by Paul Gilster | Feb 15, 2012 | Deep Sky Astronomy & Telescopes |
The Planck mission continues to peel the layers off the onion as it probes the early universe. Planck is all about the Cosmic Microwave Background (CMB), that radiation left over from the era of recombination around 380,000 years after the Big Bang. As electrons and protons began to form neutral atoms, light was freed to stream through the universe, an afterglow of the Big Bang that missions like the Wilkinson Microwave Anisotropy Probe have studied in detail, and which Planck will now observe at still better sensitivity, angular resolution and frequency range.
But the initial job for researchers is to remove sources of foreground emission to reveal the CMB itself, and that process is turning up interesting findings in its own right. The latest announcement from the European Space Agency involves a haze of microwaves that is not yet understood. Coming from the region around galactic center, the haze appears to be synchrotron emission, produced as electrons accelerated in supernovae explosions pass through magnetic fields. So far so good, but this synchrotron emission does not fall off as rapidly at increasing energies as the synchrotron emission that can be observed elsewhere in the Milky Way.
What Planck has found is an enormous field of haze spanning some 35,000 light years. The problem: Supernovae don’t make enough electrons and positrons at high energy to fill the volume taken up by the Planck haze, according to Gregory Dobler (UC Santa Barbara):
“There are many possibilities and theories, ranging from Galactic winds to a jet generated by the black hole at the center of our Galaxy to exotic physics related to dark matter. The problem is that the picture that has emerged with the Planck data, as well as the Fermi data, challenges all of the explanations. There is no Goldilocks theory yet. None of them fit the data just right.”
The fact that early explanations for the haze are all over the map tells us how little we understand what is going on here — one theory invokes the annihilation of dark matter particles, while others involve higher supernova rates in the early universe. ESA’s Jan Tauber, project scientist for Planck, will only say that the galactic haze result is ‘interesting,’ which basically says we’re still in the dark. Tauber goes on to say “The lengthy and delicate task of foreground removal provides us with prime datasets that are shedding new light on hot topics in galactic and extragalactic astronomy alike.” True enough, and the biggest Planck findings are surely ahead.
Image: This all-sky image shows the spatial distribution over the whole sky of the Galactic Haze at 30 and 44 GHz, extracted from the Planck observations. In addition to this component, other foreground components such as synchrotron and free-free radiation, thermal dust, spinning dust, and extragalactic point sources contribute to the total emission detected by Planck at these frequencies. The prominent empty band across the plane of the Galaxy corresponds to the mask that has been used in the analysis of the data to exclude regions with strong foreground contamination due to the Galaxy’s diffuse emission. The mask also includes strong point-like sources located over the whole sky. Credit: ESA/Planck Collaboration.
Planck’s carbon monoxide map is also noteworthy and a major addition to ground-based carbon monoxide surveys, which are complicated and lengthy enough to restrict our observation. Planck is scanning the entire sky for this constituent of the cold clouds, made predominately of hydrogen, that become the birthplace of stars. The spacecraft is finding previously undiscovered clouds that will all go into the overview of cosmic structure scientists anticipate from the mission. And once these foreground materials are accounted for, we will see the real prize, the Cosmic Microwave Background viewed through an instrument sensitive to temperature variations of a few millionths of a degree and capable of mapping the full sky over nine wavelength bands.
The questions Planck is investigating are among the most pressing in cosmology. Here are the mission goals as found in this European Space Agency backgrounder:
- The determination of the Universe’s fundamental characteristics, such as the overall geometry of space, the density of normal matter and the rate at which the Universe is expanding.
- A test of whether the Universe passed through a period of rapidly-accelerated expansion just after the Big Bang. This period is known as inflation.
- The search for ‘defects’ in space, for example cosmic strings, which could indicate that the Universe fundamentally changed state early in its existence.
- Accurate measurement of the variations in the microwave background that grew into the largest structures today: filaments of galaxies and voids.
- A survey of the distorting effects of modern galaxy clusters on the microwave background radiation, giving the internal conditions of the gas in the galaxy clusters.
