by Paul Gilster | Jan 26, 2008 | Deep Sky Astronomy & Telescopes |
Sorting Out Science offers the most recent Carnival of Space in a noir-ish style that recalls the detective pulps of years gone by, not to mention many a film noir itself (Out of the Past may be my favorite, but there were so many terrific movies in the genre). I always pick one blog entry with relevance for interstellar watchers, and this week it’s the work of Quasar9, with a look at Hubble images that cover one of the largest expanses of sky ever observed by the instrument. The distortion of galactic shapes revealing the presence of dark matter makes fascinating reading, said light being bent by the massive gravitational field involved in the dark matter distribution around the observed supercluster. Once again we’re in the realm of gravitational lensing, a phenomenon proving useful from the galactic cluster level to the hunt for distant exoplanets.
by Paul Gilster | Jan 25, 2008 | Asteroid and Comet Deflection |
With the news that an asteroid called 2007 TU24 will pass 538,000 kilometers from Earth on January 29, attention turns to the Catalina Sky Survey, which discovered this near-Earth object last October. The asteroid is thought to be between 150 and 600 meters in diameter, and should become visible to amateur astronomers in late January. The sky map below shows its track near Earth close approach as seen from Philadelphia, but you can generate personalized ephemeris tables here.
The Near Earth Object Program is quick to point out that 2007 TU24 poses no threat to Earth during the upcoming encounter, and also notes that objects of this size are thought to pass this close to our planet every five years or so. With an estimated 7000 discovered and undiscovered asteroids in near-Earth orbits, let’s keep the Catalina Sky Survey and other programs well funded. The next known close approach by an asteroid of this size will be in 2027, all of which should remind us of the need to get an asteroid surveying mission on our launch calendar soon.
Image (click to enlarge): The track of 2007 TU24 near the time of close approach to Earth, as seen from Philadelphia. Credit: Dr. Dale Ireland/Near Earth Object Program.
Meanwhile, speaking of a hit rather than a miss, the Chicxulub crater in the Yucatan has been the subject of three-dimensional seismic studies as part of an attempt to assess its role in the KT extinction event that caused the demise of the dinosaurs, along with so many other species. Work at the University of Texas at Austin reveals that the asteroid landed in deeper water than previously thought, releasing some 6.5 times more water vapor into the atmosphere.
The result? A need to reconsider how materials at the impact site would have been spread. UT research scientist Sean Gulick notes that “The greater amount of water vapor and consequent potential increase in sulfate aerosols needs to be taken into account for models of extinction mechanisms” (see this news release for more). Gulick refers to sulphur-rich sediments at the site, whose reaction with water vapor would produce the said aerosols. These aerosols could create a cooling effect in the upper atmosphere and also generate acid rain, effects now believed to be more intense than had once been believed.
Watch the study of impact events take a new direction as we examine the trajectory of the incoming impactor. The UT work began as an attempt to find the signature of a trajectory, and while that goal was not achieved (“We discovered that the shallow structure of the crater was determined much more by what the impact site was like before impact than by the trajectory of the impactor,” says Gulick), future attempts to determine such trajectories could prove quite useful. Most ejected materials would be flung out of the crater downrange from the impact, flagging those areas most immediately affected by the event.
The paper is Gulick et al., “Importance of pre-impact crustal structure for the asymmetry of the Chicxulub impact crater,” published online at Nature Geoscience 13 January 2008 (abstract).
by Paul Gilster | Jan 24, 2008 | Antimatter |
Those gamma rays coming out of galactic center, flagging the presence of an antimatter cloud of enormous extent, have spawned few explanations more exotic than the one we consider today: Black holes. Primordial black holes, in fact, produced in their trillions at the time of the Big Bang and left evaporating through so-called ‘Hawking radiation’ ever since. That’s the theory of Cosimo Bambi (Wayne State University) and colleagues, who are studying the same antimatter cloud we recently examined here in terms of its possible connection with low mass X-ray binary stars.
Hawking radiation offers a mechanism for small black holes to lose mass over time. But since the phenomenon has never been observed, the upcoming launch of the GLAST (Gamma-ray Large Area Space Telescope) satellite again looms large in significance. GLAST should be able to find evaporating black holes, assuming they are there, and there is even some possibility that the Pierre Auger Observatory may eventually detect tiny black holes created when high-energy cosmic rays slam into the upper atmosphere. If so, we would have a window into any evaporative effects associated with these enigmatic events.
