An Encouraging Formation Scenario for Icy Moons

It makes sense that planets in other stellar systems would have moons, but so far it has been difficult to find them. That’s why Kepler-1625b, about 8,000 light years out in the direction of Cygnus, is so interesting. As we noted last month, David Kipping and graduate student Alex Teachey have compiled interesting evidence of a moon around this gas giant, which is itself either close to or within the habitable zone of its star. The massive candidate exomoon is the size of Neptune, and if confirmed, would mark the first exomoon detection in our catalog.

As the examination of Kepler-1625b and its transit timing variations continues, we have new work out of the University of Zürich, ETH Zürich and NCCR PlanetS that adds weight to the assumption that moons around large planets should be ubiquitous. Using computer simulations run at the Swiss National Supercomputing Centre (CSCS) in Lugano, a team of researchers led by Judit Szulágyi (University of Zurich and ETH Zurich) has determined that both gas giants and ice giants like Neptune and Uranus will produce moon-bearing circumplanetary disks.

Image: One of the computer simulations on the formation of moons (white bodies) around Neptune (blue sphere). Credit: Judit Szulágyi.

The issue is given point by the difference between Neptune and Uranus when it comes to moons. The five major moons of Uranus do not seem out of place when compared to what we see around Jupiter and Saturn. But we seem to see a different formation history at Neptune, whose solitary major moon, Triton, may well have been captured from the Kuiper Belt.

Szulágyi and team wondered whether the moons of Uranus were not themselves outliers, perhaps formed through a collision in the early days of the Solar System. Our own Moon is thought to have been the result of just such an ancient catastrophe. But the simulations the researchers ran pointed to both Uranus and Neptune originally having their own moon-forming disk of gas and dust. In each case, the simulations produced icy moons. This is a useful result as it has been widely believed that the two ice giants were too light to form such a disk.

The implication: Neptune was itself once orbited by a system of icy moons much like that of Uranus, one that would have been disrupted during the capture of the massive moon Triton. Bear in mind that Triton contains 99 percent of the mass of Neptune’s entire satellite system. The authors point to an earlier study showing that the capture of Triton would only have been possible if Neptune originally had a moon system with the mass of the Uranian moons.

From the paper:

We investigated CPD [circumplanetary disk]- and moon-formation around Uranus and Neptune with combining radiative hydrodynamical simulations with satellite population synthesis. We found that both Uranus and Neptune could form a gaseous disk at the end of their formation, when their surface temperature dropped below 500 K. These disks are able to form satellites in them within a few hundred thousand years. The masses of such satellite-systems for both planets were often similar to the current one around Uranus. All the formed moons must be icy in composition, given that they formed in a CPD that has a temperature below water freezing-point.

All of this has implications for the exomoon hunt, in that the formation of moons seems to be likely across the entire range from gas giant to ice giant. Bear in mind how often we’ve found Neptune-class planets among the population of exoplanet candidates. If such worlds are producing exomoons, then such moons form a larger population than we had realized. Our studies of the Jovian and Saturnian moons have shown how interesting they are in terms of astrobiological possibilities, a realm that now widens as we expand the discovery space.

Says Szulágyi: “[A] a much larger population of icy moons in the Universe means more potentially habitable worlds out there than it was imagined so far. They will be excellent targets to search for life outside the Solar System.”

The paper is Szulágyi, Cilibrasi and Mayer, “In situ formation of icy moons of Uranus and Neptune,” Astrophysical Journal Letters 868 (2018), L13. Abstract / Preprint. Among the papers on Triton as a disrupter of Neptune’s early system of moons, see in particular Rufu & Canup, “Triton’s Evolution with a Primordial Neptunian Satellite System,” Astronomical Journal Vol. 154, No. 5 (2017). Abstract.


Puzzling Out Chariklo’s Rings

The outer system object called Chariklo doesn’t get into the news all that much, so I’m glad that this morning I have the chance to give it its place in the Sun. 10199 Chariklo is a Centaur, moving between the orbits of Saturn and Uranus. With an estimated diameter of 250 kilometers, it’s the largest Centaur known, and as far as I know, the first one known to have a ring system. Another Centaur, Chiron, is also suspected of having rings, but on the latter, researchers have not ruled out other explanations for the observed feature, like symmetrical jets of gas and dust.

With Chariklo, we have data from a 2013 occultation of a distant star that revealed the existence of two rings, one 3 kilometers and the other about 7 kilometers wide, separated by about 9 kilometers. Chariklo’s rings have even been given nicknames — Olapoque for the larger, Chui for the smaller, both the names of Brazilian rivers, though the IAU will have the final say on such matters. Of particular interest since the discovery is the question of what keeps a ring system intact around such a small object. The discovery of rings around Haumea deepens the question.

Image: An artist’s impression of the dense and narrow rings around Chariklo. The origin of these rings remains a mystery, but they may be the result of a collision that created a disc of debris. Data from a recent occultation implies that another centaur, Chiron, may also have a ring system, although other explanations are still being examined. Credit: European Southern Observatory.

