by Paul Gilster | Jun 10, 2009 | Exoplanetary Science |
Planets around binary stars continue to be a major interest here, given our fascination with nearby Alpha Centauri. Thus the recent radio interferometry images captured by the Submillimeter Array radio telescope system (Mauna Kea) come right to the top of the queue. We’re looking at a young binary system called V4046 Sagittarii, providing a glimpse of planetary system formation occurring around two stars of roughly the Sun’s mass. This system is approximately 240 light years from our own.
Image: Submillimeter Array image of the rotating, gaseous disk surrounding the young twin-star system V4046 Sagittarii (located at the white dot in the image). Note the size of the V4046 Sagittarii disk relative to the orbit of Neptune, shown to scale at the lower right (the filled oval at lower left represents the size of the smallest structures that could be detected in the image). The disk is tipped from our perspective, such that it appears as elliptical rather than circular. The image is color-coded according to the motion of the gas in the system, with blue representing material that is approaching and red representing material that is receding from us. The fastest-moving approaching and receding gas is detected closest to the central binary star system, as expected if the disk gas obeys Kepler’s laws of planetary motion. This disk gas likely represents the raw material out of which Pluto-like bodies, comets, and perhaps gas giant planets will form (or might already have formed) around the double star. Credit: Joel Kastner/SMA.
Carbon monoxide and hydrogen cyanide had been found in the circumstellar molecular gas cloud around V4046 Sagittarii, comprising raw materials for planet formation. The rotating disk found there was confirmed by the imagery following that earlier work, says Joel Kastner (Rochester Institute of Technology):
“It’s a case of seeing is believing. We had the first evidence for this rotating disk in radio telescope observations of V4046 Sagittarii that we made last summer. But at that point, all we had were molecular spectra, and there are different ways to interpret the spectra. Once we saw the image data from the SMA, there was no doubt that we have a rotating disk here.”
And quite an interesting disk at that: We’re dealing with two solar-mass stars with a separation of no more than five solar diameters, a much tighter fit than we see between Centauri A and B. The twin stars are young, at roughly twelve million years old, but the protoplanetary disk may be the oldest known. If that’s the case, we’re looking at the formation of Jupiter-class planets continuing long after the first few million years. What challenges that poses to planet formation theories remains to be seen.
Close binaries present unusual problems for radial velocity measurements, so we’re pushing into a new discovery zone for exoplanets with this work. Kastner notes the implications:
“In this case the stars are so close together, and the profile of the gas — in terms of the types of molecules that are there — is so much like the types of gaseous disks that we see around single stars, that we now have a direct link between planets forming around single stars and planets forming around double stars.”
This work was presented today at the American Astronomical Society meeting in Pasadena and draws on earlier work reporting on the molecular gas cloud around V4046 Sagittarii. The imagery now confirms the earlier study. The paper is Kastner et al., “Molecules in the disk orbiting the twin young suns of V4046 Sgr,” Astronomy & Astrophysics 492 (2008), pp. 469-473 (abstract).
by Paul Gilster | Jun 9, 2009 | Exoplanetary Science |
I love Greg Laughlin’s remark to the Washington Post‘s Joel Achenbach in last week’s article Astronomers Seek New Home Closer to Home. Having discussed Debra Fischer’s ongoing search for Alpha Centauri planets and his own theories on planet formation around binary stars, Laughlin points out where we stand today: “We have what is to all appearances by far the best planet in the galaxy. And we have no workable backup plan.”
The Washington Post article doubtless draws on Lee Billings’ earlier piece in SEED Magazine called The Long Shot, which discusses with an elegance rare in science writing the attempt to find planets around the Centauri stars by Fischer as well as Michel Mayor’s Geneva team. Mayor has been using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at La Silla, the Cadillac of radial velocity instrumentation (and boy does that auto industry reference date me!). Competition can work wonders, and having two teams on the case can only bode well for quick results.
Not that those results will necessarily reveal planets, but we can hope for the best. Laughlin writes about the search in Alpha Centauri: Market Outperform, noting that “…when HARPS is working full bore on a bright quiet star, it can drill right down into the habitable zone.” Note, too, that Alpha Centauri is visible almost year-round from La Silla. Laughlin plugs in values for Centauri B’s habitable zone and creates data sets for differing values of planetary mass in the system. He then extrapolates from this to determine where the Geneva team might be now that it has upped its frequency of observations.
