Twenty different amino acids go into making up the vast variety of proteins so essential to life. But why does life on Earth use only left-handed versions of amino acids to build them? After all, amino acids can be made in mirror images of each other. Jason Dworkin (NASA GSFC) notes the key issue. Mix left- and right-handed amino acids and "...life turns to something resembling scrambled eggs -- it's a mess. Since life doesn't work with a mixture of left-handed and right-handed amino acids, the mystery is: how did life decide -- what made life choose left-handed amino acids over right-handed ones?" Image: This artist's concept uses hands to illustrate the left and right-handed versions of the amino acid isovaline. Credit: NASA/Mary Pat Hrybyk-Keith. It's a question with ramifications for life elsewhere in the universe. Suppose the day comes when we finally get a robotic lander to Enceladus. The news flashes around the world: Life discovered on one of Saturn's moons! But is it truly...

The planets in our Solar System rotate around the Sun more or less in a plane (the ecliptic) that is tilted some sixty degrees with relation to the galactic disk. It's interesting to speculate that this could have ramifications in terms of the SETI hunt. Shmuel Nussinov (Tel Aviv University) considers the possibility that any extraterrestrial civilizations might try to contact us only after they had a fair idea we were here. And just as we are now trying, via Kepler and CoRoT, to track down small planets using the transit method, so too might extraterrestrials try to observe our transits, and having done so, to transmit a message. Targeting habitable planets should optimize chances for a successful reception. From our end, a prudent SETI strategy might then be to home in on the 'stripes' of the sky within which our system's planetary transits are detectable from other solar systems. As Nussinov writes: The thickness of the galactic disc in our neighborhood is ? 150 parsecs. With the...

George Dvorsky takes on the 'rare earth' hypothesis in his Sentient Developments blog, calling it a 'delusion' and noting all the reasons why life in the galaxy is unlikely to be unusual. The post reminds me why the book that spawned all this was so significant. Rare Earth: Why Complex Life is Uncommon in the Universe (Copernicus, 2000) is Peter Ward and Donald Brownlee's take on our place in the cosmos, concluding that complex life is rare because an incredibly fortuitous chain of circumstances must arise for it to occur. Indeed, the authors argue that large parts of our galaxy are composed of what they call 'dead zones.' The argument is complex and looks at factors ranging from a planet's place in the galactic habitable zone (itself a controversial subject), its orbit around its own star, its size, its satellites, its magnetosphere, its plate tectonics, and more. I'm surprised to realize, looking through our archives here, that I haven't managed to do a complete review of Rare...

We've already speculated here that if the Kepler mission finds few Earth-like planets in the course of its investigations, the belief that life is rare will grow. But let's be optimists and speculate on the reverse: What if Kepler pulls in dozens, even hundreds, of Earth-sized planets in the habitable zones of their respective stars? In that case, the effort to push on to study the atmospheres of such planets would receive a major boost, aiding the drive to launch a terrestrial planet hunter with serious spectroscopic capabilities some time in the next decade. Budget problems? Let's fold Darwin and whatever Terrestrial Planet Finder design wins approval into the same package, and make this a joint NASA/ESA mission. Finding numerous Earth-like planets will be a driver, as will gradual economic recovery. Finding Many Earths The discovery of numerous 'Earths' would also galvanize public interest in interstellar flight, which offers a useful educational opportunity. Even the short-lived...

If we're looking for pristine environments for life, Antarctica offers much. More than 150 subglacial lakes have been discovered beneath the ice sheet, isolated from the surface for long periods and possibly home to species that have never before been observed. From November 2007 to February 2008, a subglacial lake named Lake Ellsworth was studied by a four-person team that used seismic and radar surveys to map the lake's depth and take other measurements that made clear its potential for exploration. Their blog is archived here, and it makes for good reading. Image: A DeHavilland Dash-7 flying near the British Antarctic Survey research station at Rothera. The station is 1630 kilometers southeast of Punta Arenas, Chile, and served as a staging area for the Lake Ellsworth studies of 2007-2008. Credit: Natural Environment Research Council. Europa, anyone? Well, there are certain resemblances. If the thickness of the ice on Europa is still controversial, we know for a fact that Lake...

A Europan Scenario Between living dirigibles on gas giants and potential organisms under the ice, we've had quite a week in terms of exotic life-forms. I didn't have space in yesterday's review of Unmasking Europa to talk about the book's chapter on biology, but here's an interesting glimpse of a not implausible biosphere on that moon, as presented by physicist Richard Greenberg: Brisk tidal water sweeps over creatures clawed into the ice, bearing a fleet of jellyfish and other floaters to the source of their nourishment. As the water reaches the limits of its flow, it picks oxygen up from the pores of the ice, oxygen formed by the breakdown of frozen H2O and by tiny plants that breath it out as they extract energy from the sun. The floating creatures absorb the ocygen and graze on the plants for a few hours. The water cools quickly, but before more than a thin layer can freeze, the ebbing tide drags the animals deep down through cracks in the ice to the warmer ocean below. Most of...

