Uranus: New Work from Voyager Data

The ring system of Uranus was the second to be discovered in our Solar System. You would assume this came about because of Voyager, but the discovery was actually made in 1977 through ground-based observations involving occultations of distant stars. The rings of Uranus are narrow — between 1 and 100 kilometers in width — and many are eccentric. The fact that they are composed of dark particles makes detection and study particularly difficult.


Image: Uranus is seen in this false-color view from NASA’s Hubble Space Telescope from August 2003. The brightness of the planet’s faint rings and dark moons has been enhanced for visibility. Credit: NASA/Erich Karkoschka (Univ. Arizona).

Voyager 2’s flyby of the planet in January of 1986 gave us useful information about the rings’ structure, with three occultation experiments performed during the flyby. We learned that the moons Cordelia and Ophelia were helping to shape the eccentricity of some of the rings (deviations of tens to hundreds of kilometers in some cases). The two moons, in other words, were acting as ‘shepherd’ satellites for three of the planet’s nine rings (?, ? and ?).

In a new paper, Rob Chancia (University of Idaho), working with colleague Matt Hedman, revisits the Voyager cache to tease out further patterns in the ring data. In particular, the amount of material on the edge of the Uranian ? ring, one of the planet’s brightest, varies periodically, a pattern that also turns up in the neighboring ? ring. “When you look at this pattern in different places around the ring, the wavelength is different — that points to something changing as you go around the ring. There’s something breaking the symmetry,” says Hedman, recognizing a pattern not dissimilar from similar structures around Saturn.

Narrow rings are themselves signs of some kind of perturbation. The authors argue that a ringlet left unperturbed will spread out radially because of particle collisions, on timescales as small as several thousand years, assuming rings the size of the Uranian ? and ? rings. A small moon that confines the edges of the ring should produce structures in the form of wavy edges and wakes, and indeed, variations like these clued scientists to the position of Saturn’s moon Pan in Voyager 2 images.


Image: Shepherding Saturn’s rings. The Cassini spacecraft spies Pan speeding through the Encke Gap, its own private path around Saturn. Illumination is from the lower left here, revealing about half of Pan (26 kilometers across) in sunlight. Credit: NASA.

The Voyager data Chancia and Hedman use in this paper were produced by experiments with the spacecraft’s Radio Science Subsystem (RSS), which transmitted at microwave wavelengths (in the X band at 3.6 cm and in the S band at 13 cm) through the rings to ground stations on Earth. Other Voyager data through two other instruments were also acquired but lacked the precision to examine the rings for the presence of perturbing moonlets.

If the same phenomenon we see in Saturn’s rings is happening at Uranus, the Voyager data aren’t sensitive enough to show the culprits, but Chancia and Hedman think that similar moonlets, between 4 and 14 kilometers in diameter, could be keeping the Uranian rings narrow.

From the paper:

Our attempts to visually detect the moonlets are not exhaustive, but given the small predicted sizes of the ? and ? moonlets, a convincing detection may not be possible in the Voyager 2 images. Future earth-based observations may be more likely to detect these moons. Regardless of the current lack of visual detection, the identification of these periodic structures in the outer regions of the ? and ? rings is evidence of interactions with nearby perturbers.

How the rings of Uranus stay this narrow is a problem that has vexed scientists since 1977, but we may be teasing out the solution in data that are not a great deal younger. If Voyager 2 continues to pay off in terms of new planetary insights (not to mention both Voyagers’ continuing push into the outer Solar System), it’s a reminder that missions like New Horizons are likely to be producing discoveries for many years now that the data download is complete. I’m also reminded of how much new data we’ll acquire from Cassini when it makes its first ‘dive’ through the planet/ring gap on April 27 of next year. Thanks to Cassini, this is one ring system we can see up close, illuminating the systems around the other outer worlds.

The paper is Chancia and Hedman, “Are there moonlets near Uranus’ alpha and beta rings?” accepted for publication in the Astrophysical Journal (preprint).


New Clue to Gas Giant Formation

Just how do gas giant planets form? A team of researchers at ETH Zürich, working with both the University of Zürich and the University of Bern, has developed the most fine-grained and instructive computer simulations yet to help us understand the process. Using the Piz Daint supercomputer at the Swiss National Supercomputing Centre (CSCS) in Lugano, ETH Zürich postdoc Judit Szulágyi and Lucio Mayer (University of Zürich) can now show clear and observable differences between the two formation processes under study by theorists.

