Centauri Dreams
Imagining and Planning Interstellar Exploration
Alpha Centauri B: A Close Look at the Habitable Zone
The dreams of Alpha Centauri I used to have as a boy all focused on visual effects. After all, the distance between Centauri A and B ranges from 11.4 to 36.0 AU. What would it be like to have a second star in our Solar System, one that occasionally closed to a little more than Saturn’s distance from the Sun? What would a day be like with two stars, and even more, what would night be like with a star that close lighting up the landscape? I also wondered about how much effect a second star would have on the planets in our system, curious as I was about gravitational effects and even the possible repercussions for weather and seasonal change.
Image: The Alpha Centauri star system and other objects near it in the sky. Image copyright Akira Fujii / David Malin Images.
You can imagine, then, that Duncan Forgan’s new paper hit close to home. Forgan (University of Edinburgh) has taken discussions of habitability around Centauri B to a new level by analyzing the effect of Centauri A on habitability using latitudinal energy balance models that allow him to study how small changes in the properties of a planet can affect the overall climate there. Such models have been useful in studying things like climate variability due to orbital eccentricity and other factors, and Forgan puts them to work to chart the effect of a binary companion.
Alpha Centauri in the Last Fifteen Years
Before I get into the results of the habitability study, though, I want to go through some of the more recent work on Alpha Centauri, all summarized carefully in the Forgan paper. Indeed, I point you to this paper with great assurance that if you are interested in the Centauri stars, you’ll find a useful bibliography and summary here that will quickly get you up to speed (though the bibliography would be better if it listed paper titles along with the rest of the citations). Let’s run through some of the more salient work — in most cases I’ll skip the authors and citations in this discussion, knowing that Forgan’s work containing all of these is freely available at the arXiv site.
Centauri A and B, being high metallicity stars, are presumably prime candidates for circumstellar disks with a high solid material component, making the building blocks of planets readily available, and deepening the spectral lines for improved precision in radial velocity studies. Another useful factor for observations is that the binary is inclined by only 11 degrees with respect to our line of sight, an important fact because it means that any planets we discover through RV methods will yield a mass that is fairly accurate, assuming that the planets around these stars have formed in the same orbital plane. Without such knowledge, the mass figures from RV studies vary widely depending on assumptions about the target system’s inclination.
Studies on planet formation have shown that both Centauri A and B should be capable of forming terrestrial planets even when the perturbations caused by the binary companion are taken into account. Early studies on this question have found that the planetesimal disks seem to be stable out to about 3 AU of the parent stars, assuming a reasonable inclination of the disk relative to the binary plane, meaning something less than 60 degrees. More recent work by Thébault and colleagues has shown that the later stages of accretion may not be efficient because the binary companion can inhibit the growth of larger objects outside 0.75 AU (Cen A) and 0.5 AU (Cen B).
What does this mean? Most likely that the formation of gas giants is unlikely here (a finding that squares with previous radial velocity surveys), while if we can get past the problem of forming larger planetesimals referred to above, Earth-mass planets should be able to form in the habitable zone of Centauri B, assuming an eccentricity of no more than 0.3. A 2009 study I’m not familiar with by Michtchenko & Porto de Mello makes the case that any terrestrial planets that do form in Centauri B’s habitable zone should be dynamically stable despite perturbations from Centauri A under certain conditions of eccentricity and orbital inclination, but planets with inclinations to the orbital plane larger than about 35 percent should experience strong instability.
So where is the habitable zone around Centauri B? Kasting and team used a model that assumed Earth-mass planets with similar atmospheric composition and found a habitable zone ranging from 0.5 to 0.9 AU, although this 1993 study did not include the perturbing influence of Centauri A. But Forgan notes this with regard to the light reaching Centauri B planets:
If main sequence relations for the luminosity of each object are assumed, the insolation experienced by planets in the habitable zone of ? Cen B due to ? Cen A would be no more than a few percent of the total insolation of the ? Cen AB system at the binary’s periastron, and around one tenth of a percent at apastron. This insolation can be diminished further by eclipses of ? Cen A by ? Cen B, the duration of which is estimated to be of order a few Earth days.
Tuning the Model for Centauri B
Kasting was using a global radiative balance model (GRBM), but he and other researchers later deployed latitudinal energy balance models (LEBMs) of the kind Forgan uses in his new study, the latter being more complex and incorporating assumptions about latitude and season and other properties that would be temperature dependent. Forgan adjusts the model to include the effects of the binary (neglecting the distant M-dwarf Proxima Centauri). From the paper:
A planet in global radiative balance is not in general in local radiative balance, and by extension habitability is not a discrete concept (i.e. either habitable or uninhabitable), but a continuous one, where a certain fraction of the planet’s surface will be habitable at any given time. In the LEBM, the evolution of the planet’s temperature T (?) is described by a diffusion equation made nonlinear by the addition of the heating and cooling terms, as well as an albedo which makes a rapid transition from low to high as temperature decreases past the freezing point of water. As a result, small changes in the properties of a planet can strongly affect the resultant climate.
