing several exoplanets, two of which have Earth-like qualities and orbit within the star’s “habitable zone”?
Well, with the help of NASA’s premier exoplanet hunter Kepler, SETI has found a natural partner in the hunt for transmitting extraterrestrials, and on Thursday (April 18) Kepler mission scientists announced the discovery of a tantalizing star system named Kepler-62.
Kepler-62 is known to have at least five worlds orbiting it. Two of the exoplanets, Kepler-62e and Kepler-62f, are located within the star’s habitable zone with orbital periods of 122 and 267 days, respectively. But the groundbreaking thing about this pair is the fact they are very small and may even possess rocky surfaces. “e” is 1.6 times the size of Earth and “f” is 1.4 times the size of Earth. Sadly, at 1,200 light-years distant, that’s all we know about the alien worlds. They are too far away for followup observations by ground-based observatories to determine their masses and any hope of detecting their atmospheric chemistry will be unlikely for some time.
Still, this is exciting as Kepler-62e and f are the smallest worlds detected by Kepler to date that orbit within the habitable zone of their star, allowing liquid water to exist (if it is indeed present), thereby potentially supporting the evolution of life as we know it on their hypothetical rocky surfaces.
In an effort to refine the search for extraterrestrial intelligence(s), data from the Kepler exoplanet hunter has been used to perform directed SETI searches. This is a paradigm-shift from the days of guessing which stars might host exoplanets. Kepler is reducing the uncertainty of hunting for ETIs. So far, astronomers carrying out directed SETI searches have turned up empty handed, but these are very early days. And as Kepler-62 demonstrates, there are a lot more promising worlds out there to study.
“These discoveries move us farther down the road to discovering planets similar to Earth,” said Jon Jenkins Senior Scientist at the Carl Sagan Center of the SETI Institute and co-author of the Kepler-62 study. “While we don’t know if Kepler-62e and f are rocky or whether they have liquid water pooling on their surfaces, their existence shows that the incidence of small worlds in the habitable zone of sun-like stars is high. Thus we can look forward to the discovery and detailed characterization of Earth’s cousins in the years and decades to come by future missions and telescopes.”
“Since December of 2011, the SonATA program to search for extraterrestrial intelligence with the Allen Telescope Array has been focusing on the Kepler exoplanet candidates and especially those planets expected to be within the “Habitable Zone” of their stars,” added Gerry Harp, Director of the Center for SETI Research. “Our surveys improve on previous, generally narrowband SETI by covering the radio frequency range where Earth’s atmosphere is most transparent, including many frequencies never before observed. We expect to complete a meaningful survey of these stars in less than 1 year — be sure to check back soon.”
The discovery of one “habitable” exoplanet is exciting. An approximately Earth-sized exoplanet inside a star’s habitable zone is very exciting. But finding two? That’s the equivalent of striking exoplanetary gold. Lacking any further clues, it doubles the life-giving qualities of that star system. It also spawns some interesting ideas that would, until now, have remained purely in the realms of science fiction.
What if an advanced civilization evolved on the outer planet Kepler-62f when the host star was younger (and therefore hotter)? As the K-type dwarf (slightly smaller and dimmer than the sun) cooled, its habitable zone would have shrunk. Although the world technically remains inside the habitable zone today, would the reduction in stellar energy have forced a hypothetical alien race into extinction? Or did it motivate a planetary exodus to the warmer Kepler-62e? Might the inhabitants be gazing at the neighboring world as we are currently looking at Mars for colonization?
Perhaps, as Discovery News’ Ray Villard describes, we live in a compulsive universe. In a compulsive universe, if the ingredients for life are there, anything is possible. Therefore, life may have evolved on both worlds independently! This is purely a sci-fi notion, but it does pose an interesting thought experiment: What if both Kepler-62 alien civilizations have discovered each other and, assuming they come from the “life as we know it” mold, an interplanetary (or is that interexoplanetary?) conflict spawned rapid advances in space technology?
Perhaps, in the spirit of whittling down the best “directed SETI” targets, Kepler-62 may be an even riper target to listen out for extraterrestrial signals — the rapid advancement of technology through conflict could have accelerated space travel and interstellar communications in such star systems.
Alternatively, nothing of the sort happened in Kepler-62 — “e” and “f” could be barren, radiation-soaked worlds. They could bear little resemblance to the rich bounty of organics found in the solar system.
But then again, they might, a fact that will — rightfully — drive SETI studies for many years to come.
Image: Artist’s impression of Kepler-62f. Credit: NASA Ames/JPL-Caltech