The core accretion model begins with a massive solid core that is large enough to pull in gas from the protoplanetary disk and maintain it. The gravitational instability theory, on the other hand, presumes a massive enough disk around the young host star that spiral arms form in the disk in which gravitational collapse can occur around material that has begun to clump there. The simulations demonstrate that with either formation mechanism, a circumplanetary disk forms around the young gas giant planet out of which satellite systems can emerge.


Image: Core accretion: A 10 Jupiter-mass planet is formed and is placed at 50 AU from the star. The planet has opened a gap in the circumstellar disk. (Image: J. Szulagyi, JUPITER code).

The circumplanetary disk forming around the young planet in the last phase of giant planet formation is central to this work. The disk eventually gives birth to the planet’s moons, while also regulating the accretion of gas as the planet forms. The authors refer to such a subdisk as a circumplanetary disk. In both theories of planet formation, such disks form within the larger circumstellar disk out of which the rest of the planetary system will grow. We have not yet observed circumplanetary disks, which is why the realm of simulation is so important.

The supercomputer simulations take us into new territory in modeling these processes, demonstrating a major difference between the two formation mechanisms: temperature. Create a gas giant with disk instability and you wind up with gas near the planet remaining in the 50 Kelvin range, much colder that what emerges in core accretion, where the disk around the emerging planet is heated to hundreds of Kelvins. The simulations, according to Mayer, used tens of millions of resolution elements, marking the first time the formation of a circumplanetary disk has been simulated around protoplanets at this level of detail.


Image: Gravitational instability simulation: Two snapshots in the early and late stage of the simulation at 780 years and 1942 years. The second snapshot shows only 4 clumps remaining among those initially formed. (Image: Lucio Mayer & T. Quinn, ChaNGa code).

Here we have effects that are observable, allowing astronomers to examine newly forming planetary systems to measure the temperatures that will indicate which formation process built the planet they observe. From the paper:

…our finding is that the temperature differs by more than an order of magnitude between the GI [gravitational instability] and CA [core accretion] formed CPDs [circumplanetary disks]. According to the simulations, the bulk subdisk temperature is < 100 K in the case of disk instability, and over 800 K for all the CA computations presented in this paper. The reason for this discrepancy lies in the different gravitational potential wells and opacities. Because the protoplanet is a few AU wide extended clump in the GI simulations, while it is a fully formed giant planet with a radius of 0.17 AU (meaning the gravitational potential smoothing length) in the CA-1 simulation, the accreted gas has significantly more energy to release into heat in the latter case than in the former.

Direct imaging of young gas giants can be deceptive, because as the second of two papers on this work points out, a shock front developing in the circumplanetary disk can be luminous and extended, masking the luminosity of the protoplanet itself as it forms beneath the disk. The shock front is created as accreted gas from the outside falls into the gap between the disk’s upper layers and the surface of the disk and the polar regions of the protoplanet.

“When we see a luminous spot inside a circumplanetary disk, we cannot be sure whether we see the planet luminosity, or also the surrounding disk luminosity,” says Szulágyi. This can cause us to estimate a planet’s mass as being up to four times higher than it actually is.

But the variation in temperature between core accretion and gravitational instability models makes it clear that what is happening in these circumplanetary disks can help us distinguish between the formation mechanisms of the planets in question. Measuring temperatures near the planet will allow astronomers to tell which formation process built the planet.

No differences in circumplanetary disk mass were found between the two formation scenarios, a finding that contradicted earlier theoretical models. Even so, although the supercomputer simulations were intensive, the researchers point out that simulations of disk instability do not cover a long enough timescale. Future simulations will have to extend that range while also looking at ionization and the effects of magnetic fields, not accounted for in this work.

The papers are Szulágyi, Mayer & Quinn, “Circumplanetary disks around young giant planets: a comparison between core-accretion and disk instability,” accepted at Monthly Notices of the Royal Astronomical Society (preprint); and Szulágyi and Mordasini, “Thermodynamics of Giant Planet Formation: Shocking Hot Surfaces on Circumplanetary Disks,” accepted at MNRAS Letters (preprint).


A Renewed Look at Boyajian’s Star

It was inevitable that KIC 8462852 would spawn a nickname, given the public attention given to this mystifying star, whose unusual lightcurves continue to challenge us. ‘Tabby’s Star’ is the moniker I’ve seen most frequently, but we now seem to be settling in on ‘Boyajian’s Star.’ It was Tabetha Boyajian (Louisiana State) whose work with the Planet Hunters citizen science project brought the story to light, and in keeping with astronomical naming conventions (Kapteyn’s Star, Barnard’s Star, etc.), I think the use of the surname is appropriate.


