by Paul Gilster | May 23, 2011 | Exotic Physics |
by James F. Woodward
I first wrote about James Woodward’s work in my 2004 book Centauri Dreams: Imagining and Planning Interstellar Exploration, and have often been asked since to comment further on his research. But it’s best to leave that to the man himself, and I’m pleased to turn today’s post over to him. A bit of biography: Jim Woodward earned bachelor’s and master’s degrees in physics at Middlebury College and New York University (respectively) in the 1960s. From his undergraduate days, his chief interest was in gravitation, a field then not very popular. So, for his Ph.D., he changed to the history of science, writing a dissertation on the history of attempts to deal with the problem of “action-at-a-distance” in gravity theory from the 17th to the early 20th centuries (Ph.D., University of Denver, 1972).
On completion of his graduate studies, Jim took a teaching job in the history of science at California State University Fullerton (CSUF), where he has been ever since. Shortly after his arrival at CSUF, he established friendships with colleagues in the Physics Department who helped him set up a small-scale, table-top experimental research program doing offbeat experiments related to gravitation – experiments which continue to this day. In 1980, the faculty of the Physics Department elected Jim to an adjunct professorship in the department in recognition of his ongoing research.
In 1989, the detection of an algebraic error in a calculation done a decade earlier led Jim to realize that an effect he had been exploring proceeded from standard gravity theory (general relativity), as long as one were willing to admit the correctness of something called “Mach’s principle” – the proposition enunciated by Mach and Einstein that the inertial properties of matter should proceed from the gravitational interaction of local bodies with the (chiefly distant) bulk of the matter in the universe. Since that time, Jim’s research efforts have been devoted to exploring “Mach effects”, trying to manipulate them so that practical effects can be produced. He has secured several patents on the methods involved.
Jim retired from teaching in 2005. Shortly thereafter, he was diagnosed with some inconvenient medical problems, problems that have necessitated ongoing care. But, notwithstanding these medical issues, he passes along the good news that he remains “in pretty good health” and continues to be active in his chosen area of research. Herewith a look at the current thinking of this innovative researcher.
Travel to even the nearest stars has long been known to require a propulsion system capable of accelerating a starship to a significant fraction of the speed of light if the trip is to be done in less than a human lifetime. And if such travel is to be seriously practical – that is, you want to get back before all of your stay-behind friends and family have passed on – faster than light transit speeds will be needed. That means “warp drives” are required. Better yet would be technology that would permit the formation of “absurdly benign wormholes” or “stargates”: short-cuts through “hyperspace” with dimensions on the order of at most a few tens of meters that leave the spacetime surrounding them flat. Like the wormholes in the movie and TV series “Stargate” (but not nearly so long and without “event horizons” as traversable wormholes don’t have event horizons). With stargates you can dispense with all of the claptrap attendant to starships and get where you want to go (and back) in literally no time at all. Indeed, you can get back before you left (if Stephen Hawking’s “chronology protection conjecture” is wrong) – but you can’t kill yourself before you leave.
Starships and stargates were the merest science fiction until 1988. In 1988 the issue of rapid spacetime transport became part of serious science when Kip Thorne and some of his graduate students posed the question: What restriction does general relativity theory (GRT) place on the activities of arbitrarily advanced aliens who putatively travel immense distances in essentially no time at all? The question was famously instigated by Carl Sagan’s request that Thorne vet his novel Contact, where travel to and from the center of the galaxy (more than 20,000 light years distant) is accomplished in almost no time at all. Thorne’s answer was wormholes – spacetime distortions that connect arbitrarily distant events through a tunnel-like structure in hyperspace – held open by “exotic” matter. Exotic matter is self-repulsive, and for the aforementioned “absurdly benign” wormholes, this stuff must have negative restmass. Not only does the restmass have to be negative, to make a wormhole large enough to be traversable, you need a Jupiter mass (2 X 1027 kg) of the stuff. This is almost exactly one one thousandth of the mass of the Sun and hundreds of times the mass of the Earth. In your livingroom, or on your patio. Warp drives, in this connection at least, are no better than wormholes. Miguel Alcubierre, in 1994, wrote out the “metric” for a warp drive; and it too places the same exotic matter requirement on would be builders.
Long before Thorne and Alcubierre laid out the requirements of GRT for rapid spacetime transport, it was obvious that finding a way to manipulate gravity and inertia was prerequisite to any scheme that hoped to approach, much less vastly surpass the speed of light. Indeed, in the late 1950s and early 1960s the US Air Force sponsored research in gravitational physics at Wright Field in Ohio. As a purely academic exercise, the Air Force could have cared less about GRT. Evidently, they hoped that such research might lead to insights that would prove of practical value. It seems that such hopes were not realized.
