Habitable Planets: Working the Odds

by Administrator on March 10, 2010

Want to play around with some numbers? The process is irresistible, and we do it all the time when plugging values into the Drake equation, trying to find ways to estimate how many other civilizations might be out there. But a question that is a bit less complicated is how many terrestrial planets exist in the habitable zones of their stars? It’s a question recently addressed by Jianpo Guo (National Astronomical Observatories, Kunming, China) and colleagues via simulations. By ‘terrestrial’ world, the researchers refer to planets between one and ten Earth masses, although they note that some scientists would take this figure lower, to perhaps 0.3 Earth masses, which may be enough to retain an atmosphere over long geological timescales and to sustain tectonic activity.

Guo’s team is interested in the distribution of terrestrial planets in our galaxy, and the simulations that grew out of this study create a probability distribution of such planets in habitable zones. The paper is laced with the specifics, but let’s cut to the chase. Guo’s figures show 45.5 billion terrestrial planets in the habitable zones of host stars in our galaxy. The team also worked out the probability for planets in the habitable zones of different types of stars, concluding that M-class dwarfs host 11.5 billion such terrestrial worlds, while K-class stars are the most fecund, with 12.9 billion. G-class stars like our Sun weigh in with 7.6 billion, while F-class stars show 5.5 billion.

Image: M81. Our estimates of the habitable worlds in galaxies like these are widely variable, but they all imply countless chances for life to get its start. Credit: Jonathan Irwin, DSS2.

It’s interesting to weigh these numbers against the year-old estimates of exomoon hunter David Kipping (University College, London). Kipping starts with the galactic distribution of stellar types. He’s assuming about 300 billion stars in the Milky Way (increasingly cited as the best estimate) and noting that 90 percent of these are main sequence and thus stable for long periods of time. He goes on to whittle the number down, eliminating M-dwarfs because of tidal lock and also cutting out short-lived stars higher than F-class. 22.7 percent of main sequence stars in classes F, G and K thus remain.

Citing Michel Mayor’s Geneva team, which found that roughly 30 percent (give or take 10%) of F, G and K-class stars have super-Earth or Neptune-mass planets, Kipping narrows the field yet again:

Using 30% as a fixed value and assuming that very roughly half of this sample correspond to rocky planets and half to Neptune-like gas giants then we may write down that 15% of all F, G and K-type stars have rocky planets around them. It should be noted that this value is very likely an underestimate due to fact planets of Earth mass are currently below the detection threshold.

But how many of these planets would exist in the habitable zone? Kipping was working with 330 exoplanets then discovered, with about thirty in the habitable zone of their host star, and so he suggested a fraction of 10 percent would be a safe estimate based on current knowledge. He then factors in a galactic habitable zone, assuming that one may exist and that any value he obtains will therefore be an underestimate if it does not. This takes the number of stars with habitable planets down to 5 percent, but still leaves him with 50 million habitable-zone exoplanets in the Milky Way. We can contrast that with Alan Boss’s prediction of ten billion habitable exoplanets in our galaxy and, of course, with Guo’s team, whose whopping 45.5 billion is the largest estimate I’ve ever seen.

The weird thing, as Kipping confirmed this morning, is that his 50 million estimate was actually rounded up from 45.5 million, a figure exactly 1000 times less than Guo and team’s number. Our numbers, then, seem to be all over the map, and Kipping also notes Bond and Martin’s 1978 estimate of 10 million habitable exoplanets. But Kipping is the only one I know who takes a shot at the intriguing question of habitable moons. He is, after all, a specialist in detection methods for moons around exoplanets, studying methods that may help us detect large satellites during exoplanet transits. Noting that a large moon could be found around anything from an Earth-class planet to a gas giant, he boosts Mayor’s 30 percent figure to 50 percent, for any kind of planet. And this is interesting:

Let us also assume that the habitable zone for exomoons is extended by around 50% due to the possibility of tidal heating maintaining temperate conditions in traditionally cold-zones. This means that 15% of all planets can host a habitable exomoon.

How many planets have large moons? Kipping notes how little information we have, but using the Solar System as an example, he finds two planets out of eight where a moon has been formed through a capture/impact process, which he believes to be a requirement for a large moon. Assume, then, that 10 percent of planets host a large moon and you wind up with a figure of 25 million habitable exomoons in the Milky Way. But we have to keep these figures in context. Kipping again:

So our calculation suggests that there are roughly half the number of exomoons than exoplanets. One important thing to realize is that these calculations are based on many guesses but many of the assumptions underlying each calculation are the same. Whether the ratio is 0.5, 1, 2 or 5 is not very reliable right now, but what does seem perhaps more persuasive is that if we talk about ‘order of magnitude’ kind of figures, the number of habitable exoplanets and exomoons is ball-park equal.

Kipping’s figures are truly mind-blowing when he turns to the larger universe. A figure of roughly 100 million habitable environments per galaxy can now be turned around for an estimate of habitable worlds in the visible universe. The number works out to 1018, or 10 million trillion. Even allowing the vast play in the numbers between our low-ball and high-end estimates of habitable planets, the universe is likely to be filled with environments conducive to life. My guess is that it’s out there in fantastic abundance. But how much of it has gone on to sentience and, perhaps, technology? That’s the question SETI continues to poke at, and it’s one that’s emphatically still in play.

The Guo paper is “Probability Distribution of Terrestrial Planets in Habitable Zones around Host Stars,” Astrophysics & Space Science Vol. 323, No. 4 (October, 2009). Preprint available. David Kipping’s Web pages are packed with good information. Start with these articles.

