by Paul Gilster | Feb 23, 2018 | Exoplanetary Science |
At the last Tennessee Valley Interstellar Workshop, I was part of a session on biosignatures in exoplanet atmospheres that highlighted how careful we have to be before declaring we have found life. Given that, as Alex Tolley points out below, our own planet has been in its current state of oxygenation for a scant 12 percent of its existence, shouldn’t our methods include life detection in as wide a variety of atmospheres as possible? A Centauri Dreams regular, Alex addresses the question by looking at new work on chemical disequilibrium and its relation to biosignature detection. The author (with Brian McConnell) of A Design for a Reusable Water-Based Spacecraft Known as the Spacecoach (Springer, 2016), Alex is a lecturer in biology at the University of California. Just how close are we to an unambiguous biosignature detection, and on what kind of world will we find it?
by Alex Tolley
Image: Archaean or early Proterozoic Earth showing stromatolites in the foreground. Credit: Peter Sawyer / Smithsonian Institution.
The Kepler space telescope has established that exoplanets are abundant in our galaxy and that many stars have planets in their habitable zones (defined as having temperatures that potentially allow surface water). This has reinvigorated the quest to answer the age-old question “Are We Alone?”. While SETI attempts to answer that question by detecting intelligent signals, the Drake equation suggests that the emergence of intelligence is a subset of the planets where life has emerged. When we envisage such living worlds, the image that is often evoked is of a verdant paradise, with abundant plant life clothing the land and emitting oxygen to support respiring animals, much like our pre-space age visions of Venus.
Naturally, much of the search for biosignatures has focused on oxygen (O2), whose production on Earth is now primarily produced by photosynthesis. Unfortunately, O2 can also be produced abiotically via photolysis of water, and therefore alone is not a conclusive biosignature. What is needed is a mixture of gases in disequilibrium that can only be maintained by biotic and not abiotic processes. Abiotic processes, unless continually sustained, will tend towards equilibrium. For example, on Earth, if life completely disappeared today, our nitrogen-oxygen dominated atmosphere would reach equilibrium with the oxygen bound as nitrate in the ocean.
Image: Schematic of methodology for calculating atmosphere-ocean disequilibrium. We quantify the disequilibrium of the atmosphere-ocean system by calculating the difference in Gibbs energy between the initial and final states. The species in this particular example show the important reactions to produce equilibrium for the Phanerozoic atmosphere-ocean system, namely, the reaction of N2, O2, and liquid water to form nitric acid, and methane oxidation to CO2 and H2O. Red species denote gases that change when reacted to equilibrium, whereas green species are created by equilibration. Details of aqueous carbonate system speciation are not shown. Credit: Krissansen-Totton et al. (citation below).
Another issue with looking for O2 is that it assumes a terrestrial biology. Other biologies may be different. However environments with large, sustained, chemical disequilibrium are more likely to be a product of biology.
A new paper digs into the issue. The work of Joshua Krissansen-Totton (University of Washington, Seattle), Stephanie Olson (UC-Riverside) and David C. Catling (UW-Seattle), the paper tackles a question the authors have addressed in an earlier paper:
“Chemical disequilibrium as a biosignature is appealing because unlike searching for biogenic gases specific to particular metabolisms, the chemical disequilibrium approach makes no assumptions about the underlying biochemistry. Instead, it is a generalized life-detection metric that rests only on the assumption that distinct metabolisms in a biosphere will produce waste gases that, with sufficient fluxes, will alter atmospheric composition and result in disequilibrium.”
This approach also opens up the possibility of detecting many more life-bearing worlds as the Earth’s highly oxygenated atmosphere has only been in this state for about 12% of the Earth’s existence.
Image: Heinrich D. Holland derivative work: Loudubewe (talk) – Oxygenation-atm.svg, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=12776502
With the absence of high partial pressures of O2 before the Pre-Cambrian, are there biogenic chemical disequilibrium conditions that can be discerned from the state of primordial atmospheres subject to purely abiotic equilibrium?
The new Krissansen-Totton et al? paper attempts to do that for the Archaean (4 – 2.5 gya) and Proterozoic (2.5 – 0.54) eons. Their approach is to calculate the Gibbs Free Energy (G), a metric of disequilibrium, for gases in an atmosphere-oceanic environment. The authors use a range of gas mixtures from the geologic record and determine the disequilibrium they represent using calculations of G for the observed versus the expected equilibrium concentrations of chemical species.
