by Paul Gilster | Jul 16, 2007 | Outer Solar System |
A human presence in space is one day going to mean something more than putting a crew into low Earth orbit or even going to the Moon. But longer journeys — to Mars, to Jupiter’s moons and beyond — count among their many challenges the problem of radiation. To solve it, we’ll have to start closer to home, puzzling out our own local radiation hazards from the Van Allen belts, those regions of high-energy electrons and ions caught within the magnetic field of Earth.
Because electromagnetic waves can accelerate electrons, causing so-called ‘enhancement events’ or surges that are up to a thousand times more dense than the norm. The danger to spacecraft electronics can be acute. A powerful solar storm in 2003, for example, caused instrument damage to several spacecraft and may have been the cause of the loss of two Japanese satellites. We’re learning that we need radiation-hardened systems that can withstand such battering.
The 2003 event — actually two storms that occurred back to back in October and November — began to offer clues to the process. So intense were these storms that part of the Van Allen radiation belt was drained of electrons and reformed much closer to Earth. A theory known as ‘radial diffusion’ suggested how the radiation belts should intensify as they reformed (an effect thought to be driven by solar activity), but that didn’t happen. Scientists using data from the European Space Agency’s Cluster mission were then able to show that very low frequency electromagnetic waves can cause the particle acceleration needed to intensify the belts.
Now a new paper out of Los Alamos National Laboratory looks at how the acceleration process works. Using data from three different satellites, the team measured electron fluxes and converted physical measurements to magnetic coordinates. The study confirmed that localized peaks have to be caused by the acceleration of the electrons by electromagnetic waves. Says Los Alamos’ Yue Chen, lead author of the paper: “Debates on the source of the acceleration have lasted for at least a decade, and this paper finally settles the argument based on observations.”
Which is a start, but the exact nature of the interaction is not yet understood. NASA will launch two missions in 2012 in an attempt to probe how the radiation belts are created and decay, with the goal of producing enough empirical data to aid in the design of future radiation-hardened vehicles, and of providing better forecast models to predict geomagnetic storm activity. The paper is Yue Chen et al., “The energization of relativistic electrons in the outer Van Allen radiation belt,” Nature Physics 1 July 2007. It’s a reminder of how far we have to go in our understanding of even the near-Earth environment, much less such vast radiation wastelands as we’ll find around planets like Jupiter.
by Paul Gilster | Jul 14, 2007 | Culture and Society |
An obvious objection to the idea of human journeys to the stars is time — if we can’t find ways to reduce travel time to well within a human lifetime, so the thinking goes, then we’ll have to stick with robotics. But expand the timeframe through multi-generational ships and you change the parameters of the debate. The notion of a multi-generational ‘worldship’ whose crewmembers have long forgotten their actual circumstance is a classic trope of science fiction, with obvious references like Robert Heinlein’s story “Universe” (1941), later reprinted in Orphans of the Sky, and Brian Aldiss’ Non-Stop (1958), published in the US as Starship.
But maybe such a crew wouldn’t forget where it was going. For that matter, would the people aboard a true worldship, one that took, say, 5000 years to make the average interstellar crossing, really consider themselves a crew? They might prefer the term ‘inhabitants’ when describing themselves, because they would be living inside a structure so vast and commodious that it would simply have become a home. By the time they reached their destination, they might well decide to study it for a few generations and then move on to the next distant world.
All of which depends, of course, on our ability to build vast structures in space, huge living worlds that take their inspiration from Gerard O’Neill’s colonies, on which entire ecosystems could flourish. Such mega-engineering is hard to imagine if you’re thinking in terms of 20th Century technologies, but by the end of this century, molecular nanotechnology could conceivably be applied to the task, turning fields of building material like the asteroid belt into workable structures. If off-planet living becomes a serious option, then generations accustomed to living aboard enormous space colonies will surely find among their number some who decide to make the interstellar journey, even if thousands and thousands of years are involved.
