Centauri Dreams
Imagining and Planning Interstellar Exploration
Exoplanet Exploration Organization Proposed
We’ve recently looked at the role of small spacecraft, inspired in part by The Planetary Society’s LightSail, a CubeSat-based sail mission that launched last week. It’s interesting in that regard to consider small missions in the exoplanet realm. ExoplanetSat, for example, is a 3-unit CubeSat designed at MIT as a mission to discover Earth-sized exoplanets around nearby stars. Here the beauty of the CubeSat is obvious: The platform is low-cost, the development time is relatively short, and there are frequent launch opportunities. Up to 100 ExoplanetSats are planned.
Pulling big benefits from small packages is not new, as the example of the Canadian MOST mission (Microvariability and Oscillations of STars) reminds us. MOST was the first mission dedicated to asteroseismology, to be followed by CoRoT (COnvection ROtation and planetary Transits) and then Kepler. Now we have a proposal for what is being called the United Quest for Exoplanets (UniQuE), which grows out of work performed by an interdisciplinary team hosted by the International Space University. Meeting in Montreal last summer, the group produced a report giving its recommendations to enhance the entire field of exoplanet research.
Michael Michaud passed along an article on this work called “Going Global with Exoplanets” that ran in Space Times, which is the newsletter of the American Astronautical Society. I want to dig into it a bit because one of the larger questions here is how to generate international support for planet-hunting, and as Michaud reminds me, this fits in with what could be a useful collaboration between the exoplanet community and advocates of interstellar flight.
The Montreal team, which was sponsored by NASA and Lockheed Martin, consisted of 28 participants from twelve countries. Its report proposes the creation of the Exoplanet eXploration Organization (EXO), drawing on the fact that while other organizations include exoplanets as part of their mission, most have wider agendas, or are focused solely on a specific group of people. EXO would from the outset take an international perspective that crosses numerous areas of expertise. One part of its charter, which I’ll return to in a moment, is the above-mentioned UniQuE, the creation of small satellites to conduct exoplanet work, just the kind of thing MIT has in mind for ExoplanetSat.
Image: Kepler-16b envisioned in a JPL ‘travel poster’ showing a sky with two suns from the planet that orbits both. Part of a modest but effective public outreach effort from JPL. The EXO proposal similarly looks at ways to heighten public interest in exoplanets and deep space.
But characterizing habitable planets is a multidisciplinary endeavor whose practitioners are located around the globe. If we are to move to characterizing exoplanet atmospheres and pushing into increasingly detailed observations of these worlds, we’re probably going to be in a cost range where funding by a single nation or agency is problematic. One way to encourage international cooperation is through better information exchange and the development of better databases.
Data availability is, the article argues, an area that needs improvement. We do have rich databases like the Exoplanet Data Explorer (California Planet Survey), the Extrasolar Planets Encyclopedia (L’Observatoire de Paris), MIT’s Open Exoplanet Catalogue and NASA’s Exoplanet Archive, but the Montreal team finds the presentation of data between these groups to be inconsistent both in the data provided and the metadata used to search the material. It argues that EXO could be of service:
One EXO initiative would be to support developing and promoting a common data and metadata format for publically available data. The Extended Extrasolar Planets Encyclopedia (EEPE) aims at making new data-fields, links to other databases, and raw data easier to add to the database. The rationale behind proposing EEPE and a structured format is to maximize the use of existing infrastructure, and make it easier to access and update data.
Other possibilities include providing consulting services for exoplanet projects and pooling fundraising expertise, as well as helping researchers promote exoplanet projects to government agencies. Like The Planetary Society, EXO is envisioned as an advocate for its areas of interest, one that also serves as a bridge between the public and the scientific community. Thus educational projects to reach non-specialized audiences are a factor, and so are crowdsourcing efforts to consolidate, sort and analyze the vast amount of incoming data:
While much of it can be handled by computer algorithms, there are various phenomena that need human investigation. At present, raw data is gathered much faster than it is studied. One example is the website planethunters.com, which provides a crowdsourcing platform. After visitors complete a brief tutorial, participants analyze Kepler data by marking potential candidates while discarding uninteresting light curves. Another example is NASA-sponsored OSCAAR (Open Source differential photometry Code for Accelerating Amateur Research), which allows amateur astronomers to contribute their own light curves from amateur observations. These participative initiatives provide people with the ability to engage in the research community. Crowdsourcing, however, is only as powerful as the participants, which emphasizes the need for effective international outreach programs.
