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.
Recent estimates are that 6 percent of red dwarf stars in our galaxy should have Earth-sized worlds. Six percent! That means that our galaxy could potentially be overflowing with terrestrial planets.
Six percent may not sound like a lot, but red dwarfs are astonishingly populous. Approximately 75 percent of all stars are red dwarfs. Assuming that there are around 300 billion stars in the Milky Way, that means there should be over 13.5 billion exo-Earths orbiting the tiny red suns strewn across our galaxy.
Red dwarfs have always been a source of controversy for planet hunters and astrobiologists. They’ve been frequently ignored in exoplanet searches, largely because they’re noisy little beasts that makes finding anything out about them rather taxing. That is, until Courtney Dressing and her colleagues looked at data from Kepler in an attempt to settle the arguments. As it happens, their study found that around 60 percent of red dwarfs should have planets smaller than Neptune, which means — as some astronomers have long suspected — these tiny stars are likely to be good hunting grounds for exoplanets.
So what exactly is the deal with red dwarfs? Are they good homes for life or not?
One Angry Dwarf
Well, a big problem with red dwarfs is that many of them are ill-tempered little things; prone to violent and unpredictable outbursts.
These little stars are fully convective, meaning that material circulates all the way from the core up to their surfaces, unlike the sun that only has convection currents in its exterior layers. This gives rise to immense magnetic fields, which are responsible for Goliath temper tantrums.
Many red dwarfs are flare stars, occasionally increasing significantly in brightness. Flares seen on these little red stars can put the solar flares we see on the sun to shame. They’re also correspondingly more lethal, brightening the star’s electromagnetic output from radio waves all the way up to x rays!
Paradoxically, the same magnetic activity powering these flares also causes dramatic starspot activity — just like sunspots, only much much more so.
Because a red dwarf is so much smaller than the sun, and its magnetic fields are so much stronger, starspots can cause the brightness of these grouchy little stars to drop by up to 40 percent. I hasten to add that not all red dwarfs are quite so volatile. But many of them are.
The other problem with life under a red sun is that red dwarf stars are cool. And I don’t mean cool like Neil deGrasse Tyson, I mean literally cool. The surface temperature of an average red dwarf is around 2000-4000 Kelvin. Compared with the sun’s surface temperature of nearly 6000 Kelvin, that’s only lukewarm. At this lower temperature, most red dwarfs struggle to put out even 1 percent the luminosity of the sun.
This means that habitable planets around red dwarfs have to lie in very tight orbits to stay warm. Practically hugging their tiny red suns for warmth, they’re very much in danger from the titanic flares that these stars can belch out.
Sunny Side Up
This isn’t to say, however, that life isn’t possible under a red sun — but it would have to be very different to what we’re familiar with.
For one thing, the habitable zone around a red dwarf is so close that any of these watery Earth-like worlds would surely be tidally locked. One side would constantly be facing the warmth of their parent star, while the other side would be freezing cold and in constant night.
This may not be as bad as it sounds. Some have hypothesized that the star-facing side of such planets may have a perpetual storm, and the night side would likely be frozen solid. In between the extremes though, such planets may have a belt of warm and potentially life sustaining surface.
Geothermal energy locked in these planets could help to stabilize their temperatures, oceans would certainly help to transport warmth around a planet. A dense Earth-like atmosphere would help too, and even more of that precious red sunlight could be trapped by greenhouse gasses which might accumulate in that atmosphere. Red dwarf stars don’t emit a lot of ultraviolet light.
With less ultraviolet to break molecules apart, potent greenhouse gasses like methane could accumulate and act like planetary blankets. The chemistry of planets around red dwarfs stars is likely to be very different to anything we see in our own solar system.
A Leisurely Life
Red dwarfs have one more trick up their sleeves. They’re very very long lived. So long lived, in fact, that no red dwarf has ever died in our universe, because the universe isn’t old enough yet!
The sun formed roughly 4.6 billion years, and will keep on burning just as it is today for another 5.4 billion years. By contrast, a red dwarf born at the same time would barely be a teenager. For example, Barnard’s star — a friendly neighborhood red dwarf, just under 6 light years away — has a life expectancy of 2.5 trillion years. Over 200 times that of the sun. No one even knows if a planet can support life for that long!
While we know that life evolved here on Earth quite soon after our planet formed, we have no idea how long it would normally take for this to happen. If it’s even remotely possible for the same thing to happen on a red dwarf planet, it would have literally all the time in the universe in which to do so!
Whether or not life exists is still an unanswered question, and one that we won’t be able to answer until our telescopes are powerful enough to actually take a detailed look at planets around red dwarf stars. That said, with 13.5 billion planets to choose from, probability may well be on the side of the astrobiologists!
This is a huge subject, and I’ve barely been able to scratch the surface in this article. If you’d like some further reading, including some more technical details, information for this article was compiled from Tarter et al (2007), Pascucci et al (2009), and the TV series Alien Worlds.
Image: An image manipulation created from public domain NASA images, showing a size comparison of (from left to right) Jupiter, Proxima Centauri, Gliese 581, and the sun.