If you read through the serious scientific literature of the 20th century, until Thorne’s work in the late ’80s at any rate, you will find almost nothing ostensibly relating to rapid spacetime transport. The crackpot literature of this era, however, is replete with all sorts of wild claims and deeply dubious schemes, none of which are accompanied by anything resembling serious science. But the serious (peer reviewed) scientific literature is not devoid of anything of interest.
If you hope to manipulate gravity and inertia to the end of rapid spacetime transport, the “obvious conjecture” is that you need a way to get some purchase on gravity and inertia. Standard physics, embodied in the field equations of Einstein (GRT) and Maxwell (electrodynamics), seems to preclude such a possibility. So that “obvious conjecture” suggests that some “coupling” beyond that contained in the Einstein-Maxwell equations needs to be found. And if we are lucky, such a coupling, when found, will lead to a way to do the desired manipulations. As it turns out, there are at least two instances of such proposed couplings advanced by physicists of impeccable credentials. The first was made by Michael Faraday – arguably the pre-eminent experimental physicist of all time – in the 1840s. He wanted to kill the action-at-a-distance character of Newtonian gravity (that is, its purported instantaneous propagation) by inductively coupling it to electromagnetism (which he had successfully shown to not be an action-at-a-distance interaction by demonstrating the inductive coupling of electricity and magnetism). He did experiments intended to reveal such coupling. He failed.
The second proposal was first made by Arthur Schuster (President of the Royal Society in the 1890s) and later Patrick M.S. Blackett (1947 Nobel laureate for physics). They speculated that planetary and stellar magnetic fields might be generated by the rotational motion of the matter that makes them up. That is, electrically neutral matter in rotation might generate a magnetic field. Maxwell’s electrodynamics, of course, makes no such prediction. There were other proposals. In the 1930s and ’40s Wolfgang Pauli and then Erwin Schrodinger constructed five-dimensional “unified” field theories of gravity and electromagnetism that predicted small coupling effects not present in the Einstein-Maxwell equations. But the Schuster-Blackett conjecture is more promising as the effects there are much larger – large enough for experimental investigation. And George Luchak, a Canadian graduate student (at the time), had written down a set of coupled field equations for Blackett’s proposal.
Some worthwhile experiments can be done with limited means in a short time but only a fool tries to do serious experiments without having a plausible theory as a guide. Plausible theory does not mean Joe Doak’s unified field theory. It means theory that only deviates from standard physics in explicit, precise ways that are transparent to inspection and evaluation. (The contra positive, by the way, is also true.) So, armed with Faraday’s conjecture and then the Schuster-Blackett conjecture and Luchak’s field equations, in the late 1960s I set out to investigate whether they might lead to some purchase on gravity and inertia. The better part of 25 years passed doing table-top experiments and poking around in pulsar astrophysics (with its rapidly rotating neutron stars with enormous magnetic fields, pulsars are the ultimate test bed for Blackett’s conjecture) to see whether anything was there. Suggestive, but not convincing, results kept turning up. In the end, nothing could be demonstrated beyond a reasonable doubt – the criterion of merit in this business. However, as this investigation was drawing to a close, about the time that Thorne and others got serious about traversable wormholes, detection of an algebraic error in a calculation led to serious re-examination of Luchak’s formalism for the Blackett effect.
Luchak, when he wrote down his coupled field equations, had been chiefly interested in getting the terms to be added to Maxwell’s electrodynamic equations that would account for Blackett’s conjecture. So, instead of invoking the full formal apparatus of GRT, he wrote down Maxwell’s equations using the usual four dimensions of spacetime, and included a Newtonian approximation for gravity using the variables made available by invoking a fifth dimension. He wanted a relativistically correct formalism, so his gravity field equations included some terms involving time. They were required because of the assumed speed of light propagation velocity of the gravity field – where Newton’s gravity theory has no time-dependent terms as gravity “propagates” instantaneously. You might think all of this not particularly interesting, because it is well-known that special relativity theory (SRT) hasn’t really got anything to do with gravity – notwithstanding that you can write down modified Newtonian gravity field equations that are relativistically correct (technospeak: “Lorentz invariant”).
