by Paul Gilster | Jan 27, 2015 | Exoplanetary Science |
Might there be gas giant planets somewhere with moons as large as the Earth, or at least Mars? Projects like the Hunt for Exomoons with Kepler (HEK) are on the prowl for exomoons, and the possibility of large moons leads to astrobiological speculation when a gas giant is in its star’s habitable zone. Interestingly, we may be looking at evidence of an extremely young — and very large — moon in formation around a planet that circles the young star J1407.
That would be intriguing in itself, but what researchers at Leiden Observatory (The Netherlands) and the University of Rochester have found is an enormous ring structure that eclipses the young star in an epic way. The diameter of the ring system, based on the lightcurve the astronomers are getting, is nearly 120 million kilometers, which makes it more than two hundred times larger than the rings of Saturn. This is a ring system that contains about an Earth’s mass of dust particles, with a marked gap that signals the possibility of the large moon.
Image: Artist’s conception of the extrasolar ring system circling the young giant planet or brown dwarf J1407b. The rings are shown eclipsing the young sun-like star J1407, as they would have appeared in early 2007. Credit: Ron Miller.
The ring system itself was discovered in 2012 by Eric Mamajek (University of Rochester) and team, with Leiden’s Matthew Kenworthy and Mamajek now refining the observations and working out the details. What emerges is a ring system with over thirty separate rings. And you need to see the lightcurve, which is available below. Kenworthy’s enthusiasm about the find is evident:
“The details that we see in the light curve are incredible. The eclipse lasted for several weeks, but you see rapid changes on time scales of tens of minutes as a result of fine structures in the rings. The star is much too far away to observe the rings directly, but we could make a detailed model based on the rapid brightness variations in the star light passing through the ring system. If we could replace Saturn’s rings with the rings around J1407b, they would be easily visible at night and be many times larger than the full moon.”
Exoring model for J1407b from Matthew Kenworthy on Vimeo.
I love the many worlds presented to us in science fiction, but I’m hard pressed to come up with a depiction of anything quite like this. Says Mamajek:
“If you were to grind up the four large Galilean moons of Jupiter into dust and ice and spread out the material over their orbits in a ring around Jupiter, the ring would be so opaque to light that a distant observer that saw the ring pass in front of the sun would see a very deep, multi-day eclipse. In the case of J1407, we see the rings blocking as much as 95 percent of the light of this young Sun-like star for days, so there is a lot of material there that could then form satellites.”
The figure below, from the paper, gives a static view of the same data:
Image: From the paper. The caption reads: “Model ring fit to J1407 data. The image of the ring system around J1407b is shown as a series of nested red rings. The intensity of the colour corresponds to the transmission of the ring. The green line shows the path and diameter of the star J1407 behind the ring system. The grey rings denote where no photometric data constrain the model fit. The lower graph shows the model transmitted intensity I(t) as a function of HJD. The red points are the binned measured flux from J1407 normalised to unity outside the eclipse. Error bars in the photometry are shown as vertical red bars.” Credit: Matthew Kenworthy/Eric Mamajek.
As to J1407b, the planet these rings surround, the astronomers estimate that it has an orbital period of about a decade, with a mass most likely in the range of between ten and forty Jupiter masses. The gap in the ring structure points to a satellite in formation that has an orbital period of approximately two years around the gas giant. It becomes clear that if we can find more instances of early disks, we can begin to study comparative satellite formation around exoplanets. From the paper:
J1407 is currently being monitored both photometrically and spectroscopically for the start of the next transit. A second transit will enable a wide range of exo-ring science to be carried out, from transmission spectroscopy of the material, through to Doppler tomography that can resolve ring structure and stellar spot structure significantly smaller than that of the diameter of the star. The orbital period of J1407b is on the order of a decade or possibly longer. Searches for other occultation events are now being carried out (Quillen et al. 2014) and searches through archival photographic plates (e.g. DASCH; Grindlay et al. 2012), may well yield several more transiting ring system candidates.