The authors note that almost all the G is in our ocean compartment from the nitrogen (N2)-O2 not reaching equilibrium as ionic nitrate. A small, but very important disequilibrium between methane (CH4) and O2 in the atmosphere is also considered a biosignature.
Using their approach, the authors look at the disequilibria in the atmosphere-ocean model in the earlier Archaean and Proterozoic eons. The geologic and model evidence suggests that the atmosphere was largely N2 and carbon dioxide (CO2), with a low concentration of O2 (2% or less partial pressure) in the Proterozoic.
In the Proterozoic, as today, the major disequilibrium is due to the lack of nitrate in the oceans and therefore the higher concentrations of O2 in the atmosphere. Similarly, an excess concentration of CH4 that should quickly oxidize to CO2 at equilibrium. In the Archaean, prior to the increase in O2 from photosynthesis, the N2, CO2, CH4 and liquid H2O equilibrium should consume the CH4 and increase the concentration of ammonium ions (NH4+ ) and bicarbonate (HCO3-) in the ocean. The persistence of CH4 in both eons is primarily driven by methanogen bacteria.
Image: Atmosphere-ocean disequilibrium in in the Archean. Blue bars denote assumed initial abundances from the literature, and green bars denote equilibrium abundances calculated using Gibbs free energy minimization. Subplots separate (A) atmospheric species and (B) ocean species. The most important contribution to Archean disequilibrium is the coexistence of atmospheric CH4, N2, CO2, and liquid water. These four species are lessened in abundance by reaction to equilibrium to form aqueous HCO3 and NH4. Oxidation of CO and H2 also contributes to the overall Gibbs energy change. Credit: Krissansen-Totton et al.
Therefore a biosignature for such an anoxic world in a stage similar to our Archaean era, would be to observe an ocean coupled with N2, CO2 and CH4 in the atmosphere. There is however an argument that might make this biosignature ambiguous.
CH4 and carbon monoxide (CO) might be present due to impacts of bolides (Kasting). Similarly, under certain conditions, it is possible that the mantle might be able to outgas CH4. In both cases, CO would be present and indicative of an abiogenic process. On Earth, CO is consumed as a substrate by bacteria, so its presence should be absent on a living world, even should such outgassing or impacts occur. The issue of CH4 outgassing, at least on earth, is countered by the known rates of outgassing compared to the concentration of CH4 in the atmosphere and ocean. The argument is primarily about rates of CH4 production between abiotic and biotic processes. Supporting Kasting, the authors conclude that on Earth, abiotic rates of production of CH4 fall far short of the observed levels.
Image: Probability distribution for maximum abiotic methane production from serpentinization on Earth-like planets. This distribution was generated by sampling generous ranges for crustal production rates, FeO wt %, maximum fractional conversion of FeO to H2, and maximum fractional conversion of H2 to CH4, and then calculating the resultant methane flux 1 million times (see the main text). The modern biological flux (58) and plausible biological Archean flux (59) far exceed the maximum possible abiotic flux. These results support the hypothesis that the co-detection of abundant CH4 and CO2 on a habitable exoplanet is a plausible biosignature. Credit: Krissansen-Totton et al.
The authors conclude that their biosignature should also exclude the presence of CO to confirm the observed gases as a biosignature:
“The CH4-N2-CO2-H2O disequilibrium is thus a potentially detectable biosignature for Earth-like exoplanets with anoxic atmospheres and microbial biospheres. The simultaneous detection of abundant CH4 and CO2 (and the absence of CO) on an ostensibly habitable exoplanet would be strongly suggestive of biology.”
Given these gases in the presence of an ocean, can we use them to detect life on exoplanets?
Astronomers have been able to detect CO2, H2O, CH4 and CO in the atmosphere of HD 189733b, which is not Earthlike, but rather a hot Jupiter with a temperature of 1700F, far too hot for life. So far these gases have not been detectable on rocky worlds. Some new ground-based telescopes and the upcoming James Webb Space Telescope should have the capability of detecting these gases using transmission spectroscopy as these exoplanets transit their star.
It is important to note that the presence of an ocean is necessary to create high values of G. The Earth’s atmosphere alone has quite a low G, even compared to Mars. It is the presence of an ocean that results in G orders of magnitude larger than that from the atmosphere alone. Such an ocean is likely to be first detected by glints or the change in color of the planet as it rotates exposing different fractions of land and ocean.