But this isn’t quite right. Special relativity has inertia implicitly built right into the foundations of the theory. Indeed, SRT is only valid in “inertial” frames of reference.[ref]Inertial reference frames are those in which Newton’s first law is valid for objects that do not experience external forces. They are not an inherent part of spacetime per se unless you adhere to the view that spacetime itself has inertial properties that cause inertial reaction forces. This is not the common view of the content of SRT where inertial reaction forces are attributed to material objects themselves, not the spacetime in which they reside.[/ref] So, consider the most famous equation in all physics (that Einstein published as an afterthought to SRT): E=mc2. But write it as Einstein first did: m=E/c2. The mass of an object – that is, its inertia – is equal to its total energy divided by the square of the speed of light. [Frank Wilczek has written a very good book about this: The Lightness of Being.] If inertia and gravity are intimately connected, then since inertia is an integral part of SRT, gravity suffuses SRT, notwithstanding that it does not appear explicitly anywhere in the theory.[ref] As a technical note it is worth mentioning that in GRT inertial reaction forces arise from gravity if space is globally flat, as in fact it is measured to be, and global flatness is the distinctive feature of space in SRT. This, however, does not mean that spacetime has inherent inertial properties.[/ref] Are gravity and inertia intimately connected? Einstein thought they were. A well known part of this connection is the “Equivalence Principle” (that inertial and gravitational forces are the same thing) but there is an even deeper notion needing attention. He gave this notion its name: Mach’s principle, for Einstein attributed the idea to Ernst Mach (of Mach number fame).[ref] Another technical note: spacetime figures into the connection between gravity and inertia as the structure of spacetime is determined by the distribution of all gravitating stuff in the universe in the field equations of GRT. So if gravity and inertia are the obverse and reverse of the same coin, the structure of spacetime is automatically encompassed. Spacetime per se only acquires inertial properties if it is ascribed material properties – that is, it gravitates. Interestingly, if “dark energy” is an inherent property of spacetime, it gravitates.[/ref]
What is Mach’s principle? Well, lots of people have given lots of versions of this principle, and protracted debates have taken place about it. Its simplest expression is: Inertial reaction forces are produced by the gravitational action of everything that gravitates in the universe. But back in 1997 Herman Bondi and Joseph Samuel, answering an argument by Wolfgang Rindler, listed a dozen different formulations of the principle. Generally, they fall into one of two categories: “relationalist” or “physical”. In the relationalist view, the motion of things can only be related to other things, but not to spacetime itself. Nothing is said about the interaction (via fields that produce forces) of matter on other matter. The physical view is different and more robust as it asserts that the principle requires that inertial reaction forces be caused by the action of other matter, which depends on its quantity, distribution, and forces, in particular, gravity, as well as its relative motion. [Brian Greene not long ago wrote a very good book about Mach’s principle called The Fabric of the Cosmos. Alas, he settled for the “relationalist” version of the principle, which turns out to be useless as far as rapid spacetime transport is concerned.]
The simplest “physical” statement of the principle, endorsed by Einstein and some others, says that all inertial reaction forces are produced by the gravitational action of chiefly the distant matter in the universe. Note that this goes a good deal farther than Einstein’s Equivalence Principle which merely states that the inertial and gravitational masses of things are the same (and, as a result, that all objects “fall” with the same acceleration in a gravity field), but says nothing about why this might be the case. Mach’s principle provides the answer to: why?
Guided by Mach’s principle and Luchak’s Newtonian approximation for gravity – and a simple calculation done by Dennis Sciama in his doctoral work for Paul Dirac in the early 1950s – it is possible to show that when extended massive objects are accelerated, if their “internal” energies change during the accelerations, fluctuations in their masses should occur. That’s the purchase on gravity and inertia you need. (Ironically, though these effects are not obviously present in the field equations of GRT or electrodynamics, they do not depend on any novel coupling of those fields. So, no “new physics” is required.) But that alone is not enough. You need two more things. First, you need experimental results that show that this theorizing actually corresponds to reality. And second, you need to show how “Mach effects” can be used to make the Jupiter masses of exotic matter needed for stargates and warp drives. This can only be done with a theory of matter that includes gravity. The Standard Model of serious physics, alas, does not include gravity. A model for matter that includes gravity was constructed in 1960 by three physicists of impeccable credentials. They are Richard Arnowitt (Texas A and M), Stanley Deser (Brandeis), and Charles Misner (U. of Maryland). Their “ADM” model can be adapted to answer the question: Does some hideously large amount of exotic matter lie shrouded in the normal matter we deal with every day? Were the answer to this question “no”, you probably wouldn’t be reading this. Happily, the argument about the nature of matter and the ADM model that bears on the wormhole problem can be followed with little more than high school algebra. And it may be that shrouded in everyday stuff all around us, including us, is the Jupiter mass of exotic matter we want. Should it be possible to expose the exotic bare masses of the elementary particles that make up normal matter, then stargates may lie in our future – and if in our future, perhaps our present and past as well.
The physics that deals with the origin of inertia and its relation to gravitation is at least not widely appreciated, and may be incomplete. Therein lie opportunities to seek new propulsion physics. Mach’s principle and Mach effects is an active area of research into such possibilities. Whether these will lead to propulsion breakthroughs cannot be predicted, but we will certainly learn more about unfinished physics questions along the way.
