Space Invaders Unlikely -- For Now
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.
Ever since the rapid rise of digital special effects in the 1980s there have been a number of sci-fi Hollywood films presenting elaborately illustrated farfetched alien invasions of Earth with city-sized motherships (Independence Day, 1996), beachfront storming aliens (Battle Los Angeles, 2011) and sea attacking aliens (Balttleship, 2012).
Even esteemed astrophysicist Stephen Hawking got a lot of press attention when he warned that we should not attempt to contact alien civilizations because it may lead to an invasion of Earth.
However, in a recent paper, Janne Korhonena of the Aalto University, Espoo, Finland argues that it is unlikely that humanity would be attacked by renegade aliens. We have no interstellar “weapon of mass destruction,” that might present a future threat as possibly perceived by some extraterrestrials. Smart aliens would likely avoid unprovoked, preemptive invasions until then.
“Destroying a species that cannot harm the invader would not improve the invader’s security at all, and the gain of a single planet would seem to be a trivial advantage to a civilization that already has the capability to live in space,” writes Korhonena.
He cautions that aliens may become hostile if we reached the ability to launch Star Trek-style space battle cruisers. “Any spacecraft capable of interstellar voyages in reasonable time is by itself a weapon of mass destruction,” he writes. Even worse, a starship would show up without warning. The infrared signature from an alien vehicle heading towards the Earth at ten percent the speed of light might be detectable no farther than 2 billion miles from Earth. We’d only have 12 to 24 hours of warning before arrival (or impact!).
There is a long list of arguments for aggressive societies: eagerness to consume our resources, a belligerent ideology, a desire to be the sole galactic power, indifference to our existence, or a combination of the above. What’s more, some alien societies might just be pathologically xenophobic.
My best guess is that they could have purely unfathomable motives behind our elimination.
Korhonena says that a scenario not widely talked about is an alien first strike aimed at eliminating competitors. This buries deep in the Darwinian psyche. A lesson learned from the Cold War is that two fundamentally antagonistic civilizations with the capability of destroying each other had a rationale for launching a preemptive first strike.
In 1954, a Joint Chiefs of Staff advance study group briefed President Eisenhower on a plan to initiate a war with the USSR before the Soviets could achieve a large enough thermonuclear capability to be a real menace to the continental U.S. In the end fear of retaliation, nuclear winter, and mass deaths far exceeding those of WWII, made the plan seem suicidal. It was nicknamed MAD for Mutually Assured Destruction.
But in interstellar wars, the home base biospheres of the enemy planets are separated by light-years. Even if the defending planet had the technological prowess to strike back, it would take decades or centuries — not minutes — for a counterblow. The enemy would have plenty of time to prepare for whatever counter invasion was coming!
In practicality the attacker would be so overwhelmingly superior the defender could not retaliate in any meaningful way.
But hold on. The limitations imposed by the speed of light make attacking a very dicey gamble for a bellicose civilization. By the time the alien warships arrive at a target planet they may find themselves outclassed by decades or even centuries of accelerated technological development by the formerly “backwards” victim civilization. Imagine their surprise to be immediately decimated by some super weapon they had not anticipated. It would be like a Spanish Galleon fleet taking so long to get to Hawaii that they wind up sailing into a fleet of battleships at Pearl Harbor.
The most likely reality is that galaxy is so vast that interstellar civilizations have ample space and do not have to risk antagonizing or crushing emerging species. That is, unless they had a purely fiendish desire to do so. And, given the age of the galaxy, any Darth Vader and Imperial Battlestar would have come by this way already.
However, such civilizations might seed the galaxy with robotic sentinels that send an alert home if an emerging society crosses a significant technological threshold. This idea is rampant in UFO mythology that blames the flying saucer scares of the 1950s on the first detonation of the Trinity atomic bomb in 1945. UFO believers overlook the fact that the electromagnetic alarm signal from that blast would have only reach a range of a few light-years by the 1950s.
Even with these exceptions, Korhonena concludes that interstellar conflicts between alien civilizations are rare, but not impossible. He writes, “if interstellar travel proves to be feasible, there is always the non-zero probability of humanity harming ETIs.” But I’d say more likely vice versa!
We could never eavesdrop on star wars taking place elsewhere in the galaxy. The universe is so naturally destructive that any oddball high-energy radiation from mega-weapons would be interpreted as simply more natural chaos in the cosmos.
Publication: “MAD with Aliens? Interstellar deterrence and its implications,” arXiv:1302.0606 [physics.pop-ph]
Image credit: Discovery Channel