The inflation question is particularly interesting and much on my mind as I read Roger Penrose’s new book Cycles of Time (Knopf, 2011), which takes a controversial look at inflation that we’ll be talking about down the road. The Planck results thus far were announced this week in Bologna at a conference devoted to the mission, with publication pending in Astronomy & Astrophysics. ESA is now saying that the first Planck cosmological dataset is to be released next year.
by Paul Gilster | Feb 14, 2012 | Deep Sky Astronomy & Telescopes |
Given yesterday’s post on wandering planets, otherwise known as ‘rogue’ planets or ‘nomads,’ today’s topic falls easily into place. For even as we ponder the possibility of 105 rogue planets at Pluto’s mass or above for every main sequence star in the galaxy, we confront the fact that we still have much to learn about objects much closer to home in our own Kuiper Belt. We have yet, for example, to have a flyby, although it’s possible the New Horizons spacecraft will line up on a useful target after its encounter with Pluto/Charon (and yes, it’s conceivable that Triton is a captured KBO, and thus we have had a Voyager flyby). The discovery of objects like Sedna, Makemake and Eris makes it clear how much we may yet uncover.
We can think of the broader category of Trans-Neptunian Objects (TNOs) in terms of potential mission targets, but we should also ponder the fact that their strong dynamical connection with the planets can help us gain insight into the mechanics of Solar System and planet formation. This is what Scott Sheppard (Carnegie Institution of Washington) and colleagues have in mind as they present the results of a southern sky survey for bright Kuiper Belt objects. Centauri Dreams reader Joseph Kittle gave me the heads-up on this interesting work, which notes that the Kuiper Belt, as a remnant of the original protoplanetary disk, contains what the authors call a ‘fossilized’ record of the original solar nebula and the development of the Solar System.
We’ve had earlier TNO surveys of northern hemisphere skies using the Palomar 48-inch Schmidt telescope (hence the discovery of the aforementioned Eris, as well as Haumea, Quaoar, Orcus and others). But the southern hemisphere has been tougher to survey because of the lack of wide-field digital imagers on suitable equipment there. It was the installation of just such an imager at Las Campanas in Chile that allowed the OGLE-Carnegie Kuiper Belt Survey (OCKS) to take place, one of the first large-scale southern sky and galactic plane surveys aimed at detecting bright Kuiper Belt Objects beyond the orbit of Neptune using state of the art digital CCD detectors. A second survey began in 2009 using the Schmidt telescope at La Silla.
Image: The University of Warsaw’s 1.3-meter telescope at Las Campanas, used in the OGLE-Carnegie Kuiper Belt Survey. Credit: OGLE/University of Warsaw.
Eighteen outer Solar System objects were detected in the survey, including fourteen that were new discoveries, an indication of how little this region of the sky has been searched for TNOs in the past. It’s interesting here to recall the definition of a ‘dwarf planet,’ defined by the IAU as an object that is in hydrostatic equilibrium and has not cleared the neighborhood around its orbit of other objects of similar size. It’s an imprecise definition, say the authors, but Pluto and Eris are clearly dwarf planets, while Makemake and Haumea most likely also qualify, and we’ll probably find that Sedna, 2007 OR10, Orcus, and Quaoar fit here as well.
The trick is figuring out what the lower size limit of an object in hydrostatic equilibrium really is, with some research indicating it could be as small as 200 kilometers in radius. That would put many more outer system objects into contention as dwarf planets, including three that were discovered in this survey, although at present the actual size and shape of these bodies will require further work to pin down with precision. Newly discovered 2010 KZ39 is particularly interesting, a possible member of the Haumea family based on its orbit. The authors have this to say about larger bodies in what we can call the classic Kuiper Belt (internal citations omitted for brevity, but of course you can find them in the online paper):
The actual number of Pluto-sized bodies is now known. Previous authors have argued that the Kuiper Belt likely lost a substantial amount of its mass through collisional grinding and dynamical interactions with the planets. Observationally, many more objects appear to be required in order to produce the observed angular momentum of the largest KBOs and binaries. Detailed simulations show that Kuiper Belt formation is possible with only the small number of Pluto-sized objects observed. A significant number of Pluto sized objects likely exist in the populations beyond 100 AU such as the Sedna types and Oort Cloud objects, which are currently too faint to be efficiently detected to date. It is important to determine if the Pluto-sized objects formed in the Kuiper Belt as we see it today or if they originated much closer to the Sun and were later transported to their current orbits.
So take note: Beyond the Kuiper Belt itself, there may exist at several hundred AU or more other large objects even of Pluto or Mercury-size, and perhaps large objects in Sedna-like orbits. The OCKS survey found no Sedna-class objects within the 300 AU region to which it was sensitive, and it may be that a survey like Pan-STARRS will have a better chance to detect such objects because it has the capability of working with fainter magnitudes than OCKS. We’ll also need the services of the Large Synoptic Survey Telescope to probe the TNO population more deeply.
