Into the Interstellar Void

by Paul Gilster on September 1, 2010

We often think about interstellar probes only in the context of what they find at the end of their journeys — astrobiologically interesting planets seem to be the whole story. But not so fast. As Ian Crawford (University of London) notes in a recent paper, there are quite a few fascinating — and indeed critical — things we need to learn about interstellar space itself, in this case what is known as the Local Interstellar Medium (LISM). Crawford, who has been analyzing these matters for the Project Icarus team, notes how much we’ve learned about the LISM since the Daedalus days.

The new paper grows out of Crawford’s presentation at the British Interplanetary Society symposium ‘Project Daedalus – Three Decades On,’ which was held last September in London. It reflects his thinking on interstellar probes in relation to planetary and stellar studies and astrobiology as well as the nature of the medium through which the probe will fly. But today I want to focus on the LISM because what we might erroneously consider to be relatively empty space turns out to be challenging indeed.

Not So Empty Space

After all, at the kinds of speeds we’re discussing for an interstellar probe, somewhere in the range of 10 percent of lightspeed at minimum (Daedalus aimed at 12 percent), what seems empty void can be a minefield. Consider just a few of the issues Crawford raises here:

Direct measurements of the interstellar dust density, and the size distribution of dust particles, will be especially important because sub-micron sized dust particles will erode exposed surfaces (see the analysis performed for Daedalus by Martin [16]), and larger particles (i.e. larger than a few tens of microns) may, if present in the LISM, cause a catastrophic failure of the vehicle. Determining some of the other properties of the LISM will also be important for longer term planning of other interstellar propulsion concepts – for example, determining the LISM hydrogen density, and its ionisation state, will be important for assessing the future practicality of interstellar ramjets…

We’d better, in other words, get up to speed about the interstellar medium before we launch any true interstellar craft, and that means a series of early missions of a precursor nature that can tell us what to expect. Crawford notes that the Sun is currently located close to the boundary of a small low-density interstellar cloud known as the Local Interstellar Cloud (LIC), one of several such clouds within several parsecs of our Solar System — indeed, one study identifies seven such clouds within 5 parsecs of Sol. These are found within the Local Bubble, about which this:

These [clouds] are immersed in the very empty (nH ~ 0.005 cm-3) and probably hot (T ~ 106 K) Local Bubble (LB) in the interstellar medium, which extends for about 60-100 parsecs from the Sun in the galactic plane before denser interstellar clouds are encountered (at high galactic latitudes the LB appears to be open, forming a chimney-like structure in the interstellar medium which extends into the galactic halo…)

We can learn a lot about these matters, including the properties of the Local Interstellar Cloud, by spectroscopic studies of interstellar absorption lines toward nearby stars. Investigations into our Solar System’s interactions with the heliosphere are also useful, but we’ll need to augment these with direct measurements of the interstellar materials just beyond the heliopause, and that means developing the capability to get space probes to distances up to 1000 AU or more.

A Rationale for Probing the Void

Beyond that, and having taken these findings in account in its design, a true interstellar probe would be a priceless tool for measuring everything from dust density and composition to the interstellar magnetic field strength enroute. Crawford points out that a spacecraft moving at 0.1c could do daily measurements 17 AU apart (roughly half the radius of the Solar System) which would offer unprecedented knowledge of the structure of the Local Interstellar Medium.

Centauri A and B evidently lie beyond the Local Interstellar Cloud, although the line of sight from Earth is dominated by another interstellar cloud known as the G cloud. An interstellar mission to this system would tell us whether the Sun is actually embedded in the Local Interstellar Cloud or just outside it in the region where the LIC is interacting with the G cloud. Conceivably a Centauri mission would thus get to sample two different interstellar clouds, along with the low density material between them. As Crawford writes, we would receive a windfall of data:

If the Sun does lie within the LIC, then a mission to α Cen would sample the outer layers of the LIC, an interval of low density LB material, the edge of the G cloud, and the deep interior of the G cloud. This would sample one of the most diverse ranges of interstellar conditions of any mission to another star located with 5 pc of the Sun, as most other potential targets lie within the LIC… Even if the Sun lies just outside the LIC (as argued by Redfield and Linsky [8]), the trajectory to α Cen would still permit detailed observations of the boundary of the G cloud (and its possible interaction with the LIC), and determine how its properties change with increasing depth into the cloud from the boundary.

Shielding Against the Medium

Needless to say, such a pathfinder mission would help in the planning of all follow-up interstellar missions. At 10 percent of lightspeed, a probe would have to be shielded against damage from high speed collisions with dust in the interstellar medium, a topic both the Daedalus and Icarus designers have had to take into consideration. It will be fascinating to see how the shielding options will be modified between Daedalus (1970s) and today’s Icarus design.

As I mentioned above, Crawford’s paper delves deeply into the rationale for an interstellar mission, going beyond the interstellar medium question to address planetary and stellar studies and astrobiology. It’s a fascinating rationale for undertaking these studies and continuing to advocate precursor missions. And note this final caveat re Alpha Centauri as a destination:

The relative proximity of α Cen, together with its interesting interstellar sightline and the presence of stars of three different spectral types, makes it an attractive target for humanity’s first interstellar mission. However, as the bulk of the scientific benefits of interstellar spaceflight pertain to planetary science and astrobiology, a final prioritization must await future developments in the detection of planetary systems around the nearest stars. Fortunately, expected advances in astronomical instrumentation over the next century should ensure that a comprehensive list of prioritized targets is available well before rapid interstellar travel is technically feasible.

