Planet Formation and Interstellar Winds

Any unexpected kink in the debris disk surrounding a young star is often attributed to a planet forming amongst the gas and dust. But that may not be the only explanation. In fact, new work by John Debes and team at NASA GSFC points to an alternative: The motion of the infant system through insterstellar gas. Thus we have yet another reminder that space is not truly empty, and that patches of gas can play their role in planet formation.

Debes and colleagues have been looking at infant systems like that around the star HD 32297, some 340 light years from Earth in the constellation Orion. About 100 million years old, the star is surrounded by a dust disk that resembles what our Solar System probably looked like not long after the major planets formed. Noticing that the dust disk around the star was warped, the team was led to link the finding to the presence of nearby interstellar gas clouds. The idea of interstellar gas drag upon a stellar system moving through such a cloud seems a natural fit in this system and elsewhere.

The mechanism suggested here is that stars encounter a gas cloud and experience forces that shape their disks. Says Debes:

“The disks contain small comet- or asteroid-like bodies that may grow to form planets. These small bodies often collide, which produces a lot of fine dust. The small particles slam into the flow, slow down, and gradually bend from their original trajectories to follow it.”

Another young star, HD 61005 in the constellation Puppis, shows a disk edge that is bent slightly away from the direction of stellar motion, with a wake of fine dust trailing behind. This ‘face-on’ encounter differs from what we would expect when the disk moves edgewise through interstellar gas, causing effects that deform the disk on the side inside the cloud. While the inner portions of the disk are relativly unaffected, the outer regions show ample evidence of the drag effect. Presumably, such influences would leave a signature in outer debris rings like our own Edgeworth/Kuiper belt.


Image: The inner, yellow portion of HD 61005’s disk spans 5.4 billion miles, or about the width of Neptune’s orbit in our own solar system. This false-color Hubble view masks the star’s direct light to bring out detail in the disk. Credit: NASA/ESA/D. Hines (Space Science Inst., New Mexico) and G. Schneider (Univ. of Arizona).

This is an evanescent drag force, one that affects only the smallest particles (those about a micrometer across and smaller). Radiation pressure from the central star’s light would normally purge this dust from the system over time, but the interstellar drag can affect their position. You get the image of a filter through which some young systems pass, shedding dust, but Debes’ collaborator Marc Kuchner likens the effect to the summer breeze that scatters dandelion seeds. Either way, the more we learn about this effect, the better we’ll understand how to make sense out of what we see in other young stars’ debris disks. We’ll also have a better idea about planet formation in outer system regions where ice giants like Neptune form.

More in this GSFC news release. The paper is “Interstellar Medium Sculpting of the HD 32297 Debris Disk,” Astrophysical Journal 1 September 2009 (abstract).


Science Fiction and Interstellar Thinking

It’s easy to cite science fiction technologies that made their way into real life, starting with, say, submarines and the Jules Verne connection, and pushing on into air travel and, eventually, a spaceship to the Moon. It’s also easy to find numerous examples of science fiction being blindsided by technologies no one really predicted. I’ve read “A Logic Named Joe,” but other than Murray Leinster’s prescient 1946 tale, did anyone really predict the advent of computers small enough to fit on your desktop, or mobile devices that connect us to a worldwide network for communications and data transfer?

Predictions or Dry Runs?

This is where I think some science fiction enthusiasts make a mistake in trying to sell their genre as a predictive force. Sure, the examples are there, and we have visionaries like Arthur Clarke who, in addition to crafting spectacular novels of the future, managed to introduce communications satellites into the pages of a popular magazine (Wireless World) before anyone had really examined the idea. But by and large, science fiction’s clout doesn’t come so much from prediction as much as from the ability to try out new ideas by remodeling the world so as to accommodate them.


I collect old science fiction magazines and sometimes look at these rows of pulp and digest-sized volumes as containing dry runs on the future we’re moving into now. Their authors weren’t really making predictions as much as asking what would happen when a new idea was introduced, or a present-day trend was taken to its logical conclusion. That makes SF a golden way to explore the present dilemmas we all face as we try to build the best future we can muster. Peter Garretson talked about this recently in an interview with the Indian science fiction magazine Kalkion, from which this excerpt:

Of course there are degrees of imaginative vision vs technical vision, but the latter feeds the former. One can imagine and say, “What if we had a method of communcating remotely with just a small device in our hands.” That vision of a fulfilled need encourages others to think through how it might actually be done.