The paper also points out possible ring structures around Fomalhaut b (anomalous bright flux in optical images) and Beta Pictoris b (anomalous photometry), though neither of these has been confirmed. The scientists involved are encouraging amateur astronomers to help monitor J1407 as the attempt to constrain the mass and period of the ringed planet J1407b continues. Observations can be reported to the American Association of Variable Star Observers (AAVSO).
The paper is “Modeling giant extrasolar ring systems in eclipse and the case of J1407b: sculpting by exomoons?” accepted for publication by the Astrophysical Journal (preprint).
by Paul Gilster | May 21, 2014 | Exoplanetary Science |
A friend asked me the other day whether my interest in exomoons — moons around exoplanets — wasn’t just a fascination with the technology of planet hunting. After all, we’ve finally gotten to the point where we can detect and confirm planets around other stars. An exomoon represents the next step at pushing our methods, and a detection would be an affirmation of just how far new technology and ingenious analysis can take us. So was there really any scientific value in finding exomoons, or was the hunt little more than an exercise in refining our tools?
I’ve written about technology for a long time, but the case for exomoons goes well beyond what my friend describes. We’ve found not just gas giants but ‘super-Earths’ in the habitable zones of other stars, and it’s a natural suggestion that around one or both classes of planet, an exomoon might he habitable even if the parent world were not. It’s a natural assumption that moons exist around other planets elsewhere as readily as they do around the planets of our own Solar System, and given the sheer number of planets out there, the prospect of adding potential sites for life — perhaps more common sites than habitable planets themselves — is irresistible.
Then in catching up with work after my recent trip, I noticed René Heller’s work on the exomoon question, as reported in Astrobiology Magazine. Heller (McMaster University, Ontario) is proposing a new method of detection that involves the particular eclipsing effect of moons during the kind of transit studies that Kepler has provided so much data on. What is striking here is that if Heller is right, we may be able to detect not just moons several times larger than Ganymede — this is where the current state of the art seems to be — but moons much smaller still, on the scale of the moons we find in our own Solar System.
Heller’s paper, recently published in The Astrophysical Journal, picks up on further scientific advantages in studying exomoons. Such objects can offer us insights into how exoplanets formed. Consider this (internal references omitted for brevity):
The satellite systems around Jupiter and Saturn, for example, show different architectures with Jupiter hosting four massive moons and Saturn hosting only one. Intriguingly, the total mass of these major satellites is about 10-4 times their planet’s mass, which can be explained by their common formation in the circumplanetary gas and debris disk…, and by Jupiter opening up a gap in the heliocentric disk during its own formation… The formation of Earth is inextricably linked with the formation of the Moon…, and Uranus’ natural satellites indicate a successive ‘collisional tilting scenario’, thereby explaining the planet’s unusual spin-orbit misalignment.
And so on. Between astrobiology and planet formation, I’d say there is plenty of reason to be interested in exomoons, though my friend is right that I still get jazzed by the ability of skilled researchers to develop the strategies we need for their detection. We’ve followed the fortunes of the Hunt for Exomoons with Kepler (HEK) for some time in these pages, and it’s fascinating to me that what Heller is proposing would also make use of the abundant Kepler data. But while HEK works through the study of variations in transit timings and durations, Heller’s method, which he calls the ‘Orbital Sampling Effect,’ relies on a different kind of analysis.
Addendum: My mistake. A note from exomoon hunter David Kipping, who heads up the Hunt for Exomoons with Kepler, sets this straight. Let me quote it:
“This is not true, actually HEK uses dynamics and photometric effects in combination. We model both the moon transit effects (which Heller is talking about) plus the dynamical perturbations, such as transit timing and duration variations, simultaneously. The difference is that Heller stacks all of the transit data on top of itself and we do not. An important point to bear in mind is that this stacking process does not give you any more signal-to-noise than using the entire time series and fitting it globally, as HEK does. Since one does not actually gain any greater sensitivity by using the OSE method, one cannot finds moons smaller than the sensitivity limits we’ve achieved thus far in HEK and I can tell you that Galilean satellites are only detectable in very exceptional cases- generally Earth-like radius sensitivity is the norm.
