Ring Formation: Clues from the Late Heavy Bombardment

by Paul Gilster on November 2, 2016

Let’s circle back this morning to ring systems, which were the subject of Monday’s post. In particular, I was interested in new work on the rings of Uranus, for Voyager data, newly analyzed, has revealed patterns that indicate the presence of small ‘shepherding’ moons. We’ve seen the same phenomenon at Saturn, but what similarities exist between the two ring systems also highlight their differences. The rings of Uranus — and this holds for Neptune as well — are much darker than the rings of Saturn, which are mostly made up of icy particles.

Darker rings, so the thinking goes, are a likely indication of higher rock content. But why are these ring systems so different, and what produced them in the first place? We have another new paper on the outer systems’ rings to throw into the mix from Ryuki Hyodo (Kobe University), working with co-authors at the Université Paris Diderot and Tokyo Institute of Technology. The team developed computer simulations to construct a plausible model for ring formation that takes us back 4 billion years to the era of the Late Heavy Bombardment (LHB).


Image: A Hubble Space Telescope view reveals Uranus surrounded by its four major rings and by 10 of its 17 known satellites. This false-color image was generated by Erich Karkoschka using data taken on August 8, 1998, with Hubble’s Near Infrared Camera and Multi-Object Spectrometer.

The LHB was a time when numerous objects collided with planets in the inner system, and the Nice model, developed to explain the sudden spike in impactors, postulates that the giant planets were undergoing orbital migration at this time. That would disrupt scattered objects in the Kuiper Belt and the main asteroid belt, driving them into the inner system. It’s an attractive theory for Hyodo and team because their work calculates the probability of large objects from the Kuiper Belt passing close enough to the giant planets to be destroyed by tidal forces.

You can see where this is heading. It turns out that Saturn, Uranus and Neptune all would have had encounters with numerous objects large and small during any phase of planetary migration. Encounters with Pluto-sized objects, according to the paper, would have ranged from just a few up to several tens of encounters within each planet’s Roche limit during the Late Heavy Bombardment. The Roche limit describes the closest distance from the center of a planet that a small body can approach before being pulled apart by gravitational forces.


Image: What happens when an object gets too close to the Roche limit. This is comet Shoemaker Levy 9, seen here after fragmentation by Jupiter in 1992. The comet would go on to impact Jupiter in 1994. Credit: NASA/HST.

The team’s computer simulations then probed the question of what would happen to kilometer-sized fragments from these passing objects, the kind produced by tidal disruption. The scientists used supercomputers at the National Astronomical Observatory of Japan to show that such captured fragments would undergo repeated, high-speed collisions that would pulverize them over time, grinding them down into centimeter to meter sized particles of the sort currently seen in Saturn’s rings. In the process, their orbits would be circularized. The authors believe enough combined mass exists to explain the formation of ring systems.


Image: Schematic illustration of the ring formation process. The dotted lines show the distance at which the giant planets’ gravity is strong enough that tidal disruption occurs. (a) When Kuiper Belt objects have close encounters with giant planets, they are destroyed by the giant planets’ tidal forces. (b) As a result of tidal disruption some fragments are captured into orbits around the planet. (c) Repeated collisions between the fragments cause the captured fragments to break down, their orbit becomes gradually more circular, and the current rings are formed (partial alteration of figure from Hyodo, Charnoz, Ohtsuki, Genda 2016, Icarus).

The simulations offer a mechanism to explain why the rings of Saturn, Neptune and Uranus appear to have a different composition. The answer has much to do with the higher density of Uranus and Neptune compared to a much lower density Saturn. The case of Uranus explains how density factors into the formation scenario:

…due to its higher density, the width between [Uranus’] surface and its Roche limit is larger than in the case of Saturn. Thus, a body can pass deeper [into the] potential field of the planet. As a result, the tidal destruction could be significant enough to disrupt not only the body’s icy mantle but also its silicate core, and thus silicate components can be more efficiently captured than in the case of Saturn. This would also be applicable to Neptune since it is also denser than Saturn. Therefore, this could explain the fact that the rings of Uranus and that of Neptune are darker than that of Saturn.