Image: The cover of Analog from November, 1962, with artwork by John Schoenherr. How many minds did such covers turn to science and eventual careers in fields like astronomy or physics?

Pushing Alternative Futures

Many forays into fictional futures, then, can give us alternative ways to make a new concept real. We can try on those futures by reading stories that make them come alive, seeing what effects these changes would have on society. And we can do more. By placing futuristic concepts in a tangible, fictional context, we can encourage their growth and dissemination.

Garretson continues:

The same is happening today with interstellar travel. It starts as an imaginative SF question, “How would our horizons be different if we had an interstellar drive.” Then it gets taken up by a amateur society, then NASA ( Now there are are annual conferences ( on the subject, dedicated organizations ( and even an academic physics textbook on potentially promising avenues ( I think it is always valuable for SF authors to say (paraphrasing Joel Barker), “What is impossible today, but if it could be changed, would change everything” for the frontiers of humanity or technology. That in turn informs us both about the widened frontiers, and the dangers and new problems we might encounter. And that, in turn, creates new and interesting technical challenges, and informs enlightened policy.

How satisfying to see the Tau Zero Foundation placed in a context that builds from amateur rocketry through the modern space age to NASA’s Breakthrough Propulsion Physics project, and now on to the Frontiers of Propulsion Science volume assembled by TZF founder Marc Millis and physicist and visionary Eric Davis. I’m not familiar with Joel Barker, but I like the idea of taking what is impossible today and asking the ‘what if’ question, which propels us to study alternatives and shapes what we do next.

A Vision Realized in Fiction

Garretson is a futurist and strategic thinker whose background includes service in the US Air Force as Chief of Future Science and Technology Exploration. The reason Kalkion snared him for this interview is that he is in India as a visiting fellow at the Indian Institute for Defence Studies and Analysis (IDSA) under the sponsorship of the US-based Council on Foreign Relations (CFR). He’s absorbed by the idea of using science fiction as a way of shaping resiliency and encouraging futuristic thinking, calling SF “…a kind of marketing for a grand future for humanity. It creates a need. It creates future envy. We get hooked on a vision, whether Utopian, or just plain cooler than today, and we want to bring it into being.”

‘Future envy’ — lovely construction, that, and so true! As to the objection that science fiction is escapist in its orientation, Garretson has this to say:

I think SF is fundamentally different than other fiction in that for many of us, it is the opposite of escapist. I do not read SF to get out of the present because I am pained by my present condition, but rather to inform and give meaning to my plans for the future and action in the present because I am optimistic about either the present or the future.

If the duties of the present consumed my every minute, left me without the leisure time and resources (including literacy) to acquire such books, or discouraged me from thinking about a future different than the status quo, I might chose escapist literature if any at all, and might prefer themes that reflected useful themes for my life–acceptance of the vicissitudes of fate, and a smallness of human beings before society’s structure and forces and those of a cruel, capricious, and uncontrollable nature.

To read SF is to pre-suppose that my daily life allows for such leisure time, and that I can imagine a future different in some meaningful way than today, where both mankind in respect to nature, and individuals in respect to both society and nature can be a meaningful actor.

Science Fiction and the Long View

I think what Garretson is getting at here ties directly to our own emphasis on long-term thinking. An optimistic turn of mind is one that sees the power of the individual to shape the future, even when the results one is working toward will not be accomplished necessarily within one’s own lifetime. We all need to be reminded of that outlook, especially when the slow pace of change (and the ongoing budgetary problems involved in any space exploration) tempt us into disillusionment. This is a good interview to read when you need a bit of bucking up. It will remind you that the interstellar future, in whatever form we realize it, is an achievement worth thinking about and working for.

Let me close by quoting Garretson again, this time on where he sees science fiction going:

I am also of the opinion that the best way to predict the future is to create it. So let me attempt to create the future of science fiction with a bit of criticism. I think today’s sci-fi is too dark, it is too pre-occupied with humanity’s problems, and not sufficiently concerned with stroking its ambitions and setting new vistas. I think science fiction needs to pull back a bit from the space-opera fantasy, and transcend the cyber-punk darkness.