REFERENCES
More technical and extensive discussions of some of the issues mentioned above are available in the peer reviewed literature and other sources. A select bibliography of some of this material is provided below. Here at Centauri Dreams you will shortly find more recent and less technical treatments available. They will be broken down into three parts. One will deal with the issues surrounding the origin of inertia and the prediction of Mach effects [tenatively titled “Mach’s principle and Mach effects”]. The second will present recent experimental results [tentatively titled “Mach effects: Recent experimental results”]. And the third will be an elaboration of the modifications of the ADM model that suggest exotic matter may be hiding in plain sight all around us [tentatively titled “Stargates, Mach’s principle, and the structure of elementary particles”]. The first two pieces will not involve very much explicit mathematics. The third will have some math, but not much beyond quadratic equations and high school algebra.
Greene, Brian, The Fabric of the Cosmos: Space, Time, and the Texture of Reality (Knopf, New York, 2004).
Sagan, Carl, Contact (Simon and Schuster, New York, 1965).
Wilczek, Frank, The Lightness of Being: Mass, Ether, and the Unification of Forces (Basic Books, New York, 2008).
Alcubierre, M., “The warp drive: hyper-fast travel within general relativity,” Class. Quant. Grav. 11 (1994) L73 – L77. The paper where Alcubierre writes out he metric for warp drives.
Arnowitt, R., Deser, S., and Misner, C.W., “Gravitational-Electromagnetic Coupling and the Classical Self-Energy Problem,” Phys. Rev. 120 (1960a) 313 – 320. The first of the ADM papers on general relativistic “electrons”.
Arnowitt, R., Deser, S., and Misner, C.W., “Interior Schwartzschild Solutions and Interpretation of Source Terms,” Phys. Rev 120 (1960b) 321 – 324. The second of the ADM papers.
Bondi, H. and Samuel, J., “The Lense-Thirring effect and Mach’s principle,” Phys. Lett. A 228 (1997) 121 – 126. One of the best papers on Mach’s principle.
Luchak, George, “A Fundamental Theory of the Magnetism of Massive Rotating Bodies,” Canadian J. Phys. 29 (1953) 470 – 479. The paper with the formalism for the Schuster-Blackett effect.
Morris, M.S. and Thorne, K. S., “Wormholes in spacetime and their use for interstellar travel: A tool for teaching general relativity,” Am. J. Phys. 56 (1988) 395 – 412. The paper where Kip Thorne and his then grad student Michael Morris spelled out the restrictions set by general relativity for interstellar travel. Their “absurdly benign wormhole” solution is found in the appendix on page 410.
Sciama, D. “On the Origin of Inertia,” Monthly Notices of the Royal Astronomical Society 113 (1953) 34 – 42. The paper where Sciama shows that a vector theory of gravity (that turns out to be an approximation to general relativity) can account for inertial reaction forces when certain conditions are met.
Woodward, J.F., “Making the Universe Safe for Historians: Time Travel and the Laws of Physics,” Found. Phys. Lett. 8 (1995) 1 – 39. The first paper where essentially all of the physics of Mach effects and their application to wormhole physics is laid out.
- Other sources for Mach effects and related issues
“Flux Capacitors and the Origin of Inertia,” Foundations of Physics 34, 1475 – 1514 (2004). [Appendicies give a line-by-line elaboration of the derivation of Mach effects, and a careful evaluation of how Newton’s second law applies to systems in which Mach effects are present.]
“The Technical End of Mach’s Principle,” in: eds. M. Sachs and A.R. Roy, Mach’s Principle and the Origin of Inertia (Apeiron, Montreal, 2003), pp. 19 – 36. [Contributed paper for a commemorative volume for the 50th anniversary of the founding of the Kharagpur campus of the Indian Institute of Technology. It is the only published paper where the wormhole term in Mach effects was sought.]
“Are the Past and Future Really Out There,” Annales de la Fondation Louis de Broglie 28, 549 – 568 (2003). [Contributed paper for a commemorative issue honoring the 60th anniversary of the completion of Olivier Costa de Beauregard’s doctoral work with Prince Louis de Broglie. The instantaneity of inertial reaction forces, combined with the lightspeed restriction on signal propagation of SRT, suggest that the Wheeler-Feynman “action-at-a-distance” picture of long range interactions is correct. This picture suggests that the past and future have some meaningful objective physical existence. This is explored in this paper, for Olivier Costa de Beauregard was one of the early proposers of the appropriateness of the action-at-a-distance picture in quantum phenomena.]
Presentations at STAIF and SPESIF (most with accompanying papers in the conference proceedings) yearly since 2000.
Presentation at the Society for Scientific Exploration meeting in June, 2010, now available in video format on the SSE website.
Presentation: Why Science Fiction has little to fear from Science, at the 75th Birthday Symposium for John Cramer, University of Washington, September 2009.
The Space Show [3/20/2007]
The Space Show [3/3/2009]
by Paul Gilster | May 20, 2011 | Exotic Physics |
The far future may be a lonely place, at least in extragalactic terms. Scientists studying gravity’s interactions with so-called dark energy — thought to be the cause of the universe’s accelerating expansion — can work out a scenario in which gravity dominated in the early universe. But somewhere around eight billion years after the Big Bang, the continuing expansion and consequent dilution of matter caused gravity to fall behind dark energy in its effects. We’re left with what we see today, a universe whose expansion will one day spread galaxies so far apart that any civilizations living in them won’t be able to see any other galaxies.