The latitudinal energy balance model, then, seems the best approach for asking how the perturbations caused by Centauri A might affect planets in the habitable zone of Centauri B.
So what does Forgan find? It turns out that calculating the habitable zone of Centauri B’s inner and outer boundaries can be roughly correct if we leave Centauri A out of the picture — the dimensions of the habitable zone remain more or less the same. But adding Centauri A does create oscillations in the planet’s climate that happen when Centauri A is at its closest to Centauri B. The temperature variations caused by Centauri A are no more than several K, and could alter the fraction of habitable surface on planets at the habitable zone boundaries by about 3 percent, a figure made flexible depending on the size of oceans or planetary obliquity.
The paper goes on to note the possible effect on life (science fiction writers take note):
It is reasonable to speculate that if life were to exist on planets around ? Cen B, that they may develop two circadian rhythms (cf Breus et al. 1995) corresponding to both the length of day around the primary, and the period of the secondary’s orbit (approx 70 years). Altering the available habitat by a few percent may also in?uence migration patterns and population evolution.
Small changes over time, though, can lead to big results, as Forgan goes on to remind us:
While we have demonstrated that the temperature ?uctuations for planets around ? Cen B due to ? Cen A are relatively small, the consequences of a periodic temperature forcing of a few K to long term climate evolution cannot be fully understood from this work. To fully appreciate the impact on (for example) ocean circulation and carbonate-silicate cycles requires further investigation with more advanced climate models.
Simulations and Their Limitations
The paper analyzes the results of the simulation for different classes of planets, from fully habitable worlds to uninhabitable hot planets, uninhabitable snowball planets, and two other classes — eccentric transient planets and binary transient planets — that are both partially habitable, with the habitability oscillating according to either the planet’s orbit around Centauri B (eccentric transients) or the period of Centauri A (binary transients). Forgan is careful to comment on the limitations of the LEBM model, which is not sensitive to long-term climate processes, and he notes that adding clouds and a carbonate-silicate cycle into the mix would potentially extend the outer edge of the habitable zone. Another limitation: The model is not sensitive to planets with extremely slow rotations.
Nevertheless, previous work with such modeling has shown its effectiveness, and the picture of the potential Centauri B system that emerges is one in which habitable worlds could well flourish. On the latter score, one other note:
The inner edge of the habitable zone is less well-de?ned than the outer edge – atmospheric changes could allow liquid water above 373 K, and the runaway greenhouse effect may become important at temperatures nearer 350 K (Spiegel et al. 2008 and references within). In any case, the outer edge is likely to be more interesting from an astrobiological standpoint, as current and future instrumentation will be more capable of prob[ing] spectral features of planets at larger semi-major axes (see e.g. Kaltenegger & Selsis 2010).
What’s needed now, of course, are radial velocity results from the ongoing studies of Alpha Centauri, which will begin to tell us whether or not rocky terrestrial worlds actually exist there. This is tricky work — radial velocity methods are much happier with huge gas giants in close orbits than with small rocky planets, which demand a much longer analysis. The paper is Forgan, “Oscillations in the Habitable Zone around Alpha Centauri B,” accepted for publication in Monthly Notices of the Royal Astronomical Society (preprint) and highly recommended to anyone with an interest in planets around our nearest stellar system.
‘Light Echo’ Reveals Eta Carinae Puzzle
Luminous Blue Variables are large, bright stars that give rise to periodic eruptions, like the so-called “Great Eruption” of Eta Carinae that was first noted in 1837 and continued to be observed for an additional 21 years. Things must have been lively around the companion star thought to orbit in the nebula around Eta Carinae, for the LBV blew off about 20 solar masses in this era, mimicking a supernova as it became the second brightest star in the sky. We’ve witnessed similar ‘supernova impostor’ events in other galaxies, but at 7500 light years, the Eta Carinae system is relatively nearby, allowing close study by Hubble and other telescopes.
What brings Eta Carinae’s 1837 event back into the news is the use of so-called ‘light echoes’ to study what happened at a time when astronomy was in a much earlier state. Armin Rest (Space Telescope Science Institute) notes how useful the work is turning out to be:
“When the eruption was seen on Earth 170 years ago, there were no cameras capable of recording the event. Everything astronomers have known to date about Eta Carinae’s outburst is from eyewitness accounts. Modern observations with science instruments were made years after the eruption actually happened. It’s as if nature has left behind a surveillance tape of the event, which we are now just beginning to watch. We can trace it year by year to see how the outburst changed.”