The paper is Crawford, “The Astronomical, Astrobiological and Planetary Science Case for Interstellar Spaceflight,” published in the Journal of the British Interplanetary Society Vol. 62 (2009), pp. 415-421 (preprint).

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HR 8799b: Low Temperatures, Surprising Spectrum

by Paul Gilster on August 31, 2010

Photos of the Earth from a significant distance are always fascinating, dating back to the spectacular shot of the rising Earth over lunar mountains taken by Apollo 8 in December of 1968. The image below, showing Earth and its Moon, comes from the Messenger spacecraft, taken at a distance of some 183 million kilometers. I see things like this and think about our future imaging of exoplanets, and the possibilities of space-based missions that can study their atmospheres. Learning how we look helps us understand what to look for around other stars, and also offers a bit of the ‘wow’ factor.

We’re nowhere near this kind of imaging with exoplanets but we’re getting better all the time, and that’s providing some curious results. HR 8799 is the interesting young system some 130 light years from Earth (in Pegasus) that has yielded direct images of three planets. Some eighteen months after the discovery of the system here, we’ve now managed to get a spectrum of HR 8799b, useful for what it can tell us about temperatures, clouds, and chemical composition.

Image: The Earth/Moon system as seen from the Messenger spacecraft. Credit: NASA/JHU-APL.

HR 8799b shows little or no methane in its atmosphere, a fact that used in conjunction with models of low-temperature atmospheres yields an estimate of 1200 K (926 degrees Celsius or 1700 Fahrenheit) as the coolest possible temperature for this young object. Oddly, the planet ought to be some 400 Kelvin cooler than what the new measurements show, extrapolating from the amount of energy the planet emits and its assumed age.

The culprit? Scientists at the University of Hawaii, who made these measurements at the Keck Observatory, think dust in the planet’s atmosphere must be to blame. If you change the computer models of gas giant planets to incorporate thick dust clouds, you wind up with essentially the same result. Thus direct spectroscopy of exoplanets may be telling us that young gas giants are cloudier than we had realized. The results are a reminder, as if we needed one, that exoplanets will continue to surprise us, especially given the fact that direct imaging of these worlds has just begun. Only six planets — three in this system — have been directly imaged. Says Michael Liu, co-author of the paper on this work:

“Direct studies of extrasolar planets are just in their infancy. But even at this early stage, we are learning they are a different beast than objects we have known about previously.”

True enough, and we should consider the advantages that direct imaging puts on the table. For one thing, it gives us the opportunity to study planets in wide orbits — the planets around HR 8799, discovered by Christian Marois (Herzberg Institute of Astrophysics) and team in 2008, orbit the star with semimajor axes of 24 AU, 38 AU and 68 AU respectively, with the interesting possibility of a 1:2:4 orbital resonance (not yet confirmed). At 68 AU, HR 8799b is too far from its star for radial velocity detection, but direct imaging gives us both planet and spectrum.

Image: Keck II image of the young extrasolar planet HR 8799b, seen as the point source in center of image. The bright light from the parent star HR 8799 is seen in the background in yellow/red and has been removed in an annular region centered on the planet. Credit: Brendan Bowler and Michael Liu, IfA/Hawaii.

But are the HR 8799 objects planets or brown dwarfs? At least one recent paper has suggested the latter, but this work advocates a younger age for the system and sees the objects as hot, young planets. Even so, explicit comparisons with brown dwarfs are useful, and the authors note that these objects are massive analogs of giant planets, objects that offer up similar physics, radii, effective temperatures and cooling histories. The comparison is intriguing, and allows the researchers to peg HR 8799b as a unique object:

Altogether, our spectral and photometric comparisons to field brown dwarfs suggest a spectral type between L5 and T2 for HR 8799 b. Although peculiar compared to most L and T dwarfs in the field, the planet’s photometry is consistent with the reddest field L dwarfs. These results imply that HR 8799 b is the lowest-mass L/T transition object currently known.

And later, the paper refers to the planet this way:

These observations, combined with spectroscopy of HR 8799 c and d, will elucidate the physical properties of this emerging class of low-mass objects which are characterized by low surface gravities, low luminosities, and exceptionally cloudy atmospheres.

‘An emerging class of low-mass objects’ is a phrase that reminds us how much we’ve learned in the past decade about small, cool stars and planets that skirt the boundary between planet and star. The paper is Bowler et al., “Near-Infrared Spectroscopy of the Extrasolar Planet HR 8799b,” accepted by The Astrophysical Journal (preprint).

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Poul Anderson’s Answer to Fermi

by Paul Gilster on August 30, 2010

Enrico Fermi’s paradox has occupied us more than occasionally in these pages, and for good reason. ‘Where are they,’ asked Fermi, acknowledging an obvious fact: Even if it takes one or two million years for a civilization to develop and use interstellar travel, that is but a blip in terms of the 13.7 billion year age of the universe. Von Neumann probes designed to study other stellar systems and reproduce, moving outward in an ever expanding wave of exploration, could easily have spread across the galaxy long before our ancestors thought of building the pyramids.

Where are they indeed. Kelvin Long, one of Project Icarus’ most energetic proponents, recently sent me Poul Anderson’s thoughts on the subject. I probably don’t need to tell this audience that Anderson was a science fiction author extraordinaire. His books and short stories occupied vast stretches of my youth, and I still maintain that if you want to get not so much the tech and science but the sheer wonder of the interstellar idea, you can tap it in its pure form in his writing. Poul was also the author of Tau Zero, the novel which gave our Tau Zero Foundation its name, and we’re delighted to have Karen Anderson, Poul’s wife, as a valuable part of the organization.