“Let me say though, the OSE method is fast and computationally cheap but lacks the dynamics of our models, which ultimately allow for an exomoon confirmation. So I think this method could be useful tool for quickly scanning the Kepler data to identify interesting anomalies worthy of further analysis.”
And back to the original post:
What more can we tease out of an exoplanetary transit? Observation of a moon orbiting an exoplanet around its equatorial midline and passing in front of the planet, then behind it, should — over the course of time and numerous observations — build up a series of dots representing its position at any given moment in its orbit. With enough observations, the effect is of two ‘wings’ coming out of the sides of the planet, an effect that will appear lighter at the inner edges and darker at the outer edges. Astrobiology Magazine explains:
That’s because when the moon reaches the extent of its orbit and then starts circling back around the planet, its positions overlap more in a tighter space. As such, the “wingtips” look darker; that is, there is increased eclipsing of background starlight at the moon’s farthest apparent positions from the planet.
And of course to spot this effect requires constant observation of the star over a long period of time, allowing the moon to complete a large number of orbits “in order for its light-blocking effect to preferentially stack up at the wingtips.” And voila, what we have with the Kepler spacecraft is an observatory that gave us precisely this, observations of about 150,000 stars through its long stare of data gathering, some four years before equipment failure laid it low. Heller’s point is that we need wait for no future technology to hunt today for moons like those in our Solar System.
Image: This figure from Heller’s paper depicts the effects of a transiting exoplanet with exomoon averaged over time. We would expect a drop in starlight as the exoplanet moves in front of the host star, but Heller’s method focuses on the change to the lightcurve caused by the exomoon before and after it crosses the stellar disk, an effect which repeats on the other side. Credit: René Heller.
The ‘stacking up’ effect of these transit observations over time as the moon and planet transit the star in different configurations allows the detection. Because we need plentiful statistical data to make all this work, red dwarf stars are ideal candidates because habitable zone planets there have extremely short years and thus make many transits. According to Heller, moons down to Ganymede size should be detectable around M-dwarfs, while around warmer orange dwarf stars, exomoons about ten times Ganymede’s mass would be within range. G-class stars like the Sun are not represented well enough in the Kepler data because they lack sufficient transits.
The minute effects discernible in an exoplanet’s regular transit are what make the exomoon detection possible. Heller notes that the Orbital Sampling Effect (OSE) yields data indicative of the moons’ radii and planetary distances, while study of the planet’s transit timing variations (TTV) and transit duration variations (TDV), in conjunction with Orbital Sampling Effect, allow measurements of the moon’s mass. More complex transit signatures could, using this method, even allow the detection of multiple exomoon candidates.
The paper is Heller, “Detecting extrasolar moons akin to solar system satellites with an orbital sampling effect,” The Astrophysical Journal, Vol. 787, No. 1 (2014), abstract and preprint available. Thanks to Dave Moore for the pointer to this work.
by Paul Gilster | Jun 6, 2013 | Exoplanetary Science |
Thinking as we have been about exoplanet detection, and in particular about taking the next steps beyond the James Webb Space Telescope, I’m intrigued to see what has happened with the WFIRST mission. After all, despite the successes of Kepler and ESA’s CoRoT, we live in an era when mission cancellation is not uncommon. The Space Interferometry Mission was canceled outright, while Terrestrial Planet Finder, long touted as the way we would home in on nearby planets like our own, has been put into indefinite suspension. The JWST is on the horizon, but interesting new possibilities are now bubbling up around WFIRST, the Wide Field Infrared Survey Telescope. A mission with a dark energy pedigree could now have serious exoplanet implications.
