by Paul Gilster | Dec 12, 2008 | Exoplanetary Science |
It’s tantalizing to speculate that there might be a brown dwarf system nearer to us than the Alpha Centauri stars. The odds seem long, but the discovery of a pair of brown dwarfs that are each no more than a millionth as bright as the Sun makes for exciting reading. The objects were originally cataloged by the Two Micron All Sky Survey (2MASS) as a single brown dwarf identified as 2MASS J09393548-2448279, but Adam Burgasser (Massachusetts Institute of Technology) has been able to show that the ‘object’ is actually a pair of the faint dwarfs.
Here the Spitzer Space Telescope was the instrument of choice, showing that 2M 0939’s brightness was twice what would have been expected from its temperature, which was determined to be in the range of 565 to 635 Kelvin (560 to 680 degrees Fahrenheit). The implication was that this is a brown dwarf binary, two dwarfs each with a mass some thirty to forty times that of Jupiter. And while the objects are a million times fainter than the Sun in total light, they’re a billion times fainter in visible light alone.
2M 0939 becomes the fifth closest known brown dwarf, but what gets my attention is Burgasser’s comment that this discovery may be the tip of the iceberg. In an MIT news release, he puts it this way:
“These brown dwarfs are the lowest power stellar light bulbs in the sky that we know of. In this regime [of faintness] we expect to find the bulk of the brown dwarfs that have formed over the lifetime of the galaxy. So in that sense these objects are the first of these ‘most common’ brown dwarfs, which haven’t been found yet because they are simply really faint.”
Faint enough for there to be examples closer to Earth than this one, and indeed closer than the Centauri stars? It seems unlikely, but it’s a stimulating thought, and the work on 2M 0939 shows just how difficult it may be to be sure. After all, Burgasser’s team spent three years studying the object with data from the Anglo-Australian Observatory before they could come up with a definitive read on its distance. That work was crucial, when combined with the Spitzer infrared observations, in measuring the object’s brightness, and thus deducing its true nature as a binary from the contrast with observed temperatures.
The vastness of interstellar space naturally staggers the imagination, but it’s striking that as we learn more, we realize how many objects are out there that we know little about. A true stellar census of the Milky Way, for example, would need to include brown dwarfs, but we aren’t prepared at this point to say how common they are. We do know that the larger and hotter M-dwarfs comprise more than 70 percent of all stars in the galaxy, and we also believe there must be planets ejected from their solar systems wandering through the interstellar depths. Get us the technology to make it to the Oort Cloud and we may find that the next truly interesting destination isn’t necessarily four light years away.
The paper is Burgasser et al., “2MASS J09393548?2448279: The Coldest and Least Luminous Brown Dwarf Binary Known?” Astrophysical Journal Letters 689 (December 10 2008), pp. L53–L56 (abstract).
by Paul Gilster | Dec 11, 2008 | Exoplanetary Science |
Among the many things that boggle my mind is the fact that we can learn things about the atmosphere of planets that we can’t even see. Take well-studied HD 189733b, a gas giant in close orbit around a K2-class star some 63 light years from us. No one has ever laid eyes on this beast, either in infrared or optical light. But that’s of little moment to the Hubble telescope, among whose tools is NICMOS — the Near Infrared Camera and Multi-Object Spectrometer. It and a lot of ingenuity get results.
A transiting planet like HD 189733b moves behind its parent star every two days or so. When that happens, light from the star itself (the planet now being behind the star) can be compared to the combined light of planet and star when both are facing the Earth. Any emissions from the planet can be examined, a useful window into its atmosphere.
Using such techniques, Mark Swain (Jet Propulsion Laboratory) and team have been able to detect carbon dioxide and carbon monoxide on this world, which had previously yielded methane and water vapor in Hubble and Spitzer Space Telescope observations. HD 189733b is too hot for life, but the find is interesting nonetheless:
“The carbon dioxide is kind of the main focus of the excitement,” says Swain, “because that is a molecule that under the right circumstances could have a connection to biological activity as it does on Earth. The very fact that we’re able to detect it, and estimate its abundance, is significant for the long-term effort of characterizing planets both to find out what they’re made of and to find out if they could be a possible host for life.”
Image: An artist’s impression of the Jupiter-size extrasolar planet, HD 189733b, being eclipsed by its parent star. Astronomers using the Hubble Space Telescope have measured carbon dioxide and carbon monoxide in the planet’s atmosphere. The planet is a “hot Jupiter,” which is so close to its star that it completes an orbit in only 2.2 days. Under the right conditions, on a more Earth-like world, carbon dioxide can indicate the presence of extraterrestrial life. This observation demonstrates that chemical biotracers can be detected by space telescope observations. Credit: Credit: ESA, NASA, M. Kornmesser (ESA/Hubble), and STScI.