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Piecing Together Titan’s Landscape

by Administrator on March 9, 2010

Titan’s Sikun Labyrinthus is a an area of connected valleys and ridges that bears a certain similarity to features on Earth. The area appears analogous to what we call ‘karst topography,’ created on our planet when layers of bedrock are dissolved by water, to leave rock outcroppings and sinkholes. The Darai Hills of Papua New Guinea are an example, as are the White Canyon of Utah and the Cockpit Country of Jamaica. Liquid methane and ethane may be what is producing such landscapes on Titan, but the processes seem familiar indeed.

Which brings us to Mike Malaska, without whose insights we might not be talking about this. Malaska is an organic chemist out of Chapel Hill NC who approached Jani Radebaugh (Brigham Young University) about a potential collaboration regarding Titan. Malaska works with visualizing NASA data and shares his results with contributors on unmannedspaceflight.com, where amateur astronomers and space exploration enthusiasts regularly discuss the latest findings. The beauty of today’s world is that rich data sets become available to the public in short order, allowing for contributions from unexpected quarters.

It was Malaska’s work tracing out landscape patterns on his computer that led to the obvious question: If many of these valleys had no apparent outlets, where had the fluid that had shaped them gone? Closed valleys turn out to be typical of karst topography, as Malaska found when he examined imagery of places like Guangxi Province in China by studying imagery from Google Earth. This is top-notch work — Malaska used a color palette derived from Cassini’s imaging science subsystem as well as the descent imaging and spectral radiometer on the Huygens probe, which touched down on Titan’s surface.

The results are striking, as shown in the image below:

Image: This artistic interpretation of the Sikun Labyrinthus area on Saturn’s moon Titan is based on radar and imaging data from NASA’s Cassini spacecraft and the descent imaging and spectral radiometer on the European Space Agency’s Huygens probe. The relative elevations are speculative and organized around the assumption that fluids are flowing downhill. Image credit: NASA/JPL/ESA/SSI and M. Malaska/B. Jonsson.

Laying a radar image over an inferred topography allowed the researchers to construct a 3D map, shown in this JPL video. We see a landscape that does bear a bit of similarity to Utah, and puts me in mind of some of John Ford’s westerns, which were often shot amidst spectacular rock outcroppings in that state. Sikun Labyrinthus inescapably reminds us of our own world, as Cassini radar team associate Karl Mitchell (JPL) points out:

“Even though Titan is an alien world with much lower temperatures, we keep learning how many similarities there are to Earth. The karst-like landscape suggests there is a lot happening right now under the surface that we can’t see.”

Caves perhaps? It will be a while before we know. Meanwhile, what an example for amateur planetary scientists. Malaska met Radebaugh at last year’s Lunar and Planetary Science Conference and went on to push his fascination with Cassini’s Titan imagery to its logical limit. Just as amateur astronomers are now making serious contributions by measuring exoplanet transits, so the much closer to home task of in-system investigation has taken on a public dimension through widely distributed data.

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Novel Technologies Aboard the IKAROS Sail

by Administrator on March 8, 2010

Not long ago we looked at IKAROS, an interesting solar sail concept out of JAXA, the Japanese space agency. Osamu Mori, project leader for the sail mission, offers up further background in an interview available at the JAXA site. IKAROS is notable because rather than relying solely on photons for propulsion, it would use solar cells covering part of the sail to generate electricity. In addition, the sail will operate with a unique attitude-control system. Here’s what Osamu Mori says about the latter:

The solar-powered attitude-control system uses a technology that controls the reflectivity of the sail. It works just like frosted glass: normally, the entire area of the sail will reflect sunlight, but by “frosting” part of the film, we can reduce the reflectivity of that area. When the reflectivity is reduced, that part of the sail generates less solar power. So by changing the reflectivity of the left and right sides of the sail, we can control its attitude.

Interesting stuff, and it fits into a broader context when you think about it in terms of a Jupiter mission. IKAROS is actually meant to be a technology demonstrator for evaluating solar sail performance in interplanetary flight. It will carry an ion engine along with the solar sail because at the distance of Jupiter, solar cells will provide only four percent of the efficiency they would offer near Earth. Japan’s intention is to go to Jupiter using solar cells, so both the ion engine and sail reflectivity adjustments can be seen as ways of stretching a known technology to see what is functional at these distances.

Not that there is any intention of going for Jupiter with the first mission. JAXA has been running vibration and thermal vacuum tests to shake out the systems of a small sail, no more than 14 meters to the side, made of polyimide resin some 7.5 micrometers thick (by comparison, a human hair is about 100 micrometers thick). Rather than Jupiter, the demonstrator will be launched along with the Venus Climate Orbiter AKATSUKI, deploying its sail a month after launch. Passing by Venus, IKAROS will navigate around the Sun as its systems are tested.

And as anyone working with sail concepts knows, deployment is a huge issue. Mori says that IKAROS will launch wrapped and folded around the body of the spacecraft. Centrifugal force generated by spinning the spacecraft will unfurl the sail, which will continue to be spin-stabilized, eliminating the need for a support structure. The method is illustrated in the image below:

Image: Deployment procedure for IKAROS sail. Credit: JAXA.

Spin-stabilization is beneficial not just given the complexity of deploying a supporting truss but also in terms of keeping the sail’s weight as low as possible. Says Mori:

IKAROS’s sail is small for a solar sail, but I think sails with a diameter of 50 to 100 meters will be developed in the near future. Unfurling such a thin film by spinning is still very difficult, though. We have gone through a long process of trial and error to figure out how we should fold the film so that it spreads smoothly. We conducted many experiments on the ground, and also launched the film aboard a sounding rocket. We even sent it high up in the sky in a big balloon, to spread the film in a near-vacuum environment. We experienced many failures, but we kept searching for the most reliable deployment method, and that led us to the model we’ve now built. I believe it will be successful.

