Centauri Dreams
Imagining and Planning Interstellar Exploration
A Gas Giant Ejected from our System?
Free-floating planets — planets moving through interstellar space without stars — may not be unusual. If solar systems in their epoch of formation go through chaotic periods when the orbits of their giant planets are affected by dynamical instability, then ejecting a gas giant from the system entirely is a plausible outcome. David Nesvorny (SwRI) has been studying the possibilities for such ejections in our Solar System, using computer simulations of the era when the system was no more than 600 million years old. Clues from the Kuiper Belt and the lunar cratering record had already suggested a scattering of giant planets and smaller bodies then.
An ejected planet makes sense. Studies of giant planets interacting with the protoplanetary disk show that they tend to migrate and wind up in a configuration where pairs of neighboring planets are locked in a mean motion resonance. Such a resonance occurs when two planets exert a regular, periodic gravitational influence on each other (there is a 2:3 resonance, for example, between Pluto and Neptune, with Pluto completing 2 solar orbits for every 3 of Neptune). Current work suggests that these resonant systems then become dynamically unstable once the gas of the protoplanetary disk disappears. Just how this happens is what the new work is all about.
From Nesvorny’s paper:
To stretch to the present, more relaxed state, the outer solar system most likely underwent a violent phase when planets scattered off of each other and acquired eccentric orbits… The system was subsequently stabilized by damping the excess orbital energy into the transplanetary disk, whose remains survived to this time in the Kuiper belt. Finally, as evidenced by dynamical structures observed in the present Kuiper belt, planets radially migrated to their current orbits by scattering planetesimals…
The scattering outlined here suggests that Jupiter moved inward in its orbit and scattered smaller bodies both outward and inward, some to take up residence in the Kuiper Belt, others to cause impacts on the inner planets and Earth’s Moon. The problem: A slow change to Jupiter’s orbit based on interaction with small bodies would have thoroughly disrupted the inner system.
A faster change of orbit due to interactions with Uranus and Neptune would have caused both the latter two planets to be ejected from the system. At this point Nesvorny added to the model an additional giant planet, working with an initial state where all the giant planets reached resonant orbits in the protoplanetary disk within a range of some 15 AU from the Sun. As the gas disk dispersed, Uranus and Neptune would have been scattered by the gas giants, reaching their current orbits and in turn scattering the planetesimals in that region into the Kuiper Belt.
The final consequence is the ejection of the fifth gas giant. An additional planet between Saturn and the ice giants — a world that was ultimately ejected from the system entirely — leaves us with a simulation that models the four giant planets we see today. Nesvorny’s simulations put a range of different masses for the planetesimal disk into play, with a total of 6000 scattering simulations following each system for 100 million years, when the planetesimal disk was depleted and planetary migration at an end. And it turns out that you are roughly ten times more likely to wind up with an analog to our Solar System if you start with five giant planets rather than four.
This scenario solves a variety of problems. Understanding how Uranus and Neptune formed is difficult because at their present distances of roughly 20 and 30 AU, accretion would have required too long a timescale. Nesvorny points out that the ice giants form readily at 15 AU or less, and the five planet resonant system his work discovered accounts for their movement outward. The results shift depending on whether one assumes an initial 3:2 resonance between Jupiter and Saturn or a 2:1 resonance, with the latter pushing the outer ice giant to a problematic 18-20 AU for formation. This makes the 3:2 resonance the most likely, but the scientist notes the need for more work on the question.
In the meantime, we’re left with the vision of an even more interesting early Solar System than we thought, and the possibility that the ejection of that fifth giant planet may be what spared the inner system — and our Earth — from complete disruption. We’re also given another look at the processes that produce dark, interstellar wanderers, planets with no star to light them, as Nesvorny notes:
“The possibility that the solar system had more than four giant planets initially, and ejected some, appears to be conceivable in view of the recent discovery of a large number of free-floating planets in interstellar space, indicating the planet ejection process could be a common occurrence.”
The paper is Nesvorny, “Young Solar System’s Fifth Giant Planet?” Preprint available.