Note, too, that this is the first near-infrared emission spectrum ever obtained for an exoplanet. All of this plays nicely into plans for using the James Webb Space Telescope, scheduled for launch in 2013, for Webb astronomers plan to look for biomarkers on distant planets, some perhaps as small as the Earth, using spectroscopic techniques that work best in the near-infrared. Swain also notes that studying molecules in exoplanet atmospheres can help to reveal information about the weather on these worlds, as described in the paper on this work:
In a previous paper, we presented a transmission spectrum of this planet at the terminator, in which methane is seen to be more abundant… in the higher, cooler regions of the terminator region atmosphere. However, it is difficult to compare directly the previous terminator results with our current dayside results because they probe atmospheric regions with significantly different temperatures and altitudes on this highly irradiated planet. The development of sophisticated global circulation and chemistry models could significantly advance the state of the art in this respect.
The paper is Swain et al., “Molecular Signatures in the Near Infrared Dayside Spectrum of HD 189733b,” accepted by Astrophysical Journal Letters and available online.
by Paul Gilster | Dec 10, 2008 | Astrobiology and SETI |
Here I was all set to write about the discovery of carbon dioxide on HD 189733b when Alpha Centauri made its way back into the news. Twentieth Century Fox will be transmitting the re-make of the science fiction classic The Day The Earth Stood Still to Alpha Centauri on Friday the 12th, timing the event to coincide with the film’s opening here on Earth. The transmission is being handled by Florida-based Deep Space Communications Network, a private organization that offers transmission services to the public (not to be confused with the Deep Space Network that manages communications with our planetary probes).
Why does Deep Space Communications Network offer transmission services to the stars? From its FAQ:
For a number of reasons, one is because we have the equipment, and the know how so we can, and also because we thought it would be an interesting public service that is not currently available.
We’re doing it because we can…
This dubious news comes on the heels of the in many ways excellent National Geographic special Journey to the Edge of the Universe. I’m always fascinated with the way the media handle nearby stars and the planets that may orbit them, especially as our inventory of confirmed planets continues to grow, and the show’s graphics were superb, its narration gripping. But how puzzling to run into a fundamental misunderstanding about our nearest stellar neighbors.
The putative travelers have moved out through the Solar System, passing (an ingenious touch) the various probes and artifacts we humans have scattered from Mercury out to the Kuiper Belt. As we move to the nearest stars, we pass what is obviously the red dwarf Proxima Centauri and make for the binaries Centauri A and B. Describing them, the narrator says, “Not one but three stars, spinning around each other locked in a celestial standoff, each star’s gravity attracting the other, their blazing orbital speed keeping them apart.”
And then this: “Get between them and we’d be vaporized.”
Not a chance. We don’t yet know whether there are planets around Centauri A or B. But we do know that there are stable orbits around these stars, and that both of them could have planets in the habitable zone, where liquid water can flow on the surface. Their mean separation is 23 AU. There is, in other words, plenty of room between Centauri A and B for a spacecraft to move without danger of being vaporized. I feel like I’m nitpicking given the intense effort that went into this production, but it seems important to clear up misconceptions that are widely distributed.
As with the show’s treatment of Gliese 581 c. The National Geographic special shows the planet as a living world of continents and oceans, and indeed, the discovery announcement made it appear that 581 c was squarely in the habitable zone of this tiny red dwarf. But almost every subsequent study has shown this to be deeply unlikely — conditions on Gliese 581 c are probably much more like Venus than Earth. Moreover, although the show depicted the planet as rotating, so that the day/night terminator continued to shift, it’s much more likely that this planet is tidally locked to its primary, one side always facing the star.
We ran through the entire GL 581 c story here over the past few years — you can use the search function to pull these stories up. An initial euphoria (all too incautiously embraced in my opening story on the discovery) quickly gave way to skepticism. Indeed, if there is a habitable planet in the GL 581 system, it may (just possibly) be GL 581 d.
As far as Twentieth Century Fox goes, my thoughts on METI are no secret. But look, there are advocates who make a strong case for METI, just as there are reputable and serious scientists who question whether brightening our signature in the electromagnetic spectrum is a good idea. I can listen and learn from both, but what I find deeply troubling is the notion that we can take a serious issue — one that deserves thoughtful study in many disciplines — and casually throw it out the window by yet another fait accompli.