In Exoplanet Capabilities of WFIRST-2.4, Philip Horzempa looks at the latest design to emerge for this mission, one that takes us much deeper into exoplanet country than I had thought the mission could. After all, WFIRST was conceived as a way of studying dark energy and the universe at the largest scale, using what had been 1.2-meter optics. But Horzempa points out that the National Reconnaissance Office (NRO) has now given NASA two Hubble-class 2.4-meter telescope mirrors, a gift that will boost the mission’s capabilities dramatically.
Image: As envisaged by the Astro2010 Decadal Study, WFIRST will be a wide-field-of-view near-infrared-imaging and low-resolution-spectroscopy space telescope. WFIRST will address two of the most fundamental questions in astrophysics: Why is the expansion rate of the universe accelerating? And are there other solar systems like ours, with worlds like Earth? Credit: NASA GSFC.
Evolution of a Mission
My recent post on using gravitational lensing to search for planets around Proxima Centauri reminded me that microlensing was folded into WFIRST from the start, but boosting the optics with a 2.4-meter mirror means that three times as much light will be collectable by the space observatory. The WFIRST exoplanet program centers on 500 days of observation time spread over five years, observing a set of adjacent fields in the galactic bulge. The information thus gathered, because it is not weighted (like radial velocity methods) to large, close-in planets, will help us understand more about solar system structure around a wide variety of stars.
The Science Definition Team commissioned by NASA to study the enhanced WFIRST design talks about finding in the range of 300 Earth-mass planets and 40 Mars-sized worlds using microlensing, with as many as 1000 super-Earths also likely to be found, for a grand total of perhaps 3000 exoplanets of all kinds emerging from the mission. Moreover, these methods would allow WFIRST to detect orphan planets ejected from their solar systems, galactic wanderers we are unable to detect with other methods. These worlds would be interesting not only in themselves but also as statistical fodder for planet formation theories.
Image: A microlensing light curve (left) produced by the relative movement of a star-planet system with respect to a background source (right). Image Credit: Dave Bennett (Notre Dame).
WFIRST’s continual evolution is also leading to the addition of a coronagraph that can block out the light of a central star to make it possible to image the planets around that star directly. So now we’re talking about a plan that, unlike Kepler, can go to work on the closest planets to our own system. Perhaps hundreds of new worlds could emerge from this work, all of them relatively nearby (the thinking in the current WFIRST report from the Science Definition Team is to look at the brightest 200 stars within 30 parsecs with these methods). Terrestrial worlds are beyond the capabilities of a WFIRST-2.4 coronagraph, but Neptunes and Jupiters should be well within its reach.
And here we come to one of the main points I wanted to make with this post, keying off Lee Billings’ concerns, discussed yesterday, about how and when we would start to get exoplanet spectra for worlds in the habitable zone. WFIRST can’t manage that feat, but it’s a step in the right direction. From Horzempa’s article:
The new WFIRST telescope will be also be able to obtain spectra of those planets, telling us something about their nature. As mentioned above, this will be the next crucial step in exoplanet exploration. We now know something about the sizes and masses of exoplanets. However, knowledge of surface composition is sorely lacking. Spectra obtained by WFIRST will help remedy this. Then, scientists will have data to help answer the questions raised by these worlds. Are they rocky planets? Gas giants? Are they ocean worlds? Are they covered in clouds? Are they worlds with compositions not found in our solar system? Some of the top targets for WFIRST will be the exoplanets discovered by the ground-based Doppler method. It will be able to confirm their existence, as well as tell us something about their personalities.
Current thinking is that WFIRST would be placed in a geosynchronous orbit, for reasons explained in the SDT report:
The primary factor that drove the selection of this orbit is the ability to continuously downlink data to the ground and obtain a much higher science data rate. The SDT weighed these benefits against the higher radiation environment and slightly less stable thermal environment versus the Sun-Earth L2 orbit chosen by the previous WFIRST SDT.