A Look at Methane-Based Life
Could life exist on a world with a methane rather than a water cycle? The nitrogen-rich atmosphere of Titan, laden with hydrocarbon smog, is a constant reminder of the question. Cassini has shown us the results of ultraviolet radiation from the Sun interacting with atmospheric methane, and we’ve had radar glimpses of lakes as well as the haunting imagery from the descending Huygens probe. Our notion of a habitable zone depends upon water, but adding methane into the mix would extend the region where life could exist much further out from a star. Chris McKay and Ashley Gilliam (NASA Ames) have been actively speculating on the possibilities around red dwarfs and have published a recent paper on the subject.
It’s intriguing, of course, that with methane we get the ‘triple point’ that allows a material to exist in liquid, solid or gaseous form at a particular temperature and pressure. That makes Titan ‘Earthlike’ in the sense that our initial view showed a landscape with the clear signs of running liquid, but this is a world where temperatures dip to 94 K (-179 Celsius) and water is the local analog of rock. In a fine essay on McKay’s work in Astrobiology Magazine, Keith Cooper notes an earlier McKay paper that suggested a potential life mechanism in this kind of environment: Local methanogens would consume hydrogen, acetylene and ethane while exhaling methane. That’s a mechanism useful for astrobiologists because it would show a particular signature in the depletion of hydrogen, acetylene and ethane at the surface.
Image: This composite was produced from images returned on 14 January 2005, by ESA’s Huygens probe during its successful descent to land on Titan. It shows the boundary between the lighter-coloured uplifted terrain, marked with what appear to be drainage channels, and darker lower areas. These images were taken from an altitude of about 8 kilometres with a resolution of about 20 metres per pixel. Credits: ESA/NASA/JPL/University of Arizona.
But the fact that Titan does show signs of such depletion isn’t necessarily indicative of life, for these signs are themselves dependent on atmospheric models that are still in play, and in any case we know little about other processes that could mimic the same characteristics without implications for life. Exo-Titans may be relatively common, for all we know, but to find them we are going to have to first establish that life can exist in this environment and then work out an atmospheric signature we can search for. Cooper quotes Lisa Kaltenegger (Max Planck Institute) on the issue:
“We just don’t know what the tell-tale signs for life would be in such an atmosphere because it is so vastly different from ours. That said, it will change in a flash if Chris [McKay] finds life on Titan and can tell us what it produces and what we could look for remotely with a telescope.”
That makes future probes of Titan all the more interesting, and adds to the desirability of a long-term presence on the moon, either through a surface rover or an aerostat that could range high over the surface and give us a highly-focused look. As for those red dwarfs McKay studied in his recent paper, a methane habitable zone should exist between 0.63 and 1.66 astronomical units (99 million and 248 million kilometers) around the star Gliese 581, that frequently invoked site of habitable planet speculation. Unfortunately, while we do have four planets confirmed in the system (with two others considered controversial), none exists in the methane sweet spot.
While Gliese 581 is an M2.5V dwarf, the authors also calculate the numbers for an M4 dwarf, finding a closer habitable zone between 0.084 AU and 0.23 AU (12.6 million kilometers to 34.4 million kilometers) in methane terms. The beauty of studying habitable environments — water or methane — around M-dwarfs is that these are systems where the orbital distances involved will be small and detection of planets through radial velocity and planetary transits somewhat easier. But what happens on the surface of such a planet is another matter. Much depends on how the atmosphere is affected by stellar conditions, as Cooper notes:
Titan’s atmosphere is opaque to blue and ultraviolet light, but transparent to red and infrared light, and red dwarfs produce more of the latter than the former. If Titan orbited a red dwarf, more red light would seep through to its surface, warming the planet and extending the range of the liquid methane habitable zone. (Interestingly, a red giant, which is close to the endpoint in the life cycle of a Sun-like star, produces light of similar red wavelengths. When our Sun expands into a bloated red giant in about five billion years, engulfing all the planets up to Earth and possibly Mars, Titan may well reap the benefits – for a short while at least before the red giant puffs away to leave behind a white dwarf star.)