The initial dark energy findings, released in 1998, were based on Type Ia supernovae, using these as ‘standard candles’ which allowed us to calculate their distance from Earth. Now we have new data from both the Galaxy Evolution Explorer satellite (drawing on a three-dimensional map of galaxies in the distant universe containing hundreds of millions of galaxies) and the Anglo-Australian Telescope (Siding Spring Mountain, Australia). Using this information, scientists are studying the pattern of distance between individual galaxies. Here we have not a ‘standard candle’ but a ‘standard ruler,’ based on the tendency of pairs of galaxies to be separated by roughly 490 million light years.
A standard ruler is an astronomical object whose size is known to an approximate degree, one that can be used to determine its distance from the Earth by measuring its apparent size in the sky. The new dark energy investigations used a standard ruler based on galactic separation. Scientists believe that acoustic pressure waves ‘frozen’ in place approximately 370,000 years after the Big Bang (the result of electrons and protons combining to form neutral hydrogen) define the separation of galaxies we see. The pressure waves, known as baryon acoustic oscillations, left their imprint in the patterns of galaxies, accounting for the separation of galactic pairs. This provides a standard ruler that can be used to measure the distance of galaxy pairs from the Earth — closer galaxies appear farther apart from each other in the sky.
We’re looking, then, at patterns of distance between galaxies, using bright young galaxies of the kind most useful in such work. Galaxy Evolution Explorer identified the galaxies to be studied, while the Anglo-Australian Telescope was used to study the pattern of distance between them. Folding distance data into information about the speeds at which galaxy pairs are receding confirms what the supernovae studies have been telling us, that the universe’s expansion is accelerating. GALEX’s ultraviolet map also shows how galactic clusters draw in new galaxies through gravity while experiencing the counterweight of dark energy, which acts to tug the clusters apart, slowing the process.
Chris Blake (Swinburne University of Technology, Melbourne), lead author of recent papers on this work, says that theories that gravity is repulsive when acting at great distances (an alternative to dark energy) fail in light of the new data:
“The action of dark energy is as if you threw a ball up in the air, and it kept speeding upward into the sky faster and faster. The results tell us that dark energy is a cosmological constant, as Einstein proposed. If gravity were the culprit, then we wouldn’t be seeing these constant effects of dark energy throughout time.”
Image: This diagram illustrates two ways to measure how fast the universe is expanding. In the past, distant supernovae, or exploded stars, have been used as “standard candles” to measure distances in the universe, and to determine that its expansion is actually speeding up. The supernovae glow with the same intrinsic brightness, so by measuring how bright they appear on the sky, astronomers can tell how far away they are. This is similar to a standard candle appearing fainter at greater distances (left-hand illustration). In the new survey, the distances to galaxies were measured using a “standard ruler” (right-hand illustration). This method is based on the preference for pairs of galaxies to be separated by a distance of 490 million light-years today. The separation appears to get smaller as the galaxies move farther away, just like a ruler of fixed length (right-hand illustration). Credit: NASA/JPL-Caltech.
Dark energy is still a huge unknown, but Jon Morse, astrophysics division director at NASA Headquarters in Washington, thinks the new work provides useful confirmation:
“Observations by astronomers over the last 15 years have produced one of the most startling discoveries in physical science; the expansion of the universe, triggered by the Big Bang, is speeding up. Using entirely independent methods, data from the Galaxy Evolution Explorer have helped increase our confidence in the existence of dark energy.”
For more, see Blake et al., “The WiggleZ Dark Energy Survey: testing the cosmological model with baryon acoustic oscillations at z=0.6,” accepted for publication in Monthly Notices of the Royal Astronomical Society (preprint) and Blake et al., “The WiggleZ Dark Energy Survey: the growth rate of cosmic structure since redshift z=0.9,” also accepted at MNRAS (preprint).
by Paul Gilster | May 19, 2011 | Exoplanetary Science |
Gravitational microlensing to the rescue. We now have evidence for the existence of the rogue planets — interstellar wanderers moving through space unattached to any star system — that we talked about just the other day. It’s been assumed that such planets existed, because early solar systems are turbulent and unstable, with planetary migrations like those that lead to ‘hot Jupiters’ in the inner system. Moving gas giants into orbits closer to their star would cause serious gravitational consequences for other worlds in the system, ejecting some entirely.
But while we’ve been thinking in terms of detecting such worlds through auroral emissions like those produced by Jupiter, researchers at two microlensing projects have made a series of detections by using gravity’s effects upon spacetime. Specifically, a stellar system passing in front of a far more distant background star will warp the light of the background object. The resulting magnification and brightening flags the presence of the intermediate object, and surveys like Microlensing Observations in Astrophysics (MOA), based in New Zealand, have developed the necessary expertise to distinguish between intermediate stars and planets.