Servicing issues are also an obvious benefit of GEO. But a geosynchronous orbit rules out the use of a starshade, which may be the most potent way to block starlight and thus explore the habitable zone around target stars. There continues to be talk of using a starshade with the James Webb instrument and later missions, but it’s unlikely that WFIRST will one day be moved into position at L2 after fulfilling its earlier mission at GEO, as Horzempa suggests. Even so, WFIRST at GEO would still be able to home in on Kepler discoveries, along with whatever the Transiting Exoplanet Survey Satellite (TESS) comes up with.
Recovering Lost Mission Capabilities
A great deal of work was expended in the Terrestrial Planet Finder days to develop our coronagraph technology. So will that work now turn around to feed into WFIRST-2.4? For that matter, asks Horzempa, can WFIRST recover some of what we lost with the cancellation of the Space Interferometry Mission? The right technology could make this happen:
This could be accomplished by an ingenious method of adding a grid of micro-dots to the primary mirror. As with the other mechanical obstructions in the optical path, these dots would not interfere with images formed by the telescope. However, those dots would produce diffraction spikes that would serve as a tool for astrometry: measuring, with exquisite detail, the positions and motions of stars. What it translates to in practice is a powerful exoplanet discovery method for WFIRST-2.4. The astrometric dots would allow the telescope to measure the masses of exoplanets, even those that do not exhibit TTVs [Transit Timing Variations].
Astrometry is a powerful tool, one that could deepen our understanding of the Kepler discoveries, conceivably detecting non-transiting planets around some of these stars. Will such an astrometry option make its way into the final WFIRST design? If so, we could say a little bit of the SIM mission survives.
A fully enhanced WFIRST opens up other exoplanet possibilities as well. Adding the larger mirror would mean that WFIRST could detect TTVs several times smaller than Kepler. That should get the attention of David Kipping (Harvard-Smithsonian Center for Astrophysics), whose continuing work on exomoons relies partly on the precise detection of these kinds of variations. Could it be that WFIRST will detect our first exomoon, conceivably a large world orbiting a gas giant?
So at least one path forward after the James Webb Space Telescope begins to become apparent, in the form of a WFIRST mission enhanced with larger mirror and coronagraph and, perhaps, the grid of micro-dots discussed above. Is this the final precursor to the terrestrial planet imager we’re aiming for, the observatory capable of taking those critical spectra in the search for biosignatures? It might be if NASA decides to proceed with the WFIRST-2.4 design, a decision that should take place this month before further SDT meetings in September.
The Science Definition Team’s final report is “Wide-Field InfraRed Survey Telescope – Astrophysics Focused Telescope Assets,” available online.
by Paul Gilster | Nov 9, 2012 | Exoplanetary Science, Uncategorized |
It’s good to see that David Kipping’s work on exomoons is back in the popular press in the form of A Harvest of New Moons, an article in The Economist. Based at the Harvard-Smithsonian Center for Astrophysics, Kipping’s Hunt for Exomoons with Kepler (HEK) culls Kepler data and massages the information, looking for the tug of large moons on transiting exoplanets. The basic method will by now be familiar to Centauri Dreams readers:
Dr Kipping’s technique relies on the fact that moons do not simply revolve around their host planets; planets also revolve around their moons—or, rather, the two bodies both revolve around their common centre of mass. If a planet is large and its moon small the distinction is trivial. But if the planet is small and the moon is large, it is not. In the case of Earth and its moon, for example, the common centre lies only around 1,700km (1,100 miles) beneath the Earth’s surface. Someone looking from afar at the movement of Earth would thus be able to deduce the moon’s existence without having to see it directly.
And as we’ve discussed in previous articles, the need for a large moon is significant. Kipping has recently reckoned that a moon about one-fifth as massive as the Earth should be in range of detection, but Ganymede, the biggest moon in our Solar System, is only 1/40th as massive. The first exomoon detected, then, will likely be a very large object, big enough that its signal won’t be masked by the presence of other planets in the same system. I’ve always had a fascination with exomoon studies and thus am looking forward to the first presentation and data analysis by the HEK team, slated to occur at the American Astronomical Society meeting in January.
Image: Exomoon hunter David Kipping. Credit: CfA.
