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.
NASA’s prolific Kepler space observatory never ceases to amaze planet hunters. Last week’s announcement of two “super-Earth” type planets sharing the habitable zone around the red dwarf star Kepler-62 further ratcheted up our optimism that life-bearing planets are all over the galaxy.
Kepler scientists explained that the only data the stellar transits (when the planets pass in front of their stars) have to tell us is the planets’ orbits and diameters. Still it’s hard for experts to resist speculating that these planets “may have polar caps,” or may be “water worlds,” or, “we may have found the first ocean planet,” The scientists finally did acknowledge in the April 18th NASA press conference that they were doing “a lot of arm waving.”
The same would be true for my imaginary alien astronomer Zork, on a planet 1,000 light years away. Zork reports to colleagues that photometric observations of the yellow dwarf star Sol (a.k.a. our sun) reveals three planets inside or on the edge of Sol’s habitable zone: Venus, Earth and Mars. He considers this a great SETI target because you get three inhabitable worlds for the price of one observation!
Because we live here we know the reality is that only Earth can support advanced life. Venus and Mars could have started out habitable, with oceans on their surfaces, but both took sharp turns in planetary evolution. Venus’ oceans evaporated away and Mars’ ocean froze over.
When pondering the inhabitability of planets for advanced life, we need to consider planetary evolution and how it changes the weather and surface environments of the planets. The first question is: how much water was available to these planets when they formed? Our terrestrial planets may have been irrigated by asteroids slingshot toward the inner solar system by Jupiter’s gravity.
But what happens in systems without Jupiters? A 2010 study by Rebecca Martin of the University of Colorado reported that only a tiny fraction of planetary systems observed to date seem to have giant planets in the right location to produce an asteroid belt of the appropriate size to influence inner planets. If it’s more massive the planets are rototilled by frequent asteroid hits — and this might not be good for biological evolution.
Another study by Martin found that our terrestrial planets formed from rocky debris in a dry, hotter region, far inside of the solar system’s so-called “snow line.” The snow line currently lies in the middle of the asteroid belt. Beyond this point the sun’s light is too weak to melt the icy debris left over from the protoplanetary disk. Conditions within disks will vary from star to star and so the solar system’s comparative dryness cannot give us insights into Kepler-62.
Compounding the astrobiology question is the fact that Earth’s own evolution shows how a planet even in the habitable zone can dramatically change over time. Four billion years ago Earth had a thick carbon dioxide atmosphere and green oceans full of iron. At 700 million years ago Earth was entombed in ice because of runaway glaciation triggered by changes in ocean currents caused by the appearance of a super-continent. Earth has only had a rich surface biosphere for the past 500 million years.
Therefore, planetary evolution keeps shuffling the playing deck. The inner planet, Kepler-62e, may really be a super-Venus rather than super-Earth. It would be a hellish place with a chocking carbon dioxide atmosphere and continuous volcanism. One the other hand could “Venus be just a fluke?” speculated one Kepler scientist.
Kepler-62 is a 7 billion year-old system, it’s got the jump on us by 2.5 billion years. So any Earth-like planet may have evolved into unknown territory for astrogeologists.
What’s tantalizing is that anyone living in a 7 billion-year-old system should be a lot smarter than us! If Kepler-62f and 62e were inhabited, space-faring civilizations might visit each other. My guess is that the atmospheres could be so different that visiting species might have a hard time without wearing spacesuits, and this would preclude any attempt at a preemptive interplanetary invasion. Inhabitants of either planet would need awfully powerful rockets to escape stronger gravitational fields. They might be restricted to twittering through interstellar radio transmissions, or exchanging small payloads.
Likewise, the only way to determine habitability would be through a radio or laser beacon sent earthward by Kepler 62’s inhabitants, as Discovery News’ Ian O’Neill recently wrote. Kepler-62 is so far away there are no telescopes envisioned — or affordable — that would be capable of observations that could be done to assess the planets’ physical characteristics.
Our best bet is NASA’s recently announced Transiting Exoplanet Survey Satellite (TESS). Call it Kepler-2. The TESS is expected to identify approximately 1,000 exoplanets around nearby stars.
Planned for a 2017 launch, TESS will find targets for NASA’s James Webb Space Telescope (JWST) that will study planetary atmospheres in infrared light. Webb has the capability of offering the first observational evidence for a true extraterrestrial ocean.
The JWST will launch only a year after TESS, and so the two observatories will work together in seeking out Earth-clones where habitability can at last be assessed. But for now, it’s all just educated “arm-waving.”
Image credit: NASA