Is it too late to lock down the mania for METI? Probably, as we’re beaming everything from movies and ads for the Doritos to watch them by seemingly at will. And a case can be made that our TV and radio signals are already reaching nearby stars, and that an advanced civilization could pick them up, as well as detecting biomarkers in our atmosphere. That’s plausible, but a sudden and deliberate brightening of our signal for whatever purpose strikes me as unwise given how little we know about the conditions that surround us. I doubt seriously that such transmissions endanger us, but the point is, we don’t know, and in the absence of that knowledge, caution and further study seem a more prudent course.
by Paul Gilster | Dec 9, 2008 | Advanced Rocketry |
Back in 1966, the Jet Propulsion Laboratory’s Dwain Spencer laid out the principles of a fusion engine that burned deuterium and helium-3 (an isotope of helium with a nucleus of two protons and one neutron). Deuterium and helium-3 make a good combination for rocket propulsion because a fusion-based drive based on them releases one-hundredth the amount of radioactive neutrons than deuterium/tritium. A spacecraft using such an engine would, in other words, require far less shielding. And even more to the point, the protons and alpha particles produced by the reaction can be readily manipulated by a magnetic nozzle.
This was the background in 1971, when physicist Friedwardt Winterberg published a paper on fusion ignition using intense beams of electrons, speculating that such techniques could be used in rocket propulsion. Winterberg’s work took place in a context of energetic study, with newly declassified work becoming available that examined the use of lasers in igniting fusion. At Lawrence Livermore Laboratory, Rod Hyde was examining the use of deuterium and helium-3 in the form of frozen pellets that would be exploded at a rate of hundreds per second within an inertial confinement fusion reactor, using lasers to ignite the reaction.
All this comes to mind because of two papers recently made available by Winterberg, and the history of his own work as tapped by Project Daedalus. The latter was the British Interplanetary Society’s ambitious attempt to design a starship based on the technology of the 1970s extrapolated as far forward as was consistent with our knowledge of physics and engineering. The committees given the task of getting a payload to Barnard’s Star examined six propulsion concepts and finally decided on pulsed fusion using deuterium and helium-3. Winterberg’s work on igniting fusion via electron beams was a key factor in designing the vast Project Daedalus vehicle.
Image: The Project Daedalus craft approaches its flyby of Barnard’s Star. Credit: Adrian Mann.
The recent papers thus offer a look at still viable concepts whose evolution has played an important role in our interstellar thinking. Now at the University of Nevada (Reno), Winterberg explores how to create fusion ignition in pure deuterium without the use of a fission ‘trigger.’ To do this, he would use high-voltage pulses generated through what he describes as a ‘super Marx generator’ that ultimately creates a 107 Ampere-GeV proton beam in a compressed deuterium cylinder (a Marx generator is a type of circuit designed to produce such pulses).
Laser fusion will ultimately not work, because for a high gain the intense light flash of a thermonuclear microexplosion is going to destroy the entire optical laser ignition apparatus. The large Livermore laser is intended for weapons simulation. There, a low gain is sufficient.
It is the purpose of this communication to show, that with the proposed super Marx generator one may be able to ignite a pure deuterium thermonuclear micro-explosion.
How to adapt the concept for space propulsion? Winterberg examines a propulsion system based on deuterium fusion, one in which “…a thermonuclear detonation wave is ignited in a small cylindrical assembly of deuterium with a gigavolt-multimegampere proton beam, drawn from the magnetically insulated spacecraft acting in the ultrahigh vacuum of space as a gigavolt capacitor.” Here are some specifics as drawn from the paper:
The spacecraft is positively charged against an electron cloud surrounding the craft, and with a magnetic field of the order 104 G, easily reached by superconducting currents flowing in an azimuthal direction, it is insulated against the electron cloud up to GeV potentials. The spacecraft and its surrounding electron cloud form a virtual diode with a GeV potential difference. To generate a proton beam, it is proposed to attach a miniature hydrogen filled rocket chamber R to the deuterium bomb target, at the position where the proton beam hits the fusion explosive… A pulsed laser beam from the spacecraft is shot into the rocket chamber, vaporizing the hydrogen, which is emitted through the Laval nozzle as a supersonic plasma jet. If the nozzle is directed towards the spacecraft, a conducting bridge is established, rich in protons between the spacecraft and the fusion explosive. Protons in this bridge are then accelerated to GeV energies, hitting the deuterium explosive.
It’s an audacious concept, one that, as Winterberg says, uses the entire spacecraft for electrostatic energy storage:
There, toroidal currents flowing azimuthally around the outer shell of the spacecraft, not only magnetically insulate the spacecraft against the surrounding electron cloud, but the currents also generate a magnetic mirror field which can reflect the plasma of the exploding fusion bomb. In addition, the expanding bomb plasma can induce large currents, and if these currents are directed to flow through magnetic field coils positioned on the upper side of the spacecraft, electrons from there can be emitted into space surrounding the spacecraft by thermionic emitters placed on the inner side of these coils in a process called inductive charging. It recharges the spacecraft for subsequent proton beam ignition pulses.