But I want to focus in on something else in this article, namely the work of Mary Anne Peters and Edwin Turner, who have asked in a recent paper whether a large enough exomoon orbiting close enough to its planet (and far enough from its star) might produce an infrared signature detectable from Earth. Think Io, and ponder how tidal heating churns the insides of such a moon, creating heat-generating friction and, in the case of Io, active volcanoes.
Peters and Turner, both at Princeton, produce an acronym I had never encountered before: THEMs, or Tidally Heated Exomoons. In terms of direct imaging, a THEM is quite interesting. For one thing, a tidally heated moon may remain hot and bright for the lifetime of its star, making it visible in solar systems both old and young. For another, such a moon may orbit its planet far away from the primary star while remaining hot because of tidal heating. It is thus a luminous target at a large separation from the star whose light would otherwise drown out its signal.
The researchers calculate that tidal heating could produce terrestrial-planet-sized moons with effective temperatures as high as 1000 K, moons as much as 0.1% as bright as the system’s primary (if the central star were low in mass). This is interesting stuff: Io has the highest measured temperatures of any body in the outer Solar System because of the tidal heating effect. Now imagine the system of Galilean moons orbiting Jupiter were scaled down to orbit Neptune in more or less the same configuration. If this were the case, Io would be more luminous than Neptune itself, and if it were as massive and dense as the Earth, the authors say it would be the brightest Solar System object beyond 5 AU, outshining even Jupiter at some wavelengths.
Given all this, there is a case to be made that tidally heated exomoons may actually be easier targets for direct imaging than the kind of hot, young gas giants at large separations from their star that are most likely to be found by current imaging efforts:
Direct imaging detection of physically plausible, tidally heated exomoons is possible with existing telescopes and instrumentation. If tidally heated exomoons are common, for example if typical gas giant exoplanets are orbited by satellite systems broadly similar to those found in the Solar System, we are likely to be able to image them around nearby Sun-like stars in the midst of their main sequence lifetimes with current or near future facilities.
The paper suggests that existing instruments can detect exomoons at temperatures of 600 K or above and radii of Earth-size or larger, while future mid-infrared space telescopes like the James Webb Space Telescope will be able to directly image heated exomoons with temperatures greater than 300 K and radii of Earth size and larger as long as they are orbiting at 12 AU or more from their star. Perhaps we’ve imaged our first exomoon already: The authors think it is possible that Fomalhaut b, whose identity as a planet remains controversial, is actually a tidally heated exomoon, or a blend of the emissions from a hot, young gas giant and a heated moon.
In one figure, the researchers look at the performance of JWST’s Mid-Infrared Instrument (MIRI), charting exomoons with temperatures similar to those found on Earth. The results:
…it is plausible that some of the exomoons JWST is capable of detecting, could potentially be habitable, in the sense of having surface temperatures that would allow liquid water to be present. Some of these exomoons have comparable irradiance to the gas giants in our solar system. At ~ 14µm and 300K, an Earth-radius exomoon would be as luminous as Jupiter. However, if [the] Jupiter were colder due to being less heated by its primary and/or being older, the Earth-like moon would be much brighter than the planet.
The paper is Peters and Turner, “On the Direct Imaging of Tidally Heated Exomoons” (preprint).
by Paul Gilster | Jan 17, 2012 | Exoplanetary Science |
What to say about an extrasolar ring system that has already had its four distinct rings named? Rochester, Sutherland, Campanas and Tololo are the Earth-bound sites where the unusual system was first detected and analyzed, and the international team of researchers involved thought them suitable monickers for the four rings thus far detected. The light curve of the young, Sun-like star they’ve been studying in the Scorpius-Centaurus association — a region of massive star formation — shows what appears to be a dust ring system orbiting a smaller companion occulting the star.