In a letter to the Journal of the British Interplanetary Society in 1986, Anderson sketched the reasons why Fermi was asking his question, citing the von Neumann probes mentioned above, and noting that while interstellar travel was likely hard enough that civilizations practicing it might be rare, all it takes is one to eventually fill the galaxy with its artifacts. He found the notion that Fermi could be answered by saying we are the only high-technology civilization unlikely, but his reason for writing was to offer an entirely different suggestion based on practicality.

Let’s assume a stable civilization arises that achieves extremely long lifespans, if not physical immortality — this may be too big an assumption, but there are those arguing that our successors may be a form of artificial intelligence for whom this could apply. Such a civilization naturally would explore its neighborhood, moving out to local star systems and gradually spreading beyond. Anderson saw this as a problem: The farther from home you go, the longer it takes you to return information. The galaxy itself is 100,000 light years wide, he noted, and that means most information would be utterly outdated by the time it spread throughout the disk.

And what of this self-replication idea? Anderson saw problems there too:

…self-replication would probably already have broken down. Quantum mechanics alone guarantees gradual degradation of the programmes, an accumulation of ‘mutations’ generation by generation — without any natural selections to winnow out the unfavourable majority — until ultimately every machine is useless and every line of its descent extinct.

Can we conjecture a kind of self-healing technology that extends to fixing these errors to maintain the integrity of the expansion? Perhaps, but the data flaw remains paramount:

…long before this has happened, the sphere of exploration will include so many stars that the data flow from them saturates the processing capacity of the present civilisation. After all, with some 1012 stars in the Galaxy, a small fraction amounts to a huge number. Moreover, while they may fall into categories with predictable properties, we are learning in our own back yard that every planet any of them may have is a world, replete with mysteries and surprises. Every life-bearing planet offers endless matter for research, especially since the life will always be changing, evolving.

In short, Fermi’s ‘they’ are not here because they are kept too busy within a few score light-years of their various homes.

If Anderson is right, then we can imagine a galaxy in which technological civilizations arise here and there, each of them gradually filling a sphere of exploration and colonization until a kind of equilibrium is reached and there is no practical advantage to pushing further. Earth, then, could be seen as being in the spaces between such civilizations, not yet aware of their existence, preparing over the next few centuries to begin its own expansion to nearby stars.

Is the galactic population sufficiently dense that such ‘bubbles’ of expansion ever meet? Or is SETI our only chance to confirm the idea that the galaxy has brought forth other technological civilizations? If the latter, we may know them only by the whisperings of their local traffic, exchanging information and perhaps speculating as we do about still more distant suns.

Anderson’s letter appeared in JBIS Vol 39 No (7 July 1986), p. 327.

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SETI and the ‘Long Stare’

by Paul Gilster on August 27, 2010

It’s been a week with an exoplanet focus, what with the interesting Kepler results yesterday and the five, or perhaps seven, planets found around the same star by the HARPS instrument. But I can’t close the week without recourse to Seth Shostak’s recent comments on biological versus machine intelligence. Paul Davies took much the same tack in his recent book The Eerie Silence (Houghton Mifflin Harcourt, 2010), arguing that any civilization we encounter will likely be composed of intelligent machines. Shostak thinks SETI should take that seriously.

Searching for Doppelgängers

Right now we’re searching for what Shostak calls ‘doppelgängers of humans’ — i.e., SETI has focused on places that could support life forms that do more or less what we do, which includes not only using radio to communicate, but much broader traits like living for finite lifetimes, following basic biochemical dictates and being subject to evolution. That biases the search toward places that could sustain life as we know it, a reasonable bias in my opinion, but one that may not take our own development into consideration. After all, we may be living in the short window between developing radio and building true artificial intelligence.

Suppose, Shostak asks in this BBC story (with accompanying audio interview), we develop true AI by the end of this century. What would happen next? This is where things get interesting. Shostak:

At some point they may just pick up and leave, at least some of them, maybe most of them… If you’re a machine, you’re interested in only two things I can think of. And that is matter and energy, because those facilitate whatever it is you’re doing. And matter and energy are not in particularly great supply here.

The result: AI lifeforms go to places more suited for their kind of existence, which could include the galactic core or, perhaps, the neighborhood of a hot, young star. Shostak is arguing that we should allocate a small percentage of our observing time — perhaps up to ten percent — for searching in places AI is more likely to call home. Thus far we’ve searched fewer than a thousand star systems intensively, and our all-sky search is of necessity unable to linger on a target. We’re new at the game, in other words, but let’s tune up our target list.

Is Biology the Issue?

The problem with SETI is that we’re forced to make assumptions about how aliens would operate, an issue that continues to bedevil the field today. Recently we’ve looked at the Benford brothers’ call for a different kind of search, one homing in on the kind of interstellar beacons an alien culture would be likely to create. The discipline is rife with new ideas as we try to figure out the basic parameters that any intelligent species would have to possess in our galaxy. But getting into an alien mind, much less an artificial one, is tricky business. The best we can do is build on our knowledge of physics and extrapolate a line of rational behavior.

The Benfords extrapolate from both physics and economics to argue that an interstellar beacon will likely use short, powerful bursts rather than continuous broadcasts. But SETI has lacked the ‘long stare’ needed to find such a signal. To me, the issue is less AI vs. biology than it is continuous survey vs. pinpoint search. The SETI League’s Project Argus, discussed in these pages recently, is an attempt to set up 5000 amateur receiving stations to implement the ‘long stare.’ It would be a low sensitivity survey, but as the cost of equipment drops and its power increases, it should become possible to implement at the amateur level, and it could be a powerful adjunct to more sophisticated (and focused) searches.