Planet Hunters works with Kepler data, looking for any dimming of the 150,000 monitored stars that may have gone undetected by the automated routines that hunt for repeating patterns. Boyajian’s Star cried out for analysis, dimming in odd ways that flagged not the kind of planetary transit across the face of a stellar disk that researchers expected but something else, something that would make the star dim by as much as 22 percent, and at irregular intervals. That led to a variety of hypotheses, the best known of which is a large group of comets, but we also have evidence that the star has been dimming at a steady rate.

Image: Tabetha Boyajian, looking up, presumably at Boyajian’s Star (caption swiped from Jason Wright’s page at Penn State).

With the story this unsettled, this morning’s energizing news is that Boyajian’s Star is now being examined by Breakthrough Listen. Working with Jason Wright, now a visiting astronomer at UC Berkeley, as well as Boyajian herself, the SETI project intends to devote hours of listening time on the Green Bank radio telescope in West Virginia to the star. You’ll recall that Breakthrough Listen is the $100 million SETI effort funded by the Breakthrough Prize Foundation and its founder, investor Yuri Milner. The Breakthrough Starshot project described often in these pages is also a Breakthrough Prize Foundation initiative.

As Andrew Siemion (director of the Berkeley SETI Research Center and co-director of Breakthrough Listen) explains in the video above, the project has access to the most powerful SETI equipment available, meaning its scientists can study Boyajian’s Star at the highest levels of sensitivity across a wide range of possible signal types. But the Green Bank effort will hardly be the first, for Boyajian’s Star has already excited a great deal of interest, as Siemion explains:

“Everyone, every SETI program telescope, I mean every astronomer that has any kind of telescope in any wavelength that can see Tabby’s star has looked at it. It’s been looked at with Hubble, it’s been looked at with Keck, it’s been looked at in the infrared and radio and high energy, and every possible thing you can imagine, including a whole range of SETI experiments. Nothing has been found.”

In Green Bank, Breakthrough Listen has access to the largest fully steerable radio telescope on the planet. Observations are scheduled for eight hours per night for three nights in the next two months, the first having taken place on October 26. The plan is to gather as much as 1 petabyte of data over hundreds of millions of individual radio channels. Siemion describes a new SETI instrument that can examine “…many gigahertz of bandwidth simultaneously and many, many billions of different radio channels all at the same time so we can explore the radio spectrum very, very quickly.”

Breakthrough Listen will be observing using four different radio receivers on the Green Bank instrument in a frequency range from 1 to 12 GHz, a range beginning, Siemion says, at about where cell phones operate up through the frequencies used for satellite TV signals.


Image: The Green Bank Radio Telescope (GBT) focuses 2.3 acres of radio light. It is 148 meters tall, nearly as tall as the nearby mountains and much taller than pine trees in the national forest. The telescope is in a valley of the Allegheny mountains to shield the observations from radio interference. Credit: NRAO/AUI.

Yesterday’s live video chat from Green Bank with Tabetha Boyajian, Jason Wright and Andrew Siemion is now available online, with the trio answering questions about the ongoing study. Boyajian was asked as the session opened how many comets it would take to reproduce the effects being observed around KIC 8462852. The answer: Hundreds to thousands of “very giant comets” just to reproduce the last 30 days of the data.

The numbers give no particular credence to the idea that we may be looking at some kind of artificial construction project around Boyajian’s Star, but they do underline how mysterious are the processes, assuming they are natural, that are driving this phenomenon. Boyajian called the comet hypothesis ‘pretty outrageous,’ but went on to say that of all the explanations, it is the one she most favors, as all other explanations are likewise outrageous.

On that score, I want to mention Jason Wright’s paper, written with Steinn Sigurðsson at Penn State, looking at other possible solutions to the Boyajian’s Star puzzle. It’s particularly useful early on in a section devoted to the follow-up work that has occurred, including the SETI studies with the VERITAS gamma-ray observatory, the Allen Telescope Array and the Boquete Optical SETI Observatory, but also reprising the interesting controversy over the dimming of the star. If you need to catch up with Boyajian’s Star, this is the place.

Wright and Sigurðsson conclude that long-term dimming would not fit well with the comet hypothesis, leaving us still searching for an answer. What does work its way up the chain of plausibility? An unusually dense region of the interstellar medium or a chance alignment with a localized molecular cloud occurring between us and the star is in the mix. The latter might be a so-called ‘Bok globule,’ an isolated and dark nebula dense with dust and gas.