But assuming that black holes do evaporate, the trick is to figure out how fast, and that rate depends upon mass, with more massive black holes producing fewer evaporated particles. What Bambi’s team argues is that a mass of about 1016 grams, roughly that of a fairly common asteroid, will produce the right amount of antimatter to explain the detections. Theoretically, the signature radiation from black holes of this particular size should be observable given the right equipment, but neither the GLAST mission or ESA’s INTEGRAL satellite seems well suited for that task (more on the latter problem in this New Scientist story).
All of which is interesting it itself, but the paper offers a bonus:
We have considered evaporating primordial BHs [black holes], as a possible source of positrons to generate the observed photon 511 keV line from the Galactic Bulge. The analysis of the accompanying continuous photon background produced, in particular, by the same evaporating BHs, allows to ?x the mass of the evaporating BHs near 1016 g. It is interesting that the necessary amount of BHs could be of the same order of magnitude as the amount of dark matter in the Galactic Bulge. This opens a possibility that such primordial BHs may form all cosmological dark matter. The background MeV photons created by these primordial BHs can be registered in the near future, while the neutrino ?ux may be still beyond observation. The signi?cance of this model would be difficult to overestimate, because these BHs would present a unique link connecting early universe and particle physics.
So there’s a theory for you: Primordial black holes as the explanation for dark matter itself. But bear in mind that along with the x-ray binaries so recently considered in relation to galactic antimatter, other explanations are still in play, including type Ia supernovae and a host of far more exotic possibilities outlined in the introduction to the paper. GLAST should help, but the suspicion grows that the antimatter cloud at galactic center may remain enigmatic for some time to come.
The paper is Bambi et al., “Primordial black holes and the observed Galactic 511 keV line,” available online.
by Paul Gilster | Jan 23, 2008 | Exoplanetary Science |
The closest we’ve come so far to identifying Earth-like planets around other stars is in the identification of so-called ‘super Earths.’ Calculations designed to model the composition of such planets say that worlds up to about ten Earth masses are rocky rather than gaseous. Some of these, as we have in the case of Gliese 581, have even excited interest in their possible habitability. We’d like to find ways beyond the now conventional radial velocity and transit studies to identify more such worlds.
Now a new planet may have been found around GJ 436, a red dwarf already known to host a Neptune-mass planet in a tight 2.6 day orbit. This is interesting work because of the methods used. Ignasi Ribas (Institut de Ciències de l’Espai, Spain) and team have taken a close look at the known planet and are arguing it is possible to identify a second world, a super-Earth, through the telltale variations in the transit duration of GJ 436b, the already known ‘hot Neptune.’
Giving the game away is the fact that GJ 436b’s orbit, while scorchingly close to the primary, is not perfectly circular. Why should a planet in such an orbit show an eccentricity as high as 0.15? Possible perturbations from other objects in this system have been investigated by others, but Ribas’ team found room to work in the fact that GJ 436b barely crosses the disk of its star as seen from Earth. Tiny changes in the orbital inclination angle can readily be observed, and if that angle is indeed changing, that would explain why this transit has been so hard to confirm — the 2007 transit detection came as a surprise that contradicted earlier results. Say the authors:
Assuming this hypothesis, it is reasonable to explore the possibility of a perturber that could be responsible for both the relatively large eccentricity and the inclination change, while remaining undetected by the radial velocity measurements.
The pieces of the puzzle begin to come together in a second planet for the GJ 436 system. GJ 436c shows a minimum mass of 4.8 Earth masses, with a 0.6 Earth mass play in the numbers. The authors are the first to point out that they do not consider this an ‘extremely solid detection,’ but argue that the case is strong because the existence of GJ 436c explains the inner planet’s orbit. If the finding is borne out, that would make GJ 436c the least massive planet known to orbit a main sequence star.
The radial velocity data on this system is consistent with a planet that matches up with these properties (but see systemic for a more detailed look, and reservations on this), and may indicate still more planets:
Indeed, the system around GJ 436 shows striking resemblances to that around the M-type star Gl 581 (Udry et al. 2007), and thus its planets may experience changes in the orbital elements, perhaps eventually undergoing transits in spite of a previously null result… Our study provides yet another illustration of the variety of exoplanet systems and highlights the potential for complex dynamical histories that imply sizeable variations of the planets’ orbital elements, like the eccentricity, over timescales of decades.