Image: Superconducting “atomic” spaceship, positively charged to GeV potential, with azimuthal currents and magnetic mirror M by magnetic field B. F fusion minibomb in position to be ignited by intense ion beam I, SB storage space for the bombs, BS bioshield for the payload PL, C coils pulsed by current drawn from induction ring IR. e electron flow neutralizing space charge of the fusion explosion plasma. Credit: Friedwardt Winterberg.
Winterberg recently presented this material at a Jet Propulsion Laboratory workshop, and the papers are available at the arXiv site. They’re “Ignition of a deuterium micro-detonation with a gigavolt super marx generator,” available here, and “Deuterium microbomb rocket propulsion,” online here. Brian Wang studies these concepts on his NextBigFuture site, and I see that Adam Crowl, who passed the original URLs along, has written them up at Crowlspace. Winterberg is exploring a range of technologies whose chief interest from the interstellar perspective is that they may be achievable in the relatively near-term without assuming breakthroughs in known physics.
by Paul Gilster | Dec 8, 2008 | Research Tools |
Caleb Scharf is director of the Columbia Astrobiology Center and author of a new book I intended to mention in Saturday’s Notes & Queries section before running out of time. I want to be sure to insert it now, because if you’re getting serious about the study of astrobiology, you’ll want to know about this title. Extrasolar Planets and Astrobiology (University Science Books, 2008) is designed for university courses on the subject, with extensive background not only in the relevant physics and mathematics, but also in chemistry, biology and geophysics, studies the multi-faceted world of astrobiology melds into a complex whole.
The book is actually based on the upper-level course Scharf has been teaching at Columbia. The author tells me in an e-mail that his intent is specifically to reach students serious about moving into the discipline: “The aim is to provide the basis for students to gain a real understanding of how to actually do research on exoplanets, as well as some of the broader science encompassed by astrobiology.” Making the point are the exercises designed for each chapter to draw newcomers into research and provide examples for calculation. I also want to mention the book’s online component, where news items are cross-referenced with the book.
And I like what planet-hunter Geoff Marcy has to say in his foreword, especially in its hint of long-term interstellar travel:
For the future, NASA and the Jet Propulsion Laboratory have developed the Space Interferometry Mission that will use the interference of light waves gathered by a spaceborne pair of telescopes to detect earth-like planets, and measure their masses, around nearby stars. Just over the horizon are plans for a spaceborne telescope that blocks the glare of nearby stars, allowing us to take images of Earth-like planets and to determine their chemical composition from their spectra. Any worlds having oxygen atmospheres and surface oceans will smell fishy from 40 light years. This census of habitable earths will fill GoogleGalaxy with ports-of-call for our grandchildren who will send robotic probes and later themselves, at least those with extreme daring and patience. The urge to explore these new worlds comes from our anthropological roots at Olduvai Gorge two million years ago. What sets us apart from the stones and the stars is our insatiable desire to understand our kinship with both.
Nicely put, and I especially like that GoogleGalaxy bit, the updated and searchable version of the Encyclopedia Galactica. Scharf looks hard at how we study planet and star formation, how we observe exoplanets and undertake chemical and biological modeling. I’m glad to see that he does not claim definitive status for the book in a field as malleable as this, but treats astrobiology as an ’emerging interdiscipline’ — exactly the right phrase — while his audience is “…the student or researcher in astronomy or physics, or possibly someone from the geophysical, chemical, or biological sciences, looking for a deeper understanding of the ‘astro’ in astrobiology.” Those looking for an astrobiology career will want Extrasolar Planets and Astrobiology on their shelves.
And thanks to all who have been asking about our Frontiers of Propulsion Science book, which is now turning up on the AIAA site. Although we had hoped for publication by the end of the year, it’s now looking like February is the likely target. Edited by Marc Millis and Eric Davis, Frontiers of Propulsion Science is a graduate/professional-level text and a first-ever compilation, as AIAA points out on the site, of the emerging science behind breakthrough concepts like warp drive and faster than light travel. In these areas we’re obviously still very early in the game:
This is a nascent field where a variety of concepts and issues are being explored in the scientific literature, beginning in about the early 1990s. The collective status is still in step 1 and 2 of the scientific method, with initial observations being made and initial hypotheses being formulated, but a small number of approaches are already at step 4, with experiments underway. This emerging science, combined with the realization that rockets are fundamentally inadequate for interstellar exploration, led NASA to support the Breakthrough Propulsion Physics Project from 1996 through 2002.
The hope is that a book like this can energize research so that more studies are performed, more papers written, and new compilations can begin to emerge on a regular basis. Our universities need the reference tools to bring structure to the courses that grow out of these studies, and we hope Frontiers of Propulsion Science is a step in that direction.
