by Paul Gilster | Mar 18, 2009 | Outer Solar System |
Every now and then a new space photo completely snares the attention. This one is a Hubble shot showing four of Saturn’s moons moving in front of the planet. Note Titan at the top, while below it from left to right are Enceladus, Dione and (at extreme right) Mimas. To see the smaller moons, you’ll want to click the image, which will take you to a zoomable view that captures these tiny satellites against the immensity behind them.
Image: Saturn and four of its moons, as seen by Hubble’s Wide Field Planetary Camera 2 on February 24, 2009, when Saturn was at a distance of roughly 1.25 billion kilometers from Earth. Hubble can see details as small as 300 km across on Saturn. The dark band running across the face of the planet slightly above the rings is the shadow of the rings cast on the planet. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).
The transit of the four moons is an unusual event because the rings only become tilted edge-on to Earth every fourteen to fifteen years (they’ll be completely edge-on on August 10 and September 4). The composite image below also shows the four transits, this time with labels, but again, be sure to click the image to get past our formatting limitations.
Image: A composite of separate exposures made by the WFPC2 instrument on the Hubble Space Telescope, showing all four transiting moons. Three filters were used to sample broad wavelength ranges. The color results from assigning different hues (colors) to each monochromatic image. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA). Acknowledgment: M. Wong (STScI/UC Berkeley) and C. Go (Philippines).
Early 2009 turns out to have been an excellent viewing season for those with small telescopes, who could watch moon and shadow transits across the face of Saturn. Titan has crossed the planet on four separate occasions since January. All of which takes me back to the little three-inch reflector I had as a boy, which I turned almost immediately to Saturn for a prized view of the rings. I can still recall the otherworldly feeling that came over me as, standing in my front yard in St. Louis, Missouri I suddenly brought Saturn into focus. It was small, almost minuscule, but the image burned bright as a jewel in that frosty Midwestern night.
by Paul Gilster | Mar 17, 2009 | Exoplanetary Science |
Most stars in our region of the galaxy are low-mass M-dwarfs, making the investigation of their planetary systems quite interesting. If we learn that stars like these, which comprise over 70 percent of the galactic population, can be orbited by Earth-like planets, then the galaxy may be awash with such worlds. But some models have indicated that Earth-sized planets would be rare around these stars, working on the assumption that scaled-down versions of the Sun’s protoplanetary disk would tend to produce only low-mass planets.
Clearly, we need to know more about the masses of such inner disks, since available mass seems to be a key to the formation of habitable planets. Extrapolate the early nebula from our own Solar System to lower protoplanetary disk masses around M-dwarfs and the terrestrial worlds that form are no larger than Mars — they’re small, dry, worlds unlikely to develop life. Low-mass disks would seem to lead to low-mass planets.
But what if those M-dwarf protoplanetary disks aren’t just scaled-down versions of our Sun’s? Fortunately, we can use accretion simulations to study the possible results of varying these parameters, which is what Gregory Laughlin and Ryan Montgomery recently set out to do. From their paper:
Time and again over the past decade, nature has surprised planet hunters with the sheer diversity of planetary systems. Experience has shown that planet formation under a variety of conditions can be more efficient than the example of our own system would suggest. Our approach in this paper has been to investigate a planet building model that lies at the optimistic, yet still justifiable range of formation scenarios. Should our picture prove correct, then the prospects for discovery of truly earthlike, alarmingly nearby, and potentially habitable planets may lie close at hand.
I like that ‘alarmingly nearby’ phrase! To set about investigating this, Laughlin and Montgomery weigh different disk and star mass scenarios. They study both Io and GJ 876d for clues, looking for reasonable answers to the question of disk density at a close orbital radius from low mass stars. They draw on the formation of the moons of Jupiter as inspiration for their model.
Image: In learning more about how planets form around M-dwarfs, we’ll discover how common Earth-mass planets may be. Credit: David Aguilar/Harvard Smithsonian CfA.
And indeed, the simulated M-dwarf planets that emerge from their simulations are worlds with more similarities to Jupiter’s larger moons than to Earth or Venus. Using an M-dwarf some twelve percent as massive as the Sun as their base, the team simulates the accretion of planetary embryos through the late phases of growth, finding that systems of three to five planets could emerge with masses comparable to Earth’s in stable orbits in or near the habitable zone, along with one to two planets per system with masses comparable to Mars.
The further good news is that a terrestrial world like this around a nearby low-mass M-dwarf should be detectable with radial velocity measurements. Narrowing the sample of local stars to a prime catalog of 169 M-dwarfs, the team tests for detection possibilities from the ground and, with a larger range of targets, from space, the latter simulated by plugging in the characteristics of the proposed TESS (Transiting Exoplanet Survey Satellite) spacecraft. While ground detections are marginal, the simulation showed that a space observatory like TESS would find some seventeen of the simulated planets after a two year survey. It wouldn’t take much, in other words, to put this theory to an early test.