This is an ambitious demonstrator, and the Jupiter mission that could develop out of it would be even more interesting as Japan develops its Jupiter and Trojan Asteroids Exploration Program. Can solar cells generate enough power to drive the ion engine, and will controlling the sail’s reflectivity prove useful for navigation? We’ll know more soon, as the launch date for AKATSUKI and IKAROS has now been set for May 18. More in this JAXA news page for IKAROS.

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Notes & Queries 3/5/10

by Administrator on March 5, 2010

Nuclear Cannon A Descendant of Orion

The new Carnival of Space is now out, from which I’ll focus on Brian Wang’s interesting notions on nuclear propulsion. The power behind the indispensable Next Big Future site, Brian has been writing about an Orion variant for some time now, one that should be able to get around the nuclear testing restrictions that put Orion itself into mothballs. A 1963 treaty effectively ended Orion’s prospects, and in 1974 the Threshold Test Ban Treaty was signed, prohibiting the testing of nuclear devices with a yield exceeding 150 kilotons. What can we do with a 150 kiloton upper limit for underground devices, and how does it relate to pulsed propulsion?

Wang envisions building what he calls a ‘nuclear cannon,’ capable of launching heavy payloads into Earth orbit. A 150 kiloton nuclear device is placed at the bottom of a two-mile shaft, packed with boron and other elements that will be converted to plasma. The 3500 ton launch projectile is placed on top. The explosion of the nuke launches it, with a chemical charge being used to quickly fill in the shaft as soon as the projectile clears it, the idea being to contain contamination. Figuring $10 million for the projectile and the propellant to launch it, plus another $20 million for construction of the shaft, Wang calculates launch costs in the neighborhood of $10 per pound, far cheaper than current launch options including the low-ball Russian Dnepr, a three-stage converted ICBM.

We’re not talking human missions here (not at 5000 G’s!) but heavy lift of the basic supplies for industrialization, with our standard launch systems being reserved for more fragile supplies and astronauts. Here’s Wang’s summation of the project’s cost and potential savings:

…100,000 tons of cargo delivered to the moon would be worth $5 trillion at the best prices today. 200,000 tons delivered to orbit would be worth $1 trillion @$5000/kg. If this could be done at one tenth the cost it is still worth $100 billion to orbit and $500 billion to the moon. Getting to one tenth of current costs is an optimistic ten years away and billions in development. The cost is to find a location like another remote island to sacrifice the underground area for nuclear launch similar to the areas sacrificed for underground nuclear testing. However, with proper preparation and a dome with a door and charges to speed the collapse of the shaft, there would be no radiation into the atmosphere.

It’s an intriguing notion, and not out of line with other industrial activities:

Other industries like oil, gas and coal regularly contaminate salt domes and underground and above ground locations. This would be safer and cleaner than those continuing operations. We would use nuclear bombs that are costing money to be maintained in storage and have a risk non-peaceful use. There is no risk of damaging EMP because damaging EMP occurs when a nuclear device is exploded at high altitude.

Interesting concept! Read more about the details here. And be aware that the regular postings of the Carnival of Space, which Brian handled this past week, are a good place to keep up with insights from space bloggers. This week you’ll find, in addition to the nuclear cannon and related links, a mind-boggling look at a Martian avalanche, a discussion of bad science in the movies (Apollo 13 and Contact stand out as exceptions to the rule that Hollywood invariably botches the science in the service of dubious plot lines), and Russia’s allocation of about $16 million for nuclear space projects this year, with plans to increase to $580 million over the next nine. Is the Russian initiative a keeper, and will it inspire new nuclear technologies from the West?

An Eerie Silence Indeed

Prolific author and physicist Paul Davies (Arizona State) will be offering an online lecture on March 31 covering our current SETI work and the prospects for extending it in new directions. His new book The Eerie Silence: Are We Alone in the Universe is just out this month from Penguin. Davies offers up an overview of our quest for extraterrestrial intelligence in a thoughtful piece on physicsworld.com, one that encapsulates the history of the discipline and asks whether we shouldn’t be thinking of expanding our horizons. It’s always interesting to note that current SETI research is almost all privately funded, with the 350-dish Allen Telescope Array now under construction growing from the philanthropy of Microsoft co-founder Paul Allen, and numerous activities coordinated by the SETI Institute and other sources working the sky on a regular basis.

Davies has his doubts that a scenario like Carl Sagan’s Contact, in which a civilization elsewhere in the galaxy beams messages to establish dialogue and provide wisdom, is really credible:

A major problem with Sagan’s thesis is that if there are any aliens out there, they almost certainly have no idea that the Earth hosts a radio-savvy civilization. Suppose there is an advanced alien community 500 light-years away – close even by optimistic SETI standards – then however fancy their technology might be, the aliens will see the Earth today as it was in the year 1510, long before the industrial revolution. In principle they could detect signs of agriculture and construction works such as the Great Wall of China, and they might predict that we would go on to develop radio astronomy after a few centuries or millennia, but it would be pointless for them to start signalling us until they obtained positive evidence that we were on the air. This would come when our first radio signals reached them, which will not be for another 400 years. It would then take a further 500 years for their first messages to arrive. So Sagan’s scenario might be conceivable in another millennium or so.

More likely that we pick up a beacon, one designed to sweep the plane of the galaxy, one sending out a civilization’s last wishes, perhaps, or calling attention to anyone who receives it that there are others who have survived their technological infancy. Even so, Davies doubts we would find the brief ping of a beacon amidst the sea of incoming data from our antennae. Better, perhaps, to look for signs of technology like Dyson spheres or other large-scale astroengineering projects which might change the spectral character of a host star.