In other words, a Kuiper Belt object with a rocky core and an icy mantle can be completely disrupted by a close pass of either Uranus or Neptune because of their higher density, with the rocky core destroyed and captured into what will become the ring structure. Objects approaching Saturn are likely to shed their icy mantle, but not necessarily their rocky core — the core in this scenario might go on to collide with the planet itself.

Hyodo’s model produces initially massive rings around all the giant planets, with the paper commenting that the diversity of rings seen today is clearly the result of subsequent evolution. The authors speculate only briefly on later ring evolution, leaving the matter to future work.

The paper is Hyodo et al., “Ring formation around giant planets by tidal disruption of a single passing large Kuiper belt object,” published online by Icarus 29 September 2016 (abstract / preprint).



Untangling the Effects of the ‘Big Whack’

by Paul Gilster on November 1, 2016

Seasonal change on our planet is relatively moderate because the Earth has a small axial tilt. Just how that situation arose makes for interesting speculation, and a series of scientific papers that have been augmented by a new analysis in Nature from Matija Ćuk (SETI Institute) and Sarah Stewart (UC-Davis). Working with colleagues at Harvard and the University of Maryland, the scientists have created computer simulations showing that the early Earth experienced a day as short as two hours, and had a highly tilted spin axis.

How we get from there to here is the question, and it’s one that Ćuk and company answer by examining the collision that spawned Earth’s Moon. The impact theory sees the Moon forming from the debris of the collision between an infant Earth and a Mars-sized protoplanet.

It was Ćuk and Stewart who suggested some four years ago that following the ‘Big Whack,’ the Earth’s rotation period was closer to two hours than the five that earlier work had suggested. The Moon would have formed much closer to the Earth than it is today, with the Earth losing much of its spin and a good deal of its tilt as the Moon’s orbit widened.


Image: Artist’s depiction of a collision between two planetary bodies. Such an impact between the Earth and a Mars-sized object likely formed the Moon. Credit: NASA/JPL-Caltech.

All this fits with what we see today, with the Moon continuing to move gradually away from the Earth as our planet’s spin continues to slow. The slowdown is produced by the tides the Moon raises on our surface, which dissipate energy continually as they interact with the oceans.

If we assume a fast spinning early Earth, the ejection of material following the impact can produce a Moon similar in composition to Earth’s mantle, which is what we see in lunar rocks. Now we get into the realm of orbital interactions that determine the system’s evolution. For today’s Moon has a tilt of its own, about five degrees off from Earth’s orbital plane. This is true despite the likelihood that internal friction due to tidal effects by the Earth should have had a profound effect on the Moon’s tilt, decreasing it over a billion-year time frame.

The tilt of the Moon’s orbit, in other words, must have once been much greater than it is today. Ćuk, Stewart and colleagues Douglas Hamilton (University of Maryland) and Simon Lock (Harvard) believe we can arrive at today’s situation if we begin with an Earth that, not long after the impact, was spinning essentially on its side, with the Moon orbiting the equator. With an axial tilt of over 70 degrees, this situation will not last, with solar gravitational forces creating an eccentricity in the Moon’s orbit that produces strong tidal flexing within it.

These internal lunar tidal effects, in the view of the scientists, would have produced a counter-force against the tidal effects from the Earth that would have been pushing the Moon into a wider orbit. During this period, Earth would have continued to lose its spin, but rather than going into a wider orbit, the Moon’s orbit would have become increasingly tilted.

It would only be as the Earth’s rotation continued to slow that the Moon could break out of this deadlock and continue moving away from our planet. Its subsequent torque on the Earth’s spin axis is what Ćuk and team believe began to move the Earth’s axial tilt into more moderate territory. And tidal flexing inside the Moon would have helped to shrink its orbital inclination, so that today it is within five degrees of the orbital plane of the planets.