What is being proposed is cross-pollination between the exoplanet community and numerous other areas of public interest in space. I like the idea, for example, of offering massive open online courses (MOOCs) as a way not only of reaching the public but also of developing open source tools like lesson plans for teachers and software for amateur astronomers, who would use the EXO platform to connect with each other’s work across borders and disciplines.
But back to small satellites. The low cost and broad involvement, from space agencies to universities and small laboratories, that CubeSats offer can be leveraged here. Think of CubeSats as a route into space for countries that lack the resources for more costly missions. The UniQuE mission proposed here has much in common with ExoplanetSat in that it involves the creation not of single satellites but a constellation of low-cost small spacecraft whose mission would be characterization of the atmospheres of previously detected nearby planets:
The UniQuE team envisioned a standardized design based on a 15 kg 12 unit CubeSat layout carrying a space-proven near-IR mini-spectrometer covering the aforementioned waveband with a sensitivity range compatible with this mission. EXO would provide an overall baseline design and participating entities would have the freedom to customize and size all relevant subsystems so long as overall mission requirements are met, thereby allowing freedom for the inclusion of innovative concepts. The ideal constellation is composed of three to six pairs of satellites on a dawn-dusk sun synchronous orbit, which are launched as piggyback or secondary payloads.
What the Montreal team envisions is that all the UniQuE satellites would be required to observe the transit of an approaching planetary transit of a nearby star, while during the remaining time, the owners of the individual satellites could use them for their own scientific purposes. Results would be disclosed across the range of participating countries to provide a testbed for collaborative research and the development of ever more sophisticated spacecraft.
To read more about the thinking behind EXO, you can access the final report here. It could be argued that we have many of the tools the report recommends already in place, and that researchers currently interact through a variety of networked venues. But I think the development of an exoplanet organization with an international focus and a determined public outreach could consolidate some of these gains while providing useful collaborative tools. Moreover, the public engagement built into this kind of organization could benefit the spread of deep space ideas as we ponder future programs of exploration.
A Mass-Radius Relationship for ‘Sub-Neptunes’
The cascading numbers of exoplanet discoveries raise questions about how to interpret our data. In particular, what do we do about all those transit finds where we can work out a planet’s radius and need to determine its mass? Andrew LePage returns to Centauri Dreams with a look at a new attempt to derive the relationship between mass and radius. Getting this right will be useful as we analyze statistical data to understand how planets form and evolve. LePage is the author of an excellent blog on exoplanetary science called Drew ex Machina, and a senior project scientist at Visidyne, Inc. specializing in the processing and analysis of remote sensing data.
By Andrew LePage
As anyone with even a passing interest in planetary studies can tell you, we are witnessing an age of planetary discovery unrivaled in the long history of astronomy. Over the last two decades, thousands of extrasolar planets have been discovered using a variety of techniques. The most successful of these to date in terms of sheer number of finds is the transit method – the use of precision photometric measurements to spot the tiny decrease in a star’s brightness as an orbiting planet passes directly between us and the star. The change in the star’s brightness during the transit allows astronomers to estimate the size of the planet relative to the star while the time between successive transits allows the orbital period of the planet to be determined. Combined with information about the properties of the star being observed, other characteristics can be calculated such as the actual size of the planet and its orbit. The most successful campaign to date to search for planets using the transit method has been performed using NASA’s Kepler spacecraft, launched in 2009.