Even changes confined to a planet’s surface may be detectable in the not-too-distant future in the form of industrial pollutants or other weird molecules in the spectrum of the planet’s atmosphere. The Kepler mission should soon produce a tally of Earth-like extrasolar planets that would be a natural target list for a future space-based optical system with this capability. We must also be alert to the possibility that an alien community might produce very different by-products than humanity – perhaps ultra-energetic neutrinos in the peta-electron-volt (1015 eV) range or intense bursts of gamma-ray photons from matter–antimatter annihilation that would be too concentrated to come from any plausible natural source.

So many questions arise from all of this and Davies works over them all, from extraterrestrial artifacts (and how to discover them if they exist in our own Solar System) to post-biological intelligence and the dangerous trap of anthropocentrism, in which we use our own civilization as a model for what an extraterrestrial culture must be like. Davies wonders whether biological intelligence won’t give way to new kinds of ‘thinking systems,’ artificial intelligence and genetically modified neural networks merging to create a new kind of sentience. Physicist Frank Wilczek calls such a development ‘quintelligence,’ and Davies thinks it might be found in intergalactic space, exploiting low temperatures and all but impossible to spot via SETI.

And what about right here on Earth?

As a final example of what we might look for, an alien expedition or migration wave may have tampered with terrestrial microbiology, perhaps creating its own shadow biosphere to assist with mineral processing, terraforming or energy production. Also, if the aliens really wanted to leave a message for posterity, implanting it in the genomes of micro-organisms might be a better strategy than sending out radio signals from a beacon. Using viruses or living cells as information repositories has many advantages: biological nanosystems are self-replicating and self-repairing, and have the potential to conserve information for millions of years. Some genes, for example, have remained largely unchanged for more than a billion years.

In any case, it’s hard to disagree with Davies’ notion that we now need to widen the search beyond radio and optical methods in the new hunt for astroengineering and technological footprints beyond our own. We’ve come a long way since the 1959 paper in Nature by Giuseppe Cocconi and Philip Morrison that first advocated a systematic search for alien radio signals. Frank Drake’s use of the 26-meter dish at Green Bank (West Virginia) was the start of a hunt that may well occupy us for decades more and perhaps centuries. My guess is that it’s the longest of long-shots, but then I think intelligent life is uncommon in the galaxy. My hope, though, is that we do find it — nothing would please me more than being proven wrong by a solid SETI detection.

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Targeting Nearby M Dwarfs

by Administrator on March 4, 2010

We’ve been talking for the last six years (since Centauri Dreams‘ inception) about finding a terrestrial world in the habitable zone of another star. It’s an exciting prospect, but the reality about space missions like Terrestrial Planet Finder and Darwin, each designed to make such identifications, is that the budget ax has fallen and we don’t know when they might fly. Indeed, we still face a host of technological difficulties that call for much work if the aim is not only to find a terrestrial world but also to study its atmosphere for possible biomarkers.

Alternatives are therefore welcome, and one is to look for terrestrial worlds around nearby red dwarf stars using transit methods. Usefully, an Earth-size planet orbiting such an M dwarf would be easier to spot than the same size planet orbiting a star like the Sun, and we could use ‘eclipse spectroscopy’ with the James Webb Space Telescope to study such a planet’s atmosphere. Right now we’re making Doppler surveys of nearby M dwarfs, and to good effect, with discoveries like the ‘hot Neptunes’ GJ 436b and GJ 581b, and ’super-Earths’ like GJ 876d. We’ve also found two planets near to their star’s habitable zone in GJ 581c and d. We should be finding habitable ’super-Earths’ in the near future with these methods and some of these, let’s hope, will be transiting.

Surveys monitoring thousands of stars can pick up transiting planets (think of Kepler), and Michaël Gillon and colleagues explain in a new paper that most known transiting planets have been detected by such dedicated photometric surveys. The MEarth Project at Mt. Hopkins, AZ monitors nearby M dwarfs with small telescopes and is sensitive to transiting worlds down to a few Earth radii. Gillon’s team is interested in a third approach, one that’s based on a helpful principle. Because planets form within disks, a planet orbiting in the habitable zone of a star will be more likely to transit as seen from Earth if that star already harbors a known transiting planet. From the paper:

Depending on the orbital inclination of the known transiting planet, on the assumed distribution of the orbital inclinations of the planetary system, on the size of the star, and on its physical distance to its HZ, significantly enhanced transit probability can be expected for habitable planets. A dedicated high-precision photometric monitoring of M dwarfs known to harbor close-in transiting planets could thus be an efficient way to detect transiting habitable planets in the near future.

The fact that planets in a system should share similar orbital inclinations is especially useful for M dwarfs because their habitable zones are close to the star. As we discover more transiting planets around M dwarfs (which are currently thought to be the most common class of star in the galaxy), we may be able to use these facts to improve the likelihood of finding habitable worlds. The researchers go on to discuss the potential of this approach for two M dwarfs known to host a transiting planet, GJ 436 and GJ 1214, using a series of simulations.

It turns out that GJ 436 is not a good target compared to GJ 1214. The transit probability of planets in the habitable zone of the latter is much larger. Moreover, GJ 1214 is smaller in radius, meaning that smaller planets could be detected around it. The latter fact also makes for a smaller habitable zone, so that any planet in that zone will be orbiting closer to the star. Ground based monitoring of GJ 1214 could theoretically find a habitable planet as small as the Earth, while space-based observatories like Spitzer could spot a transiting habitable planet down to Mars size.