Thus the value of a ‘near-grazing transit’! Note what’s happening here: We’re examining what we know about one planet and using its characteristics to find the signature of a smaller world. Space-based transit studies should be quite useful in working with such tight transits, and that pushes the limit on what we can detect down to even smaller objects, at least in systems as helpful as the one around GJ 436.
The paper is Ribas et al., “A ~5 M_earth Super-Earth Orbiting GJ 436?: The Power of Near-Grazing Transits,” submitted to Astrophysical Journal Letters and available online.
by Paul Gilster | Jan 22, 2008 | Culture and Society |
The universe so frequently sends the message that we humans are not entirely special. In fact, the notion of us as ‘privileged observers’ seemed to be dead as recently as a few years ago. Over the centuries we had learned that the Sun did not revolve around us, nor was the Sun itself the center of the cosmos, and with the understanding of its true position in a galaxy of stars, Sol became just another G-type star circled by planets. The recent ‘rare Earth’ hypothesis does challenge the idea that our planet is of a kind likely to be found elsewhere, but exoplanet discoveries will soon tell us whether or not Earth-like worlds really are common.
We may be getting used to the idea of Earth as just one of the vast billions of planets that are doubtless sprinkled through the Milky Way, but we have a long way to go in terms of our thinking about the future. For the one place where that sense of privilege seems to remain is in the idea that having achieved our planetary dominance, we are simply here to stay. The history of our planet tells a different story. Mass extinctions aren’t pretty, and new work at the University of Bristol makes the case that recovering from them can take tens of millions of years.
Three periods of extinctions occurred in the boundary between the Permian and Triassic periods some 251 million years ago, a series of ecological mega-disasters that some believe was driven by volcanism. In any case, the so-called ‘Siberian Traps’ coincide with this boundary, covering a vast area of Siberia in basaltic lava in a period of volcanic activity lasting for a million years. Ninety percent of life on the Earth disappeared, and recovery was agonizingly slow.
Sarda Sahney and Michael Benton (both at Bristol) have been studying the recovery of tetrapods, animals with a backbone and four legs. While life itself rebounded quickly after the disaster, their research shows that specialized animals involved in complex ecosystems took much longer to recover. From the paper:
Though globally tetrapods recovered quickly, the dramatic restructuring that occurred at the community level was not permanent and communities did not recover numerically or ecologically in the Early and Middle Triassic. It would not be until the great diversity of the Late Triassic, which included dinosaurs, pterosaurs, crocodilians, rauisuchids, aetosaurs, rhynchosaurs, trilophosaurs, sphenodonts, amphibians and mammals, some 30 Myr after the end-Permian event, that terrestrial tetrapod community diversity was restored.
Life’s essential resilience is unquestionable, but so is the fragile nature of any specific ecological niche. The end of the Permian saw extinctions so wide-ranging that the survival of any particular species hung by a thread.
Image: The sabre-toothed Lycaenops was a top predator of the latest Permian in South Africa. Lycaenops was a gorgonopsian, one of a group of highly successful animals that dominated faunas in the Late Permian, but were wiped out, together with 90 per cent of all species, by the end-Permian mass extinction. Credit: Mike Benton, University of Bristol
A tenet of Centauri Dreams from the beginning has been that we humans must expand into the cosmos if we expect our species to survive. The Earth may never again undergo volcanism like that associated with the Permian/Triassic boundary, and perhaps the next asteroid strike won’t occur for another million years. Perhaps. Individuals might choose to play such odds, but a technological culture at the brink of expansion into its planetary system cannot. Long-term, even a relatively settled and middle-aged solar system like ours is rife with potential danger.
Unprivileged as we seem to be, let’s push into nearby space as a hedge against planetary catastrophe, a species-wide insurance policy. And if we do continue to expand, we may well wind up as Freeman Dyson has so often speculated, evolving in our own separate ways according to the ecosystems we move into. That sense of a viable future no matter how we change along the way should drive us to look at all our options, both near the Earth and gradually outward as we expand into the Orion Arm.
The paper is Sahney and Benton, “Recovery from the most profound mass extinction of all time,” in Proceedings of the Royal Society B, published online January 15, 2008 (abstract).