Countering this warming is the effect of large stellar flares on evolving life, frequent on younger red dwarfs. McKay’s work suggests that such active M-dwarfs would dissociate atmospheric molecules on a Titan-like world, making the place more and more smoggy and reducing the surface temperature. The net effect would be to move the methane habitable zone closer to the star. Clearly we have a long way to go to be able to actively search for methane-based life outside our own Solar System, and probably decades to go before we get back to Titan.
For the time being, then, a methane habitable zone is sheer speculation, but it’s interesting to ponder the life that might appear on such worlds. One thing seems sure: The temperatures at which liquid methane exists would produce creatures with slow metabolisms. Will a future Titan probe find life? Given our relatively greater understanding of life’s relation to liquid water, we’re obviously going to keep the focus there, but a ‘second genesis’ on Titan would change the equation considerably as we ponder how frequently life can form and with what constituents.
The paper is McKay and Gilliam, “Titan under a red dwarf star and as a rogue planet: requirements for liquid methane,” Planetary and Space Science Volume 59, Issue 9, pp. 835-839 (July 2011). Abstract available.
Pluto/Charon: A Dangerous Arrival?
We’ve often considered the effect of interstellar dust on a spacecraft moving at a substantial percentage of the speed of light. The matter becomes even more acute when we consider an interstellar probe arriving at the destination solar system. A flyby mission moving at ten percent of the speed of light is going to encounter a far more dangerous environment just as it sets about its critical observations, which is why various shielding concepts have been in play to protect the vehicle. But even at today’s velocities, spacecraft can have unexpected surprises when they arrive at their target.
We’re now looking toward a 2015 encounter at Pluto/Charon. New Horizons is potentially at risk because of the fact that debris in the Pluto system may not be found in a plane but could take the form of a thick torus or even a spherical cloud around the system. We don’t yet know how much of a factor impactors from the Kuiper Belt may be, but strikes at 1-2 kilometers per second would kick up fragments moving at high velocity, generating debris rings or clouds we have yet to see. For that matter, are there undiscovered satellites in this system that could pose a threat?
Image: Pluto’s newest found moon, P4, orbits between Nix and Hydra, both of which orbit beyond Charon. Finding out whether there are other moons or potential hazards near Pluto/Charon was the subject of a recent workshop that is gauging the dangers involved in the encounter. Credit: Alan Stern/New Horizons.
Working on these questions is the job of a team that met at the Southwest Research Institute (Boulder, CO) in early November. The New Horizons Pluto Encounter Hazards Workshop had plenty on its agenda. As principal investigator Alan Stern notes in this report on the New Horizons mission, the group was composed of about 20 of the leading experts in ring systems, orbital dynamics and the astronomical methods used to observe objects at the edge of the Solar System.
The Hubble Space Telescope will play a role in the search for undiscovered moons and possible rings, aided by ground-based telescopes that will study the environment between Pluto and Charon, space through which New Horizons is slated to move. Stern also notes that the ALMA (Atacama Large Millimeter/submillimeter Array) radio telescope will be able to make thermal observations of the system, all of which should give us a better idea of the situation ahead even as plans go forward to consider alternate routes in case the current trajectory starts to look too dangerous. From Stern’s report:
Studies presented at the Encounter Hazards Workshop indicate that a good ‘safe haven bailout trajectory’ (or SHBOT) could be designed to target a closest-approach aim point about 10,000 kilometers farther than our nominal mission trajectory. More specifically, a good candidate SHBOT aim point would be near Charon’s orbit, but about 180 degrees away from Charon on closest-approach day. Why this location? Because Charon’s gravity clears out the region close to it of debris, creating a safe zone.
New Horizons is now approaching 22 AU out and has been brought out of hibernation until November 15 for regular maintenance activities. Tracking a spacecraft on its way to a dwarf planet we have never visited is intriguing enough, but the recent workshop was inspired at least partially by discoveries made after launch, such as the existence of the moon P4, which was found this summer. Stern mentions some evidence for still fainter moons that have not yet been confirmed, but it’s clear that space ahead may have more surprises in store.