Methods like these could detect an alien beacon, whether built by machine or biological beings, out to several hundred light years, with the sphere of detection growing as we replace older stations with newer technology. They’re a great complement to higher-powered instruments. If we’re looking for beacons, a continuous, high-sensitivity stare along the galactic plane is a sensible way to proceed. But there’s a place for minimal assumptions and broad coverage too, and the advantage of an all-sky survey is that it takes what it finds, which might involve the kind of surprises SETI is made for.

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New Kepler Planets in Resonance

by Paul Gilster on August 26, 2010

Somewhere around 2000 light years away in the direction of the constellation Lyra is a Sun-like star orbited by at least two Saturn-class planets. What’s interesting about this news, as just discussed in the Kepler press conference I’ve been listening to this afternoon, is that for the first time we’ve detected and confirmed more than one planet around a single star using the transit method. But much more important, transit timing variations — the leads and lags of the two planets as they transit the star as seen from Kepler — can be used to tease out new and significant information.

Kepler-9b and 9c mark the first clear detection of transit timing variations by Kepler, allowing us to study the gravitational interactions between the planets involved. And that’s useful stuff: We see two planets in a 2:1 orbital resonance, one with a 19.2-day orbit, the other with a 38.9-day orbit. As the inner planet completes two orbits, the outer planet completes one. The variations in transit time (TTV) help us establish the mass of the two planets, showing that both have a mass and radius slightly smaller than Saturn. Peg size and mass and you can derive planetary density.

Image: Lightcurves of the Kepler 9 system. Credit: Matthew Holman (Harvard-Smithsonian Center for Astrophysics).

We’ve known for some time that transit timing variations should be a useful part of the exoplanet toolbox, but seeing them working in practice is a powerful proof of concept. The hope, of course, is that similar TTV methods can be put to work on smaller worlds down to terrestrial mass planets in the habitable zones of their stars. In fact, in this case, the Kepler 9 system sports a candidate that is 1.5 times the radius of Earth, a possible super-Earth whose barely detectable signature is not yet sufficient for us to declare the planet confirmed.

The other aspect of the Kepler 9 system that received discussion at the news conference was what it can tell us about planetary formation theories. These planets orbit well within the orbit of Mercury in our own system, and the assumption is that they could not have formed there. Planetary migration resulting in a 2:1 orbital resonance is something that points back to an earlier set of conditions whose nature will take many more precision measurements — in this and many other systems — to understand. But ultimately, the way a planetary system looks today can reveal much about its history.

Is there good news in this for terrestrial worlds? Not yet, but there is at least a hint. Alycia Weinberger (Carnegie Institution of Washington) made this case at the conference:

Ultimately Kepler will find many multiple planet systems. We will know how many systems show these resonances, how often and when different kinds of migration occurred while planets were forming. Our 1.5-Earth radius candidate, if real, survived whatever migration it and the other planets went through, a fact that bodes well for systems with substantial migration.

In other words, the Kepler 9 system may eventually tell us that the movement of gas giants into the inner system does not necessarily spell the doom of smaller worlds there, if and when we confirm the existence of the super-Earth. Weinberger goes on:

It would be interesting to know that planetary systems with different histories can produce low mass planets or planets more Earth-like in size. Resonances like those Kepler-9b and 9c demonstrate can ensure stability and produce planetary systems that last billions of years. Frequent resonances, in other words, are good news for low mass planets, giving them stable orbits. And transit timing variations can help us deduce the masses of the planets involved.

Not a bad haul for this interesting system, about which we’ll learn more as the Kepler mission progresses. As I said above, produce figures on the size and mass of a planet and you can derive its density, helping astronomers understand its composition, from gaseous to rocky or water planets. We’ll see what transpires with that candidate super-Earth — if it’s there, it’s in a scorching 1.6-day orbit, another hellish world singed by its star. But perhaps Kepler 9 helps point the way to a future news conference when an Earth-like planet in the habitable zone will be announced.

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HD 10180: A Planetary Harvest

by Paul Gilster on August 25, 2010

In a sense the planets discovered around the Sun-like star HD 10180 are no surprise. We’ve long assumed that planetary systems with numerous planets were common. We lacked the evidence, it’s true, but that could be put down to the limitations of the commonly used radial velocity method, which favors massive worlds close to their stars. But we’re getting much better at radial velocity work and, using instruments like the HARPS spectrograph at the European Southern Observatory’s La Silla (Chile) telescope, we’re teasing out ever more exquisite signals from distant systems. More and more multiple-planet scenarios are in our future.

Noting that high-precision radial velocity surveys are now able to detect planets down to roughly 1.9 Earth masses, the paper on the HD 10180 work frames the situation this way:

Preliminary results from the HARPS survey are hinting at a large population of Neptune-like objects and super-Earths within ∼0.5 AU of solar-type stars (Lovis et al. 2009). Moreover, hundreds of small radius candidate planets have been announced by the Kepler Team (Borucki & the Kepler Team 2010). Clearly, the exploration of the low-mass planet population has now fully started, and will become the main focus of the field in the coming years.