Rest is referring to light bouncing off dust clouds, echoing the cataclysm of 170 years ago so that astronomers can study it with the latest technology. The light echo technique has been employed before in the study of supernovae, culling information about the speed and temperature of the material ejected from the star. You may recall, too, that the anomalous object called Hanny’s Voorwerp, discovered by a Dutch school teacher using the Galaxy Zoo project, turned out to be a gas cloud illuminated by a beam of intense optical and ultraviolet emission from the center of a nearby galaxy, an event researchers dubbed the ‘first quasar light echo.’
Image: The color image at left shows the Carina Nebula, with the massive double-star system Eta Carinae near the top of the image. The star system, about 120 times more massive than the Sun, produced a spectacular outburst that was seen on Earth from 1837 to 1858. But some of the light from the eruption took an indirect path and is just now reaching our planet. The light bounced off dust clouds (the boxed region about 100 light-years away at the bottom of the image) and was rerouted to Earth, a phenomenon called a light echo. The three black-and-white images at right show light from the eruption illuminating dust clouds near the doomed star system as it moves through them. The effect is like shining a flashlight on different regions of a vast cavern. Credit: NASA, NOAO, and A. Rest (Space Telescope Science Institute, Baltimore, Md.).
In the case of Eta Carinae, the echo showed up when Rest and colleagues compared visible-light observations of the system with earlier observations from the Cerro Tololo Inter-American Observatory (CTIO) in Chile. An intense spectroscopic follow-up allowed the astronomers to measure the speed of the outflow (about 195 kilometers per second), and to determine its temperature (5000 K). Among Luminous Blue Variables, Eta Carinae seems unusual in that the outflow from the central region is cooler than observed in other erupting stars, a fact we’ll return to in a moment.
Having this kind of cosmic play-back gives us all kinds of interesting possibilities, including the fact that we already know that a year after its 1843 peak in brightness (which is what the current work measures), another brightening was observed in 1844. Rest’s team is thus awaiting the light echo of that event, which should be observable in about six months and will offer a more complete view of the eruption. Massive Eta Carinae, thought likely to explode as a supernova within the next million years, has much to teach us about the behavior of Luminous Blue Variables. “This star really seems to be an oddball,” Rest adds. “Now we have to go back to the models and see what has to change to actually produce what we are measuring.”
A ‘Lesser Eruption’ occurring around 1890 was observed spectroscopically and shows a strikingly different light spectrum, indicating to the authors of the paper on this work that two distinct physical processes may have been involved in the two events. Whatever the case, Eta Carinae is extraordinarily useful to astronomers because LBV giant eruptions are a rarity, with only the Great Eruption of this star and a giant eruption of the star P Cygni in the 17th Century recorded in our galaxy in the last 400 years. All other supernova ‘impostor’ events have been extragalactic. The challenges posed by this intriguing star are made clear in the paper:
? Car’s Great Eruption has been considered the prototype of the extragalactic SN imposters or ? Car analogues, even though it is actually an extreme case in terms of radiated energy (1049:3 erg), kinetic energy (>1050 erg), and its decade-long duration. The spectra of the light echo indicates now that it is not only extreme, but a different and rather unique object. It is dif?cult to see how strong emission lines could be avoided in an opaque wind where the continuum photosphere is determined by a change in opacity, and its temperature and broad absorption lines are more consistent with the opaque cooling photosphere of an explosion. The cause that triggered such an explosion and the mass-loss without destroying the star is still unknown, but predictions from future radiative transfer simulations trying to explain ? Car and its Great Eruption can now be matched to these spectral observations. Other alternative models that were proposed, e.g. the ones that use mass accretion from the companion star during periapsis passage as a trigger for the eruption, can be either veri?ed or dismissed.
The work points to the conclusion that Eta Carinae is too cool to qualify as the same kind of supernova impostor observed in other galaxies because such stars are thought to be far hotter. This article in Nature quotes Augusto Damineli (University of São Paulo) as saying the findings have surprised everyone. “All well educated astronomers would have bet that they would find the spectrum of a 7,000-kelvin star.” Rest and colleagues are now looking for further Eta Carinae light echoes in different parts of the sky to build up a fuller picture of the eruption.
The paper is Rest et al., “Light echoes reveal an unexpectedly cool ? Carinae during its 19th-century Great Eruption,” Nature 482, 375–378 (16 February 2012). Abstract / Preprint.
A Haze at Galactic Center
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.
KBOs: Surveying the Southern Skies
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).
‘Island-Hopping’ to the Stars
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).
Of Ice and the Planetesimal
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).
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