Fact of the day!
Today’s science fact is about the Texan Horned Lizard.
This lizard deters predators by shooting its own blood into their face. Out of its eyes. Apparently the blood taste......Learn More!
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| This chart shows star temperatures vs. stellar flux showing various zones including Earth. Credit: Chester Harmon |
Scientists should take the conservative approach when searching for habitable zones where life-sustaining planets might exist, according to James Kasting, Evan Pugh Professor of Geosciences at Penn State, including when building Terrestrial Planet Finders. That conservative approach means looking for planets that have liquid water and solid or liquid surfaces, as opposed to gas giants like Jupiter or Saturn. The habitable zone in a solar system is the area where liquid water, and by extension life, could exist. Defining the habitable zone is key to the search for life sustaining planets in part because the idea of a habitable zone is used in designing the space-based telescopes that scientists would use to find planets where metabolism -- and potentially life -- life might exist.
"It's one of the biggest and oldest questions that science has tried to investigate: is there life off the earth?" Kasting said. "NASA is pursuing the search for life elsewhere in the Solar System, but some of us think that looking for life on planets around other stars may actually be the best way to answer this question."
Recent research by Ravi Kopparapu, a post-doctoral researcher working with Kasting, suggests that the frequency of Earth-like planets in the habitable zones of stars known as M-dwarfs is 0.4 to 0.5. To find four potential Earth-like candidates, scientists would need to survey the habitable zones of about 10 cool stars. This data came from NASA's Kepler Space Telescope, which collected information on transiting exoplanets for almost four years before being partially disabled. Previous estimates put this frequency at 0.1, which would have forced scientists using planet finders to survey more stars, searching farther away from our Solar System.
An even more recent estimate of the frequency of Earth-like planets was announced by Eric Petigura and colleagues at the Kepler Science Conference in early November. They calculated the figure at 0.22 around stars more similar to the Sun. But Kopparapu and Kasting think Petigura and colleagues' estimate could be too high by a factor of two because they used an overly optimistic estimate for the width of the habitable zone. If so, then the old value of 0.1 may be closer to the truth.
The ability of a planet to sustain liquid water is traditionally part of the criteria when searching for life-sustaining planets. While some have argued that subsurface water would be enough to sustain life, testing that hypothesis remotely would be virtually impossible, so the focus for astronomers should remain on surface water, Kopparapu and Kasting note in a special issue of the Proceedings of the National Academies of Science.
"All life that we know of is carbon-based and depends on the presence of liquid water during at least part of its life cycle," Kasting notes in the paper. "Hence, if we see a planet that shows evidence for liquid water, we can immediately think about the possible presence of carbon-based life."
While no federal funding to build a Terrestrial Planet Finder is currently in place, the amount of research related to exoplanets is strengthening. A TPF would allow for the detection of gases -- or lack thereof -- in planets' atmospheres. If, for example, no signs of life are found after searching the habitable zones of 30 stars, that could be a reason for pessimism, said Kasting, who is also part of Penn State's Earth and Environmental Systems Institute.
And, while it may be more appealing to know that there is evidence of life on other planets, learning that there is not would have scientific implications.
"Maybe every planet out there that has the right conditions develops life," Kasting said. "We don't really know the answer to that. But, it could be. If you're an optimist, you think it just takes the right conditions. It happened on Earth, why wouldn't it happen somewhere else?"
It is possible that initial observations of Earth-like exoplanets could give an ambiguous answer, Kasting added. For example, oxygen might be found, but not methane. But even that could open the door to further exploration.
While the pursuit of life in the outer reaches of the sky might seem far-fetched at first glance, Kasting noted that astronomers have talked about it as a second Copernican revolution.
"Did it make any difference when we figured out that the Earth was going around the sun rather than vice versa? If you're just a practical-minded person, it made absolutely no difference to your life because life goes on Earth just the way it did," Kasting said.
"But if you expand your mind a little bit, it helped us figure out our place in the universe -- that we're actually on a little planet going around a rather normal star amongst many other stars in the galaxy, and there are many galaxies out there. It's been one of the most profound changes ever in human thought. We think of TPF as the next step in the Copernican revolution, to figure out if there are other Earths out there and if there is life on those planets."
Source: Penn State
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| ESO’s Very Large Telescope (VLT) tested a new laser on Feb. 14, 2013. To the right of the center of the image, just below the Small Magellanic Cloud, streaks comet Lemmon. |
When it comes to detecting laser pulses aliens might shoot at Earth to attract our attention, scientists now find they can detect signals as faint as a single photon of light every few tiny fractions of a second.