Both MOA, which scans the galactic center for these microlensing events, and the Optical Gravitational Lensing Experiment (OGLE), using a 1.3 meter telescope in Chile, have studied and built a case for the existence of up to 10 rogue planets of roughly Jupiter mass. Microlensing because of its nature picks up objects a long way from our stellar neighborhood — these average between 10,000 and 20,000 light years from Earth. Extrapolating from the lensing probabilities, the efficiency of their equipment and the rate of lensing, the researchers now conclude that there could be as many as 400 billion rogue planets in the Milky Way.
How Do Rogue Planets Form?
That’s a big number, but at this point we’re still shooting in the dark. After all, lower-mass planets should be ejected from young solar systems more frequently than the gas giants this work has detected, which is why planet hunter Debra Fischer (Yale University) told Nature News in a related story that lighter planets “…might be littering the galaxy.” What a scenario, particularly given the possibility that a hydrogen atmosphere could trap enough heat to allow the presence of liquid oceans. Unfortunately, the current survey was not sensitive enough to detect planets smaller than Saturn.
This work is getting huge play in the press, but I think it raises as many questions as it answers, for the scenario shifts depending on how these wandering worlds were formed. The current work draws on the idea that they were the result of ejection from solar systems. In fact, David Bennett (University of Notre Dame), a co-author of the study in Nature, assumes ejection as the primary mechanism:
“If free-floating planets formed like stars, then we would have expected to see only one or two of them in our survey instead of 10,” Bennett said. “Our results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth.”
And if ejection is the driver here, then we should assume a huge population of low-mass planets moving through space without any star, just like these gas giants. But if there is another formation mechanism at work (Greg Laughlin speculates about this in the Nature News article I linked to above), then the low-mass wanderers are much less prevalent. Right now we just don’t know, because we would need a formation mechanism that would account for objects not much larger than Jupiter, “…something more similar to that of a tiny star than a giant planet,” Laughlin adds. Whether or not ejection is the mechanism thus becomes crucial for any hypothesis about rogue ‘Earths.’
Outer Orbits and Unseen Hosts
Also in play is the question of whether the ten detections could be of gas giants in planetary orbits around stars that were simply not detected. The study sees no host stars within 10 AU, a figure that remains relatively close to any potential host. We don’t have a firm answer, and I see that Alan Boss (Carnegie Institution) told the New York Times‘ Dennis Overbye that this scenario is the more likely one. If that’s the case, then we should look with even greater interest at data from the WISE (Wide-Field Infrared Survey Explorer) mission, which should have been able to spot any gas giant lurking in the distant regions of the Oort Cloud. Ten detections like this would imply such outer orbits may be common around stars.
Where we go from here seems obvious: We need to confirm there are no host stars. If we do, then the presence of twice as many rogue gas giants as there are stars in the galaxy is enough to take the breath away, whether or not they’re in the company of rogue ‘Earths.’ The planned Wide-Field Infrared Survey Telescope (WFIRST) might be able to make a detection of such Earth-mass rogue planets and give us some constraints on their numbers. We’ll also learn, as we continue the study of galactic wanderers, to tighten up our theories of planet formation and migration to account for the suddenly increased population of sub-stellar objects among the stars.
The paper is Sumi et al., “Unbound or distant planetary mass population detected by gravitational microlensing,” Nature 473 (19 May 2011), 349-352 (abstract).
by Paul Gilster | May 18, 2011 | Outer Solar System |
We looked recently at Titan Mare Explorer (TiME), a mission to land a probe on Titan’s Ligeia Mare, a methane-ethane sea that would be observed for an extended period by this floating observatory. But I don’t want to pass too quickly over Comet Hopper, one of the other missions being considered by NASA’s Discovery Program. This one is a proposal out of the University of Maryland that would land on comet 46P/Wirtanen not once but multiple times, observing the changes on the comet and in its innermost coma as it interacts with the Sun.
The innermost coma is the comet’s atmosphere immediately above the nucleus, where cometary jets and outgassing originate. Jessica Sunshine, principal investigator for Comet Hopper, says the idea is to watch how surface and coma change through a solar approach:
“We’ve had some amazing cometary flybys but they have given us only snapshots of one point in time of what a comet is like. Comets are exciting because they are dynamic, changing throughout their orbits. With this new mission, we will start out with a comet that is in the cold, outer reaches of its orbit and watch its activity come alive as it moves closer and closer to the Sun.”