Stern adds “it is not lost on us that there is a certain irony that the very object of our long-held scientific interest and affection may, after so many years of work to reach her, turn out to be less hospitable than other planets have been.” Indeed. Then factor in how much work went into getting this mission funded, built and flown — the payload aboard New Horizons is a precious thing indeed. We can only hope — and assume — that the observing campaign to verify the path ahead will be successful.
2005 YU55 Closest Approach Today
2005 YU55, an asteroid roughly the size of a city block, makes its closest pass today, approaching within 325,000 kilometers, closer than the distance between the Earth and the Moon. It will be another seventeen years before we get an asteroid as substantial as this in such proximity. That one is 2001 WN5, which will pass halfway between the Moon and the Earth in 2028. Today’s object of interest, 2005 YU55, isn’t in danger of hitting the Earth on this pass, but astronomers track these objects closely because over time their trajectories are known to change.
Image: This radar image of asteroid 2005 YU55 was obtained on Nov. 7, 2011, at 11:45 a.m. PST (2:45 p.m. EST/1945 UTC), when the space rock was at 3.6 lunar distances, which is about 860,000 miles, or 1.38 million kilometers, from Earth. Image credit: NASA/JPL-Caltech.
The asteroid’s discoverer, Robert McMillan (University of Arizona) calls it “…one of the potentially hazardous asteroids that make close approaches from time to time because their orbits either approach or intersect the orbit of the Earth,” all of which reminds us of the need to keep an eye on it and other Earth-crossing objects. McMillan is co-founder and principal investigator for SPACEWATCH, whose job it is to find and track objects that might pose a threat to the Earth. Both it and the Catalina Sky Survey are based at the University of Arizona’s Lunar and Planetary Laboratory, where the CSS has NASA support to discover potentially dangerous asteroids.
SPACEWATCH uses charge-coupled devices (CCDs) and specialized software to study the images it generates, passing suspect objects on to the Minor Planet Center at the Smithsonian Astrophysical Laboratory. It was on one of these searches that 2005 YU55 turned up six years ago, an apparent Near Earth Object. Subsequent investigation refined its trajectory:
“The MPC posted it on their confirmation page, which is monitored by everybody who follows up newly discovered Near Earth Objects,” McMillan said. “So we followed it up on subsequent nights and over the following month. Over time, we refined its orbit to the point that NASA’s Jet Propulsion Laboratory listed a large number of potential close encounters with the Earth. Now, after 767 observations by ground-based observers, we have the orbit of that asteroid really nailed down, so we know it’s not going to hit the Earth on Nov. 8.”
What we know about this object is that it is roughly spherical and measures about 400 meters in diameter, with a complete rotation every 18 hours. The asteroid is a carbonaceous chondrite, but in the absence of more detailed information about its physical properties, its long-range trajectory is hard to predict, especially given variables like the Yarkovsky Effect, which results from uneven heat distribution on the surface as the object radiates sunlight back into space. All the more reason, then to look forward to the OSIRIS-REx mission, a sample return of the carbonaceous chondrite 1999 RQ36 that will bring material from the asteroid back to Earth in 2023.
The Near-Earth Object Observations Program at JPL known as Spaceguard is also part of the detection program for nearby asteroids, discovering and tracking them to analyze their trajectories for possible danger to Earth. What we know of 2005 YU55 is that its orbit regularly brings it close not only to the Earth but also to Venus and Mars, but the 2011 encounter with our planet is the closest it has come to us for the last 200 years. We should have new Arecibo radar images of the asteroid after closest approach at 1828 EST (1128 UTC). More in this NASA news release, and see this University of Arizona page for further background on the 2005 YU55 encounter. Finally, amateur astronomers equipped to do accurate photometry are being sought to help observe the object. If you’re interested, this Sky & Telescope article has the details.