The paper summarizes the situation, with interesting exoplanet implications. Note the reference in the passage below to the ‘Cassini state,’ which is a system that obeys the laws of the Moon’s motion with respect to the Earth that were originally stated by the Italian astronomer Giovanni Domenico Cassini (for whom our Saturn orbiter is named):

Our high-obliquity model is at present the only model we are aware of to explain the origin of large past lunar inclination, which was subsequently reduced by strong obliquity tides at the Cassini state transition… [O]ur results support high-AM [angular momentum] giant-impact scenarios for lunar origin. An initially high-obliquity Earth is consistent with the expectation of random spin-axis orientations for terrestrial planets after giant impacts, and the dynamics discussed here naturally reduces Earth’s obliquity to values that are low to moderate.

Which is interesting indeed, because here we have a mechanism that can gradually bring a high obliquity exoplanet to a much lower axial tilt, thereby making moderate seasons possible. Is a large moon critical for planetary habitability? Ćuk comments on the same point:

“This work shows that there are multiple ways a planet could get a small axial tilt, making moderate seasons possible. We thought Earth was this way because of the direction of the giant impact 4.5 billion years ago, but it looks like Earth achieved this state later through a complex interaction with the Moon and the Sun. I wonder how many habitable Earth-like extrasolar planets also have a large Moon.”

Centauri Dreams’ take: Interacting gravitational influences within the Solar System have a great deal to do with habitability, as the work of the Serbian astronomer Milutin Milanković has demonstrated. Milanković (working while a prisoner of war during World War I) found rhythmical climate cycles that deep sea core analysis confirmed in the 1970s, all related to gravitational nudges involving the motions of Jupiter, Saturn and the Moon.

David Grinspoon writes about Milanković in his fine new book Earth in Human Hands (2016), pointing out that while our Moon has kept Earth’s rotational axis stable at a 23.5 degrees tilt from the Sun, Mars (currently at about 25 degrees of tilt) varies from 15 to 35 degrees and sometimes more over a period of 120,000 years. Rhythmical climate torques also show up on Titan. Clearly such interactions will have to be taken into account as we examine young systems around other stars and their likely evolution.

The paper is Ćuk, “Tidal evolution of the Moon from a high-obliquity, high-angular-momentum Earth,” published online by Nature 31 October 2016 (abstract).



Uranus: New Work from Voyager Data

by Paul Gilster on October 31, 2016

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 (ε, δ and γ).

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 α ring, one of the planet’s brightest, varies periodically, a pattern that also turns up in the neighboring β 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 α and β 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 α and β 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 α and β 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

by Paul Gilster on October 28, 2016

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

by Paul Gilster on October 27, 2016

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?

by Paul Gilster on October 26, 2016

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

by Paul Gilster on October 25, 2016

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 α 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 α 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 α Cen, and for the preparation of the recently announced Breakthrough Starshot initiative to send ultra-fast light-driven nanocrafts to α Centauri.”

The Kervella paper is “Close stellar conjunctions of α 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.



Red Dwarfs: Oldest Known Circumstellar Disk

by Paul Gilster on October 24, 2016

Determining the age of a star is not easy, but one way of proceeding with at least some degree of confidence is to identify the star as a member of a stellar association. Here we’re talking about a loose cluster of stars of a common origin. Over time, the stars have begun to separate, but they still move together through space. It was the Armenian astronomer Viktor Ambartsumian, the founder of the Byurakan Observatory, who discovered the nature of these associations and demonstrated that they were composed of relatively young groups of stars.

Stellar associations, or young moving groups (YMGs), provide an outstanding place to study the evolution of protoplanetary disks around young stars, for all associated stars have a similar age. Indeed, their galactic motion can be traced back to their place of origin. Another benefit: Exoplanets in such infant systems are often still hot, well within the capabilities of our near-infrared direct imaging techniques. Many direct imaging and disk evolution surveys in recent years have focused on the members of young moving groups.

Without Ambartsumian’s discovery of these groups, we would be hard pressed to come up with the age of the interesting red dwarf tagged AWI0005x3s. We’ve just learned, through a team led by Steven Silverberg (University of Oklahoma), working with a group of citizen scientists using the Disk Detective site, that this star has a warm circumstellar disk, an interesting find in its own right because we haven’t found many disks around red dwarfs.

