by Paul Gilster | Mar 10, 2010 | Exoplanetary Science |
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.
by Paul Gilster | Mar 9, 2010 | Outer Solar System |
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.
by Paul Gilster | Mar 8, 2010 | Sail Concepts |
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.
by Paul Gilster | Mar 5, 2010 | Culture and Society |
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.
by Paul Gilster | Mar 4, 2010 | Exoplanetary Science |
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?