By Larry Klaes 'The Gasbags of Jupiter' sounds for all the world like the title of an early 1930s novel that would have run in a venue like Science Wonder Stories. In fact, as Larry Klaes tells us below, the idea grew out of Carl Sagan's speculations about free-floating life-forms that might populate the atmospheres of gas giant planets like Jupiter. Cornell physicist Edwin Salpeter had much to do with the evolution of that concept, helping Sagan produce a paper that was a classic of informed imagination (and one that led to numerous science fiction treatments as the idea gained currency). Larry's celebration of Salpeter's life gives a nod to that paper but also notes his deep involvement in the study of the most distant celestial objects. On March 14, the Department of Astronomy at Cornell University will commemorate the life of one of their most prestigious faculty members: Edwin E. Salpeter, the James Gilbert White Distinguished Professor of Physical Sciences Emeritus. Salpeter...

Just how many forms of life are there? We often speculate here about life on other worlds, but Paul Davies (Arizona State) is currently exploring the question from a different perspective entirely. Davies would like to know whether a 'second genesis' might have occurred, producing a fundamentally different form of life that would have evolved right here on Earth and might still occupy our planet. Life may, in other words, have started many times, perhaps with significantly different results we just haven't uncovered yet. Call it a 'shadow biosphere,' a concept the physicist calls for exploring: "...[It] is still just a theory. If someone discovers shadow life or weird life it will be the biggest sensation in biology since Darwin. We are simply saying, 'Why not let's take a look for it?' It doesn't cost much (compared to looking for weird life on Mars, say), and, it might be right under our noses." Finding these alternate life forms, if they exist, may be tricky, as they could be...

I promised a quick return to recent work on the Drake Equation, which helps us estimate the number of communicating civilizations in the galaxy, but a BBC story on Duncan Forgan has me back at it even sooner than I had intended. It's no surprise that the matters encapsulated in Drake's thinking should be in the news. After all, the era of Fermi and Drake was without firm knowledge of extrasolar worlds, of which we now know over three hundred. For that matter, the concepts of habitable zones around both stars and the galaxy itself had not come to fruition, nor had anyone ever heard of the 'rare Earth' hypothesis. We also work today with knowledge of Charles Lineweaver's studies of the median age of terrestrial planets in the Milky Way, which point to civilizations around other stars having had as much as two billion years-plus to emerge before our own Earth had even coalesced. Until we know more, I suspect we'll be adjusting Drake parameters for some time, as Duncan Forgan (University...

Centauri Dreams' recent post on the Drake Equation triggered a broad range of response, both in comments and back-channel e-mails, the latter of which produced a note from Kelvin Long quoting a rather controversial position on Drake by one leading scientist. Here it is. See if it raises your hackles: "I reject as worthless all attempts to calculate from theoretical principles the frequency of occurrence of intelligent life forms in the universe. Our ignorance of the chemical processes by which life arose on earth makes such calculations meaningless." The words are Freeman Dyson's, from his essay "Extraterrestrials" in Disturbing the Universe (Harper & Row, 1979), a book I re-read every few years as much to admire the author's rhetorical skills as to draw again on his insights. Kelvin has differing views on Drake and so do I, but I'm going to quote Marc Millis' reaction to the Dyson statement, reflecting as it does an approach toward scientific method that I share. Marc writes: "On...

The Drake Equation in its various forms has been tormenting us for decades, raising the question of how to adjust variables that range from astronomical (the abundance of terrestrial planets) to biological (the probability of life's emergence) and even sociological (the average lifetime of a technological civilization). Wildly optimistic estimates of the number of technological civilizations in our galaxy are now giving way to more sober reflection. Now Reginald Smith (Bouchet-Franklin Institute, Rochester NY) offers up a new analysis looking at how likely radio contact is given a civilization's lifetime, and how widely that civilization's signals can be clearly received. The key question: What if there is a reasonable horizon for the detection of a signal from an extraterrestrial sender? Signals and Their Lifetime This is useful stuff, because contact depends not just upon the density of communicating civilizations (CC) but their average lifetime and the maximum detectable distance...