Michael Anissimov discusses space colonization in terms that relate to this in a fascinating post on his Accelerating Future weblog. Noting Marshall Savage’s projection that the asteroid belt could theoretically house 7,500 trillion people if exploited in its entirety (this is drawn from the latter’s The Millennial Project), Anissimov goes on to ponder the motivations for space exploration itself. Here’s one relevant bit:
Why expand into space? For many, the answers are blatantly obvious, but the easiest is that the alternatives are limiting the human freedom to reproduce, or mass murder, both of which are morally unacceptable. Population growth is not inherently antithetical to a love of the environment – in fact, by expanding outwards into the cosmos in all directions, we’ll be able to seed every star system with every species of plant and animal imaginable. The genetic diversity of the embryonic home planet will seem tiny by comparison.
Thus one model for interstellar colonization. Rather than building a starship capable of carrying a tiny crew to another star in less than a century, we first turn to exploiting the resources of our own Solar System. Eventually we make a wide variety of space-based habitats available, where colonies of all sizes and descriptions can flourish. Our expertise with nanotech engineering will teach us how to make the most of the abundant resources we have within reach of our planet. At some point, pushing ever deeper and farther out, we may well see the first enterprising colonists depart Sol forever.
Of course, such a perspective demands a different view of time than anything we’re familiar with today. Those of us who believe our expansion into the stars is inevitable also know that the time scale is unknowable. The one thing we can predict with certainty is that it will take the efforts of generations to build the necessary base for such journeys, and to put the technologies available at each step of the way to wise use. Worldships are only one possible outcome of this work, but the reason Centauri Dreams supports continuing research into all aspects of interstellar flight is that building that base is critical if we ever hope to make the interstellar vision a reality. Ad astra incrementis.
by Paul Gilster | Jul 13, 2007 | Exoplanetary Science |
Centauri Dreams sometimes gets e-mail from readers asking how research results can be so contradictory. We’ve discussed gas giants around red dwarf stars, for example, noting theories that such planets are rare in this environment. And then we come up with stars like Gliese 876 and GJ 317, both red dwarfs, and both sporting not one but two gas giants as companions. But stand by, for in a moment we’ll look at new evidence that outer gas giants are indeed rare, and not just around M dwarfs.
What’s going on? The answer is that exoplanetary studies are a work in progress, and will continue to be as far into the future as I can see. We have identified over 200 exoplanets in a galaxy of several hundred billion stars. You bet we’re going to find anomalous situations that challenge every theory we have. And the idea is to put hard scientific work out there for review and critique, noting methodologies and explaining conclusions, thus letting other scientists have a go at the same data.
Those seeking conclusive answers this early in the game are going to find this frustrating, but that’s how science works, and it’s a measure of the complexity of what we’re studying that exoplanetary systems yield their secrets only slowly and over time. The new work for today is an example, a collaboration between US and European astronomers that surveyed 54 young, nearby stars thought to be candidates for Jupiter-class planets at distances beyond Jupiter’s own 5 AU from their star.
Radial velocity techniques are great for finding planets close to the stars they orbit, but much more problematic when dealing with outer planets. So the survey team worked with direct-imaging methods instead, and methane-sensitive imagers specifically designed for this operation. Their conclusion is a bit startling: The survey failed to find a single extrasolar planet in the outer parts of any of the nearby systems it studied. Says graduate student Eric Nielsen (Seward Observatory), “There is no ‘planet oasis’ between 20 and 100 AU. We achieved contrasts high enough to find these super Jupiters, but didn’t.”
One thing scientists will now look at as they probe this work is the imaging technique involved in the survey, based on an instrument called the Simultaneous Differential Imager (SDI) that has been used with both the ESO Very Large Telescope 8.2-meter instrument in Chile and the 6.5-meter telescope at the UA/Smithsonian MMT Observatory on Mount Hopkins, Arizona. The SDI camera splits the light from a single object into four images, which are then sent through methane-sensitive filters to a detector array. Ideally, the bright star disappears while the methane-laden companion comes into view, as shown below.