Not only that, but a low-mass star offers up a readily detectable field for a transit of a planet of this size. And because the Laughlin/Montgomery model would produce planets that are poor in volatiles (and thus smaller in radius for a given mass than their volatile-rich counterparts), a Doppler wobble combined with a transit could determine whether this scenario is viable.
All of this used to seem so utterly theoretical, but with both CoRoT and Kepler in space, we’re getting used to the idea that finding Earth-class planets may not take more than another couple of years. Learning that habitable worlds are possible around the numerous red dwarfs in our neighborhood is clearly something we can accomplish, with the added advantage of learning much about how these planets form. The paper is Montgomery and Laughlin, “Formation and Detection of Earth Mass Planets Around Low Mass Stars,” accepted by Icarus and available online.
by Paul Gilster | Mar 16, 2009 | Culture and Society |
Tau Zero practitioner Kelvin Long has organized an interstellar session at the forthcoming 2009 UK Space Conference, which will take place from April 1 to 4 at Charterhouse School, near Godalming Surrey. The overall conference looks to be an excellent one, with symposia on rocket technology, panels and presentations on astronomy and space science, much educational material for teachers and students, and the presentation of the Arthur Clarke Awards on the evening of the 4th.
From our perspective, of course, it’s good to see the Tau Zero logo up on the site’s interstellar page, with links to all presentations. Long is a scientist in the plasma physics industry who will address inertial confinement fusion and antimatter-catalyzed fusion for space propulsion. You’ll recall that inertial confinement was the propulsion system of choice for the Project Daedalus starship design created by members of the British Interplanetary Society. Antimatter-catalyzed fusion interests me in light of recent work on harvesting antimatter in space, which suggests a way of augmenting our tiny stores of the stuff.
The agenda for the interstellar session should be firm by now, but you can keep up with any last-minute changes on the site. Here’s what is currently listed:
- Claudio Maccone will be discussing ‘Realistic Targets for Early Interstellar Missions,’ with an eye on the Sun’s gravity focus as a sensible first step before launching probes toward nearby stars. The huge magnification offered by using the gravity lens would make it possible to study and characterize any target system long before a star probe would be designed, much less launched, to that destination.
- Aerospace engineer Luca Derosa (CEO of the Italian company iMEX.A) will be examining velocity profiles for a relativistic space journey in ‘Relativistic Engineering for Interstellar Missions.’
- My interstellar bet partner, the physicist Tibor Pacher, will present ‘Unconventional Thinking in Interstellar Spaceflight Research: Practical Approaches,’ which gets into challenging questions on how to present propulsion research to the public while ensuring scientific rigor.
- Space engineer Mike McCulloch (University of Plymouth) will look at the Pioneer anomaly and other spacecraft trajectory questions, discussing a model that could allow for reduction of the inertial mass of a spacecraft. The paper is ‘The Possibility of Inertial Reduction for Interstellar Travel.’
- Remo Garattini (University of Bergamo) will address the concept of wormholes and the question of their stability in the context of Casimir forces in ‘The Use of Casimir Energy for Traversable Wormholes.’
I’m unable to be in the UK for this conference, but here’s hoping those with an interstellar interest in Britain and nearby countries will be able to swell the ranks to hear topics that range from established physics to cutting-edge speculation. I’m hoping that, as occurred at last year’s IAC conference in Glasgow, some or all of these presentations will be made available through the Net.
by Paul Gilster | Mar 14, 2009 | Astrobiology and SETI |
The planets in our Solar System rotate around the Sun more or less in a plane (the ecliptic) that is tilted some sixty degrees with relation to the galactic disk. It’s interesting to speculate that this could have ramifications in terms of the SETI hunt. Shmuel Nussinov (Tel Aviv University) considers the possibility that any extraterrestrial civilizations might try to contact us only after they had a fair idea we were here. And just as we are now trying, via Kepler and CoRoT, to track down small planets using the transit method, so too might extraterrestrials try to observe our transits, and having done so, to transmit a message.
Targeting habitable planets should optimize chances for a successful reception. From our end, a prudent SETI strategy might then be to home in on the ‘stripes’ of the sky within which our system’s planetary transits are detectable from other solar systems. As Nussinov writes:
The thickness of the galactic disc in our neighborhood is ? 150 parsecs. With the ecliptic at 60° relative to the disc the radial extent of the above slices where some eclipsing in the solar system is observable is typically ? 100 parsecs. This distance spikes at ? 10 K-parsecs towards the intersection of the ecliptic with the galactic plane.
Thus, if we consider only those stars (and prospective ITS’s thereabout) from which at any specific time eclipse by the inner planets can be seen, we restrict to 1.5%-2.5% of all candidates and to ? 7% if we use the broader +/- 3.4° stripe.