An interesting observation of this approach is that a waterworld or ocean exoplanet might not show these biosignatures as the lack of weathering blocks the geologic carbon cycle and may preclude life’s emergence or long term survival. This theory might now be testable using spectroscopy and calculations for G.
This approach to biosignatures is applicable to our own solar system. As mentioned, Mars’ current G is greater than Earth’s atmosphere G. This is due to the photochemical disequilibrium of CO and O2. The detection of CH4 in Mars’ atmosphere, although at very low levels, would add to their calculation of Mars’ atmospheric G. In future, if the size of Mars’ early ocean can be inferred and gases in rocks extracted, the evidence for paleo life might be inferred. Fossil evidence of life would then confirm the approach.
Similarly, the composition of the plumes from Europa and Enceladus should also allow calculation of G for these icy moons and help to infer whether their subsurface oceans are abiotic or support life.
Within a decade, we may have convincing evidence of extraterrestrial life. If any of those worlds are not too distant, the possibility of studying that life directly in the future will be exciting.
The paper is Krissansen-Totton? et al., “Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life,?” (2018)? Science Advances 4 ? (abstract? / ?full tex?t).
by Paul Gilster | Jan 29, 2018 | Exoplanetary Science |
Are there large moons — perhaps Earth-sized or even bigger — around gas giant planets in habitable zones somewhere in the Milky Way? It’s a wonderful thought given how it multiplies the opportunities for life to find a foothold even in systems much different from our own. Centauri Dreams regular Andrew Tribick recently passed along a new paper that addresses the question in an interesting way, by modeling moon formation and orbital evolution under widely varying conditions of circumplanetary disk composition and evolution.
We’re entering new terrain from this site’s perspective, because I can’t recall going deeply into circumplanetary disks before, at least not in the exoplanet context. But Marco Cilibrasi (Università di Pisa, Italy) and colleagues take us through the necessary background issues. We have two primary models for giant planet formation inside a protoplanetary disk, one being core accretion, when collision and coagulation occurs among dust particles to build up a planetary embryo. The other is gravitational instability, when a gaseous clump collapses into a planet.
We learn from Cilibrasi et al. that both of these approaches to gas giant formation predict that circumplanetary disks will form around planets in the latter stages of their formation. Here we have to be cautious, because circumplanetary disks do not track precisely with the behavior of their larger cousins around stars. The circumplanetary disk (CPD) receives a constant influx of gas and associated dust from the upper layers of the protoplanetary disk, the result of gas accretion onto the forming gas giant. We can take the moons of Jupiter as our local analog of this process. Indeed, understanding a CPD is crucial to make sense of moon formation.
Image: An artist’s rendition of a sunset view from the perspective of an imagined Earth-like moon orbiting the giant planet, PH2 b (a planet confirmed by the Planet Hunters project). The scene is spectacular, but how likely is it that gas giants would have moons beyond Mars size? The answer to the question awaits further work in exomoon detection, but the Cilibrasi paper sees massive moons as a serious possibility. Credit: H. Giguere, M. Giguere/Yale University.
A few interesting things about the Galilean moons: We know that Io is rocky, while the three outer satellites contain a good deal of water ice. The paper points out that icy satellites can only emerge from a circumplanetary disk with temperatures that have dropped below the water freezing point. But simulations of such CPDs show temperatures as high as several thousand Kelvins. This is an indication that the Galilean moons must have formed very late in the process, when Jupiter had substantially cooled and the circumplanetary disk was beginning to dissipate.
Cilibrasi and colleagues performed hydrodynamic simulations that varied the conditions in the circumstellar as well as circumplanetary disks. 20,000 systems were put into play to study the formation, migration and further accretion of the resulting ‘satellitesimals.’ What happens is striking, for because of the continuous influx of dust from the CPD, high-mass satellites form and migrate into the planet, polluting its atmosphere with metals. From the paper:
Our results show that the moons are forming fast, often within 104 years (20 % of the population), which is mainly due to the short orbital timescales of the circumplanetary disc. Indeed the CPD completes several orders of magnitude more revolutions around the planet than the protoplanetary disc material can do around the star at the location of Jupiter. Due to the short formation time, the satellites can form very late, about 30% after 4 dispersion timescales, i.e. when the disc has ? 2% of the initial mass.