The Light of Alien Cities
If you’re looking for a new tactic for SETI, the search for extraterrestrial intelligence, Avi Loeb (Harvard-Smithsonian Center for Astrophysics) and Princeton’s Edwin Turner may be able to supply it. The duo are studying how we might find other civilizations by spotting the lights of their cities. It’s an exotic concept and Loeb understates when he says looking for alien cities would be a long shot, but Centauri Dreams is all in favor of adding to our SETI toolkit, which thus far has been filled with the implements of radio, optical and, to a small extent, infrared methods.
Image: If an alien civilization builds brightly-lit cities like those shown in this artist’s conception, future generations of telescopes might allow us to detect them. This would offer a new method of searching for extraterrestrial intelligence elsewhere in our Galaxy. Credit: David A. Aguilar (CfA).
Spotting city lights would be the ultimate case of detecting a civilization not through an intentional beacon but by the leakage of radiation from its activities. In my more naïve days when I just assumed such civilizations filled the galaxy, I imagined someone making an accidental detection of an errant radio signal with a desktop radio receiver, the ultimate ham radio DX catch. As I learned more and came to realize how attenuated such signals would be at these distances, it seemed more likely that we’d need to listen for directed signals, or at best the kind of accidental transmission that might indicate something like one of our own planetary radars.
Then, too, we have to take into account how much our own use of radio has changed, so that because of fiber optic cables and other technologies, we’re lowering our visibility in many wavelengths. An advanced civilization would presumably do the same, but Loeb and Turner figure lighting is something intelligent creatures are going to have no matter what they’re listening to or how they’re listening to it. This is from the recent paper on their work:
Our civilization uses two basic classes of illumination: thermal (incandesent light bulbs) and quantum (light emitting diodes [LEDs] and fluorescent lamps). Such artificial light sources have different spectral properties than sunlight. The spectra of artificial lights on distant objects would likely distinguish them from natural illumination sources, since such emission would be exceptionally rare in the natural thermodynamic conditions present on the surface of relatively cold objects. Therefore, artificial illumination may serve as a lamppost which signals the existence of extraterrestrial technologies and thus civilizations.
The first order of business is to show that searching for artificial lighting is possible within the Solar System, which Loeb and Turner approach by looking at objects in the Kuiper Belt. The technique is “…to measure the variation of the observed flux F as a function of its changing distance D along its orbit.” Working the math, they conclude that “…existing telescopes and surveys could detect the artificial light from a reasonably brightly illuminated region, roughly the size of a terrestrial city, located on a KBO.” Indeed, existing telescopes could pick out the artificially illuminated side of the Earth to a distance of roughly 1000 AU. If something equivalent to a major terrestrial city existed in the Kuiper Belt, we would be able to see its lights.
Objects of interest could be followed up with long exposures on 8 to 10 meter telescopes to examine their spectra for signs of artificial lighting, while radio observatories like the Low Frequency Array (LOFAR) or the Precision Array for Probing the Epoch of Reionization (PAPER) could be used to check for artificial radio signals from the same sources. Interestingly, the Large Synoptic Survey Telescope (LSST) survey will be obtaining much data on KBO brightnesses of the sort that could be plugged into Loeb and Turner’s methodology. Thus running a KBO survey as a tune-up of their methods would involve no additional observational resources.
The researchers aren’t expecting to find cities on KBOs, but they do point out that the next generation of telescopes, both space- and ground-based, is going to be able to reach much further into the universe for signs of artificial lighting. An exoplanet can be examined for changes to the observed flux during the course of its orbit. When it’s in a dark phase, we should see more artificial light on the night side than what is reflected from the day side. A signature like this would have to be bright — the night side would need to have an artificial brightness comparable to natural illumination on the day side — but an advanced civilization might have such cities. For now, the Kuiper Belt provides a handy set of targets we can use to test the technique.
The paper is Loeb and Turner, “Detection Technique for Artificially-Illuminated Objects in the Outer Solar System and Beyond,” submitted to Astrobiology (preprint).