But five planets at one go is still an eye-opener, especially when you consider that two others are also possible here. It took six years of study of this star, some 127 light years away in the constellation Hydrus, to bag the five leading signals, representing planets like Neptune in being between 13 and 25 Earth masses. These worlds circle their star in orbits that range from six to 600 days at distances between 0.06 and 1.4 AU. Some accounts are citing similarities with our Solar System because of the number of worlds, but we might just as well note the differences, including the crowding of the inner system and the presence of massive planets there.

Image: This wide-field image shows the sky around the star HD 10180, which appears as a fairly bright star just below the centre. The picture was created from photographs taken through red and blue filters and forming part of the Digitized Sky Survey 2. The field of view is approximately three degrees across. The coloured halos around the stars are artifacts of the photographic process and are not real. The remarkable planetary system around this star is far too faint and close in to be visible in this image. Credit: ESO, Digitized Sky Survey 2. Acknowledgement: Davide De Martin.

The paper notes that systems like this open up new realms of study:

It is expected that the characterization of planetary system architectures, taking into account all objects from gas giants to Earth-like planets, will greatly improve our understanding of their formation and evolution. It will also allow us to eventually put our Solar System into a broader context and determine how typical it is in the vastly diverse world of planetary systems. The characterization of a significant sample of low-mass objects, through their mean density and some basic atmospheric properties, is also at hand and will bring much desired insights into their composition and the physical processes at play during planet formation.

Those two additional worlds, whose existence Christophe Lovis (Observatoire de Genève), lead author of the cited paper, says is supported by solid evidence, include a 65 Earth-mass gas giant in a 2200-day orbit and a world that, if confirmed, would be the least massive exoplanet yet discovered, with a mass of about 1.4 times that of the Earth. This one is not exactly a candidate for astrobiology, though, orbiting the host star at a distance so close (0.02 AU) that a planetary year lasts a mere 1.18 Earth days. This ESO news release likens the possible world to the rocky inferno CoRoT-7b.

If there were a gas giant like Jupiter in this system, we should have evidence of it. And note that the orbits of all these planets seem to be almost circular. Says Lovis:

“Systems of low-mass planets like the one around HD 10180 appear to be quite common, but their formation history remains a puzzle.”

Although HD 10180 presents us with one of fifteen planetary systems known to have at least three worlds, that number will grow quickly. The new planets were found in a radial velocity survey of about 400 bright FGK stars in the solar neighborhood using HARPS, and the paper notes that ‘many new systems are about to be published.’ We’re homing in on the ability to derive statistical properties of the low-mass planet population, a new phase in the exoplanet hunt, one that focuses on complex planetary systems rather than individual planets.

From the paper:

The HD 10180 system shows the ability of the RV technique to study complex multi-planet systems around nearby solar-type stars, with detection limits reaching rocky/icy objects within habitable zones. Future instruments like VLT-ESPRESSO will build on the successful HARPS experience and carry out a complete census of these low-mass systems in the solar neighborhood, pushing towards planets of a few Earth masses at 1 AU.

The paper, submitted to Astronomy & Astrophysics, is Lovis et al., “The HARPS search for southern extra-solar planets. XXVII. Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems” (full text).

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Twin Suns May Spell Disaster

by Paul Gilster on August 24, 2010

The image of double suns rising over the planet Tatooine from the first Star Wars movie never quite goes away. I remember watching the film in a theater about a week after its release, being dazzled by the visuals but thinking that a planet in an orbit around both stars of a binary would have to be well outside the habitable zone. I didn’t believe in Tatooine, in other words, though now I’m a bit more circumspect. A couple of years ago Cheongho Han (Chungbuk National University, Korea) wrote a paper suggesting that microlensing might be of use in finding a planet fitting this description, if indeed such a planet exists.

Then yesterday Massimo Marengo dropped me a note about new work he has been involved in that puts a damper on the idea of terrestrial worlds in such settings. Long-time Centauri Dreams readers will remember Marengo, whose fascinating work on Epsilon Eridani we’ve covered in these pages on several previous occasions. Now at Iowa State University, the astrophysicist has been studying tight double-star systems, using data harvested from the Spitzer Space Telescope.

Working with principal investigator Jeremy Drake (Harvard-Smithsonian Center for Astrophysics), Marengo and team are learning that the problem with tight binaries is the chance for planetary collisions. We’re talking about a class of binaries called RS Canum Venaticorums (RS CVns) that are separated by something on the order of 3.2 million kilometers, roughly two percent of the distance between the Earth and the Sun. That produces orbits of just a few days and tidal lock, with each star presenting the same face to the other.

Image: This artist’s concept illustrates a tight pair of stars and a surrounding disk of dust — most likely the shattered remains of planetary smashups. Using NASA’s Spitzer Space Telescope, the scientists found dusty evidence for such collisions around three sets of stellar twins (a class of stars called RS Canum Venaticorum’s or RS CVns for short). Credit: NASA/JPL-Caltech.

Imagine two stars similar to the Sun in size and about as old as the Sun when life first evolved on the Earth. They’re possessed of strong magnetic fields and giant spots, the result of their fast spin, and the magnetic fields, in turn, drive powerful stellar winds that slow the stars and pull them closer together over time. Now things get tricky, for the new work suggests that the gravitational influences of the stellar pair continually change as the stars approach each other, causing planets and other objects circling the stars to experience collisions.

Says Marc Kuchner (NASA GSFC):

“These kinds of systems paint a picture of the late stages in the lives of planetary systems. And it’s a future that’s messy and violent.”