During this period of formation, the circumplanetary disk continues to cool off, finally allowing icy moons to form. In fact, the Cilibrasi simulations show that 85 percent of surviving moons contain water ice. The authors found that the satellites lost to the forming planet account for 0.3 Earth masses on average, up to 10 Earth-masses in some cases, thus contributing to the abundance of heavy metals found at Jupiter. But the high mass of the satellites formed by these simulations is an encouraging result.
Even though surviving satellites form late, when the circumplanetary disk has dwindled to about 2 percent of its initial mass, the moons that form out of this late process can be massive, as the paper notes, a distribution “peaking slightly above Galilean masses, up until a few Earth-masses.” This should put the moons around close-orbiting gas giants within range of projects like the Hunt for Exomoons with Kepler. As for exomoons in a habitable zone and the interesting astrobiological implications they offer, we can hope that future breakthroughs in instrumentation will allow us to test these findings.
The paper is Cilibrasi et al., “Satellites Form Fast & Late: a Population Synthesis for the Galilean Moons,” submitted to Monthly Notices of the Royal Astronomical Society (preprint).
by Paul Gilster | Aug 1, 2017 | Uncategorized |
Exomoons — moons around planets in other star systems — are an exhilarating and at the same time seemingly inevitable prospect. There is little reason to assume our Solar System is unique in its menagerie of moons, with the gas giants favoring us particularly with interesting mission targets, and then there’s that fascinating double system at Pluto/Charon. If we visualize what we expect to find in any given stellar system, surely moons are part of the mix, and investigations like the Hunt for Exomoons with Kepler will doubtless find them.
An actual exomoon detection would be a triumph for exoplanet science, especially given how recently it was that we nailed down the first confirmed exoplanet, 51 Pegasi b, in 1995 (or, if you prefer, the 1992 detection of terrestrial-mass planets orbiting the pulsar PSR B1257+12). We’re new at this, and what huge strides we’ve made! Given the small size of the transit signal and its changing relation to the body it orbits, exomoons offer a particularly difficult challenge, although David Kipping’s team at HEK has plenty of Kepler data to work with.
Image: A star with a transiting planet and its moon. The angled area shows the inclination of the moon orbit. Orbit positions beyond the dashed line are not undergoing transit, and are thus not observable. Credit: Michael Hippke.
With all this in mind, every paper that comes out of HEK gets my attention. Kipping (Columbia University), working with graduate student Alex Teachey and citizen scientist Allan Schmitt, has now produced a paper that takes a significant step as the investigation proceeds. We have no detection yet — more about that in a moment — but we do have a broader result showing that exomoons are unusual in the inner regions of the systems surveyed.
Kipping and Teachey looked at 284 viable moon-hosting Kepler planetary candidates to search for moons around planets from Earth to Jupiter in size and distances from their stars of 0.1 to 1 AU. This finding seems to be getting less attention in the press than it deserves, so let’s dig into the paper on it:
Our results place new upper limits on the exomoon population for planets orbiting within about 1 AU of their host star, upper limits that are remarkably low. We have also analyzed subsets of the ensemble to test the effect of various data cuts, and we have identified the regime in which the OSE model presented in Heller (2014) breaks down, which we call the “Callisto Effect” — beyond 20 planetary radii, discrepancies appear in the results.
OSE stands for Orbital Sampling Effect, developed by René Heller in 2014 and described by Michael Hippke in Exomoons: A Data Search for the Orbital Sampling Effect and the Scatter Peak. OSE stacks multiple planet transits to search for an exomoon signature. What the paper is referring to as the ‘Callisto effect’ is the disagreement between OSE predictions and moons like Callisto. Even so, the authors continue to see OSE as a useful tool, and learning about an area in which it breaks down is helpful as we fine-tune our capabilities.
Back to the paper:
Our analysis suggests that exomoons may be quite rare around planets at small semi-major axes, a finding that supports theoretical work suggesting moons may be lost as planets migrate inward. On the other hand, if the dearth of exomoons can be read as a reliable indicator of migration, our results suggest a large fraction of the planets in the ensemble have migrated to their present location.
And that is a pointer to which we need to pay attention. Is a lack of exomoons a marker for planetary migration? If further analysis determines that it is, then we’ve found an extremely handy tool for studying the formation history of other stellar systems.