One of the other important bulk properties of a planet that is of interest to scientists is its mass. Unfortunately, the transit method is typically unable to supply us with this information except in special circumstances where planets in a system strongly interact with each other to produce measurable variations in the timing or duration of their transits. The transit timing variation (TTV) or transit duration variation (TDV) methods can be used to estimate the masses of the planets of a system including non-transiting planets that might be present. Based on an analysis of Kepler results to date, however, this method can be used in only about 6% of planetary systems that produce transits.
A more widely applicable method to determine the mass of an extrasolar planet is through the precision measurement of a star’s radial velocity to detect the reflex motion caused by the orbiting planet. Combined with information from transit observations as well as the star’s properties, it is possible to calculate the actual mass of a planet and further refine its orbital properties. Unfortunately, NASA’s Kepler mission has discovered thousands of planets and making precision radial velocity measurements takes a lot of time on a limited number of busy telescopes that are equipped to make the required observations. In addition, many of the stars observed by Kepler are too dim or their planets too small for the current generation of instruments to detect radial velocity variations above the noise. This is especially a problem for sub-Neptune size planets including Earth-size terrestrial planets. Taken as a whole, only a small minority of all of Kepler’s finds currently have had their masses measured.
Puzzling Out a Planetary Mass
While astronomers continue to struggle to measure the masses of thousands of individual extrasolar planets found by Kepler, there have been efforts to derive a mass-radius relationship so that the mass of a planet with a known radius can at least be estimated. In addition to being useful for evaluating the level of accuracy required for detection using radial velocity measurements or other methods, such mass estimates are also valuable for scientists wishing to use Kepler radius and orbit data in statistical studies of planetary properties, dynamics, formation and evolution. Over the past few years, there have been various investigators who have attempted to derive a planetary mass-radius relationship as information on the mass and radius of known planets has expanded. These relationships have taken a mathematical form known as a power law such as M = CRγ where M is the mass of the planet (in terms of Earth mass or ME), R is its radius (in terms of Earth radii or RE) and C and γ are constants determined by analysis.
The latest work to derive a mass-radius relationship for sub-Neptune size planets (i.e. planets whose radii are less than 4RE) is a paper by Angie Wolfgang (University of California – Santa Cruz), Leslie A. Rogers (California Institute of Technology), and Eric B. Ford (Pennsylvania State University), which they recently submitted for publication in The Astrophysical Journal. These sub-Neptune size worlds are of particular interest to the scientific community since they span the size range between the Earth and Neptune where no Solar System analogs exist to provide guidance for deriving a mass-radius relationship.
Earlier work over the last few years on the planetary mass-radius relationship relied on least squares regression analysis of a set of planetary radius and mass measurements – a fairly straightforward mathematical method used to determine the constants of an equation that provides the best fit to a set of data points. Unfortunately, this classic method has some drawbacks. It does not properly take into account the uncertainty in the independent variable (i.e. the planet radius, in this case) or instances where the planet has not been detected using precision radial velocity measurements and only an upper limit of the mass can be derived. Another issue is that the least squares regression method assumes a deterministic relationship where a particular planetary radius value corresponds to a unique mass value. In reality, planets with a given radius can have a range of different mass values, in part reflecting the variation in planetary composition running from massive rocky planets with large iron-nickel cores to less massive, volatile-rich planets with deep atmospheres. These variations are expected to be especially important in sub-Neptune-class worlds.
A Bayesian Approach to the Mass/Radius Problem
Instead of using the least squares regression method, Wolfgang, Rogers and Ford evaluated their data using a hierarchal Bayesian technique which allowed them not only to derive the parameters for a best fit of the available data, but also to quantify the uncertainty in those parameters as well as the distribution of actual planetary mass values. Using their approach, they have derived a probabilistic mass-radius relationship where the most likely mass and the distribution of those values are determined. The team considered a total of 90 extrasolar planets with known radii less than 4RE whose masses have been measured or constrained using radial velocity or TTV methods. Neither unconfirmed planets nor circumbinary planets were considered to keep the sample as homogeneous as possible. The team also truncated the mass distribution to physically plausible values that were no less than zero (since it is physically impossible to have a negative mass) and no greater than the mass of a planet composed of iron (since it is unlikely for a planet to have a composition dominated by any element denser than iron).