It’s heartening to ponder the authors’ conclusion that we are making serious progress on the Kuiper Belt, with complete surveys now for objects 80 kilometers in radius out to 30 AU, and 225 kilometers in radius out to 50 AU. Beyond a few hundred AU, much remains to be done. Given the gaps in our knowledge as we move out toward the inner Oort Cloud, it’s clear how many discoveries await us in this region of space comparatively close to home compared to the interstellar distances we one day hope to travel. The relatively simple vision of the Solar System many of us grew up with has rapidly given way to a system surrounded and permeated by rocky and cometary debris, with dwarf planets in unknown numbers far from the central star.
The paper is Sheppard et al., “A Southern Sky and Galactic Plane Survey for Bright Kuiper Belt Objects,” Astronomical Journal 142, (October, 2011) p. 98 (preprint).
by Paul Gilster | Feb 13, 2012 | Outer Solar System |
We tend to think of interstellar journeys as leaps into the void, leaving the security of one solar system to travel non-stop to another. But a number of alternatives exist, a fact that becomes clear when we ponder that our own cloud of comets — the Oort Cloud — is thought to extend a light year out and perhaps a good deal further. There may be ways, in other words, to take advantage of resources like comets and other icy objects for a good part of an interstellar trip. That scenario is not as dramatic as a starship journey, but it opens up possibilities.
Let’s say, for example, that we only manage to get up to about 1 percent of lightspeed (3000 kilometers per second) before we run into technical challenges that are at least temporarily insurmountable. Speeds like that take well over 400 years to get a payload to Centauri A and B, but they make movement between planets and out into the Kuiper Belt and Oort Cloud a straightforward proposition. A civilization content to create way-stations and take its time could establish habitats all along the way, its distant descendants reaching the next solar system.
The idea takes me back to the island-hopping of Polynesian cultures as they pushed ever deeper into the Pacific, which is sometimes invoked to describe a civilization expanding from star to star. But the ‘island-hopping’ may actually involve small, dark objects exploited step by step all the way across to the target star, a process that could take millennia. A space-faring culture at home in the dark outer regions emerges. All of this depends, of course, upon the resources available, but the Oort Cloud is thought to be vast, comprising perhaps trillions of icy and rocky objects, a supply of raw materials on which such a culture could thrive.
Nomads Between the Stars
Adam Crowl recently passed along a new paper that takes this idea to another level. Louis Strigari (Stanford University) and colleagues have been looking at unbound objects, free-floating planets formed either directly in the collapse of a molecular cloud or ejected due to gravitational interactions in a solar system. Right now we know little about such rogue planets — Strigari and team call them ‘nomads’ — but they are quite interesting from the interstellar expansion standpoint as they, too, could provide even more stepping stones to distant destinations. Moreover, they cannot be ruled out as worthwhile targets on their own, as the paper suggests:
The name “nomad” is invoked to include that allusion that there may be an accompanying “?ock,” either in the form of a system of moons (Debes & Sigurdsson 2007) or in its own ecosystem. Though an interstellar object might seem an especially inhospitable habitat, if one allows for internal radioactive or tectonic heating and the development of a thick atmosphere e?ective at trapping infrared heat (Stevenson 1999; Abbot & Switzer 2011), and recognizes that most life on Earth is bacterial and highly adaptive, then the idea that interstellar (and, given the prevalence of debris from major galaxy mergers, intergalactic) space is a vast ecosystem, exchanging mass through chips from rare direct collisions, is intriguing with obvious implications for the instigation of life on earth.
It’s a dizzying thought when you couple this with the paper’s estimates on the number of free-floating planetary objects. The authors estimate there may be up to 105 compact objects per main sequence star in the galaxy that are greater than the mass of Pluto. The mass function of the lowest-mass nomads is modeled from what we see in the Kuiper Belt and the distribution of diameters in KBOs, while at the higher end (corresponding to masses several times that of Jupiter), evidence exists that nomads in open clusters follow a smooth continuation of the brown dwarf mass function. Drawing in evidence from microlensing as well as direct imaging, the paper goes on to suggest a galaxy in which the space between the stars is well populated with objects of planetary mass, most relatively small but some larger than Jupiter.
The authors acknowledge that much uncertainty exists about the mass function as we move from larger to smaller nomads, which makes space-based observations critical for refining these estimates. One way to move forward is through a survey of the inner galaxy (the proposed Wide-Field Infrared Survey Telescope, or WFIRST, could be significant here), while large scale galaxy surveys like the Gaia mission and the Large Synoptic Survey Telescope (LSST) should be sensitive to nomads greater than Jupiter mass. Even Kepler may come into play, as any anomalous microlensing events it encounters could imply a high value for the number of nomads between the stars. From the paper:
…we note that an additional outcome of the observational approach discussed above, especially regarding the detection of short timescale microlensing events, is that upper limits may be set on the density of nomads. This could set very interesting constraints on the population of planetesimals in nascent planetary systems.