The results: A 4.6 Earth-mass planet in an optimally habitable orbit around Centauri B might be “…on the verge of current ‘announceability.'” A smaller 2.3 Earth-mass planet in the same zone would not be visible yet, requiring another year and a half of observations. If we’re anxious to find not a ‘super Earth’ but a true Earth analog, then, silence on the Centauri front for another eighteen months may, as Laughlin suggests, be good news.
But back to Laughlin’s comment to Joel Achenbach. At present, Earth’s lack of a backup plan is obvious, a matter of little concern to most unless we one day discover an asteroid in a dangerous, intersecting orbit. What technologies do we have today that could move quickly to reach an object potentially far out in the Solar System, allowing us to change its trajectory? Making sure we have the tools is part one of the backup plan, and that puts an emphasis on observation and propulsion studies.
Part two is building the infrastructure for a continuing human presence off-planet, and for ensuring that future asteroid or comet encounters are better anticipated. Part three is finding out whether continuing work on interstellar propulsion will one day produce the energies we need to push a payload to the nearest stars in time frames that human crews can survive. The answer will determine whether our backup plan is system-wide or extends to habitable planets around suns other than our own.
by Paul Gilster | Jun 8, 2009 | Advanced Rocketry |
Catching my eye in the latest Carnival of Space, hosted by Brian Wang at Next Big Future, is Adam Crowl’s write-up of a rethinking of an exotic ramjet technology. Robert Bussard put the interstellar ramjet into the public eye back in 1960 in a paper proposing that a starship moving fast enough would be able to use the hydrogen between the stars as a source of fuel, enabling a constant acceleration at one g. You’ll recognize the Bussard ramjet in Poul Anderson’s classic novel Tau Zero (originally published in Galaxy in 1967 as To Outlive Eternity).
The Problem with Slow Fusion
Anderson’s ‘Leonora Christine’ was a runaway starship, accelerating ever closer to lightspeed until she was punching through entire galaxies in times experienced by the crew as mere minutes. But we don’t have to get quite that extreme with the Bussard idea. It’s built around the premise of gathering fuel along the way so as to avoid the vast mass ratio problems of conventional rocketry. We can imagine an enormous magnetic scoop that might gather up this interstellar hydrogen, but the problem is how to burn it.
Image: The Bussard ramjet concept, as envisioned by the space artist Adrian Mann.
Adam’s discussion nails the problem: Bussard relied on proton to proton fusion, but the problem is that it’s too slow. Assuming we could gather the hydrogen in the first place, we face the fact that proton burning is a kind of reaction that works in stars (like our Sun) because of the sheer size of the stars themselves. Adam describes this well:
The reaction rate of proton-proton fusion at “low” (i.e. an achievable 100 million degrees) temperatures is essentially negligible and only powers the stars because they’re so gigantic. The Sun’s energy production rate is a bit more than 10 Watts per cubic metre of the fusion part of its core, which is far less than the power packed into a battery, for example. Unlike a battery, of course, that energy can trickle out for billions of years – but that’s no good for propelling a starship.
Lighting the Fire: The CNO Alternative
Physicist Daniel Whitmire tackled this problem in a 1975 paper that proposed using hydrogen for fuel but exploiting a catalytic nuclear reaction chain instead of straight proton burning. The so-called CNO Bi-Cycle becomes dominant in sufficiently hot main sequence stars (usually those about 1.5 times the mass of the Sun), while the proton-proton chain is more significant in smaller stars. Here’s Adam’s description:
Basically a hydrogen fuses to a carbon-12, then another is fused to it to make nitrogen-14, then two more to make oxygen-16, which is then highly ‘excited’ and it spits out a helium nucleus (He-4) to return the nitrogen-14 back to carbon-12. Since the carbon-12 isn’t consumed it’s called a “catalytic” cycle, but it’s not chemical catalysis as we know it. Call it “nuclear chemistry.”