Astronomers have gazed at the skies for decades searching for signs of extraterrestrial intelligence. Lasers can in principle help transmit messages over extraordinary distances, but while scientists have monitored a large number of stars looking for alien laser signals — for instance, facilities at Harvard and Princeton scanned more than 10,000 sun-like stars for several years — no evidence for any have been found yet.
Prior attempts to look for extraterrestrial laser signals concentrated on isolated bursts of light, ones so extraordinarily intense they are likely artificial. In contrast, laser scientist Walter Leeb at the Vienna University of Technology in Austria and his colleagues are focusing on repetitive, faint laser signals received over a sufficiently long amount of time.
"We assumed that aliens would use the simplest possible way of attracting our attention, one already implemented in seafaring since ancient times using lighthouses — that is, periodic light pulses," Leeb said.
The kinds of strings of pulses the researchers are looking for, each roughly nanoseconds or billionths of a second long, are not likely found in nature. "Such signals can, however, be generated by lasers," Leeb said, thus hinting at an extraterrestrial intelligent origin.
The advantage of a repetitious signal is that it can readily be distinguished from random noise, even if a large portion of transmitted pulses are lost before they are received. Leeb did note that astronomical objects known as pulsars also give off repetitive light pulses.
"However, their signals have a rather low repetition frequency, and do not consist of nanosecond pulses an extraterrestrial intelligence will possibly use," he said.
Since current technology on Earth is capable of powerful visible and near-infrared lasers, the scientists assumed extraterrestrials might be as well. They reasoned aliens would choose wavelengths that would stand out from the light from their home stars as well.
Although Leeb and his colleagues assumed that aliens would use high-energy lasers to fire these pulses, they also reasoned the vast distances these signals traveled means these pulses might be very faint by the time they reached Earth. As such, their strategy employs devices known as single-photon detectors, which as their name implies can detect signals even as dim as a single photon, the particle making up light.
"We are designing a detection system for an unknown and at the same time most likely very faint optical signal — a formidable task indeed," Leeb said.
In a lab test of their strategy, the researchers simulated an extraterrestrial laser signal with 10,000 near-infrared pulses per second from an exoplanet orbiting a sun-like star 500 light-years away. Assuming a receiver 5.5 feet (1.7 meters) wide and a transmitter about 33 feet (10 m) in diameter with a 42-kilojoule laser — one potentially technically within reach of modern technology — Leeb and his colleagues found their technique was capable of clearly detecting as little as one photon on average per pulse.
Leeb hopes the simplicity of this method will spur other groups to use this method "when searching for signals sent by extraterrestrial intelligence. This will increase the chance to provide evidence that we are not alone."
The researchers have already begun trying out their method on distant stars with the 80-centimeter telescope at the University of Vienna, targeting a number of recently detected alien planets that supposedly lie within the habitable zones of their host stars — the areas where that star's light makes temperatures just right for keeping oceans of liquid water on the surfaces of those planets.
"Examples were Kepler-22b, HD 33564b, HD 87883b and the exoplanets of Gliese 581," Leeb said. They plan a more systematic survey as soon as they have improved the pointing and tracking accuracy of their telescope.
Leeb cautioned, "We have yet not actually received the signals for which we have designed our method."
"They show that current detector technology can detect individual photons and process their arrival times to sense the expected faint illumination from even a distant alien transmitter," said astronomer Geoffrey Marcy at the University of California, Berkeley, who did not take part in this study. "The key now is to implement their SETI technique on a large telescope having a mirror of diameter 1 to 3 meters (3 to 10 feet) to begin searching for periodic laser transmissions."
Space instrumentation engineer Felix Hormuth at the Max Planck Institute for Astronomy in Heidelberg, Germany, who did not participate in this study, said this latest work showed "a very clear, sound, and technically proven way" to help discover alien signals, and was suitable "for deployment at many of the small- to mid-sized telescopes becoming less and less used in the times of extremely large telescopes.
“It is not science fiction, but a reasonable approach with the right amount of boldness to be both fascinating and realistic," Hormuth said.
The scientists detailed their findings in the June issue of the journal Astrobiology.
This story was provided by Astrobiology Magazine, a web-based publication sponsored by the NASA astrobiology program.