The Kepler data did yield one exomoon candidate in the Kepler-1625 system for which the authors have set up plans for follow-up observations with Hubble this fall. There is no way to know at this point whether we’ve got a genuine exomoon here or not. And I much appreciate the thorough job that Alex Teachey did in getting this point across to the public in his article Are Astronomers on the Verge of Finding an Exomoon? We learn here that the authors put their paper online earlier than intended because a media outlet was going to publish news about the upcoming Hubble study (Hubble proposals are publicly posted online).
And Teachey’s point is sound at a time when ideas whip around the Internet at lightspeed:
Peer review is a critical part of the scientific process, and we are not terribly comfortable putting out our results before they have been examined by a qualified referee. Unfortunately, we feel the circumstances have forced us to make our results freely available to the public before such a review, so that everyone may see for themselves what we are claiming and what we are not. While David and I are both big proponents of engaging with the public and boosting interest in the incredible things happening every day in astronomy, we have serious concerns about the potential for sensational headlines misleading the public into thinking a discovery has been made when it is really too early to say that for sure.
It’s a solid point. But I also want to emphasize that this paper’s findings about the apparent rarity of exomoons in the inner systems of the stars being studied is quite significant. To my knowledge this is the first time we’ve developed a constraint on exomoon formation. We doubless have moons hiding in the data (recall that the authors are looking for analogs to the Galilean moons of Jupiter), and we can also suspect they are going to be much more common in outer stellar systems, which is certainly the case in our own Solar System.
Don’t expect an immediate result from the Hubble observations. According to this article in Nature, Kipping and team will take about six months to analyze the work before making any announcements. Steady, painstaking effort is how this job gets done.
The paper is Teachey, Kipping & Schmitt, “HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I,” submitted to AAS journals and available as a preprint. For helpful background, check Kipping, “The Transits of Extrasolar Planets with Moons,” PhD thesis, University College London (March 14, 2011), available online.
by Paul Gilster | Jan 13, 2017 | Astrobiology and SETI |
A recent snowfall followed by warming temperatures produced a foggy night recently, one in which I was out for my usual walk and noticed a beautiful Moon trying to break through the fog layers. The scene was silvery, almost surreal, the kind of thing my wife would write a poem about. For my part, I was thinking about the effect of the Moon on life, and the theory that a large single moon might have an effect on our planet’s habitability. Perhaps its presence helps to keep Earth’s obliquity within tolerable grounds, allowing for a more stable climate.
But that assumes we’ve had a single moon all along, or at least since the ‘big whack’ the Earth sustained from a Mars-sized protoplanet that may have caused the Moon’s formation. Is it possible the Earth has had more than one moon in its past? It’s an intriguing question, as witness a new paper in Nature Geoscience from researchers at the Technion-Israel Institute of Technology and the Weizmann Institute of Science. The paper suggests the Moon we see today is the last of a series of moons that once orbited the Earth.
“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth,” says co-author Assistant Prof. Perets (Technion). “It’s likely that such moonlets were later ejected, or collided with the Earth or with each other to form bigger moons.”
To explore alternatives to giant impact theories, the researchers have produced simulations of early Earth impacts, varying the values for the impactor’s velocity, mass, angle of impact and the initial rotation of the target. The process that emerges involves multiple impacts that would produce small moons, whose gravitational interactions would eventually cause collisions and mergers, to produce the Moon we see today. Here’s how the paper describes the process:
… we consider a multi-impact hypothesis for the Moon’s formation. In this scenario, the proto-Earth experiences a sequence of collisions by medium- to large-size bodies (0.01–0.1M?). Small satellites form from the impact-generated disks and migrate outward controlled by tidal interactions, faster at first, and slower as the body retreats away from the proto-Earth. The slowing migration causes the satellites to enter their mutual Hill radii and eventually coalesce to form the final Moon. In this fashion, the Moon forms as a consequence of a variety of multiple impacts in contrast to a more precisely tuned single impact.
Here’s a graphic from the paper (listed as Figure 1) that shows the process at work:
Image (click to enlarge): a,b, Moon- to Mars-sized bodies impact the proto-Earth (a) forming a debris disk (b). c, Due to tidal interaction, accreted moonlets migrate outward. d,e, Moonlets reach distant orbits before the next collision (d) and the subsequent debris disk generation (e). As the moonlet–proto-Earth distance grows, the tidal acceleration slows and moonlets enter their mutual Hill radii. f, The moonlet interactions can eventually lead to moonlet loss or merger. The timescale between these stages is estimated from previous works.