Getting the Vessel Up to Speed
Whitmire points out in his paper that using this method, the catalyst fuel, carried along with the spacecraft, is not depleted. Interstellar hydrogen, however, remains the ultimate source of the energy. We still run into the issue of the density of the interstellar medium, which is tricky business. Whitmire comments on the problem of gathering enough fuel:
…independent of other restrictions, nuclear powered ramjets may require nebular regions of space for take off if accelerations greater than 1g are desired. However, once the desired velocity is obtained in a nebula runway, it will be maintained and even increased as the ship moves into less dense regions of space… Nebulae are also required for stopping interstellar ramjets since deceleration requires as much energy as acceleration.
Image: Science fiction has worked often with the ramjet concept, as seen here in this Analog cover from 1978. Artwork by Rick Sternbach.
What do we do about getting up to speed? There being no nearby sufficiently dense nebula, we on planet Earth might find ourselves unable to light the fire. But Whitmire goes on to speculate that technological civilizations living within or near a dense nebula would have no such constraints, and even goes to far as to outline the possibility of searching for the signature of extraterrestrial ramjets that might be tracked while using a nebula ‘runway.’
The Ramjet as Brake
Bussard’s ramjet ideas, followed by Whitmire’s modifications, really did open up the idea of practical interstellar flight some fifty years ago, but recent studies have shown that the ramjet idea has a key obstacle. The ramscoop needed to collect all that hydrogen acts more effectively as a brake. Indeed, we’re now considering similar magnetic scoop ideas as possible braking systems for decelerating a probe into a destination solar system. Is the ramjet through, or are there ways around the braking problem, including other ways of hydrogen collection?
The paper is Whitmire, “Relativistic Spaceflight and the Catalytic Nuclear Ramjet,” Acta Astronautica Vol. 2 (1975), pp. 497-509 (available online, and well worth reading). The original Bussard ramjet paper is “Galactic Matter and Interstellar Flight,” Acta Astronautica Vol. 6 (1960), pp. 179-194. Adam Crowl also points out that Gregory Benford’s starship ‘Lancer’ in Across the Sea of Suns used the CNO cycle, an apparent nod to Whitmire.
by Paul Gilster | Jun 5, 2009 | Astrobiology and SETI |
Imagine an extraterrestrial civilization that manages to colonize the entire galaxy. Then imagine the colonizing civilization collapsing so definitively that no trace of its existence has yet been detected, at least from our planet. We can call it, as Jacob Haqq-Misra and Seth Baum (Pennsylvania State University) do in a recently released paper, a ‘graveyard civilization,’ one whose remains might still be accessible provided we know where and how to look.
Pushing the Limits of Growth
What could bring down such a civilization? The idea here is that we can explain the Fermi paradox (‘Where are they?’) by assuming that exponential growth is not a sustainable development pattern for intelligent civilizations. The authors draw on human experience in analyzing this possibility. Here’s the gist of it:
The consequences of unsustainable development are often dire. In many documented cases, resource depletion caused by human activities has led to the permanent collapse of human populations, and resource depletion and environmental degradation can also cause or exacerbate violent conflict. Note that collapsed human populations do not necessarily disappear —they may persist in diminished numbers. This is particularly evident in the case of Easter Island, where resource depletion is believed to have caused or significantly contributed to a major population decline. Some analysts are concerned that the unsustainable practices of human civilization could lead to a global-scale collapse. Should such a collapse occur, human civilization would not be able to colonize the galaxy.
Fermi’s question, of course, was built around the assumption that alien civilizations would do more or less what we would do if we had their technology, which is to explore and colonize the galaxy the way we have done these things on our own world. Thus one answer to Fermi is that the reason we do not see evidence of extraterrestrials is that exponential growth patterns throughout the galaxy are not feasible. This does not rule out the existence of ETI, but does establish some constraints:
…the Paradox can only conclude that other intelligent civilizations have not sustained exponential growth patterns throughout the galaxy. It is still possible that slower-growth ETI civilizations exist but have not expanded rapidly enough to be easily detectable by the searches humans have yet made.
Such a slower growth pattern resonates in an era when ‘sustainable development’ in everything from technology to agriculture and environmental protection is much in the air. Civilizations moving beyond their growth limits, under this scenario, might find themselves in a state of retreat:
It is also possible that faster-growth ETI civilizations previously expanded throughout the galaxy but could not sustain this state, collapsing in a way that whatever artifacts they might have left have also remained undetected. Both of these growth patterns can be observed in human civilization, suggesting that they may be possible for ETI civilizations as well.