Another good phrase: ‘transcend the cyber-punk darkness.’ I hear similar sentiments from many long-term science fiction fans (Les Johnson and I had this discussion at the Aosta conference on a walk before dinner one day). Garretson continues:

I think right now we most need science fiction that creates a compelling vision of where we can take humanity over perhaps 3 generations using real, not just imagined technology… I believe there is a real world of the future that could involve a sustainable, developed world getting its energy from Space Solar Power, protecting Earth from asteroids, mining the sky for valuable minerals, and protecting our climate. Where access to space through space planes and other new innovations is common. I think we could use that as a stepping stone to free-flying space colonies. How different would that be? What would it be like to live in that world? What kind of institutions would make it work? How can we hook kids on the science it takes to put it all together? How do we get them to decide: “I want to solve that problem”,”I want to live in that world!”

If we let it tap its deepest roots, science fiction can indeed be the stuff that dreams are made of. Be sure to read all of this absorbing interview.


Rethinking Planets and Stellar Metals

We often speak about planets migrating from the outer to the inner system of a star, something that helps us put ‘hot Jupiters’ in context. But what about migration within the galactic disk? It’s an idea under continuing investigation. In the absence of direct observational evidence, we infer migration and assume that older stars often come from regions with significantly different metallicity than stars in their current environment. The presumed origin would be the inner disk, which Misha Haywood defines as that part of the galaxy inside the radius from galactic center to our Sun.

Dave Moore sent me Haywood’s latest paper a few months back and I’ve been slow in getting to it because I wanted to give its conclusions further thought. It’s intriguing stuff. Haywood (Observatoire de Paris) takes note of the fact that we tend to find gas giants around stars that are rich in metals (here a pause to remind newcomers that by ‘metals,’ we mean elements higher than helium). And he wants to answer a key question: How do we know that this higher percentage of Jovian worlds detected around metal-rich stars is the result of metallicity, and not some other factor linked with their origin in the inner disk? The question is relevant, Haywood writes:

…because any measurable property of inner disk stars other than metallicity would be correlated with the presence of planet. The obvious a priori response is that metallicity is a measurable parameter, and intrinsic to the star. But there could be others however, which, although not measurable on the stars, could be no less important, such as, for example, the surface density of molecular hydrogen in the inner galactic disk regions.

The obvious next step is to find exceptions to the giant planet/metallicity correlation, and Haywood notes that we don’t see the same metallicity connection among giant stars hosting planets that we do around smaller stars. Moreover, at intermediate metallicities, giant planets seem to favor thick disk stars rather than thin disk objects.

Here we’re talking about different and distinct star populations. The ‘thin disk’ we see edge-on in images of spiral galaxies is complemented by the more diffuse ‘thick disk,’ containing older stars. The thick disk population, thinks Haywood, comes from migrating stars from the inner disk, while the metal-poor group derives from stars from the outer disk.

This takes us to an interesting place:

We are now facing the following picture: stars that come from the inner disk are noticeably rich in giant planets, while stars that come from the outer disk seems to be less favored in this respect. This new information changes considerably how we envisage the correlation between metallicity and the presence of giant planets. For the surprising point here is not the fact that most host-planet stars are metal-rich, since they come from a region where most stars are metal-rich, but the very fact that most would come from the inner disk. We are led to conclude that the distance to the galactic center must somehow play a role in setting the percentage of giant planets

The italics above are mine, because the statement is the core of the argument. Looked at from this perspective, the correlation between metals and the presence of giant planets turns out to reflect the galactic origin of the stars. It does not imply that metallicity is the necessary cause for the formation of these planets.

But if not metallicity, what other factors can we link to the galactocentric distance of a star? One possibility is dust density, which would favor the development of planetesimals. But Haywood prefers molecular hydrogen as the answer. It is the basic ingredient for the formation of giant planets, the principal consituent of stellar disks. Moreover, we have to think in terms of where it is most abundant:

Its main structure in the Galaxy, the molecular ring, is thought to contain 70% of H2 gas inside the solar circle…, thereby providing a huge reservoir for star (H2 is known to be directly linked to star formation…) and planet formation. The most interesting aspect however, is the fact the molecular ring reaches a maximum density at 3-5 kpc from the sun, corresponding to the distance where stars with metallicity in the range (+0.3,+0.5) dex are expected to be formed preferentially.

