by Paul Gilster | Jun 8, 2006 | Exoplanetary Science |
Beta Pictoris, an A5 dwarf star some 63 light years from the Earth, is well known to exoplanet hunters, some of whom have been studying its circumstellar dust disk since its discovery by the Infrared Astronomical Satellite (IRAS). That disk was first detected way back in 1983, and is thought to be perhaps 1100 AU wide and much more massive than the disk from which our own Solar System formed. The disk and possible planetary formation going on there has always been tantalizingly like our own system’s, but now we get a surprise.
For as a new paper in Nature suggests, this young system (between eight and twenty million years old) contains much more carbon gas than expected. This work comes courtesy of the Far Ultraviolet Spectroscopic Explorer satellite (FUSE), along with Hubble’s imaging spectrograph. The presence of carbon may solve at least one Beta Pictoris mystery: why didn’t the star’s radiation reduce the gas orbiting it? A hidden mass of hydrogen had been suspected as blocking the outflow, but the authors believe that ionized carbon is the cause.
From the paper:
“Neither oxygen nor carbon feels strong radiation pressure in the ? Pic disk, since the star is relatively faint in the far-UV where the strong absorption lines of these species lie. By contrast, metals such as Na and Fe feel extremely strong radiation pressure and could be blown out of the system, producing apparent C and O overabundances relative to these elements…”
But the larger questions remain: what put the carbon into this system, and why is it overabundant relative to other elements, like oxygen, that also feel weak radiation pressure? The answer will demand new work, and the authors suggest the B9 primary star of the binary system Sigma Herculis might be a place to start, since circumstellar dust is being blown away from this UV-bright star in ways that offer interesting parallels as well as differences from Beta Pictoris. But until then, two intriguing ideas emerge from the current work:
Planets may already be forming within the Beta Pictoris disk. Since the comets and asteroids around Beta Pictoris might contain large amounts of graphite and methane, carbon-rich materials, any planets that are evolving here will have a similar composition and probably methane-rich atmospheres.
The Beta Pictoris asteroids and comets might be just like those of the early Solar System, suggesting that our young system might have had far more organic material available to it than today’s studies of asteroids and comets suggest. “We might be observing processes that occurred early in our solar system’s development,” said Nature co-author Alycia Weinberger (Carnegie Institution).
The paper is Roberge, Feldman, Weinberger et al., “Stabilization of the disk around Beta Pictoris by extremely carbon-rich gas,” in Nature 441 (8 June 2006), pp. 724-726 and also available here.
by Paul Gilster | Jun 7, 2006 | Deep Sky Astronomy & Telescopes, Exotic Physics
Nobody can see dark matter, but the mysterious stuff can be detected because it influences large-scale structures like galaxies and galactic clusters. As far as we know, galaxies wouldn’t look the way they do without it. And studies of the cosmic microwave background lead to the belief that dark matter is five times more common than the normal matter we see around us in the form of stars, gas and dust. But that’s about all we know, and we’re therefore left with a problem. How do we study the accelerating expansion of the universe without being able to measure its effects on dark matter?
For that expansion is considered to be the result of an equally mysterious ‘dark energy’ that may well interact with both visible and dark matter, an interaction we need to know more about. A solution that may allow us to study this effect is being developing by Marc Kamionkowski (California Institute of Technology) and Michael Kesden (University of Toronto), who are studying the way dark matter in galaxies disrupts the satellite galaxies near them. Kesden reported on this work at the AAS Calgary meeting today.
To get a handle on it, think about tidal forces, as in the Moon’s effects in raising tides on Earth’s oceans. In a similar way, the Milky Way is disrupting the stars and gas of nearby satellite galaxies, in some cases even pulling stars away from these smaller galaxies to create streams of stars that lead or trail the satellite in its orbit around the Milky Way.
These stellar streams can be useful indeed, if Kamionkowski and Kesden are correct. In their scenario, a satellite galaxy continues to be dominated by dark matter and the forces acting upon it, but the stars that have been disrupted begin to orbit the parent galaxy solely under the influence of gravity. The relative actions of each make for a provocative comparison. An attractive dark matter force should pull the satellite galaxy around its orbit faster than would be expected under the influence of gravity alone. A repulsive dark matter force should slow the satellite down.