The planet we already know about here, GJ 1214b, is a super-Earth about 6.6 times the mass of Earth, with a radius somewhat less than three times our planet’s, and it orbits its star every 1.6 days. Roughly 40 light years from the Sun, this system would seem to be ideal for pushing the search for a smaller companion world. The team finds that probing the habitable zone of GJ 1214 would require three weeks of constant monitoring, whereas GJ 436 would require a full two months. That three week run would allow for two transits and could lead to the detection of smaller planets than we’ve hitherto found. The paper confirms the viability of transit surveys like MEarth and offers what may be the shortest course to detecting habitable planets as small, or even smaller, than the Earth. The authors continue:

…we advocate the development of the approach used by MEarth (other facilities spread in longitude, a similar survey observing from the Southern hemisphere, larger telescopes and IR cameras to monitor cooler M dwarfs), but also an intense and high-precision photometric monitoring of GJ 1214 and of the other transiting systems that MEarth (or similar projects) will detect. This two-step approach targeting nearby M dwarfs makes possible the detection in the near-future of transiting habitable planets much smaller than our Earth that would be out of reach for existing Doppler and transit surveys.

The paper is Gillon et al., “Educated search for transiting habitable planets: Targeting M dwarfs with known transiting planets,” submitted to Astronomy & Astrophysics (preprint available). The betting here is what it has always been, that our first detection of a terrestrial exoplanet that is unequivocally in the habitable zone of its star will be around an M dwarf. We’re likely to spot a growing number of habitable ’super-Earths’ in coming years, so methods that will allow us to extend our discoveries to Earth-size planets are all to the good. After all, who knows how long it will be until funds become available for the kind of terrestrial planet hunter mission we’ve long wished for?

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Human Compulsions Among the Stars

by Administrator on March 3, 2010

What are the odds for survival of a technological society? We don’t know yet, having but one example to work with, but it’s interesting to speculate, as Ray Villard does in a recent online post, about the kinds of intelligence that may evolve in the universe. All too often we equate technology with intelligence, which may skew our view of projects like SETI. Energized by the American Association for the Advancement of Science meeting in San Diego last week, Villard is thinking that intelligent life may have appeared on our planet not once but twice, and one of those life-forms is never going to be found by listening to radio wavelengths.

The case for cetaceans seems strong. Here’s Villard on the matter:

Physiologically, dolphins have a brain architecture and brain mass-to-body mass ratio that is closer to that of humans than for any other species on Earth. Many years of experiments on captive dolphins show that they are self-aware, have a sense of self-identity, do detailed problem solving, interpret symbolic language, and exhibit empathy. Dolphins form complex societies with groups segregated by sex and age, alliances, and conduct long-term nurturing of the young.

And whereas apes and humans appear closely in evolutionary time, cetaceans do not, making the case for independent emergence of a far different kind of intelligence than humans possess, one adapted for life in the ocean. The argument is interesting on its own merits because the emergence of dolphins and whales as self-aware beings implies that evolution has established two different routes to intelligent life on the same planet. That would make a strong case that self-awareness is a common feature on any planets where complex life-forms establish themselves, and would seem to bode well for extraterrestrial civilizations.

SETI, of course, is quite another matter. A world populated only by dolphins and whales is not one that is going to be sending strong beacon signals at 1420 MHz to nearby worlds. The Fermi paradox? Maybe the ‘where are they’ question is answered by the thought that they’re on many nearby worlds, but don’t necessarily have the technological means to tell us so. Villard goes a step farther still and asks whether creatures that do develop technologies aren’t the most hubristic, the builders of guns, cars and refrigerators also being capable of creating thermonuclear devices and bacteriological weapons to destroy themselves.

At the AAAS meeting, Seth Shostak opined that we would have an interstellar greeting from another civilization within the next twenty-five years. He bases this on the fact that we’re reaching so many stars now that within two years, we’ll have surveyed as many stars as we did in the past fifty years, since Frank Drake first fired up Project Ozma to listen to Epsilon Eridani and Tau Ceti. Villard notes that this exponential rise offers the best chance for success when it reaches the top of the slope and begins to flatten out. He quotes Shostak as saying “If we don’t have a detection by the year 2035 then something is wrong with our fundamental assumptions.”

But our fundamental assumptions are constantly being challenged with every new discovery in planetary science and astrophysics. Why should SETI be any different? The possibility of intelligence evolving in such a way that it has no technology seems clearly demonstrated here on Earth. But we should also be asking whether even technological societies necessarily have the same urge to communicate that seems to drive us. Is reaching out across the stars a fundamental impulse of intelligent life, or is it a trait of our species alone, and if the latter, what is the impulse behind it? If we can’t assume alien civilizations will share our technologies, neither should we assume they would share our compulsions. A lack of SETI success by 2035 may simply tell us that the quest for knowledge of the wider universe may be a human philosophical quirk.

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Gravitational Lensing Measures the Universe

by Administrator on March 2, 2010

Data from the Keck telescope (Mauna Kea), the Hubble Space Telescope and the Very Large Array have been used in conjunction with the findings of the Wilkinson Microwave Anisotropy Probe to offer up a new way to measure the size of the universe, as well as how rapidly it is expanding and how old it is now. By determining a value for the Hubble constant, the work confirms the age of the universe within a span of 170 million years as 13.75 billion years old.

I’m always fascinated with work involving gravitational lensing — just yesterday we looked at using the Sun’s lensing effects for potential SETI investigations — and here we have a classic case of measuring how light traveled from a bright, active galaxy along different paths to reach the Earth. A strong gravitational lens like the one used in this study, called B1608+656, creates multiple images of the same galaxy lying behind the lensing object. Studying the time the light took along each path, it was possible to gather information about the distance of the galaxy as well as the age of the universe and details about its expansion.