The Hill radius mentioned above describes the gravitational sphere of influence of an object; in this case, meshing Hill radii can produce interactions that sometimes lead to mergers. The paper notes that in head-on impacts, the rotation of the planet is important because the disk needs angular momentum resulting from the rotation to stay stable. With increased rates of rotation, the angular momentum of the disks increases. Moons like ours emerge from many of the simulations:
We find that debris disks resulting from medium- to large-size impactors (0.01–0.1M?) have sufficient angular momentum and mass to accrete a sub-lunar-size moonlet. We performed 1,000 Monte Carlo simulations of sequences of N = 10, 20 and 30 impacts each, to estimate the ability of multiple impacts to produce a Moon-like satellite. The impact parameters were drawn from distributions previously found in terrestrial formation dynamical studies. With perfect accretionary mergers, approximately half the simulations result in a moon mass that grows to its present value after ~20 impacts.
If the multi-moon hypothesis proves credible, how would it affect the larger astrobiology question? In Ward and Brownlee’s Rare Earth (Copernicus, 2000), after a discussion of obliquity and the Moon’s effect on the Earth’s early history, the authors say this:
If the Earth’s formation could be replayed 100 times, how many times would it have such a large moon? If the great impactor had resulted in a retrograde orbit, it would have decayed. It has been suggested that this may have happened for Venus and may explain that planet’s slow rotation and lack of any moon. If the great impact had occurred at a later stage in Earth’s formation, the higher mass and gravity of the planet would not have allowed enough mass to be ejected to form a large moon. If the impact had occurred earlier, much of the debris would have been lost to space, and the resulting moon would have been too small to stabilize the obliquity of Earth’s spin axis. If the giant impact had not occurred at all, the Earth might have retained a much higher inventory of water, carbon and nitrogen, perhaps leading to a Runaway Greenhouse atmosphere.
The idea of a series of impacts eventually leading to a larger moon significantly muddies the waters here. It is true that in our Solar System, the inner planets are nearly devoid of moons, but we have no way of extending this situation to exoplanets without collecting the necessary data, which will begin with our first exomoon detections. Certainly if numerous collisions in an early planetary system can produce a large moon, as this paper argues, then we can expect similar collisional scenarios in many systems, making such moons a frequent outcome.
The paper is Rufu, Oharonson & Perets, “A Multiple Impact Hypothesis for Moon Formation,” published online by Nature Geoscience 9 January 2017 (abstract).
by Paul Gilster | Aug 12, 2016 | Exoplanetary Science |
I often work out my thoughts on the topics we discuss here while taking long walks. I try to get in five miles a day but more often it’s about three. In any case, these long, reflective walks identify me as the neighborhood eccentric, an identity that is confirmed by the things I write about. What’s interesting about that is that so many people have a genuine interest in the stars and how we might get there. Some of the best questions I’ve ever had have been from people whose interest is casual but persistent, and one good question usually leads to another.
Hence I wasn’t surprised on yesterday’s walk to find myself talking with a neighbor about exomoons and why we study them. After all, we have a Solar System in which moons are commonplace. Isn’t it perfectly obvious that different solar systems would have planets with moons?
The answer is yes, but it also follows that things that seem perfectly obvious still have to be confirmed. But let’s unpack it a bit more than that. We’re familiar with our own system’s configuration, in which moons of astrobiological interest are orbiting gas giants a long way from the inner system. But we know from our exoplanet work that large planets like these can exist in warmer places. Thus the notion of habitable moons around gas giants, or perhaps double planets in the habitable zone, something like a larger version of Pluto and Charon in a comfortable orbit.
Popular films like Avatar keep the exomoon theme in front of the public, whose interest is understandable. After all, could anything be more exotic than a warm gas giant orbited by something a bit like the Earth? From an astrobiological perspective, the thought of Europa or Titan analogs in warm orbits is thrilling, a reminder that life may have gained many footholds in the galaxy. The Hunt for Exomoons with Kepler project is all about figuring out the occurrence rate of large moons so we can learn whether such moons are common.