Comet Hopper is being portrayed as a reconnaissance mission for an even more ambitious effort, the Comet Nucleus Sample Return mission, proposed to the European Space Agency’s Cosmic Vision program as a way to retrieve three sample cores from different locations on a cometary nucleus. We’ll see how Comet Hopper fares as it works its way through the Discovery Program process. Titan Mare Explorer and the Mars-oriented Geophysical Monitoring Station (GEMS) have also received funding from the program to develop their respective mission concepts, with one of the three to be chosen after a 2012 review.
Homing in on Hartley 2
Meanwhile, comets remain in the news in the form of new findings about Comet 103P/Hartley 2, recently studied by the EPOXI spacecraft in a 2010 flyby. One research effort draws on imagery from the 2.1-meter instrument at Kitt Peak in Arizona, using a blue filter to isolate the light emitted by cyanogen (CN) molecules in the coma of the comet. The variations over short time scales were striking, according to Nalin H. Samarasinha (Planetary Science Institute):
“The rotational state of a comet’s nucleus is a basic physical parameter needed to accurately interpret other observations of the nucleus and coma. Analysis of these cyanogen features indicates that the nucleus is spinning down and suggests that it is in a state of a dynamically excited rotation. Our observations have clearly shown that the effective rotation period has increased during the observation window.”
Image: An image of comet Hartley 2 taken on Sept. 3, 2010 at the 2.1 meter telescope at the Kitt Peak National Observatory near Tucson, Ariz. In this image, red denotes regions where CN gas is more abundant. The image is enhanced by removing the underlying background in order to highlight the jet feature present. North is up and east is to the left. The image is nearly 50,000 miles across and the comet nucleus which is not resolved is at the center. The red streaks are star trails. Credit: PSI.
The findings indicate just how active Hartley 2 is. Its nucleus is a scant two-kilometers long, but its rotational changes are clearly being caused by jets of gases emitted from the icy body. This is exactly the kind of information we need as we continue to study both comets and asteroids to learn how to deal with any future objects that may be on a collision course with the Earth. Samarasinha points out that the material properties of comets are crucial for such purposes:
“…fortunately for the first time, we are on the threshold of our technical knowhow to mitigate such a hazardous impact,” Samarasinha said. “In order to do that we need to know the material properties of comets. The most appropriate mitigation strategy for a strong rigid body is different from that for a weakly bound agglomerate.”
Exploring a Cometary Core
At NASA GSFC, Michael Mumma and team collected data from telescopes in Hawaii and Chile as part of the worldwide effort to add to our knowledge of Hartley 2, folding in their findings with images taken by EPOXI. The combined work indicates that the comet’s core is not uniform, but appears to contain two and possibly three different kinds of ice. This is where a mission like Comet Hopper could be valuable, with its ability to make in situ investigations at multiple sites on the surface of the nucleus. The ices in Hartley 2 are primarily made of water, along with traces of other kinds of molecules. These remain frozen until warmed by the Sun, when they become swept up into the coma.
Knowing of the comet’s rapid rotation, the researchers studied how these molecules were detected during each rotation, finding not only swift changes in the amount of water but equivalent changes in the amounts of the other gases. “This is the first time anyone has seen an entire suite of these gases change in the same way at the same time,” says Mumma. It’s a significant result because cometary gases are usually studied one at a time. Examining the simultaneous behavior of a whole range of gases helps scientists get a better idea of the comet’s composition.
But EPOXI had shown a large variation in the release of carbon dioxide relative to water. What seems to be happening is that chunks of water ice are glued in the cometary core by frozen carbon dioxide. The latter evaporates before the water ice, dragging ice grains out of the comet that later evaporate to produce water vapor in the coma. The process hasn’t been seen in other comets. Moreover, EPOXI found variations in the location of carbon dioxide, showing that the composition of the core changes from one region to another.
Image: Jets spew out ice and carbon dioxide from one end of comet Hartley-2 in this EPOXI image, while water vapor gets released from the middle region. The differences suggest that the comet’s core is made of at least two different ices. Ground-based measurements suggest the presence of a third ice. Credit: NASA/JPL-Caltech/UMD.
Mumma’s team confirms the finding, examining four types of gas to see how they were produced. Water and methanol came off the comet in all directions. Says Mumma:
“Because they are found together, we infer that they come from the same chunks of ice. So, we have water ice with methanol in it, and we have carbon dioxide ice. Both are in the comet’s core. We may also have a third type of ice, made from ethane [the ethane was released strongly in one direction]. This is actually rather profound. It suggests that some molecules, such as methanol, may be mixed with water, while others, such as ethane, are not. This isn’t the way we’ve thought of comets, before now.”
Mumma says Hartley 2 could be “…the first of a new breed.” But learning more about how comets are formed and how they behave as they make their approach to the Sun is clearly high on the priority list for near-term missions, whether Comet Hopper flies or not. Comets offer not only a window into the early history of the Solar System but a possible source of materials as we look toward the human exploration of the planets. Knowing their properties will also help us map strategies in the unlikely but not inconceivable event that a cometary impact with the Earth looms in our future.