Image: This plot shows the available mass and radius data (and associated error bars) used in the latest analysis of the mass-radius relationship for sub-Neptune size planets. Various fits to these data are shown including an earlier analysis by Lauren Weiss and Geoffrey Marcy (black dashed line) as well as fits for radii <8 RE, <4RE and <1.6RE (solid colored lines). (credit: Wolfgang et al.)
The detailed analysis of the dataset by Wolfgang, Rogers and Ford found that the subset of extrasolar planets whose masses were measured using the TTV method has a definite bias towards lower density planets. This bias had been suspected since a low density planet will have a larger radius than a denser planet with the same mass. And all else being equal, a larger planet is more likely to be detected using the transit method than a smaller planet. When only considering the sample of extrasolar planets with masses determined using precision radial velocity measurements, this team found that the best fit for the data set was a power law with C = 2.7 and γ = 1.3 (i.e. M = 2.7R1.3). Based on their statistical analysis, Wolfgang, Rogers and Ford found that the data were consistent with a Gaussian or bell-curve distribution of actual planet masses with a sigma of 1.9ME at any given radius value. Just as has been suspected, planets with radii less than 4RE display a range of compositions that is reflected as a fairly broad distribution of actual mass values.
In earlier work by Rogers, it was found that there seems to be a transition in planet composition at a radius no larger than 1.6 RE, above which planets are unlikely to be dense, rocky worlds like the Earth and much more likely to be less dense, volatile-rich planets like Neptune (see The Transition from Rocky to Non-Rocky Planets in Centauri Dreams for a full discussion of this work). For the sample of planets considered here with radii less than 1.6 RE, the team found that C = 1.4 and γ = 2.3. Unfortunately, the sample considered by Wolfgang, Rogers and Ford has little good data for planets in this size range and the masses with their large uncertainties tend to span the full range of physically plausible values. As a result, this analysis can not rule out the possibility of a deterministic mass-radius relationship where there is only a very narrow range of actual planet masses for any particular radius value. Recent work by others suggests that these smaller planets tend to have a more Earth-like, rocky composition which could be characterized with a more deterministic mass-radius relationship (see The Composition of Super-Earths in Drew Ex Machina for a discussion of this work).
This new work by Wolfgang, Rogers and Ford represents the best attempt to date to determine the mass-radius relationship for planets smaller than Neptune. While more data of better quality for planets in this size range are needed, it does appear that sub-Neptunes can have a range of different compositions and therefore possess a range of mass values at any given radius. This new relation will be most useful to scientists hoping to get the maximum benefit out of the ever-growing list of Kepler planetary finds where only the radius is known. Much more data will be required to determine more accurately the mass-radius distribution of planets with radii less than 1.6 RE and more precisely characterize the transition from large, rocky Earth-like planets to larger, volatile-rich planets like Neptune.
The preprint of the paper by Wolfgang, Rogers and Ford, “Probabilistic Mass-Radius Relationship for Sub-Neptune-Sized Planet”, can be found here.
LightSail Aloft!
One of the joys of science fiction is the ability to enter into conjectured worlds at will, tweaking parameters here and there to see what happens. I remember talking a few years ago to Jay Lake, a fine writer especially of short stories who died far too young in 2014. Jay commented that while it was indeed wonderful to move between imagined worlds as a reader, it was even more wondrous to do so as a writer. I’ve mostly written non-fiction in my career, but the few times I’ve done short stories, I’ve experienced a bit of this ‘world-building’ sense of possibility.
Even so, it’s always striking how science and technology keep moving in ways that defy our expectations. Take yesterday’s launch of The Planetary Society’s crowd-funded LightSail, which went aloft thanks to the efforts of a United Launch Alliance Atlas V from Cape Canaveral. LightSail violates expectations on a number of fronts. For one thing, the crowd-funding thing, which is a consequence of an Internet era that science fiction writers lustily engaged, but which enters homes on desktop computers that SF had trouble anticipating.