But AWI0005x3s is found in the Carina association, on the order of 200 light years away in the Carina nebula, and appears to be moving with it. That pegs the young red dwarf as highly unusual, as Silverberg explains:

“Most disks of this kind fade away in less than 30 million years. This particular red dwarf is a candidate member of the Carina stellar association, which would make it around 45 million years old [like the rest of the stars in that group]. It’s the oldest red dwarf system with a disk we’ve seen in one of these associations.”


Image: Artist’s concept of the newly discovered disk. Credit: Jonathan Holden.

Looking through the paper, I learned that the intriguing AWI0005x3s disk would be the oldest ever observed around an M-dwarf, assuming the star can be confirmed as a member of Carina (the authors argue that the star has a probability of over 90 percent of being part of the association). Astronomers have found a disk frequency of about 6 percent around M-dwarfs less than 40 million years old, dropping to 1.3 percent around older members of this stellar class.

So where does AWI0005x3s fit in? The paper contrasts what we see in M-dwarfs with other types of star:

…debris disks are detected around 32 ± 5% of young A stars with Spitzer/MIPS (Su et al. 2006), and around 1−6% of old (∼ 670 Myr) Sun-like (F5-K9) stars with Spitzer/MIPS (Urban et al. 2012). Survival models predict that M dwarf debris disks occur at a similar frequency as disks around Sun-like stars, and that the dearth of detections to date is either due to systems having blackbody-like dust close to their central star, or due to systems having a smaller amount of dust distributed over a larger orbital separation (Heng & Malik 2013).

But other possibilities are still in play, including accelerated disk dissipation through interactions with a young stellar wind. Its age places AWI0005x3s in a potentially useful place in relation to other M-dwarfs, as the paper makes clear:

Our new M dwarf debris disk would bridge the gap between YMG and field M dwarf disks. Given their common spectral type (both M5.5V), this system could be a young analog for the Proxima Centauri system (Anglada-Escudé et al. 2016), as well.

The authors believe that AWI0005x3s is a potential target for study via adaptive optics on large telescopes and should be within range for high-contrast imaging, which could allow us to resolve the structure of the disk and potentially identify exoplanet candidates.

As for Disk Detective, it’s well worth a look. In fact, some 30,000 people have gotten involved in viewing short videos from surveys like the Wide-field Infrared Survey Explorer mission (WISE) and Two-Micron All Sky Survey (2MASS) projects, with some two million classifications of celestial objects now achieved. Eight of the citizen scientists involved are listed as co-authors on the AWI0005x3s paper, which is now available online.

The paper is Silverberg et al., “A New M Dwarf Debris Disk Candidate in a Young Moving Group Discovered with Disk Detective,” Astrophysical Journal Letters Vol. 830, No. 2 (14 October 2016). Abstract / preprint.



Witnessing Titan’s ever-changing seasons has been a major payoff of the Cassini mission, whose end is now close enough (September, 2017) to cause us to reflect on its accomplishments. We now see winter settling in firmly in the southern hemisphere, along with a strong vortex now developing over the south pole. When Cassini arrived in 2004, we saw much the same thing, only in the northern hemisphere. Athena Coustenis (Observatoire de Paris) is presenting results on Titan’s climate at the ongoing joint meeting of the American Astronomical Society’s Division for Planetary Sciences and 11th European Planetary Science Congress.

“Cassini’s long mission and frequent visits to Titan have allowed us to observe the pattern of seasonal changes on Titan, in exquisite detail, for the first time,” says Dr. Coustenis. “We arrived at the northern mid-winter and have now had the opportunity to monitor Titan’s atmospheric response through two full seasons. Since the equinox, where both hemispheres received equal heating from the Sun, we have seen rapid changes.”