The Samarasinha paper is “Rotation of Comet 103P/Hartley 2 from Structures in the Coma,” Astrophysical Journal Letters 734 No. 1 (16 May 2011), L3 (abstract). See also other papers in the same issue, including Weaver et al., “The Carbon Monoxide Abundance in Comet 103P/Hartley 2 During the EPOXI Flyby” (abstract), Dello Russo et al., “The Volatile Composition and Activity of Comet 103P/Hartley 2 During the EPOXI Closest Approach” (abstract) and Meech et al., “EPOXI: Comet 103P/Hartley 2 Observations from a Worldwide Campaign” (abstract)
by Paul Gilster | May 17, 2011 | Exoplanetary Science |
Imagine a planet far more massive than Jupiter and spinning faster than Jupiter’s 10 hour rotation. Throw in a large nearby moon and the associated auroral effects that would occur as the moon moved through fields of plasma trapped in the planet’s magnetic field. The scenario isn’t all that different from what we see happening between Jupiter and Io. But here’s the kicker: Put planet and moon far away from any star, a rogue planet scenario of the kind recently discussed by Dorian Abbot and Eric Switzer, who called such rogue planets ‘Steppenwolfs.’ I jumped on that idea in a Centauri Dreams post last February because interstellar planets have always fascinated me.
Abbot and Switzer were interested in whether a rogue planet could support life, finding in their paper that a planet just 3.5 times as massive as the Earth, and with the same basic composition and age, could sustain a liquid ocean under layers of insulating water ice and frozen atmosphere. But our rogue gas giant offers something the smaller world cannot. The auroral effects created by planet and moon could be detected out to a distance of 185 light years, provided the planet were big enough and we had the appropriate equipment, which in this case means the Square Kilometer Array radio telescope. So says Heikki Vanhamaki of the Finnish Meteorological Institute in Helsinki.
Image: Aurora on Jupiter, as detected by the Hubble Space Telescope. Can we use the emissions of such events to detect a planet wandering between the stars? Credit: NASA and the Hubble Heritage Team (STScI/AURA) Acknowledgment: NASA/ESA, John Clarke (University of Michigan).
You may recall that we also looked recently at Jonathan Nichols’ work at the University of Leicester on how an exoplanet aurora could be used as a detection tool. Vanhamaki’s calculations show that such an aurora could be produced by a rogue planet in two different ways. Movement through interstellar plasma could generate an aurora, but the scenario above, in which a moon moves through magnetically trapped plasma, produces a signature 100 times stronger. The scientist calls detection of a rogue Jupiter extremely unlikely — “perhaps nearly impossible” — in the forseeable future, but he adds that there may be 2800 interstellar planets within 185 light years, the distance within which he calculates a Square Kilometer Array might be effective.
One or more of them will have to be large enough, at least eight Jupiter masses, and orbited by a moon inducing auroral activity, for any of this to work. It’s the slimmest of chances, but earlier studies of known ‘hot Jupiters’ have predicted that some of them should produce cyclotron radiation of between 1014 and 1016 W, orders of magnitude higher than the values Vanhamaki discusses for typical interstellar planets, although these emissions have not yet been detected. Nonetheless, exoplanet aurorae seem like a promising area of research, and the suspicion that biological processes could go to work on a rogue planet makes the hunt more enticing. Says Vanhamaki:
“It has been speculated that Earth-like rogue planets could have very thick atmosphere that keeps them relatively warm, or moons of giant rogue planets could experience tidal heating and have oceans beneath their icy surface.”
It’s a scenario that once again turns our notion of the habitable zone on its head. We’ve gone from describing a habitable zone as a region around a star where liquid water could be maintained at the surface to seeing the possibilities of life under frozen oceans, a zone that could extend all the way to the Kuiper Belt. A rogue planet would be the ultimate habitable zone extension. If it has no reflected starlight by which to spot it, will the auroral method come into its own with a breakthrough detection?
Recall, too, that Christopher McKay (NASA Ames) has looked at a variety of scenarios involving Titan-like worlds, placing them around several red dwarf stars to see where they could host oceans of liquid methane. One idea his team looked at was a rogue Titan in interstellar space. The conclusion: A Titan with 20 times the geothermal heat of Earth could keep its current surface temperature in the absence of any star. Alternatively, an atmosphere 20 times thicker than Titan’s could retain enough heat to make a surface ocean viable. These are tall orders, but a larger Titan with a thicker atmosphere might sustain a liquid methane sea.
The Vanhamaki paper is “Emission of cyclotron radiation by interstellar planets,” Planetary and Space Science, published online 17 April 2011 (abstract). Christopher McKay’s paper is “Titan under a red dwarf star and as a rogue planet: requirements for liquid methane,” Planetary and Space Science, published online 2 April 2011 (abstract). Astrobiology Magazine covers the story here.