The data here come from SuperWASP (Wide Angle Search for Planets) and the All Sky Automated Survey (ASAS) project. The star in question is 1SWASP J140747.93-394542.6, which displays a complex eclipse event with, at some points, 95 percent of the light from the star being blocked by dust. Similar in mass to the Sun, the star is only about 16 million years old, and lies about 420 light years away from the Solar System. Eric Mamajek (University of Rochester) thinks the object at the center of the system is either a low-mass star, a brown dwarf, or a planet, but it will take follow-up studies to determine the answer. Says Mamajek:
“When I first saw the light curve, I knew we had found a very weird and unique object. After we ruled out the eclipse being due to a spherical star or a circumstellar disk passing in front of the star, I realized that the only plausible explanation was some sort of dust ring system orbiting a smaller companion—basically a ‘Saturn on steroids.’ This marks the first time astronomers have detected an extrasolar ring system transiting a Sun-like star, and the first system of discrete, thin, dust rings detected around a very low-mass object outside of our solar system.”
Image: Rings found in a young stellar system may offer clues to planet formation, including the moons around gas giants. Credit: Michael Osadciw/University of Rochester.
The discovery is unusual but it’s not likely to be our last look at such systems. A circumstellar disk can have a huge radius, and we can imagine seeing one star in a binary system with such a disk that would regularly eclipse the other star. The same is true of a giant planet in a young system that is building moons out of its own circumplanetary disk. Seeing such disks in eclipse could tell us much about the era of planet formation, and we already have long-period eclipsing systems like EE Cephei, Epsilon Aurigae and OGLE-LMC-ECL-17782 to build on. In fact, the eclipses associated with Epsilon Aurigae last almost 2 years, and we may be seeing “…rings and gaps in a forming planetary system around a lower mass secondary,” as the paper on this work notes.
But we could also be looking at moons in formation around a gas giant, a subject that calls up our recent discussion of the HEK (Hunt for Exomoons with Kepler) project and the detectability of these objects. The exomoon discussion in the paper is worthy of an extended quote:
A simple thought experiment illustrates the potential observability of moon-forming circumplanetary disks around young gas giants (and indeed this was the back of the envelope calculation that spawned our interest in the interpretation of the eclipsing star)… If one were [to] take the Galilean satellites of Jupiter and grind them up into dust grains, and spread the grains uniformly between Jupiter and Callisto’s orbit, one would have a dusty disk of optical depth O(105). The size of such a proto-moon disk in this case would be a few solar radii – i.e. large enough and optically thick enough to potentially eclipse a star’s light. Of course such a disk need not be face on – more likely the disk would have a nonzero inclination with respect to the planet-star orbital plane, so the star need not be completely geometrically eclipsed by such a circumplanetary disk.
Relating this to our own system, the paper goes on:
The rings of Saturn have optical depth near ?1 even at a relatively old age (4.6 Gyr), however the vast majority of mass orbiting Saturn is locked up in satellites (Mrings ? 10?4 Msatellites). Presumably a disk of much higher optical depth and signi?cant radial substructure existed during the epoch of satellite formation. While there have been studies investigating the detectability of thin, discrete planetary rings similar to Saturn’s… there has been negligible investigation of the observability of the dense proto-satellite disks that likely existed during the ?rst ?107 years. Relaxing the assumptions about the size, mass, composition and structure of the disk in our back-of-the-envelope calculation has little impact on the feasibility of the idea that dusty disks of high optical depth may be a common feature of young gas giant planets, and such objects may be observable via deep eclipses of young stars.
Putting all this together, our knowledge of the moons of gas giants in our own Solar System tells us that when they were in formation, circumplanetary disks would have produced equally complex eclipses if seen in transit by distant astronomers, showing dense areas alternating with gaps where the young moons were in the process of formation. Viewing such events around exoplanets, then, we may be seeing a testbed for moon and planet formation theories that will become increasingly more valuable as the number of such disk observations increases.
The paper is Mamajek et al., “Planetary Construction Zones in Occultation: Discovery of an Extrasolar Ring System Transiting a Young Sun-like Star and Future Prospects for Detecting Eclipses by Circumsecondary and Circumplanetary Disks,” in press at the Astronomical Journal (preprint).