Indeed. And the evidence from Spitzer seems tight. The instrument can see the glow of hot dusky disks around three tight binary systems matching this description. The thinking is that the dust found here would normally have dissipated from stars at this level of maturity. Something, in other words, is causing fresh dust to be created, implying a chaotic process. Planetary collisions are the most likely candidate.

Image: Spitzer’s cameras, which take pictures at different infrared wavelengths, observed the signatures of dust around three close binary systems. Data for one of those systems are shown here in orange. Models for the stars and a surrounding dusty disk are shown in yellow and red, respectively. The disk reveals that some sort of chaotic event — probably a planetary collision — must have generated the dusty disk. Credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA.

We know that planets can exist around closely-spaced binaries — a tight eclipsing binary system called HW Virginis, for example, is known to be orbited by two gas giants. But HW Virginis c has a semi-major axis of 3.6 AU, while HW Vir b is at 5.3 AU. Both are well outside the habitable zone of the stars they orbit. In any case, this binary system involves a B-class and an M-class star, not the kind of system depicted in Tatooine or examined in the current work.

With the stars under study, we have more of a When Worlds Collide scenario than anything from Star Wars. Here’s Jeremy Drake on the matter:

“This is real-life science fiction. Our data tell us that planets in these systems might not be so lucky — collisions could be common. It’s theoretically possible that habitable planets could exist around these types of stars, so if there happened to be any life there, it could be doomed.”

Another exoplanet orbiting twin stars is found around the binary PSR B1620-26, but here again, we’re not exactly dealing with Sun-like stars. The planet involved orbits a pulsar and a white dwarf. And back to Cheongho Han for a moment. The scientist believes that if a terrestrial world did exist in a stable orbit around two stars similar to our Sun, the only way to find it would be through microlensing. Radial velocity studies avoid short-period binaries, but the microlensing signature should be detectible. Marengo and Drake’s work suggests that if such a world is found, it may be a rarity indeed.

The paper is Matranga et al., “Close Binaries with Infrared Excess: Destroyers of Worlds?” Astrophysical Journal Letters 720 (August, 2010), L164 (preprint). Cheongho Han’s paper is “Microlensing Search for Planets with Two Simultaneously Rising Suns,” Astrophysical Journal Letters 676, No. 1 (20 March 2008), L53 (abstract).

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Pulsar Timing: An Outer System Tool

by Paul Gilster on August 23, 2010

The ways astronomers find to wrest new findings from raw data never ceases to amaze me. This news release from the Max-Planck-Institut für Radioastronomie focuses on a new way to weigh the planets in our Solar System by using signals from pulsars. The method flows out of work on pulsar timing that has been used in the hunt for gravitational waves and has implications not just for the known planets but for detecting hitherto unknown objects in our system.

Pulsar timing supplements earlier ways of weighing planets by measuring their effect on spacecraft flown past them, or extrapolating information from the orbits of their moons. And it seems to be hugely sensitive, to just 0.003 percent of the mass of the Earth and one ten-millionth of Jupiter’s mass.

“This is first time anyone has weighed entire planetary systems – planets with their moons and rings,” says team leader Dr. David Champion (MPIfR). “In addition, we can provide an independent check on previous results, which is great for planetary science.”

So how does this work? The reception of pulsar signals is affected by the Earth’s movement around the Sun, an effect that can be removed by calculating when the pulsar signals would have reached the Solar System’s center of mass, or barycenter, which is the rotation center for all the planets. As you would imagine, the barycenter itself moves as the planetary positions change over time. An ephemeris charting the position of the planets can be used to work out the barycenter’s position according to the values for their masses that have been measured.

Any error in the calculation of the barycenter causes repeating timing errors in the pulsar data. Dick Manchester (CSIRO/Australia) says that “…if the mass of Jupiter and its moons is wrong, we see a pattern of timing errors that repeats over 12 years, the time Jupiter takes to orbit the Sun.” Correct the mass of Jupiter and its moons and the timing error disappears.

Image: Planets in the solar system with their masses determined by means of pulsar timing observations. Credit: David Champion.

Thus far the method has been used to weigh Mercury, Venus, Mars, Jupiter and Saturn (with moons and rings). The mass of the Jovian system can be calculated to a value far more accurate than the best Pioneer and Voyager spacecraft results, though not quite as accurate as the value measured by the Galileo spacecraft. Spacecraft will continue to be the best source of planetary weight measurements, but for planets not being probed by spacecraft, the pulsar measurement will provide accurate data, becoming more accurate as measurements are repeated over time.

This story takes us deep into the Solar System and involves more than the known planets. The kind of data being used may help us with a variety of unknown bodies. From the paper:

The pulsar timing technique is also sensitive to other solar system objects such as asteroids and currently unknown bodies, e.g., trans-Neptunian objects (TNOs). Measurements of anomalous period derivatives and binary period derivatives for a number of millisecond pulsars have already been used to place limits on the acceleration of the Solar System toward nearby stars or undetected massive planets… Pulsar timing array experiments with a wide distribution of pulsars on the sky will be sensitive to the dipolar spatial dependence resulting from any error in the solar system ephemeris, including currently unknown TNOs.

Thus exquisitely sensitive timing data gathered at Parkes, Arecibo and the German Effelsberg instrument, useful in the search for gravitational waves, opens up a new investigative tool for planetary measurement and the hunt for unknown bodies in our system. It’s easy to see why this emerged: The kind of tiny changes in pulsar timing that could flag gravitational waves has to be distinguished from timing error caused by objects in our Solar System. And this is interesting:

Limits for unknown masses have also been placed by spacecraft using deviations from their predicted trajectories. Doppler tracking data from the two Pioneer spacecraft were searched for accelerations due to an unknown planet. The anomalous acceleration detected in these data… is attributed to non-gravitational sources (Anderson et al. 2002) and is not detected in planetary measurements (Folkner 2010).