My old saying applies: It’s the business of the future to surprise us, even those of us who keep thinking about the future every day. Another LightSail surprise is its size. Many science fiction tales have covered solar sails dating back to the wondrous “The Lady Who Sailed the Soul,’ from Cordwainer Smith, and Arthur C. Clarke’s “The Wind from the Sun.” We’ve looked at a number of the early stories in these pages over the years. But imagined sails in those days were vast, just like Robert Forward’s gigantic designs, and I can’t think of anyone in those days who anticipated matching up sails with tiny satellites — CubeSats — which have brought space capabilities down from the level of government organizations to small university groups.
Image: The launch of LightSail aboard an Atlas V, as captured by remote camera on May 20. Credit: Navid Baraty / The Planetary Society.
So we have a CubeSat about the size of a loaf of bread that is about to deploy a sail measuring 32 square meters. CubeSats are cheap, and while they can’t mount missions of the complexity of a Juno or a Cassini, I can see a robust future for them. The beauty of The Planetary Society’s effort here is that while CubeSats can be readily orbited, they’ve had no real propulsion capabilities. Until now. So we’re not testing just one sail. We’re testing a broader concept.
Can we get a CubeSat to another planet? I can see no reason why not if it turns out that the solar sail strategy employed here does the job. And if we can get one CubeSat to another planet, we can surely get more. Thus the possibility of future missions designed around ‘swarms’ of CubeSat descendants, deployed on missions in which the components of a much larger spacecraft are effectively distributed among a host of carriers, all driven by solar photon momentum. Perhaps LightSail is the first step in making such a vision a reality.
Remember, too, that LightSail was launched as only one payload among many. Much media attention went into the launch of the X-37B, understandable because the small space plane has been operated with relative secrecy. But the Atlas V carrying LightSail also carried several other CubeSats into space. Contrast this with the early days of the space program, when each rocket lifted a single payload, and consider where miniaturization and improved design have begun to take us. With Mason Peck’s ‘sprites,’ we’re now exploring an even smaller realm some call ‘satellites on a chip,’ where the idea of swarm operations takes on a whole new luster.
We have about four weeks to wait before LightSail attempts deployment of its mylar sail. Even then the craft will quickly be pulled back into the Earth’s atmosphere, returning along the way images and data on spacecraft performance that will flow to the ground stations at Cal Poly San Luis Obispo and Georgia Tech (LightSail was designed by San Luis Obispo firm Stellar Exploration, Inc.) Data return has already begun. You’ll want to follow Jason Davis’ updates on The Planetary Society’s site as this story unfolds. LightSail’s first telemetry file can be downloaded — according to Jason, the early values appear to be ‘nominal or near predicted ranges.’ Here’s the one item that could be problematic:
The team’s only major concern is a line of telemetry showing the indicator switches for solar panel deployment have been triggered. (Look for line 77 in the telemetry file—the “f” is a hexidecimal value indicating all switches were released.) Under normal circumstances, the solar panels do not open until the sail deployment sequence starts, because the sails have a tendency to start billowing out of their storage cavities.
This telemetry reading, however, does not necessarily mean the panels are open. The switches were once inadvertantly triggered during vibration testing, so it’s possible they popped loose during the ride to orbit. We’ll know for sure after flight day four, when we test out the camera system. This is one time we don’t want to see a pretty picture of Earth—it would mean the panels are open.
I’ll be checking in with Jason’s blog frequently during the mission as we get closer to sail deployment. Meanwhile, be aware that the second iteration of LightSail is scheduled for a 2016 flight, this one a full demonstration of solar sailing in Earth orbit, with launch aboard a SpaceX Falcon Heavy to an orbit of about 720 kilometers. The KickStarter campaign supporting the LightSail project can be accessed here. The level of support that has emerged is encouraging indeed, as success with LightSail will energize the entire community of sail researchers.
Enter the ‘Warm Titan’
Our definition of the habitable zone is water-based, focusing on planetary surfaces warm enough that liquid water can exist there. New work by Steven Benner (Foundation for Applied Molecular Evolution) and colleagues considers other kinds of habitable zones, specifically those supporting hydrocarbons, which can be liquids, solids or gases depending on the ambient temperature and pressure. Benner’s work focuses on compounds called ethers that can link together to form polyethers, offering life a chance to emerge and adapt in hydrocarbon environments.