By ‘ITS’ Nussinov means Intelligent Technological Societies, cultures able to become aware of us through their own planet-finding technologies and thus more likely to transmit a signal in our direction. And as the author speculates, the notion might come into play with regard to the Fermi paradox. After all, if planetary transits are the primary detection methods at work around the galaxy, then the reason we may not be aware of extraterrestrials is that they simply haven’t found us yet:
This is due to the confluence of 1) our ecliptic plane being inclined by 60% to that of the galaxy—hiding us from most potential ITS’s which are actively searching; and 2) a noisy Sun surface further impeding discovery via the transit method.
Suppose Kepler flags fifty or so terrestrial planets, some of them in the habitable zone of their stars. Would interest in sending signals toward these planets result in actual transmissions? The answer is clearly yes, based upon what we’ve seen in recent years here on Earth, when signals have been transmitted to promote movies and snack foods. We’ve considered whether such messages are wise many times in these pages (search the site under ‘METI’) — my opposition to such transmissions is on record — but our culture show no sign of putting on the brakes. It’s interesting to speculate that an alien culture might act the same.
The paper is Nussinov, “Some Comments on Possible Preferred Directions for the SETI Search” (available online). I was sure I had run into this concept before and, after working on it subconsciously over night, finally recalled Richard Conn Henry (Johns Hopkins), who spoke on this topic at a recent AAS meeting. Here’s my post on that 2008 suggestion, and I’ll quote Henry from it:
“If those civilizations are out there — and we don’t know that they are — those that inhabit star systems that lie close to the plane of the Earth’s orbit around the sun will be the most motivated to send communications signals toward Earth, because those civilizations will surely have detected our annual transit across the face of the sun, telling them that Earth lies in a habitable zone, where liquid water is stable. Through spectroscopic analysis of our atmosphere, they will know that Earth likely bears life.”
This is clearly an idea that is coming into play. Henry notes the particular interest that Taurus and Sagittarius should have for this search, being intersections of the ecliptic with the galactic plane. Observatories like the Allen Telescope Array (ATA) may find this a profitable place to look.
by Paul Gilster | Mar 13, 2009 | Culture and Society |
I love the London Underground and have a great fondness for wandering about the city with a tube map stuck in my pocket. My wife and I last did this a few years back, making an early March trip in which we rented a Bloomsbury apartment for ten days and hopped all over the area, station to station, emerging for blustery walks to various historical sites (we were both, at one time, medievalists), then ducking into nearby restaurants for tea and warming up, talking about what we had seen and examining the map for our next stop.
A map of the London Underground is a schematic diagram that has a beauty of its own, reducing a city beyond its topography to a sequence of formalized connections and zones. The fascination is in the abstraction of the familiar, rendering distance and space intelligible. Now look at what we might call a ‘tube map’ of the Milky Way, as produced by Samuel Arbesman, a postdoc at Harvard with an interest in computational sociology and, obviously, big maps.
Click on the image to see Arbesman’s larger version of it, which in turn links to a downloadable PDF. This is the route map of what Arbesman calls the Milky Way Transit Authority, inspired by his recent re-reading of Carl Sagan’s Contact and the notion of a vast subway handling galactic traffic. Remember its main hub?
And swimming into her field of view as the dodec rotated was… a prodigy, a wonder, a miracle. They were upon it almost before they knew it. It filled half the sky. Now they were flying over it. On its surface were hundreds, perhaps thousands, of illuminated doorways, each with a different shape. Many were polygonal or circular or with an elliptical cross section, some had projecting appendages or a sequence of partly overlapping off-center circles. She realized they were docking ports, thousands of different docking ports — some perhaps only meters in size, others clearly kilometers across, or larger. Every one of them, she decided, was the template of some interstellar machine like this one. Big creatures in serious machines had imposing entry posts. Little creatures, like us, had tiny ports. It was a democratic arrangement, with no hint of particularly privileged civilizations. The diversity of ports suggested few social distinctions among the sundry civilizations, but it implied a breathtaking diversity of beings and cultures. Talk about Grand Central Station! she thought.
Well, the London Underground was never like what Ellie Arroway saw, but I still love it. Ramping up to a galactic scale reminds me, too, of Jon Lomberg’s Galaxy Garden, which renders our vast city of stars in the form of a gorgeous botanical display. I think Arbesman is right in saying “…there is power in creating tools for beginning to wrap our minds around the interconnections of our galactic neighborhood.”
Maybe we’re at the dawn of the era when the galaxy will begin to be represented as a more or less familiar place, rather than a vague stripe of stars across the sky, all but unnoticeable unless you get away from city lights in many parts of the world. That sense of context, of our place within that part of the universe that is immediately around us, is one we need to explain and enhance for our children. Astronomy education can do this, and it doesn’t always hurt to include a bit of whimsy — check out Arbesman’s MWTA tote bags!