Haywood argues that stars hosting ‘super Earths’ or Neptune-class worlds with no accompanying gas giants are less likely to have had an origin in the inner disk, and thus form in an environment less dense in molecular hydrogen. We would, then, expect no predominance of metal-rich stars among this population. Surveying twelve systems that house super-Earths or Neptune-class planets, Haywood finds that the seven with no Jovian planets have low metallicity, fitting his theory, while the five that do contain gas giants indeed show a higher proportion of metals, “…amply confirming the possibility that the first group of stars could be genuine solar radius objects, and the second wanderers from inside the Galaxy.”

The paper is Haywood, “On the Correlation Between Metallicity and the Presence of Giant Planets,” accepted at Astrophysical Journal Letters and available as a preprint.


?Earth: Crash Course in Building a World

?by Larry Klaes

Tau Zero journalist Larry Klaes here gives us a quick overview of the history and future of the Earth, so vital for understanding not only how life emerged here but how it may appear around other stars. It’s good to keep this background in mind as Kepler and COROT go planet-hunting. Thus far we’ve had our share of surprises as we’ve explored other systems, and doubtless there will be many more as future instruments come online, both in space and on the ground. And as Larry reminds us, there is much we still have to learn about our own planet.

Let’s look at our celestial home’s place in time as well as space, namely the long and ancient history of its cosmic birth and development. This story includes a general history of the wide variety of living beings that dwell just about everywhere on this planet.

Planet Formation and the Big Collision

Earth’s geological history began about five billion years ago, roughly eight billion years after the Universe got its start in the Big Bang. Back then, our planet was just another disembodied collection of dust and gas along with the rest of the Solar System in what astronomers call a nebula. It took the dramatic death of a nearby and unknown star to get our system and world started via the shockwaves of the exploding sun, also known as a supernova.

Not only did the supernova impact start condensing the interstellar dust and gas cloud that would eventually become us, that star’s death throes also infused our nebula with heavier elements that helped produce the various worlds and eventually all life on Earth. As the late Cornell astronomer Carl Sagan once famously said: “We are made of star stuff.”

The cosmic cloud that would become our Solar System began condensing and collapsing into many individual worldlets called planetesimals. As the planetesimals collided with each other as they looped around the newborn Sun amidst the nebula that was turning into a flattened disk of debris, the ones that were not destroyed by the impacts slowly built up into larger worlds. One particular collection became our Earth, though at this early stage it was essentially a sphere of molten rock, with no water and nothing living on it.


There were more planets in our Solar System during this era than there are now. Many of them were in chaotic orbits around the Sun. One of these early planets, about the size of Mars (but not the current Red Planet, please note) smashed into the young and still molten Earth, becoming almost totally vaporized in the process. Massive chunks of our world were thrown into space, then began circling Earth and eventually condensed into our Moon.

Image: Heavy bombardment from space may have caused life to ‘reboot’ multiple times. Credit and copyright: Julian Baum.

Over time our planet cooled enough so that the surface, or crust, hardened into solid rock. Water began to appear both from condensation and various comets that struck Earth, as there was still plenty of nebula debris left over from the Solar System smashing into itself and into our planet. Our Moon bears the scars of that ancient bombardment period to this day: We call them craters.

Let There Be Life

Though solid evidence for the first signs of life on Earth currently go back over 3.5 billion years, scientists think simple organisms may have started when our planet cooled and water formed on it over four billion years ago. However, these earliest living natives may have been wiped out by the relentless impacts of space debris, causing life on Earth to restart more than once. The fact that life did begin on our planet so relatively soon after its violent formation could mean that life may also have begun and exists on many other worlds throughout the Universe.


For most of the time that organisms have been on Earth, native creatures had been no more sophisticated than microorganisms, some of which gathered themselves into colonies called stromatolites. Then just over half a billion years ago, Earth’s climate underwent changes, perhaps in part due to these ancient microbes giving off oxygen for ages, which brought about what scientists call the Cambrian Explosion or Radiation. In a relatively short time geologically speaking, life grew and developed into multicellular forms which became the ancestors of the wide variety of organisms that crawl, walk, fly, and swim all over our planet, including us, humanity.