So the team hopes to compare the leading and trailing stars with the movement of the satellite galaxies, contrasting observation to simulation in hopes of detecting a dark matter force even if it is only a fraction of that of gravity. “What we’re doing here is a twenty-first century equivalent of Galileo’s Leaning-Tower experiment,” says Kamionkowski. “Galileo demonstrated there that terrestrial materials all fall in the same way in a gravitational field, and we’re trying to figure out whether his conclusion applies to dark matter as well.”
Centauri Dreams‘ take: This is a long and demanding project. Kamionkowski and Kesden will use the Sagittarius dwarf spheroidal galaxy some 78,000 light years from Earth as the testbed, and their simulations indicate that dark matter forces just a few percent of that of gravity should be detectable with their methods. What we can say about the fundamental forces that govern our universe — and indeed about the ultimate fate of that universe — depends heavily on our learning how dark matter operates and what effects the so-called dark energy exerts on it. We can’t expect quick breakthroughs here, but the idea that there is a fifth fundamental force affecting matter we cannot see should put to rest the notion that we are anywhere near a true ‘theory of everything.’
by Paul Gilster | Jun 6, 2006 | Exoplanetary Science |
It’s shaping up to be a good week for exoplanet findings, with yesterday’s intriguing work on ‘planemos’ and their disks and now, also presented at the AAS Calgary meeting, word of new findings on planetary migration. This is a significant issue, because so many of the exoplanets we know about are huge ‘hot Jupiters’ in tight orbits around their star. The effects such planets would have on smaller worlds in the habitable zone could be devastating if the gas giants migrated through that region early in the system’s life.
And migration is assumed to be what happens. The assumption is that such planets form a long way from their stars, as much as 20 AU out, and move to their present positions as the planet interacts tidally with the surrounding gas disk. But migration is tricky business, implying that most planets would fall into their stars within a million years. Preserving a solar system with gas giants and low-mass terrestrial worlds becomes challenging business (and recall that it wasn’t so long ago that ‘hot Jupiters’ were considered more or less an impossibility, a reminder of the nascent state of our migration theories).
Perhaps ‘dead zones’ can save the day. They were the topic of a presentation by Ralph Pudritz (McMaster University) at Calgary yesterday, reporting on a theoretical case that could give us more leeway in the formation of planetary systems. Extending out to about 13 AU, a dead zone is a region of low viscosity gas that can slow planetary migration. Moving inward toward its star, a gas giant opens a gap in the circumstellar disk; its migration speed then becomes locked to the inward drift of the gas. After entering the dead zone, the planet opens a much wider gap, and its migration is substantially slowed by the gas within the zone.
In contrast, low mass planets do not open gaps in the disk as they migrate inward, but their migration can be reversed if they encounter a steep gradient in gas density, as would be found at the edge of the dead zone. Lighter worlds that formed within the dead zone in the first place can open gaps in the zone and have their inward migration slowed.
From the paper on this work, which has been submitted to The Astrophysical Journal, one of several interesting conclusions: “Jovian or super Jovian planets are likely to be formed beyond a dead zone. Inside dead zones, a gap opens for smaller mass planets – ice giants or even terrestrial planets.” What follows is interesting indeed: most massive planets in other solar systems remain undiscovered, but will be found in orbits at 5 AU and greater from their parent star. That’s close enough to Solar System parameters to fuel Centauri Dreams‘ continuing interest in this work. The paper is Matsumura and Pudritz, “Dead Zones and Extrasolar Planetary Properties,” now available here.
by Paul Gilster | Jun 5, 2006 | Exoplanetary Science |
Centauri Dreams marvels at the growth of the new lexicon whose definitions routintely fill these pages. Just the other day we encountered ‘mascon’ — a concentration of mass denoting the presence of a long-obscured crater. Today we get ‘planemos’ — planetary mass objects that float freely through space rather than orbiting a star. The latter come from new findings being discussed at the American Astronomical Society’s Calgary meeting that started yesterday and runs through Thursday. We’ll have a good deal to say about that meeting as the week progresses.
But back to planemos, whose existence was suggested by earlier work on brown dwarfs, many of which are known to be surrounded by potentiallly planet-forming disks of material. “Now that we know of these planetary mass objects with their own little infant planetary systems, the definition of the word ‘planet’ has blurred even more,” says Ray Jayawardhana (University of Toronto), who presented the findings in Calgary today. “In a way, the new discoveries are not too surprising – after all, Jupiter must have been born with its own disc, out of which its bigger moons formed.”