Out of this comes the notion of ‘warm Titans,’ moons with hydrocarbon seas that are not made up of methane. We have no such worlds in our Solar System, and they needn’t be moons of gas giants to fit the bill. Think of them, as this Astrobio.net news release does, as being oily Earths drenched in hydrocarbons like propane or octane. Although they do not appear in any genetic molecules on Earth, ethers may be the key to fill the function of DNA and RNA on such worlds.
The nucleobases in the four-letter code of DNA and RNA can mutate even as the molecule’s form is retained, and out of this come the proteins that help life interact and adapt with its environment. Like DNA, ethers show repeating elements, in this case of carbon and oxygen, in their chemical backbones. But unlike DNA and RNA, they have no outward negative charge of the kind that lets them dissolve and float freely so they can interact with other biomolecules. Says Benner:
“This is the central point of the ‘polyelectrolyte theory of gene,’ which holds that any genetic biopolymer able to support Darwinian evolution operating in water must have an ever-repeating backbone charge. The repeating charges so dominate the physical behavior of the genetic molecule that any changes in the nucleobases that influence genetic information have essentially no significant impact on the molecule’s overall physical properties.”
Molecules like DNA and RNA cannot dissolve in a hydrocarbon ocean, making them unable to provide the necessary interactions on worlds like Titan. But ethers, strung together in complex polyethers, while they lack an outward charge, do have internal charge repulsions that allow small parts of the molecule to function in ways similar to DNA and RNA nucleobases.
Image: An artist’s impression of the low-lit surface of Titan under the moon’s thick, orange haze, with liquid hydrocarbons pooling and eroding the surface much like water on Earth. Credit: Steven Hobbs (Brisbane, Queensland, Australia).
Benner’s experiments with ethers show that they are not soluble when we get down to temperatures as low as Titan’s, making Saturn’s largest moon an unlikely venue for such life. But while methane has a narrow liquid range (between -184 and -173 degrees Celsius), we can still put ethers to work in warmer hydrocarbon oceans. Thus the emergence of the ‘warm Titan,’ a world perhaps covered with propane instead of methane oceans that can stay liquid over a broad range (-184 degrees Celsius to -40 degrees). Octane turns out to be even better, not freezing until it reaches -57 degrees Celsius or vaporizing until it hits a temperature of 125 degrees.
Thus hydrocarbon molecules larger than methane come to the rescue. Once again we reconsider the notion of a habitable zone. Certainly in terms of life that we are familiar with, liquid water at the surface is a prerequisite. But as we’ve seen on the icy moons of our system’s gas giants, oceans can provide subsurface environments where life could conceivably emerge. Now we have to consider a hydrocarbon habitable zone where propane or octane can exist in a liquid state. “Virtually every star,” says Benner, “has a habitable zone for every solvent.”
The paper is Christopher et al., “Solubility of Polyethers in Hydrocarbons at Low Temperatures. A Model for Potential Genetic Backbones on Warm Titans,” Astrobiology Vol. 15, Issue 3 (11 March 2015). Thanks to Ivan Vuletich for the pointer to this one.
Exoplanets: The Hunt for Circular Orbits
If you’re looking for planets that may be habitable, eccentric orbits are a problem. Vary the orbit enough and the surface goes through extreme swings in temperature. In our own Solar System, planets tend to follow circular orbits. In fact, Mercury is the planet with the highest degree of eccentricity, while the other seven planets show a modest value of 0.04 (on a scale where 0 is a completely circular orbit — Mercury’s value is 0.21). But much of our work on exoplanets has revealed gas giant planets with a wide range of eccentricities, and we’ve even found one (HD 80606b) with an eccentricity of 0.927. As far as I know, this is the current record holder.