Indeed. If resources like these are available in quantity between the stars, then a pattern of slow expansion would make interstellar migration almost inevitable if humans (or their machine surrogates) can adapt to life in the outer Solar System and beyond. Propulsion is always a huge issue, but in this scenario we also focus on the ability to build and maintain habitats on distant objects, exploiting their raw materials and preparing for the next leap outwards. Long-haul technologies would surely arise from a culture capable of these things, but the possibility exists that interstellar travel will mean slow and steady outpost building before the target is reached.
The paper is Strigari et al., “Nomads of the Galaxy” (preprint).
by Paul Gilster | Feb 10, 2012 | Outer Solar System |
Mindful of the recent work on axial tilt I’ve reported in these pages, I was interested to learn that Vesta’s axial tilt is just a bit greater than the Earth’s, about 27 degrees. We’ve been pondering the consequences of such obliquity on planets in the habitable zone, but in Vesta’s case, the issue isn’t habitability but water ice. For spurred by the Dawn mission, scientists are looking at whether permanently shadowed craters on the asteroid’s surface would allow water to stay frozen all year long. Unlike the situation on the Moon, the answer on Vesta (on the surface at least) seems to be no.
Earth’s axial tilt is 23.5 degrees, but the Moon’s is a scant 1.5 degrees, making the shadow in some lunar craters permanent, a fact that has led to speculation that ice in these locations could be of use to future manned missions there. In contrast, Vesta’s obliquity means that it has seasons, so that every part of the surface becomes exposed to sunlight at some point in the year. Even so, says Timothy Stubbs (NASA GSFC), “Near the north and south poles, the conditions appear to be favorable for water ice to exist beneath the surface” [italics mine].
Image: New modeling shows that, under present conditions, Vesta’s polar regions are cold enough (less than about 145 K) to sustain water ice for billions of years, as this map of average surface temperature around the asteroid’s south pole indicates. (The white dashed line marks Vesta’s south polar circle.) Figure reprinted from the paper in Icarus by T.J. Stubbs, T.J. and Y. Wang. Image credit: NASA/GSFC/UMBC.
The polar regions on Vesta are under scrutiny as the Dawn mission continues its close look. Observations from the Earth have suggested a bone-dry Vesta, but in its current low orbit, Dawn is using its gamma ray and neutron detector (GRaND) spectrometer to look for hydrogen-rich deposits that might flag the presence of water ice below the surface. Because Dawn’s targets — Vesta and Ceres — are both considered remnant protoplanets, it will be important to learn whether there is sub-surface water on Vesta. The next issue to ponder will be whether any water that is discovered has arrived recently or goes back to the earliest days of the Solar System.
Modeling has shown that water ice should be able to survive in the top few meters of regolith when surface temperatures are less than 145 K or so, and while Vesta’s equator sees average yearly temperatures around 150 K — too warm to sustain water ice within meters of the surface — the regions near the asteroid’s north and south poles are cold enough to allow its survival for billions of years. The temperature dividing lines show up at about 27 degrees north latitude and 27 degrees south latitude.
As for the surface, this JPL news release quotes Stubbs on the matter:
“The bottoms of some craters could be cold enough on average — about 100 kelvins — for water to be able to survive on the surface for much of the Vestan year [about 3.6 years on Earth]. Although, at some point during the summer, enough sunlight would shine in to make the water leave the surface and either be lost or perhaps redeposit somewhere else.”
Ceres, the second of Dawn’s destinations, should prove a study in contrasts. As compared with Vesta’s dry surface, Ceres may have seasonal polar caps of water frost and even a thin atmosphere. Some models show a layer of 100 kilometers of icy material including water and ammonia, and allow the presence of liquid water beneath. Recall that this is the most massive body in the main asteroid belt, making up perhaps as much as a third of the mass of the entire main belt. Hubble observations have suggested that the topography here is low, with a lack of deep craters that indicates flow in the crust to even out the landscape. Dawn will be able to give us better estimates of Ceres’ mass and help us understand how that mass is distributed.