The paper is Champion et al., “Measuring the Mass of Solar System Planets Using Pulsar Timing,” to be published in The Astrophysical Journal. Also interesting (in terms of distributed science) is Knispel et al., “Pulsar Discovery by Global Volunteer Computing,” Science Express (12 August 2010). Abstract here.

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A Near-Term Read on Life in the Galaxy

by Paul Gilster on August 20, 2010

Although he doesn’t post nearly as often as some of us would like, Caleb Scharf’s Life, Unbounded site is always worth reading. Scharf, author of the textbook Extrasolar Planets and Astrobiology (University Science Books, 2008) is the director of the Columbia University Astrobiology Center. As such, he’s positioned to offer valuable insights into our investigations of the forms life might take on other worlds. Not long ago he wrote a fascinating post for Scientific American on a statistical approach to astrobiology, a timely idea as we discuss ongoing missions like Kepler and proposed space telescopes like WFIRST.

Natural Selection on a Galactic Scale

Scharf’s latest is a quick take on panspermia, the idea that viable organisms may be exchanged between planets as various early impacts spread debris through a planetary system. We know that surface material moves continually between the rocky moons and planets of our own system, and we’ve also come to understand that microbial organisms of great hardiness might survive such extreme journeys, even though they involve millions of years of exposure to interplanetary and even interstellar space. Life may indeed be seeded on a galactic level.

But if this is the case, what about the role of natural selection? Scharf writes:

Although it’s a complex problem, it seems likely that life driven by cosmic dispersal will end up being completely dominated by the super-hardy, spore-forming, radiation resistant, rock-eating (endolithic) type of critters. There will be no advantage to a particularly diverse gene pool. Billions of years of galactic transferral will have whittled it down to only the most indelicate and non-fussy microbes – super efficient, super persistent, and ubiquitous – the galactic top dogs.

All of this would fit with what we see on Earth, for we know about numerous organisms in extreme environments here that do indeed survive under conditions most living things would consider hostile. Scharf’s point, though, is that if panspermia is true on a galactic level, then tough organisms like these should be just about everywhere. As our robotic probes grow in sophistication, they should start finding life’s tenacious foothold throughout the Solar System, from the ancient seabeds of Mars to the smog-choked surface of Titan. A galactic panspermia would know no favorites, and it has had billions of years to work.

Galactic panspermia, in other words, is going to make itself apparent in the not distant future. If we find that this is not the case, that life doesn’t pop up just about everywhere we look, then the case for panspermia at this level is vastly weakened, although we can still see a role for panspermia between planets. The larger question of life around other stars, in that case, will remain as intractable as it does today, and will require our most advanced instrumentation to detect in the form of atmospheric biomarkers on Earth-like planets near enough to study.

From Sagan to Drake

All of which reminds me of a recent interview with Seth Shostak. Asked about Carl Sagan’s estimate that there might be one million intelligent species in the galaxy and Frank Drake’s speculation that the number was closer to 10,000, Shostak comes down on the side of Drake, noting that if 3 percent of the solar systems in the Milky Way have Earth-like planets, then 10 billion such planets must exist. Assume just one in a million to have intelligent life and you still end up with 10,000 civilizations. Kepler will let us tighten these numbers within a few years.

Shostak takes note of the debates he has had with Rare Earth author Peter Ward, who argues in his book with Donald Brownlee that intelligent life must be scarce due to the huge number of factors — Jupiter-like planets, a nearby moon, the tight parameters of habitable zones — that would make it possible. Shostak:

I’m not at all convinced that moons are needed to support life. Without the moon, the tides would be different and the poles would migrate every so often. But that wouldn’t wipe out life. Regarding gravity, there are already two planets with earthlike gravity in our solar system, and even Mars could probably have supported life earlier in its history (and maybe even today). And we are now fairly certain that Jupiter sized planets are commonplace – we have already located hundreds of them. So I just don’t find the argument that complex life must be rare in the galaxy to be compelling.

The encouraging thing about these discussions is that we are dealing with issues about which we will have preliminary answers within a matter of years. Kepler will be able to give us statistical answers about the prevalence of Earth-like worlds in the galaxy, and using Scharf’s reasoning, we can draw extrapolations on the likelihood of panspermia based on what we find in our own system fairly soon, just as long as it takes to get complex robotics to environments like Europa.

True terrestrial planet hunter spacecraft — the kind that can make spectroscopic analyses of exoplanet atmospheres on worlds this small — are at least a decade and perhaps much more away depending on funding issues, but they represent logical extrapolations of near-term technology. In 25 years, we will have not just a philosophical view of life in the universe but a practical knowledge based on data that can tell us whether we’re likely to be alone or simply one among many galactic species.

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Star Wars? Not at NASA

by Paul Gilster on August 19, 2010

I had started today’s entry — on dark energy — only to be sidetracked by a short piece in Space.com that almost had me spewing my morning coffee all over my keyboard. Here’s a quote from the story, which focuses on a Star Wars convention in Florida held last weekend:

“‘Star Wars’ filmmakers and fans asked NASA representatives to develop a hyperdrive that can transport astronauts through space at light speed. And to make it snappy.”