These values have been measured using radial velocity techniques that most readily detect large planets close to their stars, although there is some evidence for high orbital eccentricities for smaller worlds. Get down into the range of Earth and ‘super-Earth’ planets, however, and the RV signal is tiny. But a new paper from Vincent Van Eylen (Aarhus University) and Simon Albrecht (MIT) goes to work on planetary transits. It’s possible to work with Transit Timing Variations to make inferences about eccentricity, but these appear only in a subset of transiting systems.
Instead, van Eylen and Albrecht look at transit duration. The length of a transit can vary depending on the eccentricity and orientation of the orbit. By measuring how long a planetary transit lasts, and weighing the result against what is known about the properties of the star, the eccentricities of the transiting planets can be measured, as explained in the paper:
Here we determine orbital eccentricities of planets making use of Kepler’s second law, which states that eccentric planets vary their velocity throughout their orbit. This results in a different duration for their transits relative to the circular case: transits can last longer or shorter depending on the orientation of the orbit in its own plane, the argument of periastron (ω)… Transit durations for circular orbits are governed by the mean stellar density (Seager & Mallen-Ornelas 2003). Therefore if the stellar density is known from an independent source then a comparison between these two values constrains the orbital eccentricity of a transiting planet independently of its mass…
Using these methods, the researchers have measured the eccentricity of 74 small extrasolar planets orbiting 28 stars, discovering that most of their orbits are close to circular. The systems under study were chosen carefully to avoid false positives — the team primarily used confirmed multi-transiting planet systems around bright host stars, and pulled in asteroseismological data — information on stellar pulsations — to help determine stellar parameters. Asteroseismology can refine our estimates of a star’s mass, radius and density. The stars in the team’s sample have all been characterized in previous asteroseismology studies.
Image: Researchers measuring the orbital eccentricity of 74 small extrasolar planets have found their orbits to be close to circular, similar to the planets in the Solar System. This is in contrast to previous measurements of more massive exoplanets where highly eccentric orbits are commonly found. Credit: Van Eylen and Albrecht / Aarhus University.
No Earth-class planets appear in the team’s dataset, but the findings cover planets with an average radius of 2.8 Earth radii, while orbital periods range from 0.8 to 180 days. Van Eylen and Albrecht conclude that it is plausible that low eccentricity orbits would be common in solar systems like ours, a finding that would have ramifications for habitability and the location of the habitable zone.
Interestingly, when weighed against parameters like the host star’s temperature and age, no trend emerges. But in systems with multiple transiting planets on circular orbits, Van Eylen and Albrecht believe that the density of the host star can be reliably estimated from transit observations. This information can help to rule out false positives, a technique they use to validate candidate worlds in several systems — KOI-270, now Kepler-449, and KOI-279, now Kepler-450, as well as KOI-285.03, now Kepler-92d, in a system with previously known planets.
The work has helpful implications for upcoming space missions that will generate the data needed for putting these methods to further use:
We anticipate that the methods used here will be useful in the context of the future photometry missions TESS and PLATO, both of which will allow for asteroseismic studies of a large number of targets. Transit durations will be useful to confirm the validity of transit signals in compact multi-planet systems, in particular for the smallest and most interest[ing] candidates that are hardest to confirm using other methods. For systems where independent stellar density measurements exist the method will also provide further information on orbital eccentricities.
The TESS mission (Transiting Exoplanet Survey Satellite) is planned for launch in 2017, and is expected to find more than 5000 exoplanet candidates, including 50 Earth-sized planets around relatively nearby stars. PLATO (PLAnetary Transits and Oscillations of stars) will likewise monitor up to a million stars looking for transit signatures, with launch planned by 2024.
The paper is Van Eylen and Albrecht, “Eccentricity from transit photometry: small planets in Kepler multi-planet systems have low eccentricities,” accepted for publication at The Astrophysical Journal (preprint). An Aarhus University news release is available.
Spacecoach on the Stage
I’m glad to see that Brian McConnell will be speaking at the International Space Development Conference in Toronto this week. McConnell, you’ll recall, has been working with Centauri Dreams regular Alex Tolley on a model the duo call ‘Spacecoach.’ It’s a crewed spacecraft using solar electric propulsion, one built around the idea of water as propellant. The beauty of the concept is that we normally treat water as ‘dead weight’ in spacecraft life support systems. It has a single use, critical but heavy and demanding a high toll in propellant.