Millis: Of Time and the Starship
What next for the 100 Year Starship Study? NASA and the Defense Advanced Research Projects Agency will make the call, as Tau Zero founder Marc Millis told Alan Boyle in his recent interview. To talk to Boyle, Millis donned virtual garb and appeared in Second Life in robotic form, but the interview is now available as a podcast on BlogTalkRadio and iTunes. I’ll send you there for the discussion in full, but do note that Boyle talked to Millis about starflight before the show and has made an edited transcript of that conversation available on Cosmic Log. The quotes I use below are from the earlier talk, but do pick up the podcast and listen to the whole thing.
Where DARPA goes next is to make a decision about awarding the funds remaining from the $1 million originally put into the project. About $500,000 is available, and Centauri Dreams speculates that DARPA will be more than happy to allocate the funds and be done with them, thus removing ‘starship funding’ from a budget always sensitive to congressional oversight. The plan all along has been to put these funds forward as seed money to the organization that can carry forth the 100 Year Starship idea. In other words, DARPA is hoping to boost interstellar flight by supporting a long-term effort that will find financing and develop technology that one day leads to the stars.
Millis has previously calculated that an actual starship is as much as two centuries away considering historical patterns of energy production and use, but projections like these are always approximations. The important point is that work on a starship has to be approached rationally. Given that we have numerous propulsion options, all of them with huge engineering issues, we have to investigate which of them might evolve to the point where a deep space mission is possible. And we have to take into account huge peripheral matters like equipment reliability over long time-frames, communications at interstellar distances and the matter of human longevity.
Research is invariably incremental, and considering the early state of our knowledge, it will proceed by identifying the key issues and laying a foundation now to chip away at them:
The important issue to figure out today is to make sure we have a sane comparison of the real challenges and the real state of the art, so we’re proceeding wisely here. Then, from that, ask, “OK, if that’s where we are, what can we start tomorrow to chip away at those issues?” We can’t build the starship tomorrow, but we can identify the correct questions to ask, and begin seeking answers to those questions. When it looks more promising, and the advancements are there, fine.
Those advancements may be a long time coming, and there is an inherent danger in impatience. An organization built to last a century or more crosses generations and requires a sustained vision (I’ll be talking about exactly that in an upcoming dialogue with Michael Michaud, to be published here within the next two weeks or so). A public saturated with immediate gratification will not support such projects, but a public educated in the problems involved and the magnitude of their solutions may begin to see things in a long-term context. Amortize interstellar research over two centuries and the costs are more manageable and can grow with the economy. Along the way, as DARPA keeps emphasizing, we should be looking for tangible, near-term spinoffs.
Lately I’ve been reading the new edition of Robert Zubrin’s The Case for Mars, which energetically describes the thinking behind ‘Mars Direct,’ a way to reach Mars with existing technology and with far less expense than many government projections have indicated. But we all know that even Mars, that close and astrobiologically tantalizing world, is still out of reach for the present because of political and economic factors. An interstellar mission is a step so far beyond these tentative steps in our Solar System as to dwarf them entirely. Why, then, look ahead to traveling between the stars when we’re still so early in the game in our own system?
The ultimate, highest-priority benefit of star flight is the survival of the human species beyond the fate of our own solar system and our home planet. In the meantime, the progress we make to try to turn all this stuff into a reality will result in profound improvements in energy conversion, transportation, self-supporting life support — things that would be very useful for life on Earth. And then there’s the social aspect. This effort can give us hope for a better future, expand our opportunities — and hopefully give people a frontier to conquer, rather than being left with no option other than to conquer each other.
Can human cultures pull together to manage major questions of species growth and survival? It’s only one of the questions interstellar flight raises that Boyle and Millis tackle in the longer interview. What we can say is that building a space-based infrastructure is an obvious precursor to an interstellar probe, and getting economically to low-Earth orbit is an obvious precursor to moving outward into the Solar System. Each of these challenges has its advocates and interstellar flight will depend on a satisfactory resolution of all of them. But the challenges of starflight are so immense that chipping away at them now helps to lay a groundwork that will become the roadmap to follow when we do achieve the technologies needed to reach a star.
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