If you're not a member of the Science Fiction and Fantasy Writers of America (still commonly known as the SFWA from the days before the 'fantasy' bit was added), you may not see the group's regular bulletin. That would be understandable, given that although it can be found on newsstands, the SFWA Bulletin now costs a solid $6.95 per copy. Nonetheless, keeping up with Robert Metzger's 'State of the Art' science column would keep me buying this journal even if it didn't come as part of my membership. Metzger, the author of the 2002 novel Picoverse and 2005's CUSP as well as a variety of short fiction in addition to his science writing (some of which is available online), speculates in his most recent column on a subject we've recently treated here. Would a species capable of star travel actually need to make the journey, given the advances in technology that would surely make it possible to learn more and more about exoplanets from its own star system? Metzger reviews current exoplanet...

By Larry Klaes Tau Zero journalist Larry Klaes gives us a look at the immense contribution of physicist Giuseppe Cocconi to SETI. It's sobering to realize how new a study SETI really is. Frank Drake's Project Ozma began less than fifty years ago, while estimates of the number of extraterrestrial civilizations are just now scaling back dramatically from the numbers Drake himself and Carl Sagan once used (Claudio Maccone's recent work on the Drake Equation arrives at an estimate of 250 such civilizations in the Milky Way -- more on this soon). If it weren't for the efforts of Cocconi and Philip Morrison, the theorizing behind the Drake Equation and the development of SETI itself might have been slowed for years, as Larry points out so ably below. On November 9, the world said farewell to physicist Giuseppe Cocconi, who passed away at the age of 94. Although his life's work was in particle physics and cosmic ray science, Cocconi will always be best known for co-authoring the paper with...

Here I was all set to write about the discovery of carbon dioxide on HD 189733b when Alpha Centauri made its way back into the news. Twentieth Century Fox will be transmitting the re-make of the science fiction classic The Day The Earth Stood Still to Alpha Centauri on Friday the 12th, timing the event to coincide with the film's opening here on Earth. The transmission is being handled by Florida-based Deep Space Communications Network, a private organization that offers transmission services to the public (not to be confused with the Deep Space Network that manages communications with our planetary probes). Why does Deep Space Communications Network offer transmission services to the stars? From its FAQ: For a number of reasons, one is because we have the equipment, and the know how so we can, and also because we thought it would be an interesting public service that is not currently available. We're doing it because we can... This dubious news comes on the heels of the in many ways...

A Dyson Sphere makes an extraordinary setting for science fiction. In fact, my first knowledge of the concept came from reading Larry Niven's 1970 novel Ringworld, a book that left such an impression that I still recall reading half of it at a sitting in the drafty little parlor of a house I was renting in Grinnell, Iowa. Ringworld had just come out as a Ballantine paperback with the lovely cover you see below. I was hooked after about three pages and read deep into a night filled with wind and snow. It could be argued, of course, that a ring made out of planetary material, a habitat so vast that it completely encircles its star, is actually one of the smaller Dyson concepts. It was in 1960 that Freeman Dyson suggested how a civilization advanced to the point of such astro-engineering might use everything it found in its solar system to create a cloud of objects, a swarm that would make the most efficient use of its primary's light. And as you keep adding objects, you point to the...

Now here's a comprehensive task for you. Take about a dozen primordial minerals found in interstellar dust grains and figure out what processes -- physical, chemical, biological -- led to the appearance of the thousands of minerals we find on our planet today. The job was undertaken by Robert Hazen and Dominic Papineau (Carnegie Institution Geophysical Laboratory) and colleagues, and it produced startling results: Of the roughly 4300 known types of minerals on Earth (fifty new types identified each year), up to two-thirds can be linked to biological activity. Mineral evolution? In a sense, although Hazen is quick to qualify the statement: "It's a different way of looking at minerals from more traditional approaches. Mineral evolution is obviously different from Darwinian evolution — minerals don't mutate, reproduce or compete like living organisms. But we found both the variety and relative abundances of minerals have changed dramatically over more than 4.5 billion years of...

Using eleven of the Allen Telescope Array's 6.1-meter dishes, the SETI Institute and the Radio Astronomy Laboratory at the University of California (Berkeley) have detected the New Horizons spacecraft on its way to Pluto/Charon. New Horizons transmits an 8.4 GHz carrier signal that showed up readily on the SETI Prelude detection system. What I hadn't realized was that snagging distant spacecraft transmitters is a standard part of SETI operations, as Jill Tarter notes in this brief article on the event posted at the New Horizons site: "We look forward to checking in with New Horizons as a routine, end-to-end test of our system health. As this spacecraft travels farther, and its signals grow weaker, we will be building out the Allen Telescope Array from 42 to 350 antennas, and thus can look forward to a long-term relationship." Image: New Horizons as tracked by the Allen Telescope Array. This plot shows 678 hertz (Hz) of spectrum collected over 98 seconds. The New Horizons signal can...