Finding ET May Be Harder Than Thought
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.
Hopes of finding evidence of life on far off alien worlds by studying their atmospheric chemistry have been dashed by a new study that concludes it's almost impossible.
The research, reported in the Proceedings of the National Academy of Sciences, found atmospheric spectral readings from distant exoplanets will never be good enough to be useful in the search for life.
The findings support an underlying view among astronomers that it was always going to be difficult to take the spectrum of an Earth like exoplanet, according to the study's lead author Hanno Rein of the University of Toronto.
"I was a bit pessimistic when I calculated these numbers for the first time, they were not what I was expecting," said Rein. "We're not going to get any useful spectra at all."
Astronomers determine the chemical composition of a gas, by looking for specific signatures in light from a star shining through the atmosphere of a planet passing in front of it. The strongest indicators of life on another planet would be chemical signatures for molecules of methane and oxygen in that planet's atmosphere.
"We think these two molecules are likely to be produced by life on Earth and also on other planets," said Rein. "There are few geological mechanisms which produce molecules of methane and oxygen in large quantities. To be really sure, we want both molecules together at the same time, then we can be more certain that there's life on that planet."
Too far away
However, most of the exoplanets being found by astronomers are too far away to provide useful spectra.
"The spectral resolution is so low, that we can hardly say anything about them," said Rein. "It's a fundamental physical limit, even if we had perfect technology that measures every single photon we receive from an exoplanet, there's just not enough photons reaching us to say whether there's life on the planet or not."
The problem would be even worse if the exoplanet was orbited by a moon.
"From a long way away an exoplanet and moon system would appear so close together that they couldn't be separately resolved, and would look like a single atmosphere to us," said Rein. "If one had oxygen and the other had methane, it would suggest an oxygen-methane atmosphere, providing a false positive for signs of life when none exists."
Astronomers have successfully collected spectra from distant exoplanets, but these were hot Jupiters, gas giants very close to their host stars, nothing that looks similar to the Earth.
"Earth is much smaller than a gas giant, so the combination of finding an Earth-sized planet, in the habitable zone, and then taking its spectra, is just very hard," said Rein.
On the positive side, Rein believes looking for the right objects would allow good atmospheric spectra to be taken.
"Planets that NASA's Kepler spacecraft detects are hundreds and thousands of light years away, but if we search for planets closer to our solar system, we might be able to take spectra with a good enough resolution," said Rein.
Another option involves looking for planets around small red dwarf stars.
"These stars would be fainter and easier to take a spectrum because the light from such a star is not as bright," said Rein.
This article originally appeared on ABC Science Online.