Super-Earths with shallower bodies of water could be more habitable for life than Earth.
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.
Earth may be a nice and cozy place for life as we know it to evolve, but is it really the best place for life to thrive? Probably not, say two researchers. In fact, Earth may be one of the more extreme examples of a “habitable” world where life was lucky to survive.
The search for extraterrestrial life is fraught with uncertainly. Faced with a seemingly infinite Universe and an assumption that life beyond our planet is an inevitability, we focus on nooks and crannies that have similar habitable environments to Earth and exoplanets that resemble our world orbiting stars that resemble our sun. But finding these special places feels like we’re looking for a very specific needle in a very big haystack. What if our definition of “habitable” is just not all that, well, habitable?
“The Earth just scrapes the inner edge of the solar system’s habitable zone — the area in which temperatures allow Earth-like planets to have liquid surface water,” said René Heller of McMaster University, Ontario, Canada. “So from this perspective, Earth is only marginally habitable. That led us to ask: could there be more hospitable environments for life on terrestrial planets?”
Looking at the history of our planet, it may seem amazing that life was ever able to gain a foothold. Between the asteroid and comet bombardments, intense volcanic activity, frigid ice ages and often poisonous atmospheres, how were the conditions ever ripe for single celled lifeforms to form? Perhaps life was just really lucky to have found its way in such an inhospitable place.
Heller and colleague John Armstrong of Weber State University think that we may be looking for life in all the wrong places and suggest that we should be looking not for “second-Earths” but a class of planet that is superhabitable.
In an article published in Astrobiology in January, the researchers describe some of the characteristics a superhabitable planet may have. Some of the features — such as the necessity for a global magnetic field to protect life from ionizing solar wind particles — sound very familiar. But Heller and Armstrong highlight the need for a more efficient global “thermostat” that would avoid damaging ice ages. Also, a more massive planet with shallower oceans may be a more habitable solution.
In their research, they highlight the nearby star Alpha Centauri B as an ideal candidate that could support a superhabitable world. Slightly smaller than our sun, Alpha Centauri B would be able to incubate hypothetical lifeforms on a superhabitable world for much longer owing to its longer lifespan. "You want to have a host star that can keep a planet in the habitable zone for 7 to 10 billion years," which would allow enough time for life to evolve and ecosystems to flourish, Heller told New Scientist.
“We propose a shift in focus,” said Heller. “We want to prioritize future searches for inhabited planets. We’re saying ‘Don’t just focus on the most Earth-like planets if you really want to find life.’”
The researchers believe our hunt for extraterrestrial life is too set in its ways and blinkered toward worlds that we consider to be habitable because they resemble Earth. It’s a grand observational bias that may ultimately mean that while looking for second-Earths, we overlook far more habitable planets.
While discussions like this are valuable, I think they can be counterproductive.
Although it is very important to have “out of the box” thinking when it comes to the search for extraterrestrial life (whether that life be single-celled organisms hidden deep inside Mars’ crust or highly intelligent alien civilizations), using life as we know it as a template is no bad thing. After all, we are the only lifeforms we know of in the entire Universe, why would we ever consider Earth “barely habitable”?
How life is sparked remains one of the biggest questions hanging over modern science and the search for extraterrestrial life is an offshoot from that. Perhaps Earth, in its barely habitable state, is actually the perfect environment for life to gain a foothold?
Lacking evidence to the contrary, seeking out second-Earths probably isn’t such a bad idea.