The method has had success in the past, discovering a brown dwarf around the star SCR 1845-6357, some 12.7 light years away. And Laird Close (University of Arizona), one of the developers of the SDI, finds it powerful enough to say this: “We certainly had the ability to detect outer super Jupiter planets at 10 AU, and farther out, around young sun-like stars.”
Image: Comparison of images taken with SDI on and off. A number of fake planets (at separations of 0.55″, 0.85″, and 1.15″ from the star) were added in to this data, which was then analyzed first using the SDI method and second, using standard adaptive optics techniques. The simulated planets, each seen as a pair of black-and-white dots 33 degrees apart in the SDI image, are easily detected yet are 10,000 times fainter than the central star in the standard adaptive optics analysis. Credit: Laird Close/University of Arizona.
But they didn’t. What would be the constraints on outer gas giant formation around these stars, and how does the survey result affect our current notions of planet formation? Work like this is interesting precisely because it targets filling in the gaps in our knowledge of outer exoplanetary systems, helping us ultimately to learn whether our own Solar System is somewhat average or a departure from the norm. And the answers to the questions it raises will be worked out over time and with the contribution of further surveys using a wide variety of technologies.
Surprises, then, are the nature of the game, and should be considered as opportunities to refine existing theories or suggest new ones. We’ll all watch this process at work as researchers study the two papers involved. They’re Biller et al., “An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with SDI at the VLT and MMT,” accepted by the Astrophysical Journal (abstract available) and Nielsen et al., “Constraints on Extrasolar Planet Populations from VLT NACO/SDI and MMT SDI and Direct Adaptive Optics Imaging Surveys: Giant Planets are Rare at Large Separations,” submitted to the Astrophysical Journal (abstract).
by Paul Gilster | Jul 12, 2007 | Exoplanetary Science |
Probing planetary atmospheres is tricky business at the best of times, but when you’re limited to planets you can’t even see, the project seems well nigh insurmountable. Nonetheless, astronomers using the Spitzer space telescope are having some success working in the infrared. They focus on transiting hot Jupiters, and earlier this year were able to obtain spectra of exoplanetary light from two such worlds, HD 189733b and HD 209458b.
We discussed that work earlier and noted that no water vapor was found in the atmosphere of either planet, despite earlier predictions that it would be. Now a team led by Giovanna Tinetti (Institute d’Astrophysique de Paris) has made further observations of HD 189733b, studying changes in the infrared light from the star as the planet transits, and thus filters the light through its own planetary atmosphere. Working at three different wavelengths, the study showed the clear signature of water.
Image: This plot of data from NASA’s Spitzer Space Telescope tells astronomers that a toasty gas exoplanet, or a planet beyond our solar system, contains water vapor. Spitzer observed the planet, called HD 189733b, cross in front of its star at three different infrared wavelengths: 3.6 microns, 5.8 microns, and 8 microns (see lime-colored dots). For each wavelength, the planet’s atmosphere absorbed different amounts of the starlight that passed through it. The pattern by which this absorption varies with wavelength matches known signatures of water, as shown by the theoretical model in blue. Credit: ESA, NASA/ JPL-Caltech/G. Tinetti (Institute d’Astrophysique de Paris, University College London).
“Water is the only molecule that can explain that behavior,” said Tinetti. “Observing primary eclipses in infrared light is the best way to search for this molecule in exoplanets.”
By ‘primary eclipse,’ Tinetti refers to the planet crossing directly in front of the star. The earlier Spitzer work on HD 189733b and HD 209458b was performed during ‘secondary’ eclipses when the planets moved back behind their stars after the transit. Tinetti clearly believes the primary eclipse is the way to go, a thought backed by visible-light studies of HD 209458b, in which astronomers using the Hubble Space Telescope found hints of water there by studying the planet during a primary eclipse.
But clearly, that water is not liquid. The average temperature on HD 189733b is, at best estimate, 1000 Kelvin (1340 degrees Fahrenheit, or 727 degrees Celsius). We’re talking about a planet that orbits its star in a mere two days. Even so, this hot Jupiter, some 63 light years away in the constellation Vulpecula, may be telling us that water is as abundant in at least one other solar system as it is in our own. And that finding may one day extend to smaller rocky worlds around stars like this one.