Image: When a large nearby object, such as a galaxy, blocks a distant object, such as another galaxy, the light can detour around the blockage. But instead of taking a single path, light can bend around the object in one of two, or four different routes, thus doubling or quadrupling the amount of information scientists receive. As the brightness of the background galaxy nucleus fluctuates, physicists can measure the ebb and flow of light from the four distinct paths, such as in the B1608+656 system imaged above. Credit: Sherry Suyu/Argelander Institut für Astronomie, Bonn.

The lensing effect produced four images of the background galaxy. What’s fascinating about lensing is that the time it takes a light ray to travel a short path can be longer than the time it takes to travel a longer path due to the gravitational time delay caused by the lensing object. This short video with physicist Sherry Suyu (University of Bonn) discusses the effect by analogy with travel times on Earth, and explains how the scientists were able to use the multiple images of the background galaxy to compute the tightened value for the Hubble constant — 21 kilometers per second per million light years. In other words, a galaxy that is a million light years away is moving away from us at about 21 kilometers per second.

An international team is behind this work, which is just out in the Astrophysical Journal. Having made a physical measurement of Hubble’s constant, says Phil Marshall (Stanford University), gravitational lensing “has come of age as a competitive tool in the astrophysicist’s toolkit.” The new value for Hubble’s constant is considered the best estimate of the uncertainty in the constant. Beyond that, however, is the fact that lensing is producing an estimate for the universe’s age that gibes well with other methods of analysis, meaning that we’re learning how to harness this remarkable natural tool for future investigations.

The paper is Suyu et al., “Dissecting the Gravitational Lens B1608+656. II. Precision Measurements of the Hubble Constant, Spatial Curvature, and the Dark Energy Equation of State. Astrophysical Journal 711 (1 March 2010), pp. 201-221 (abstract).

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SETI: The Solar Sail Perspective

by Administrator on March 1, 2010

I love what Dan Wertheimer, a Berkeley astronomer and one of the powers behind the SETI@Home distributed computing project, told a session at the recent AAAS meeting in San Diego. Wertheimer was talking about the possibility of using the Sun’s gravitational lens for SETI purposes, and as quoted by Alan Boyle, said that such an observatory could “read the license plates on an extrasolar planet.” That reminded me of Claudio Maccone’s whimsical but mind-boggling remark at the interstellar conference in Aosta last July, which went in much the same direction. What could lensing do? “We could see the roads of their cities. We could see the cars they are driving.”

Drake has made the case for using the Sun’s gravitational lens for SETI purposes for a long time now, and he repeated it at the TED 2010 conference in Long Beach. As to Maccone, he has long championed the FOCAL mission to the gravitational lens that would exploit the fantastic magnifications available at 550 AU and beyond. But it was Drake who first acquainted him with the topic back in 1987 at a conference on Lake Balaton in Hungary. Maccone then worked hard on both the equations and the mission possibilities, submitting a proposal to the European Space Agency in 2000 that the agency chose not to finance, although he was complimented for his vision.

Image: IAA Secretary General Jean-Michel Contant (left) with Frank Drake (center) and Claudio Maccone. Taken in London at the Royal Society Meeting of January 25-26, 2010. Credit: Claudio Maccone.

SETI and the gravitational lens make for fascinating possibilities. The 1992 Conference on Space Missions and Astrodynamics which Maccone led in Turin was the first time I am aware of that scientists and engineers began to study the possibilities of a mission. Solar sails were the propulsion method of choice, and Italy was the home of considerable work on sail concepts at the time, beginning with Quasat, a concept coming out of aerospace firm Alenia Spazio which would have been an inflatable radio telescope in Earth orbit. Quasat was never launched, but Maccone continued studying inflatable technologies with applications to extrasolar studies.

At one point, working with Jean Heidmann, Maccone suggested two kinds of FOCAL mission, one that would target astrophysical objects of interest, to be called ASTROsail, the other to study suspected artificial radio signals and to be called SETIsail. The later Aurora Project was conceived as a somewhat less ambitious solar sail attempt to reach the heliopause, with results of preliminary studies being presented at the International Academy of Astronautics meeting in Turin in 1996. Both Giovanni Vulpetti and Giancarlo Genta offered up impressive analyses of Aurora.

But back to Drake, who as the first SETI experimentalist (through his Project Ozma efforts in 1960) can be considered the godfather of the discipline. He and Nathan Cohen (Boston University) presented the case for using the gravitational lens for SETI at the 1987 bioastronomy conference in Hungary referenced above, and both have gone on to write non-technical accounts of lensing and its possibilities for SETI. SETIsail, meanwhile, grew from a targeted SETI mission to the lens to the ongoing FOCAL study, which could be used for many observations besides those involved in SETI itself.

For a look at this bit of solar sail and SETI history in context, see Heidmann and Maccone, “AstroSail and FOCAL: Two extrasolar system missions to the Sun’s gravitational focuses,” Acta Astronautica, Vol. 35 (1994), pp. 409-410. Maccone’s book on the mission is Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens (Springer/Praxis, 2009). And if you really want to dig, read A. Einstein, “Lens-like Action of a Star by the Deviation of Light in the Gravitational Field,” Science Vol. 84, (1936), pp. 506-507.