Image: This view of the shallows of Shark’s Bay, Australia, shows a colony of living stromatolites. ©Isao Inouye (University of Tsukuba), Mark Schneegurt (Wichita State University), and Cyanosite.

Though impacts from large space objects in later epochs did disrupt ecosystems and cause many extinctions – the most famous being the death of the dinosaurs 65 million years ago – life on Earth has managed to survive and thrive just about everywhere in numerous forms.

The Final Days

There will come a time in the far future, however, when all life on this planet will have to either leave or perish. Several billion years from now, our Sun will begin to expand from a yellow dwarf star into a red giant sun. Earth may escape being engulfed and vaporized by the bloating Sun, but it will become very hot, enough to evaporate the oceans and turn our planet’s surface into molten slag.


Image: A possible end for the Earth. Credit: fsgregs/Wikimedia Commons.

Eventually, the Sun will shrink and cool down into a white dwarf star and later a dead cinder of itself. Earth will become a barren, frozen, and dark place and may one day drift off into interstellar space, no longer held in place by the remnant mass of the Sun. Hopefully long before our Sun comes to the end of its life, our very distant descendants will have left Earth and the Solar System to start new lives in other parts of the galaxy.


Building Infrastructure: The Tether Option

Conservation of energy means we never really get something for nothing. Nonetheless, the idea of propellantless propulsion is profoundly important for our future in space. A solar sail uses momentum from solar photons to get its boost, letting the Sun serve as the energy source so we don’t have to carry heavy fuel tanks and can maximize payload. So propellantless propulsion really means finding sources outside the spacecraft itself to do the work.

The Interplanetary Gambit

Recently I’ve finished Michel Van Pelt’s book Space Tethers and Space Elevators (Copernicus/Praxis, 2009), a treatment of a technology we seldom consider in these pages because it’s more practical in terms of near-Earth solutions. But Van Pelt surveys tethers — and the space elevator idea, which is built around what could be considered a giant tether — so comprehensively with regard to the implications of leaving the propellant behind that his book is a must read for those of us interested in deep space development. After all, building a space-based infrastructure will demand cheap access to the outer system, and it turns out tethers have interplanetary possibilities.


Consider the principles of momentum exchange tether systems of the kind known as ‘bolos.’ Here we’re talking about a rotating system that could be used to transfer spacecraft to higher orbits. A 100 kilometer cable in an elliptical orbit can be set to spinning vertically like a sling. Imagine it with a ballast mass on one end and a spacecraft catching and docking device on the other. The center of mass (and thus the center of rotation) will lie close to the ballast mass. Timing is all — the tether’s rotation can be timed so that when the bolo reaches its perigee, the tether is vertical and swinging backward, capable of matching the velocity of a slower moving satellite.

As the catching device swings past, the payload spacecraft can be hooked to its docking clamp. At the other side of the rotation, the docked spacecraft will now be at maximum altitude, and can be released at a higher velocity. We’ve given our spacecraft a cheap ride to a new orbit. Now imagine a larger system of this kind, as Van Pelt does:

A series of bolo tethers, each tether passing a spacecraft onto the next, could be used to achieve even larger orbit changes than a single system. For example, one tether system could catch a spacecraft from a very low orbit and swing it into a somewhat higher orbit. Another bolo picks it up from there and puts the satellite into a geosynchronous transfer orbit (GTO). A third tether catches the load again and imparts sufficient velocity to it so that it reaches escape velocity. A satellite initially orbiting just above the atmosphere could thus be slung all the way into an interplanetary orbit around the Sun, and all this without using any rocket propulsion and propellant…

Reverse the process and you can catch a spacecraft at the top of the tether rotation and release it into a lower orbit. We can imagine, then, a system that brings interplanetary missions returning to Earth into a series of orbits around the Earth that culminate in a reentry and landing that use no propellant at all. All those kilograms of fuel that would otherwise have had to make the interplanetary journey so as to be available for return are thus no longer needed, and once again we’ve flown with a much larger payload.