The paper is Tinetti et al., “Water vapour in the atmosphere of a transiting extrasolar planet,” Nature 448, (12 July 2007), pp. 169-171, with abstract available.
by Paul Gilster | Jul 11, 2007 | Exoplanetary Science |
Since we’ve just been looking at stellar metallicity and planet formation, news from the European Southern Observatory catches my attention. A new paper from ESO astronomers discusses the question of planetary debris falling onto the surface of stars, and its effects on what we observe. Evidence has been accumulating that planets tend to be found around stars that are enriched in iron. On average, stars with planets are almost twice as rich in metals as stars with no known planetary system.
But what exactly does this result mean? On the one hand, it’s possible that stars that are rich in metals naturally enhance planet formation. But the reverse is also possible: It could be that debris from the planetary system could have polluted the star itself, so that the metals we see aren’t intrinsic to the star. Bear in mind that a stellar spectrum shows only the star’s outer layers, so we can’t be sure what’s at the core. And in-falling planetary debris would stay in the star’s outer regions.
In other words, observed metallicity could actually be caused by the planetary system itself, and not by the star. The ESO team, led by Luca Pasquini, approached the question by studying red giant stars that have exhausted hydrogen in their core. And in the fourteen planet-hosting red giants under investigation, a clear difference appeared between these and normal planet-hosting stars. “We find that evolved stars are not enriched in metals, even when hosting planets,” says Pasquini. “Thus, the anomalies found in planet-hosting stars seem to disappear when they get older and puff up!”
Now it gets interesting, because we’re gaining insights that could affect the evolution of planet formation theories. The ESO astronomers think the difference between red giants and stars like our own in terms of metallicity studies is the size of the convective zone (see image and caption below). In a star like the Sun, this region is about two percent of the star’s mass, but the convective zone in red giants is 35 times larger. Any polluting metals would thus be 35 times more diluted in a red giant, and correspondingly that much more difficult to observe.
Image (click to enlarge): Artist’s impression of the structure of a solar-like star and a red giant. The two images are not to scale – the scale is given in the lower right corner. It is common to divide the Sun’s (and solar-like stars’) interior into three distinct zones. Here, note the uppermost, called the Convective Zone. It extends downwards from the bottom of the photosphere to a depth of about 15% of the radius of the Sun. The energy in the Convective Zone is mainly transported upwards by (convection) streams of gas. In red giants, the convection zone is much larger, encompassing more than 35 times more mass than in the Sun. Credit: ESO.
Artie Hatzes (Thüringer Landessternwarte Tautenburg) puts it starkly: “Although the interpretation of the data is not straightforward, the simplest explanation is that solar-like stars appear metal-rich because of the pollution of their atmospheres.” A co-author of the paper, Hatzes illustrates the tricky nature of metallicity studies. We may be seeing metal excesses resulting from heavy elements falling onto the star from its proto-planetary disk, making the case that how metals function in planet formation is something our theories are a long way from explaining.
And note this: The core accretion model of planet formation seems to be challenged by this finding. Core accretion assumes that planets ‘grow’ as protoplanetary materials bang together and accumulate until, gaining enough mass and forming a solid core, they are able to capture a gas atmosphere. The model depends on dust content to function, and implies that the host stars should show high metallicity down to their core.
The gravitational instability model is different. Let me quote from the Pasquini paper on this mechanism: “…a gravitationally unstable region in a protoplanetary disk forms self-gravitating clumps of gas and dust within which the dust grains coagulate and sediment to form a central core.” Alan Boss (Carnegie Institution of Washington), the major proponent of this theory, has argued that gravitational instability has little dependence on metallicity. The current ESO work seems to be a point in gravitational instability’s favor. The situation is still in flux, however, with more results being gathered from sub-giant star planet searches in other venues. The metallicity findings of these surveys should provide valuable new clues.
The paper is Pasquini et al., “Evolved stars hint to an external origin of enhanced metallicity in planet-hosting stars,” to be published in Astronomy & Astrophysics, with preprint available.