The comet hypothesis is still in play, but a number of other explanations are problematic:

Less compelling, but difficult to rule out, are intrinsic variations due to spots, a “return to normal” from a temporary brightening (due to, perhaps, a stellar merger) and a cloud of material in the outer solar system. We find instrumental effects, other intrinsic variation in Boyajian’s Star, and obscuration by a disk around an orbital companion to Boyajian’s Star very unlikely to be responsible.

Read the paper for the entire list, which includes, with plausibility listed as unclear, the idea of artificial structures (“Would find support if all natural hypotheses are ruled out, we detect signals, or if star suffers significant achromatic extinction.”) The paper is Wright and Sigurðsson, “Families of Plausible Solutions to the Puzzle of Boyajian’s Star,” accepted at the Astrophysical Journal (preprint). But see also Jason Wright’s 10-part popular summary on Boyajian’s Star, which goes through all the options.


Are Planets Like Proxima b Water Worlds?

Those of us fascinated by dim red stars find these to be exhilarating days indeed. The buzz over Proxima b continues, as well it should, given the fact that this provocative planet orbits the nearest star. We also have detections like the three small planets around TRAPPIST-1, another red dwarf that is just under 40 light years out in the constellation Aquarius. These are small stars indeed, just 8 percent the mass of the Sun in the case of the latter, while Proxima Centauri is about 10 times less massive (and 500 times less luminous) than the Sun.

But just what might we find on planets like these? A new paper from Yann Alibert and Willy Benz (University of Bern) drills down into their composition. The researchers’ goal is to study planet formation, with a focus on planets orbiting within 0.1 AU, a range that includes the habitable zone for such stars. While a forthcoming paper will look at the formation process of these planets in greater detail, the present work studies planetary mass, radius, period and water content.

To do this, Alibert and Benz have developed computer simulations that model red dwarf planetary systems, assuming a central star with a tenth the mass of the Sun and a protoplanetary disk around each modeled star. Putting the model into motion, the scientists studied a series ranging from a few hundred to thousands of such stars, with 10 planetary embryos in each disk — each embryo was modeled as having an initial mass equal to that of the Moon, and the initial location of each planetary embryo was drawn at random.

The results, according to Alibert:

“Our models succeed in reproducing planets that are similar in terms of mass and period to the ones observed recently. Interestingly, we find that planets in close-in orbits around these type of stars are of small sizes. Typically, they range between 0.5 and 1.5 Earth radii with a peak at about 1.0 Earth radius. Future discoveries will tell if we are correct!”


Image: Artist’s impression of Earth-sized planets orbiting a red dwarf star. Credit: @ NASA, ESA, and G.Bacon (STScI).

The most striking aspect of this work is likely to be Alibert and Benz’ findings on the water content of small planets in the habitable zone. The amount of water is found to depend upon the location at which the planet has accreted planetesimals, their composition being dictated by the thermal structure of the disk and the location of the snowline, which varies depending on disk mass. Note this: A significant fraction of the planets modeled show more than 10 percent water. Contrast this with the Earth, whose fraction of water is roughly 0.02%.

The study shows a correlation between the mass of a planet and the water fraction, with planets that do not contain a high degree of water being lower in mass (generally below one Earth mass), while planets totally devoid of water are all less massive than one Earth mass. Alibert and Benz see this as the result of migration, with more massive planets migrating from further out in the system, thus collecting water-rich material from beyond the snowline.

Water may be a key to habitability, and the ‘habitable zone’ is defined as the zone in which liquid water can exist on the surface. But water to the extent of deep global oceans is problematic. A large enough water layer can produce high pressure ice at the bottom, preventing the carbonate-silicate cycle that regulates surface temperature over long timescales from operating. Without this mechanism, atmospheric CO2 cannot cycle through the weathering of rocks on the Earth’s surface and the eventual subduction of calcium carbonate. High temperatures and pressures eventually return CO2 to the atmosphere by processes like volcanism, regulating global temperatures.

We don’t know how much of a factor the loss of this process might be on planets around red dwarf stars, as the paper takes pains to note:

In the case of low mass stars, which evolve on much longer timescales, this may not be a major problem, as the stellar flux varies on timescales much longer than in the case of the Sun. In this situation, a process that stabilizes the surface temperature may not be necessary. The second reason [why large amounts of water may be detrimental for habitability] is connected to the fact that for planets with too much water an unstable CO2 cycle destabilizes the climate making habitability more challenging… Again, this was demonstrated for solar-type stars and a similar process may or may not exist for low mass stars.

So we have a computer model that produces planets similar in mass and radius to the interesting worlds we’re finding around some nearby red dwarfs, with a peak in the distribution of radius at about one Earth radius. The authors argue that the properties of the disk potentially correlate with the mass of the star, thus determining the water content of emerging planets. Deep oceans on planets in the habitable zone of red dwarfs may be the norm, in which case we need to know a great deal more about climate on such exotic worlds.