In response, the story quotes NASA’s Joseph Tellado, a logistics manager for the International Space Station, who says this:

“We need better propulsion systems. Right now I’d say that would be the one invention that would really help us out a lot. It’d be great if our astronauts could go at hyperspeed…. I believe ‘Star Wars’ and NASA have a lot in common. We’re looking to the future. NASA is like the first stepping stone to ultimately get to that ‘Star Wars’ level.”

And the story adds this:

The inspiration works both ways, with NASA and “Star Wars” inspiring each other to stretch out and envision the future and then fill in details of what that future might look like.

NASA in the Hunt for Breakthroughs?

Astounding. Here’s why I burned my tongue on a cup of Sumatra Mandheling this morning: Despite what convention-goers may now believe, NASA has no involvement whatsoever in the kind of technologies these people are talking about. True, the agency once funded the Breakthrough Propulsion Physics project, run out of Glenn Research Center by Marc Millis. BPP’s charter was to investigate the kind of technologies that might one day lead to deep space and interstellar flight, among them so-called ‘warp drive’ and other possibilities. But the agency stopped funding BPP in 2002.

NASA’s Institute for Advanced Concepts, not as ‘breakthrough’ oriented as BPP but a potent force for showcasing new ideas, was cut off from its funding in 2007. In short, the idea that NASA is conducting serious research on any aspect of advanced propulsion — I am talking here about the kind of concepts this convention glories in — is completely false. That work is now off the table. Marc Millis himself has left NASA and works on breakthrough concepts through the Tau Zero Foundation he founded, for which I toil on a daily basis in writing these posts. TZF has no NASA connection whatsoever and proceeds through private funding. The relevant links on the home page here give you the background on TZF.

So while I agree with NASA’s Joseph Tellado that hyperspeed is a desirable outcome, it should be added that it’s not one that NASA is engaged in studying. This is not to say that potential near-term technologies like solar sails may not be revived within the agency — the NASA solar sail is up to a Technological Readiness Level of 6 and a demonstrator sail like NanoSail-D should be launched within a year. But if you’re talking futuristic concepts like warp drive and the study of potential breakthroughs, NASA is no longer the place to be.

Pushing into Dark Territory

With that off my chest, let’s proceed to dark energy, which I want to discuss because utterly unexpected scientific results may offer us useful clues to future technologies. We’re only beginning to learn about dark energy, but the notion that the universe should be expanding at an accelerating rate has flowed out of supernova observations that have been supported by later studies like the Supernova Legacy Survey. Using the Hubble Space Telescope, the Higher-Z team concludes that dark energy has been a factor for at least nine billion years.

All of which is strange and wondrous, as it implies that a hugely important component of our universe only became known within the last twelve years, when the first supernova work was reported. If there is a factor that causes space to expand — one that seems to make up about 72 percent of the mass-energy of the universe — it must exert a strong negative pressure to account for its effects. The fact that it is so hard to detect and is not thought to interact with the fundamental forces other than gravity means that studying it in the laboratory is, to say the least, problematic.

And as far as harnessing its powers for future propulsion systems, the idea is far-fetched in the extreme — at our current technology level. We can’t, however, rule it out for the far future. And that’s the thing about the future. It plays out according to the inputs we give it, meaning that if in some future century our science progresses to the point that what we now know as dark energy becomes something we can engineer, it will be because a long line of scientists, starting now, have put in the groundwork to get us to that destination.

That’s why I keep an eye on dark energy studies in these pages, suggesting only that the more unexplained things we gradually master in the universe, the more likely we are to make genuine breakthroughs. The Breakthrough Propulsion Physics project used to make this sort of thing its bread and butter, but private organizations like the Tau Zero Foundation now have to continue that work without help from government agencies.

The Geometry of Spacetime

And there is some interesting news about dark energy as we continue to pursue this odd effect. One reason that dark energy work is so absorbing is that it tackles the very geometry of the universe. Findings from a team including Yale University physicist Priyamvada Natarajan, reported in the August 20 issue of Science, are based on gravitational lensing of 34 extremely distant galaxies situated behind the massive galactic cluster Abell 1689. Astronomers can study how the images are distorted by intervening mass. Says Natarajan:

“The content, geometry and fate of the universe are linked, so if you can constrain two of those things, you learn something about the third.”

Image: The massive gravitational force of the dark matter (shown in blue) in giant galaxy cluster Abell 1689 bends the light from distant background galaxies, giving astronomers clues to the nature of dark energy. Credit: NASA, ESA, Eric Jullo/JPL, Priyamvada Natarajan/Yale University, Jean-Paul Kneib/Universite de Provence.

As I said, we’re in early days when it comes to the study of dark energy, and if there are actually ways to harness it, such developments may well be centuries away. But it’s useful to know that Natarajan and team have been able to narrow the range of current estimates about dark energy’s effect on the universe — denoted by the value w — by some thirty percent. They did this by combining the gravitational lensing studies with new data from supernovae, X-ray galaxy clusters and related data from the Wilkinson Microwave Anisotropy Probe (WMAP).

We learn from all this that the dark energy work thus far confirms previous findings that we do indeed live in a flat universe, one in which the expansion will continue to accelerate and the universe will expand forever. Assuming, that is, that dark energy’s effects remain constant over cosmological time scales. We have much to do to understand how dark energy works, but with NASA out of the hunt when it comes to examining it and other components of far future propulsion engineering, we shouldn’t expect that hyperdrive any time soon.

The paper is Natarajan et al., “Cosmological Constraints from Strong Gravitational Lensing in Clusters of Galaxies,” Science Vol. 329, No. 5994 (20 August 2010), pp. 924-927 (abstract).

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