It’s fascinating to speculate on how a SETI mission to the gravitational lens might actually be used. As Drake says, a kind of galactic Internet might be built up using lenses in different systems, but given the cost and time involved in reaching lensing distances, when would a mission be contemplated? Surely it would be after the reception of signals so promising that they left little doubt of their origin in another civilization. At that point, the prospect of using the lens to examine the system in question might prove irresistible, driving mission design and advancing our propulsion technologies. A FOCAL-style SETI mission could take us from the simple knowledge that we are not alone to a rich understanding of a culture on another world.

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Enceladus Hotspots, and Memories of Orion

by Administrator on February 26, 2010

Although we’ve been preoccupied largely with theoretical matters this week, I don’t want it to close without reference to the new Cassini imagery of Enceladus. This shot was made at a phase angle of 145 degrees when Cassini was about 14,000 kilometers from Enceladus, during the flyby of November 21. The remarkable jets spraying from the fractured surface in the south polar region are clearly visible.

Image: Dramatic plumes, both large and small, spray water ice out from many locations along the famed “tiger stripes” near the south pole of Saturn’s moon Enceladus. The tiger stripes are fissures that spray icy particles, water vapor and organic compounds. More than 30 individual jets of different sizes can be seen in this image and more than 20 of them had not been identified before. At least one jet spouting prominently in previous images now appears less powerful. Credit: NASA/JPL/SSI.

I keep thinking about Project Orion, back in the crazy days before the Test Ban Treaty of 1963 closed down the nuclear option. The prospect of taking a huge vessel with a crew of 100 all the way to Saturn as early as 1968 was much in the air at Los Alamos, and Enceladus was to have been the ultimate destination, chosen because observations of the distant moon seemed to show plenty of ice on the surface. So gleefully did the Orion team ponder its propulsive capabilities that project leader Ted Taylor wanted to install a two-ton barber’s chair on the ship, a poke in the eye to chemical rocketry and all its limitations. The atomic spaceship was going to be big.

As to Enceladus, Dyson recalled:

“We knew very little about the satellites in those days. Enceladus looked particularly good. It was known to have a density of .618, so it clearly had to be made of ice plus hydrocarbons, really light things, which were what you need both for biology and for propellant, so you could imagine growing your vegetables there…”

The quote is from George Dyson’s Project Orion: The True Story of the Atomic Spaceship (New York: Henry Holt, 2002), and it always makes me wonder what Dyson and crew would have thought of Enceladus, with its extraordinary sprays of fine material showing geological activity and the possibility of liquid water, when they actually arrived. The new Cassini imagery shows us more jets than ever before, more than thirty individual geysers in one mosaic, with changes to previously seen jets that are telling:

“This last flyby confirms what we suspected,” said Carolyn Porco, imaging team lead based at the Space Science Institute in Boulder, Colo. “The vigor of individual jets can vary with time, and many jets, large and small, erupt all along the tiger stripes.”

The section of Baghdad Sulcus shown below combines heat data with visible-light images for a 40-kilometer stretch of this, the longest of the so-called ‘tiger stripes.’ Peak temperatures along Baghdad Sulcus reach 180 kelvin, perhaps as high as 200 kelvin, which the Cassini team believes is the result of heating by upwelling water vapor. This is an intense effect — the heat is confined to a narrow region about a kilometer wide along the fracture, and its strength varies along the length of the fissure.

Image: A mosaic combining high-resolution data from the imaging science subsystem and composite infrared spectrometer aboard NASA’s Cassini spacecraft. Pockets of heat appear along one of the mysterious fractures in the south polar region of Saturn’s moon Enceladus. The fracture, named Baghdad Sulcus, is one of the so-called “tiger stripe” features that erupt with jets of water vapor and ice particles. It runs diagonally across the image. This mosaic, obtained on Nov. 21, 2009, shows a 40-kilometer (25-mile) segment of Baghdad Sulcus and illustrates the correlation between the geologically youthful surface fractures and anomalously warm temperatures recorded in the south polar region. It shows the highest-resolution data yet of the heat leaking from the moon’s interior along the tiger stripes.

This JPL news release goes into more detail:

While the heat appears to emanate mostly from the main Baghdad tiger stripe, some of the fractures branching off or parallel to it also appear warmer and active to varying degrees, though this needs to be confirmed by further analysis. The total amount of infrared energy and the relative amounts given off at different wavelengths show that the highest temperatures along Baghdad Sulcus are limited to a region no more than tens of meters (yards) across. Most of the heat measured by the infrared spectrometer probably arises from the warm flanks of the active fractures, rather than their central fissures. The narrow central fissure is probably even warmer than the 180 Kelvin (minus 140 degrees Fahrenheit) detected – possibly warm enough for liquid water in the fractures to be the source of the observed jets.

Carolyn Porco refers to Enceladus’ “organic-rich, liquid sub-surface environment” as “the most accessible extraterrestrial watery zone known in the Solar System.” The temperature differential between places like Baghdad Sulcus and the 50 kelvin reading of the surrounding surface is fascinating, and tells us that melting underground ice in these regions may not be all that difficult. Too bad we missed out on Orion’s 1968 journey, but the eight Cassini flybys thus far are telling us much about this unexpectedly interesting moon, surely the target of a future mission of its own.

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Other Life in the Multiverse?

by Administrator on February 25, 2010

What conditions would you say are ‘congenial to life’? For physicist Robert Jaffe and colleagues at MIT, the phrase refers to places where stable forms of hydrogen, carbon and oxygen can exist. Jaffe explains why:

“If you don’t have a stable entity with the chemistry of hydrogen, you’re not going to have hydrocarbons, or complex carbohydrates, and you’re not going to have life. The same goes for carbon and oxygen. Beyond those three we felt the rest is detail.”