…our models show that the properties of the disk and their potential correlation with the mass of the star are the most important parameters determining the characteristics, in particular the water content, of the emerging planet population. In this context, observational constraints on mass and lifetime of discs in orbit of low-mass stars become of paramount importance.

The paper is Alibert and Benz, “Formation and composition of planets around very low mass stars,” accepted at Astronomy & Astrophysics (preprint).


A Microlensing Opportunity for Centauri A

First light for the European Extremely Large Telescope (E-ELT) is scheduled for 2024, a useful fact given that a few years later, we may be able to use the instrument in a gravitational lensing opportunity involving Alpha Centauri. Specifically, Centauri A is expected to align with the star 2MASS 14392160-6049528, thought to be a red giant or supergiant and far more distant than Alpha Centauri. This will create an event that not just the E-ELT but other instruments, like the GRAVITY instrument on the Very Large Telescope Interferometer (VLTI), will be able to study — GRAVITY is capable of extremely high accuracy astrometry.

A team of French astronomers led by Pierre Kervella (CNRS/Universidad de Chile) is behind this new study, which involved fine-tuning our knowledge of the trajectories of Centauri A and B. Remember that we see gravitational lensing when a massive object like a star distorts the spacetime around it, so that light from the more distant object must follow a curved path to reach us. The amount of mass in the closer star affects the extent of this deflection, and when one or more planets orbit the star, they become theoretically detectable.


Image: The predicted trajectory of Alpha Centauri A and B. Credit: ESO.

The Centauri A event is to occur in 2028, by which time we may well have knowledge of other planets around one or more of the primary Alpha Centauri stars. But a particularly useful aspect of microlensing is that it is not reliant on proximity to the star. Unlike transit studies or radial velocity analysis, which can produce evidence for a close-in planet quickly but require lengthy study for planets further out in the system, microlensing allows us to spot planets in wider orbits, even if the observation in which we find them is transitory and does not repeat.

So this lensing event is another potential window into this intriguing system, around which at present we know only of the world orbiting Proxima Centauri. Recall that the latter has also been studied in two microlensing events, one in October of 2014, the second in February of 2016. We found Proxima b through radial velocity methods (although the star was also the subject of transit studies), but microlensing is useful even when it fails to turn up planets, since it gives us a way of refining our estimates of a given star’s mass.

Alpha Centauri gives us serious opportunities for microlensing because the star field behind it is densely populated, thanks to the system’s location near the plane of the galaxy as seen from Earth. The Kervella paper studies conjunctions with background stars that will occur in coming decades, based on observations of the field surrounding Alpha Centauri that Kervella and co-author Fréderic Thévenin (Observatoire de la Côte d’Azur) began over a decade ago.


Image: An enlargement of the conjunction that will occurs in 2028, with the Einstein ring of Alpha Cen A represented in cyan color. Credit: Pierre Kervella.

The conjunction of Centauri A with 2MASS 14392160-6049528 is the most favorable of the conjunctions involving Alpha Centauri over the next three decades. From the paper, which references the background star as S5; i.e., one of the background stars numbered in the survey:

During the approaches, astrometric measurements will reveal the relativistic deflection of the background star light with a high signal to noise ratio. For the conjunction with S5, we may be able to directly observe the gravitational splitting of the distant source image using the E-ELT, VLTI/GRAVITY or ALMA. The astrometric monitoring of the relative positions of ??Cen and the S stars may reveal the presence of planets through secondary gravitational lensing. In addition, the light from the background star will possibly be subject to photometric variations induced by transits of low mass objects present in the ??Cen system.

Potential results include, in other words, everything from high and low mass planets to asteroids and comets. The authors believe the conjunctions in coming decades will give us highly accurate information about the Alpha Centauri stars’ proper motion, orbital parameters and parallax values. And note this: “This accuracy will be valuable for high-precision modeling of the two stars of ??Cen, and for the preparation of the recently announced Breakthrough Starshot initiative to send ultra-fast light-driven nanocrafts to ??Centauri.”

The Kervella paper is “Close stellar conjunctions of ? Centauri A and B until 2050,” Astronomy & Astrophysics 594 (2016), A107 (abstract). On microlensing and Proxima Centauri, see Sahu et al., “Microlensing Events by Proxima Centauri in 2014 and 2016: Opportunities for Mass Determination and Possible Planet Detection,” Astrophysical Journal Volume 782, Issue 2 (2014). Abstract available.