The overall cycle of heat circulation on Titan is clearly defined. Warm gases rise at the summer pole as cold gases subside at its winter equivalent. The equinox occurred on Titan in 2009, and since then Cassini has observed a reversal of the system. A strong, revolving pattern of circulation, or vortex, has developed in the stratosphere over the south pole, one that is enriched in trace gases that are otherwise rarely found in Titan’s atmosphere. Cassini also revealed an atmospheric hot spot developing at high altitudes within months of the equinox, while its counterpart in the northern hemisphere had greatly diminished two years later.


Image: Slipping into shadow, the south polar vortex at Saturn’s moon Titan still stands out against the orange and blue haze layers that are characteristic of Titan’s atmosphere. Images like this, from NASA’s Cassini spacecraft, lead scientists to conclude that the polar vortex clouds form at a much higher altitude — where sunlight can still reach — than the lower-altitude surrounding haze. This view looks towards the trailing hemisphere of Titan (5,150 kilometers across). North on Titan is up and rotated 17 degrees to the left. Images taken using red, green and blue spectral filters were combined to create this natural-color view. The image was taken with the Cassini spacecraft narrow-angle camera on July 30, 2013. Credit: NASA/JPL-Caltech/Space Science Institute.

Within the polar vortex over the south pole, trace gases are accumulating as sunlight diminishes. Here again the parallel is direct. We now see, according to this Europlanet news release, the appearance of complex hydrocarbons and nitriles like methylacetylene and benzene, which were before observed only at high northern latitudes. Coustenis again:

“We’ve had the chance to witness the onset of winter from the beginning and are approaching the peak time for these gas-production processes in the southern hemisphere. We are now looking for new molecules in the atmosphere above Titan’s south polar region that have been predicted by our computer models. Making these detections will help us understand the photochemistry going on.”

While the onset of winter led to a swift temperature drop of 40 degrees Celsius in the stratosphere over the southern pole, the warming effects in the northern hemisphere as the seasons change have been much more gradual, with a 6-degree rise since 2014. In these northerly regions, Cassini has found trace gases that persist into the summer. Although these should eventually disappear, Coustenis says an area of depleted molecular gas and aerosols has emerged across the entire northern hemisphere at an altitude of 400-500 kilometers.

High altitudes on Titan are, in other words, complicated, and while we’re developing a consistent picture thanks to Cassini’s twelve years of observations, these complex effects bear further study. Remember that although we’re entering Cassini’s last year, we have a two-part endgame to go through that involves a final close flyby of Titan to reshape the spacecraft’s orbit. In its new trajectory, Cassini will make 22 passes through the gap between the rings and the planet.

The so-called Grand Finale begins in April of 2017 and takes us to a first dive through the ring/planet gap on April 27. It should be quite a ride, with the closest observations ever made of Saturn, including mapping the planet’s magnetic and gravity fields at high precision, along with samples of particles in the main rings and gases from Saturn’s outer atmosphere. In addition, we should get spectacular views of the rings when, in November of this year, Cassini begins a series of 20 passes just beyond the outer edge of the main rings. Cassini has not gotten this close to the rings since its arrival at Saturn in 2004; we’ll see the ring structure at high resolution. The spacecraft’s final dive into Saturn is planned for September 15, 2017.

“While it will be sad to say goodbye, Cassini’s final act is like getting a whole new mission in its own right,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, California. “The scientific value of the F ring and Grand Finale orbits is so compelling that you could imagine an entire mission to Saturn designed around what we’re about to do.”



New Work on Planet Nine

by Paul Gilster on October 20, 2016

Considering how long we’ve been thinking about a massive planet in the outer Solar System — and I’m going all the way back to Percival Lowell’s Planet X here — the idea that we might find the hypothetical Planet Nine in just three years or so is a bit startling. But Caltech’s Mike Brown and colleague Konstantin Batygin, who predicted the existence of the planet last January based on its effects on Kuiper Belt objects, are continuing to search the putative planet’s likely orbital path, hoping for a hit within the next few years, a welcome discovery if it happens.

The duo are working with graduate student Elizabeth Bailey, lead author of a new study being discussed at the American Astronomical Society’s Division for Planetary Sciences meeting in Pasadena, which is occurring in conjunction with the European Planetary Science Congress. The new paper is all about angles and alignments, focusing on the fact that the relatively flat orbital plane of the planets is tilted about six degrees with respect to the Sun. That’s an oddity, and Planet Nine, hypothesized to be about ten times the mass of the Earth and in an orbit averaging 20 times Neptune’s distance from the Sun, just may be the cause.

