Efrain Morales Rivera / Astronomical Society of the Caribbean / PHL / UPR Arecibo
Artistic representation of the potentially habitable Super-Earth Gliese 832c with an actual photo of its parent star, center, taken on June 25, 2014 from Aguadilla, Puerto Rico.
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.
One of the key incentives behind hunting down exoplanets is to find alien worlds with qualities similar to Earth. But in the case of a newly-discovered exoplanet orbiting a star only 16 light-years away, although astronomers may call it ‘habitable’ and a ‘super-Earth,’ it’s likely anything but.
Gliese 832c orbits a red dwarf star and it was discovered by the international Anglo-Australian Planet Search team led by Robert Wittenmyer of the University of New South Wales, Australia. The discovery has been accepted for publication in the Astrophysical Journal.
Red dwarfs are small, dim stars that generate far less energy than our sun. Therefore, for a red dwarf-orbiting planet to maintain water in a liquid state on its surface, it must orbit much closer to the star. In the case of Gliese 832, its ‘habitable zone’ is very compact and Gliese 832c has an orbital period of just under 36 days. The possibly-rocky world, which is around 5 times the mass of Earth, is therefore considered ‘habitable.’ In fact, Gliese 832c is considered to be the third-most habitable world known so far on the Earth Similarity Index (ESI).
But don’t go having dreams of blue skies, opal oceans and lush, alien forests — this world would likely choke any life (well, life as we know it).
“Given the large mass of the planet, it seems likely that it would possess a massive atmosphere, which may well render the planet inhospitable,” said co-investigator Chris Tinney, also of UNSW. “A denser atmosphere would trap heat and could make it more like a super-Venus and too hot for life.”
Like Venus, Gliese 832c is probably enduring intense warming caused by a runaway greenhouse effect. In this case, although the planet’s orbital location should allow liquid water to persist, any water would likely be ripped apart on a molecular level by intense atmospheric heating and ultraviolet light from the star, a process known as dissociation.
Of course, the astronomers have no idea what chemicals are contained within Gliese 832c’s atmosphere. The world was discovered through its gravitational pull on its parent star, so no information about its atmosphere (if it indeed has one) and any water it contains is known. The wobbling effect (which can be detected through precise radial velocity measurements) was detected by combining observations by the Anglo-Australian Telescope (AAT) at Siding Spring Observatory, Australia, the 6.5 meter Magellan Telescope and the European Southern Observatory’s 3.6 meter telescope (both located in Chile).
In 2009, the same team detected another planet around Gliese 832. Thought to be a large gas giant like Jupiter, Gliese 832b has a 9 year orbit around the star. It is for this reason that astronomers believe the system to resemble the multi-planetary structure of our solar system, only more compact. And more planets could be discovered in the future.
“With an outer giant planet and an interior potentially rocky planet, this planetary system can be thought of as a miniature version of our Solar System,” added Tinney.
So beware the headlines that suggest Gliese 832c is ‘Earth-like’ — it is more likely ‘Venus-like’ and very alien to us terrestrial lifeforms.