The other aspect of exomoon detection has to do with increasing our expertise. It wasn’t so long ago that we had yet to detect our first exoplanet. Now we’re delving into planetary atmospheres and working out the orbital dynamics of multi-planet systems. An exomoon detection would be a major proof of concept, demonstrating the growth of our skills. It would also begin to build an exomoon catalog that will help us understand how important exomoons may be to planetary habitability. How big a role does our own moon play in keeping our planet habitable?
There’s also plenty to learn about how planetary systems form in the first place. We now think our Moon formed in a massive collision (the Big Whack) with a Mars-sized object in the early days of our planet’s history. How likely an event is this, and how often does it happen in other Solar Systems? We still have a lot to learn about how the satellite systems around various planets emerge, especially when we consider the wild variety of moons we see in our Solar System. Building the exomoon catalog will help answer these questions.
The Joys of Beta Pictoris b
I hadn’t planned to get into exomoons today, but serendipity struck. After yesterday’s conversation I ran across Phil Plait’s latest essay for Slate. The popular astronomer and science popularizer (author of Death from the Skies! and, of course, Bad Astronomy), now explains that because of an unusual alignment beginning in 2017, we may be able to detect an exomoon, if there is one, around the planet Beta Pictoris b.
We’re dealing with a system far different from our own. Some 60 light years away, the star Beta Pictoris is more massive than the Sun and a mere infant, at 25 million years old, compared to our own star (around 4.5 billion years). This is a solar system in formation. Moreover, it has been under intensive study since scientists realized it was surrounded by a large circumstellar disk. The planet Beta Pictoris b was first imaged in 2003, a world more massive than Jupiter that orbits its host every 20 years. You can see its movement in the time-spaced images below.
Image: Infrared images of the planet ? Pictoris b obtained in 2003 (a), 2009 (b) and 2010 (c), showing the planet’s movement in an orbital plane that is nearly edge-on as seen from Earth. The host star is in the central part, but its light has been suppressed to show the fainter planet. The white dots in b and c denote previous positions of the planet. Faint blobs are optical effects. It is not possible to tell from these images whether the planet is orbiting towards or away from us, but {Ignas] Snellen and colleagues’ spectroscopic observations clearly indicate that the planet is currently in a part of its orbit where it is moving towards us. Credit: ESO.
Note in the description above that the planet’s orbital plane is close to edge-on from our perspective. It’s not close enough to make a transit possible, but what Plait talks about is
the next best thing. Drawing on a paper by Jason Wang (UC-Berkeley) and colleagues, Plait explains that the region around the planet called its Hill Sphere will pass in front of the star from our perspective. The Hill Sphere is the area around an astronomical body in which its gravity dominates. In other words, within the Hill Sphere, a moon could be retained by the planet.
Nobody explains such concepts as well as Phil Plait, so I’ll give him the floor here, drawing directly from his essay:
The size of the sphere depends on the mass of the planet, the mass of the star, and the distance between them. For example, the Earth’s Hill sphere reaches out to about 1.5 million kilometers. The Moon, orbiting 380,000 km away, is well inside that, so its motion is mostly influenced by the Earth (some people like to say the Moon orbits the Sun more than it does the Earth, but those people are wrong). Weirdly, Pluto’s Hill sphere is much larger than Earth’s, but that’s because it’s so far from the Sun that an object can orbit Pluto from farther away and still be heavily influenced by it.
What emerges with regard to Beta Pictoris b is that its Hill Sphere is 160 million kilometers in radius. We get no transit of the star by the planet itself, but by August of 2017, the planet will be at its closest approach to the star and the Hill Sphere region will transit. We’ll be able to look for debris or exomoons. A large moon passing in front of the star would be the first entry in the exomoon catalog.
But even if we get no exomoon detection, bear in mind that we may make other interesting observations. This young planet is still being born, and it may well contain a circumplanetary disk of its own, or even a ring system that is the residue of planet formation. “The transit of ? Pic b’s Hill sphere,” Wang et al. write, “should be our best chance in the near future to investigate young circumplanetary material.” We’ll also learn a lot more about how Beta Pictoris b perturbs the circumstellar disk, a window into early solar system formation.
All this is good material for my next walk and the conversations sure to follow. The paper is Wang et al., “The Orbit and Transit Prospects for ? Pictoris b constrained with One Milliarcsecond Astrometry,” accepted at the Astrophysical Journal (preprint).