The calculations on display in the new paper depict a planet some 30 degrees out of alignment with the orbital plane of the other planets. That can help to explain orbital observations of Kuiper Belt objects, but also the unusual system-wide tilt, which stands out because of the assumed formation of the planets through the collapse of a spinning cloud into a disk and, eventually, a collection of planets orbiting the Sun. We would expect the angular momentum of the planets to maintain a rough alignment with the Sun along the orbital plane.

Unless, of course, something is disrupting the system. Throw in the angular momentum of Planet Nine, based on its assumed mass and distance from the Sun, and profound effects on the system’s spin become evident, creating a long-term wobble that shows up in the system’s tilt. As Bailey puts it, “Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the Solar System has no choice but to slowly twist out of alignment.”

And this from the paper:

… a solar obliquity of order several degrees is an expected observable effect of Planet Nine. Moreover, for a range of masses and orbits of Planet Nine that are broadly consistent with those predicted by Batygin & Brown (2016); Brown & Batygin (2016), Planet Nine is capable of reproducing the observed solar obliquity of 6 degrees, from a nearly coplanar configuration. The existence of Planet Nine therefore provides a tangible explanation for the spin orbit misalignment of the solar system.


Image: This artistic rendering shows the distant view from Planet Nine back towards the sun. The planet is thought to be gaseous, similar to Uranus and Neptune. Hypothetical lightning lights up the night side. Credit: Caltech/R. Hurt (IPAC).

The six-degree tilt we see between planetary disk and Sun thus fits into the team’s calculations regarding Planet Nine’s size and distance from the central star. And if this does indeed turn out to be the explanation, speculation will then center on how Planet Nine came to be so far out of line with the other planets. We know that gravitational interactions in young planetary systems can sometimes result in disruption, causing some planets to be thrown out of their systems, and others to be moved into distant orbits. Such gravitational byplay may well be the reason for Planet Nine’s unusual position. Now we just need to discover the planet.

I also want to mention that Renu Malhotra (University of Arizona) and team have continued their analysis of a possible Planet Nine, likewise presenting their results at the AAS/EPSC meeting in Pasadena. Through analysis of what they call ‘extreme Kuiper Belt Objects’ —on eccentric orbits with aphelia hundreds of AU out — the team finds a clustering of orbital parameters that may point to the existence of a planet of 10 Earth masses with an aphelion of more than 660 AU. Two orbital planes seem possible, one at 18 degrees offset from the mean plane, the other inclined at 48 degrees.

Dr. Malhotra confirmed in an email this morning that her own constraints on the current position of this possible planet line up with Mike Brown and team at Caltech. But her team continues to point out that we have no detection at this point, and much to learn about the orbits of the Kuiper Belt objects under study. From her paper:

…we note that the long orbital timescales in this region of the outer solar system may allow formally unstable orbits to persist for very long times, possibly even to the age of the solar system, depending on the planet mass; if so, this would weaken the argument for a resonant planet orbit. In future work it would be useful to investigate scattering efficiency as a function of the planet mass, as well as dynamical lifetimes of non-resonant planet-crossing orbits in this region of the outer system. Nevertheless, the possibility that resonant orbital relations could be a useful aid to prediction and discovery of additional high mass planets in the distant solar system makes a stimulating case for renewed study of aspects of solar system dynamics, such as resonant dynamics in the high eccentricity regime, which have hitherto garnered insufficient attention.

The Bailey, Batygin & Brown paper is “Solar Obliquity Induced by Planet Nine,” accepted for publication in the Astrophysical Journal (preprint). The Malhotra paper is Malhotra, Volk & Wang, “Coralling a Distant Planet with Extreme Resonant Kuiper Belt Objects,” Astrophysical Journal Letters Vol. 824, No. 2 (20 June 2016). Abstract / preprint.