Image (click to enlarge): Astronomers have found disks of dust and gas, the raw material for planet making, around objects that are only a few times heftier than Jupiter. These findings suggest that miniature versions of the solar system may circle “planemos” that are some 100 times less massive than our Sun. Credit: Jon Lomberg (www.jonlomberg.com).
But Jupiter is a mini-solar system orbiting within a larger one. What Jayawardhana and team are looking at are six newly formed objects somewhat larger than Jupiter that are located in star-forming regions some 450 light years from Earth. Their infrared signatures, detected by European Southern Observatory telescopes, lead the astronomers to believe they will evolve planetary systems of their own. All are larger than Jupiter, but range between five and fifteen times Jupiter mass, so they’re clearly not brown dwarfs.
Backing up these conclusions is a second study by Jayawardhana, Subhanjoy Mohanty (Harvard-Smithsonian Center for Astrophysics) and colleagues that targets the brown dwarf 2M1207, known to have a planetary companion discovered some two years ago and dubbed, logically enough, 2M1207B. That planet, some eight times the mass of Jupiter, orbits the brown dwarf at about 40 AU. It has now been shown to be orbited by a disk of its own, an indication that objects not much larger than Jupiter may be prolific in producing small planets.
The size of this planet in relation to the brown dwarf it orbits, and its unusually large separation from the dwarf, tell us something about how 2M1207B probably formed. “Mass ratios of that size are more typical for binary stars than for planetary systems,” said Mohanty. “2M 1207B probably formed like a star, together with the brown dwarf, rather than from core accretion like giant planets around other stars.”
Centauri Dreams‘ take: These findings should strengthen our interest in brown dwarfs, around which planetary systems now seem more and more likely to be found, and suggest intriguing new candidates for potential biospheres as we tune up future life-finder space telescope missions. Indeed, the last five years have begun to shift attention from a narrow focus on Sol-type stars to potential life-bearing worlds around M-class red dwarfs, and it is clear that we may have to keep other options open in a universe that seems to produce planetary systems wherever and whenever it gets the slightest chance.
by Paul Gilster | Jun 3, 2006 | Exotic Physics, Missions, Outer Solar System |
New Scientist is running an interesting piece [subscription required for full access] on Slava Turyshev (JPL), who plans to investigate the so-called Pioneer Anomaly by re-flying the mission virtually. It’s a fascinating tale for various reasons, not the least of which is how close we came to losing much if not all of the precious Pioneer data. For one thing, 400 reels of magnetic tapes housing information about the trajectories of the two spacecraft had to be saved from years of neglect and transferred to DVD.
And that was just the beginning. When Turyshev visited NASA’s Ames Research Center, his search for project records from the 114 onboard sensors that recorded the Pioneers’ spin rate and other data turned up the floppy disks that mission engineer Larry Kellogg had saved. But Ames managers were close to destroying the disks because of lack of space. Having interceded to save this material, Turyshev then turned to programmer Viktor Toth to write a program to extract 40 gigabytes of data from the old floppies, to be recorded like the tapes onto DVD.
So now we can re-fly the missions, correlating each spacecraft event with tracking data in hopes of finding something that might tell us why Pioneer 10, when last heard from, was off-course by about 400,000 kilometers, and why its sister spacecraft is also not quite where it ought to be. A new challenge for Einsteinian gravity, or an effect created within the spacecraft themselves? Prime candidates for scrutiny are the radioisotope thermoelectric generators (RTGs) that power the Pioneers, and the possibility that their waste heat could raise the temperature of one side of the spacecraft, producing a tiny thrust.
The data should tell us whether we can rule out onboard effects and start contemplating a tweak to current gravitational theories. Such a tweak seems unlikely to Centauri Dreams given the lack of evidence of any such effect on other objects in the outer Solar System, but the investigation is well worth making, and could be further developed in a future space mission. On that score, be aware of Dario Izzo (European Space Agency), whose paper “Options for a nondedicated mission to test the Pioneer anomaly,” written with Andreas Rathke and scheduled to appear in the Journal of Spacecraft and Rockets, is available here.
And get this comment from Izzo on the size of the malfunction (if it exists) aboard the Pioneers: “The leak of a single molecule of gas from the spacecraft will give a momentary acceleration similar in size to the Pioneer anomaly.” That’s not much to work with, but as New Scientist writer Stuart Clark points out, the force acting on the spacecraft is a persistent one, so any malfunction would have to be similarly long-lived. The article is “Have We Got Gravity All Wrong?” in the magazine’s June 3 issue.