The paper on Vesta is Stubbs and Wang, “Illumination conditions at the Asteroid 4 Vesta: Implications for the presence of water ice,” Icarus Vol. 217, Issue 1, pp. 272-276 (January, 2012).
by Paul Gilster | Feb 9, 2012 | Deep Sky Astronomy & Telescopes |
Not long ago we looked at comet C/2011 N3 (SOHO), discovered last July just two days before it plunged into the Sun, evaporating some 100,000 kilometers above the solar surface. It was startling to learn that the SOHO observatory is tracking numerous ‘Sun-grazers,’ comets whose fatal encounters with our star are occurring roughly once every three days. Now comes news that Sagittarius A*, the supermassive black hole at the center of the Milky Way, is producing X-ray flares about once a day, thought by some to be the result of similar debris in the process of destruction.
The flares last just a few hours, according to researchers working with data from the Chandra X-Ray Observatory, and can reach brightness levels up to 100 times what is normally observed in the black hole region. Kastytis Zubovas (University of Leicester) thinks we’re seeing asteroids and comets passing within 160 million kilometers of the black hole (roughly 1 AU), at which point they would likely be broken apart by tidal forces and eventually vaporized as they move through the hot, thin gas surrounding and feeding Sgr A*.
“As a reality check, we worked out that a few trillion asteroids should have been removed by the black hole over the 10-billion-year lifetime of the galaxy,” said co-author Sera Markoff of the University of Amsterdam in the Netherlands. “Only a small fraction of the total would have been consumed, so the supply of asteroids would hardly be depleted.”
Image (click to enlarge): The panel on the left is an image containing nearly a million seconds of Chandra observing of the region around the black hole, with red representing low-energy X-rays, green as medium-energy X-rays, and blue being the highest. An asteroid that undergoes a close encounter with another object, such as a star or planet, can be thrown into an orbit headed towards Sgr A*, as seen in a series of artist’s illustrations beginning with the top-right panel. If the asteroid passes within about 100 million miles of the black hole, roughly the distance between the Earth and the Sun, it would be torn into pieces by the tidal forces from the black hole (middle-right panel). These fragments would then be vaporized by friction as they pass through the hot, thin gas flowing onto Sgr A*, similar to a meteor heating up and glowing as it falls through Earth’s atmosphere. A flare is produced (bottom-right panel) and eventually the remains of the asteroid are swallowed by the black hole. Credit: X-ray: NASA/CXC/MIT/F. Baganoff et al.; Illustrations: NASA/CXC/M.Weiss.
To generate flares large enough for Chandra to see, the objects in question would have to be roughly 10 kilometers in radius or larger. These are relatively small events compared to what might happen when an entire star passed too close to a supermassive black hole, which would power a spectacularly bright flare of the kind that has been observed in other galaxies. But disintegrating stars are thought to be rare, while the Chandra flares are common, and last only hours compared to the larger events, which can be studied for a period of months.
Zubovas and team have developed a model that looks at the asteroid population passing near Sagittarius A*, one that offers criteria for testing whether a given flare is the result of an external object encountering the black hole or the result of instabilities within the accretion disk already around it. From the paper:
The least constrained parts of the model have to do with the exact distribution of asteroids and their orbits in the hypothesised ”Super-Oort cloud” around Sgr A* and with conversion of the bulk kinetic energy of the asteroids into electromagnetic radiation. However, there almost certainly are asteroids in the central few pc of the Galaxy and the processes described here must occur. Our paper makes several estimates of the e?ects that asteroids have on the luminosity of Sgr A* and suggests a method to distinguish between such externally caused ?ares and accretion-instability caused ones. If future observations reveal that asteroid disruptions are responsible for at least a fraction of the ?ares, this would be an important step in understanding the accretion processes in Sgr A*. In addition, further investigation may help constrain the size of the asteroid population in the Galactic centre.
So we’re looking at what can be called a ‘Super-Oort’ cloud, a thick torus of asteroids stripped from their parent stars that surrounds our galaxy’s supermassive black hole. Interestingly enough, the Chandra data fall in line with earlier work suggesting that this cloud of asteroids and planets may have formed through star formation episodes inside the massive accretion disk itself, during phases when the black hole grew rapidly. The asteroids would be drawn from this cloud of stars whenever one of the stars passed too close to the black hole. Where Zubovas’ team differs from some earlier theorists is in seeing the Chandra flares as the result of asteroid interactions with the black hole rather than stars, the latter being of insufficient density to produce the small and frequent flares we see happening in the region of Sgr A*.
The paper is Zubovas et al., “Sgr A* flares: tidal disruption of asteroids and planets?” accepted for publication in Monthly Notices of the Royal Astronomical Society (preprint).