The spacecoach, on the other hand, can use the water it carries for radiation shielding and climate control within the vessel, while crew comfort is drastically enhanced in an environment where water is plentiful and space agriculture a serious option. Along with numerous other benefits that Brian discusses in his recent article A Stagecoach to the Stars, mission costs are sharply reduced by constructing a spaceship that is mostly water. McConnell and Tolley believe that cost reductions of one or two orders of magnitude are possible. Have a look, if you haven’t already seen it, at Alex’s Spaceward Ho! for an imaginative look at what a spacecoach can be.
ISDC is a good place to get this model before an audience of scientists, engineers, business contacts and educators from the military, civilian, commercial and entrepreneurial sectors. ISDC 2014 brought over 1000 attendees into the four-day event, and this year’s conference brings plenary talks and speakers from top names in the field: Buzz Aldrin, Charles Bolden, Neil deGrasse Tyson, Peter Diamandis, Lori Garver, Richard Garriott, Bill Nye, Elon Musk and more. My hope is that a concept as novel but also as feasible as the spacecoach will resonate.
Image: Ernst Stuhlinger’s concept for a solar powered ship using ion propulsion, a notion now upgraded and highly modified in the spacecoach concept, which realizes huge cost savings by its use of water as reaction mass. This illustration, which Alex Tolley found as part of a magazine advertisement, dates from the 1950s.
Towards Building an Infrastructure
We have to make the transition from expensive, highly targeted missions with dedicated spacecraft to missions that can be flown with adaptable, low-cost technologies like the spacecoach. Long-duration missions to Mars and the asteroid belt will be rendered far more workable once we can offer a measure of crew safety and comfort not available today, with all the benefits of in situ refueling and upgradable modularity. Building up a Solar System infrastructure that can one day begin the long expansion beyond demands vehicles that can carry humans on deep space journeys that will eventually become routine.
The response to the two spacecoach articles here on Centauri Dreams has been strong, and I’ll be tracking the idea as it continues to develop. McConnell and Tolley are currently working on a book for Springer that should be out by late summer or early fall. You can follow the progress of the idea as well on the Spacecoach.org site, where the two discuss a round-trip mission from Earth-Moon Lagrange point 2 (EML-2) to Ceres, a high delta-v mission in which between 80 and 90 percent of the mission cost is the cost of delivering water to EML-2.
The idea in this and other missions is to use a SpaceX Falcon 9 Heavy to launch material to low-Earth orbit, with a solar-electric propulsion spiral out to EML-2 (the crew will later take a direct chemical propulsion trajectory to EML-2 to minimize exposure time in the Van Allen belts). The water cost is about $3000 per kilogram. The Falcon 9 Heavy should be able to deliver 53,000 kilograms to low-Earth orbit per launch. McConnell and Tolley figure about 40,000 kilograms of this will be water, while the remainder will be other equipment including the module engines and solar arrays. From EML-2, various destinations can be modeled, with values adjustable within the model so you can see how costs change with different parameters.
The online parametric model has just been updated to calculate mission costs as a function of the number of Falcon Heavy 9 launches required. You can see the new graph below (click on it to enlarge). At a specific impulse of 2000s or better for the solar-electric power engines, only two launches are required for most missions, one taking the crew direct to EML-2, the other carrying the water and durable equipment on a spiral orbit out from LEO. It is only the most ambitious destinations like Ceres that require three launches. At $100 million per launch, even that mission is cheap by today’s spaceflight standards.
Brian notes in a recent email that the launches do not need to be closely spaced, because the spiral transfer from LEO to EML-2 takes months to complete. The crew only goes when everything else is in place at EML-2. For more on this model, see spacecoach.org. I’ll be interested to hear how the idea is received at ISDC, and how the upcoming publication of the spacecoach book helps to put this innovative design for interplanetary transport on the map.
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