A ‘Rotavator’ in Lunar Orbit

This is absorbing stuff, and Van Pelt goes on to discuss Hans Moravec’s idea of a lunar ‘skyhook’ or ‘rotovator, a notion that, in turn, draws on John McCarthy’s work in the 1950s.’ Here we have two long tether arms in rotation and a massive central facility. The tethers would have the same length as the orbital altitude of the central unit, meaning that as the system rotates, their tips would periodically reach the surface of the Moon. Now this is important: The rotovator would rotate in the same direction as its orbit, and at a rate so that the velocity of each tether’s tip would equal the orbital velocity of the system’s center of mass. That means that when the tips reach the lunar surface, their velocity relative to the Moon would be zero.

We’ve built a system — call it a ‘lunavator’ — that can place an object on the surface or pick up something that needs to get into space. What a scenario. Here’s how Van Pelt describes it, noting that people on the surface would not see it as a rotating system at all:

To them it will look like a long cable reaching straight down vertically from the sky, then retreating back up exactly the same way. Depending on the tether length and the rotation speed (the combination always needs to be selected so that the tip speed at the lunar surface is zero), lunavators can be made to periodically touch down at the same single spot, at several fixed spots or at any number of varying locations every orbit. However, a rotovator that makes multiple rotations per orbit could service more than one lunar station. We may also use the tether as a kind of helicopter that picks up a payload at one site and then, after one or multiple rotations, drops it at another surface location without releasing it into space.

Tuning Up (and Reboosting) the Assembly

Now with a system like this, you have to assume that the amount of time available to pick up payloads or deposit them on the lunar surface would be short, but additional tether deployed from reels at the tether tips could lessen that problem. Van Pelt figures the unreeling tether mechanism could lengthen the time the end of the tether spends on the surface to several minutes, a more practical solution. It’s clear in this system that the central tether facility will have to be quite massive to prevent individual payloads from stealing too much momentum from the rotating tether system.

How to prevent orbital decay is one of many problems Van Pelt addresses in this volume — he goes on in the chapter I’m dealing with here to discuss the Momentum-eXchange/Electrodynamic-Reboost (MXER) tether system invented in the 1980s by Robert Hoyt of Tethers Unlimited. Here, a tether system in orbit around the Earth replenishes its transferred orbital energy by using solar arrays, running electrical energy through a metal wire in the tether. The Lorentz force caused by the interaction of the electric tether’s magnetic field with the Earth’s magnetic field then provides a steady push which can be used to propel the entire tether system back up to a higher orbit.


Image (click for a larger image at the Tethers Unlimited site): Momentum-Exchange/Electrodynamic-Reboost (MXER) tether systems can provide propellantless propulsion for a wide range of missions, including: orbital maneuvering and stationkeeping within Low Earth Orbit (LEO); orbital transfer of payloads from LEO to GEO, the Moon, and Mars; and eventually even Earth-to-Orbit (ETO) launch assist. By eliminating the need for propellant for in-space propulsion, MXER tethers can enable payloads to be launched on much smaller launch vehicles, resulting in order-of-magnitude reductions in launch costs. In order for MXER tethers to achieve their potential in real-world application, several key technologies must be developed and demonstrated, including space-survivable tethers incorporating both high-strength and conducting materials, technologies for rendezvous with and grappling of payloads, and techniques for predicting and controlling tether rotation and dynamics. Credit: Tethers Unlimited.

Making the Concept Credible

Who knew so many tether experiments had already been performed as are revealed in this book, going back to the days of the Gemini program? This is enlightening reading, and I’m glad to see a book focused on tethers (and in one chapter, on the space elevator concept) coming onto the popular science market. Van Pelt does a fine job acquainting us with the history and principles of tether systems and their possible uses in making near-Earth and interplanetary operations far cheaper than they are today. We may also find them useful for creating artificial gravity on long space missions and, interestingly, sweeping away dangerous charged particles around a spacecraft.

Credibility is a key to getting tether systems accepted as a viable technology, but it’s disheartening to see that they do not factor into the current planning of NASA, ESA or any other space agencies. “It will require considerable advocating, publicizing, convincing and lobbying,” writes Van Pelt, “to keep development going, and that may turn out to be even harder than meeting the technical challenges.” That’s a sentiment that scientists on numerous projects, from solar sails to nuclear propulsion, will understand.