It’s an important issue in Jaffe’s work because he wants to see whether other universes could harbor life. We know that slight changes to the laws of physics would disrupt the evolution of the universe we live in. The strong nuclear force, for example, could have been just a bit stronger, or weaker, and stars would have been able to produce few of the elements needed to build planets. Remove the electromagnetic force and light would not exist, nor would atoms and chemical bonds.

Nudging Nature’s Parameters

Run through the constants of nature and you’ll find many that have to show precise values for life as we know it to have formed. Thus the idea that there may be not one but many universes, each with its own laws, and the thought that we happen to occupy a universe where the conditions that make life a possibility managed to fall into place.

Anthropic reasoning like this — things have to be this way because otherwise we couldn’t be here to think about all this — suggests that multitudes of universes exist, a multiverse in which almost all the universes would be devoid of life and, indeed, matter as we know it. Jaffe is interested in finding out whether universes with different physical laws might not be so inhospitable to life after all. His team focused on universes with nuclear and electromagnetic forces that allow atoms to exist. Another stipulation: Universes that allowed stable forms of hydrogen, carbon and oxygen.

Then it became a matter of playing with nature’s building blocks. Take quarks: In our universe, the ‘down’ quark is roughly twice as heavy as the ‘up’ quark, so that neutrons are 0.1 percent heavier than protons. Jaffe’s team lightened up the down quark so that protons were up to one percent heavier than neutrons. According to this modeling, hydrogen would no longer be stable, but the heavier isotopes deuterium and tritium would be. Carbon-14 could exist and so would a form of oxygen. It’s a different universe than ours, but the models say life could emerge in it.

Other quark variations, including one where the ‘up’ and ’strange’ quarks have roughly the same mass, unlike in our universe, produced atomic nuclei made up of neutrons and a hyperon called the ’sigma minus,’ which would replace protons. The fact that we have a reasonable understanding about quark interactions makes them useful for studies of this kind, but changing other physical laws is even trickier business.

Into a ‘Weakless’ Universe

Nonetheless, Lawrence Berkeley National Laboratory researchers have modeled universes that lack one of the four fundamental forces of ours. Without the weak force big bang nucleosynthesis — turning groups of four protons into helium 4 nuclei of two protons and two neutrons — would not have been possible. But when the team at LNBL removed the weak nuclear force in their models, they were able to tweak the other three forces to compensate. Stable elements could form in this universe as well.

Note what’s happening here. Rather than changing a single constant, the LBNL researchers tweaked several. After all, in a multiverse that can keep spewing out universe after universe, all combinations would seem to be possible and you can keep trying until you get it right. This Scientific American article by Alejandro Jenkins (MIT) and Gilad Perez (now at the Weizmann Institute) gets into the specifics:

In the weakless universe, the usual fusing of protons to form helium would be impossible, because it requires that two of the protons convert into neutrons. But other pathways could exist for the creation of the elements. For example, our universe contains overwhelmingly more matter than antimatter, but a small adjustment to the parameter that controls this asymmetry is enough to ensure that the big bang nucleosynthesis would leave behind a substantial amount of deuterium nuclei. Deuterium, also known as hydrogen 2, is the isotope of hydrogen whose nucleus contains a neutron in addition to the usual proton. Stars could then shine by fusing a proton and a deuterium nucleus to make a helium 3 (two protons and one neutron) nucleus.

But would these stars be anything like what we are familiar with? The article continues:

Such weakless stars would be colder and smaller than the stars in our own universe. According to computer simulations by astrophysicist Adam Burrows of Princeton University, they could burn for about seven billion years—about the current age of our sun—and radiate energy at a rate that would be a few percent of that of the sun.

A Strange But Living Universe

A strange place, this ‘weakless’ universe. Supernova explosions of the kind that synthesize and distribute heavy elements in our universe would not occur, at least not from the same causes, but a different kind of supernova caused by accretion rather than gravitational collapse would be possible, allowing elements to seed interstellar space. A planet like ours circling one of the weakless stars would need to be six times closer to the Sun to stay habitable. And check this out:

Weakless Earths would be significantly different from our own Earth in other ways. In our world, plate tectonics and volcanic activity are powered by the radioactive decay of uranium and thorium deep within Earth. Without these heavy elements, a typical weakless Earth might have a comparatively boring and featureless geology—except if gravitational processes provided an alternative source of heating, as happens on some moons of Saturn and Jupiter.

Chemistry, on the other hand, would be very similar to that of our world. One difference would be that the periodic table would stop at iron, except for extremely small traces of other elements. But this limitation should not prevent life-forms similar to the ones we know from evolving. Thus, even a universe with just three fundamental forces could be congenial to life.

Accounting for the Cosmological Constant

Still tantalizing is the cosmological constant, a measure of the amount of energy found in empty space. The discovery of the continuing acceleration of the universe’s expansion has brought ‘dark energy’ into the picture, implying a cosmological constant that is positive as well as minute, allowing the universe to form structure. It’s a constant that seems fine-tuned to a remarkable degree, and as the article notes, “…the methods our teams have applied to the weak nuclear force and to the masses of quarks seem to fail in this case, because it seems impossible to find congenial universes in which the cosmological constant is substantially larger than the value we observe. Within a multiverse, the vast majority of universes could have cosmological constants incompatible with the formation of any structure.”

All of this is almost joyously theoretical, basing itself on a theory of inflation that conceives of small pockets of spacetime that inflate so rapidly that it is impossible to travel between them. Inflation is highly regarded but not definitively understood, but different values for the constants of nature in the universes it produces seem like a reasonable conjecture. And the cosmological constant itself is an example of fine-tuning on such a scale that it may require the existence of a multiverse to give us a rational explanation for how we lucked into this one.

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