Recalibrating the SETI Search
You can see the effect of this on SETI strategy. Haqq-Misra and Baum suggest that SETI be recalibrated to focus on slow-growth civilizations, and those that have already endured their collapse. After all, a society that has chosen a sustainable, slow growth path may still broadcast a signal detectable by SETI. So, too, might a post-collapse culture, which might beam the story of its downfall out to the broader universe, or perhaps use automated systems to send out a galactic requiem.
But the authors’ first choice for SETI is to follow up on the possibility that slow-growth cultures might still be able to send small, long-duration star probes, in the absence of true colonizing forays into the universe. If so, we might consider complementing the SETI search with SETA (Search for Extraterrestrial Artifacts), which has been considered at both visible and radio wavelengths. The paper goes so far as to suggest that “…a survey of the solar vicinity may be more pragmatic than an all-sky search for encoded messages.”
What we don’t know is how representative we are. Nor do we know the limits of exponential growth, for they may lie not at the planetary but the solar system level, assuming they’re not fully surmountable in the first place (by some future civilization if no one has done it in the past). A success at finding some kind of artifact here in our own system would at least tell us that an interstellar crossing is not out of the question, but how much further do we want to take these conclusions? The authors raise the question themselves, and point out that “…we cannot rule out the possibility that ETI civilization may follow a development pattern sufficiently different that we wouldn’t recognize it even if we detected its signal.”
The paper is Haqq-Misra and Baum, “The Sustainability Solution to the Fermi Paradox,” Journal of the British Interplanetary Society 62 (2009), pp. 47-51 (available online).
by Paul Gilster | Jun 4, 2009 | Outer Solar System |
With a dense atmosphere of nitrogen and methane, Titan is the only moon in our Solar System that shares Earth-like characteristics in climate. But Titan’s climate, receiving one hundred times less sunlight at ten times Earth’s distance from the Sun, operates at a much slower pace. The seasons on the distant moon last more than seven Earth years, and the motion of its clouds is slow and deliberate.
We’ve had a good look via the Cassini spacecraft at the movement of those clouds, some two hundred of them being examined between July 2004 and December 2007 in a study of global circulation patterns. Summer changes to fall at the equinox in August of this year. We’re at a time when the circulation models say clouds in the southern latitudes should have already disappeared, but it’s clear from the Cassini imagery that many clouds remained as late as 2007.
Image: This infrared image of Saturn’s moon Titan shows a large burst of clouds in the moon’s south polar region. These clouds form and move much like those on Earth, but in a much slower, more lingering fashion, new results from NASA’s Cassini Spacecraft show. Titan’s southern hemisphere still shows a very active meteorology (the cloud appears in white-reddish tones) even in 2007. According to climate models, these clouds should have faded out since 2005. Credit: NASA/JPL/University of Arizona/University of Nantes.
Sebastien Rodriguez (University of Paris Diderot), who has been working with the Cassini visual and infrared mapping spectrometer team, has this to say about the phenomenon:
“Titan’s clouds don’t move with the seasons exactly as we expected. We see lots of clouds during the summer in the southern hemisphere, and this summer weather seems to last into the early fall. It looks like Indian summer on Earth, even if the mechanisms are radically different on Titan from those on Earth. Titan may then experience a warmer and wetter early autumn than forecast by the models.”
Rodriguez’ comment reminds this jazz buff of the lush Woody Herman tune (with lyrics by Johnny Mercer) called ‘Early Autumn.’ I’ve always loved the Stan Getz version of this standard, but I’m listening to Jo Stafford’s rendition as I write, enjoying the juxtaposition of a jazz classic and imagery from an exotic moon around a ringed world. One of the pleasures of digital music is being able to pop up favorites on a whim.
Cassini’s extended mission will run until the early autumn of 2010, which will offer plenty of opportunities to monitor climate change on Titan — the spacecraft makes its next flyby of the moon on June 6. So expect to learn much more about sluggish weather on Titan, including whether it’s the result of a slow rate of temperature change at the surface and in the low atmosphere. Until then, I can’t help thinking that an autumn that lingers is something Johnny Mercer would have appreciated.