An interesting new paper speculates that gravity coupled with the odd shapes of both Chariklo and Haumea is the stabilizing factor. At Saturn, we know that so-called ‘shepherd moons’ play a role in stabilizing ring structures. The new work, led by Bruno Sicardy (Observatoire de Paris), focuses on topographical anomalies on the small outer system objects themselves. Co-author Maryame El Moutamid, a research associate in the Cornell Center for Astrophysics and Planetary Science and a member of Cornell’s Carl Sagan Institute, explains:

“Rings appear around Saturn, Jupiter, Neptune and Uranus, but scientists found rings around Chariklo and Haumea within the last few years. Chariklo and Haumea were the first small objects known to have rings, and we think that rings throughout the solar system are more common than we thought. In the case of small bodies Chariklo and Haumea, gravity shepherds the rings. The rings are confined by the gravity because of the shape irregularity of their bodies.”

Thus Chariklo, whose evidently elongated shape includes a large mountain-like feature. The authors believe such topographical anomalies play a role similar to Saturn’s shepherding moons in preventing the ring structure from dissipating. Couple this with the fast rotation of both Chariklo and Haumea, itself an unusually shaped, elongated Kuiper Belt object, and according to the researchers’ simulations, you have all the factors in place to account for ring stability.

So we have a novel mechanism at work here that explains the first ring systems ever found other than those around the giant planets. In contrast to the latter, notes the paper:

…gravitational fields of small bodies may exhibit large non-axisymmetric terms that create strong resonances between the spin of the object and the mean motion of ring particles. Here we show that modest topographic features or elongations of Chariklo and Haumea explain why their rings are relatively far away from the central body, when scaled to those of the giant planets.

The team’s simulations show that these resonances quickly clear out what would have been an early ‘collisional disk’ in the region where the mean motion of the particles matches the object’s spin. Disk material inside this radius falls onto the surface, while material outside the corotation radius is pushed just outside the 1/2 resonance (one rotation of ring particles for every two rotations of the parent body).

Consequently, the existence of rings around non-axisymmetric bodies requires that the 1/2 resonance resides inside the Roche limit of the body, favouring faster rotators for being surrounded by rings.

The Roche limit referenced above refers to the closest a small object can approach the parent body it orbits without being disrupted by tidal forces. The rings, then, stay in place just beyond the Roche limit thanks to the resonances induced partly by unusual topography at the surface.

The paper is Sicardy et al., “Ring dynamics around non-axisymmetric bodies with application to Chariklo and Haumea,” Nature Astronomy 19 November 2018 (abstract). On the original discovery of rings around Chariklo, see Braga-Ribas et al., “A ring system detected around the Centaur (10199) Chariklo,” Nature Vol. 508, Issue 7494 (2014), pp. 72-75 (preprint).


AAVSO Exoplanet Archive for Amateur Astronomers

Even today, I can well understand the reaction that Dennis Conti had when confronted with the prospect of finding a planet around another star with nothing more than an amateur instrument. Conti, who founded and now chairs the Exoplanet Section of the American Association of Variable Star Observers, was a newcomer to the transit method just a few years ago. “I thought, there’s no way for someone with a backyard telescope to detect a planet going around a distant star,” he says, looking back from the vantage of one now immersed in such observations.

My boyhood 3-inch reflector was not a backyard instrument — too many trees back there. So it became a front-yard telescope. Absent the technological innovations of the past five decades, I could only imagine vast instruments for studying objects around other stars. The transit method in exoplanet detection was a long way off, but the idea of seeing not a planet itself but a change in starlight as the planet crossed the face of its host now seems intuitively obvious. It takes a good deal more than a 3-inch reflector to get into the game, but today’s more sophisticated amateur telescopes can definitely make a contribution, as Conti has demonstrated.

Consider: In 2016, Conti coordinated more than 40 amateur astronomers who would assist a professional scientist in characterizing the atmospheres of 15 different exoplanets. A year later, he would begin teaching a course in observing exoplanets through the AAVSO, one that has taught more than 120 students ranging from professional scientists to high-school teachers. His Practical Guide to Exoplanet Observing, now in its 4th revision, is in use in many countries.

The AAVSO is now building its own Exoplanet Database, a useful entry into the field because the increasing amount of data gathered by amateurs should have a unified home. Many existing databases are built around specific space missions or ground-based surveys. The AAVSO’s entry will be a place where amateur astronomers can archive their exoplanet transit observations to make them available to the broader community. Long-term archiving of observations may turn up interesting features in lightcurves like transit timing variations, that could potentially identify the presence of a second planet in the same observed system.

Image: This artist’s concept shows the Kepler-444 planetary system, in which five small planets orbit a distant star. These five planets were detected by the transit method, which involves recording the periodic dimming of a star as a planet transits across its face. Amateur astronomers have been using the same technique to successfully and accurately detect exoplanets for more than a decade, and their observations can now be recorded in the AAVSO’s Exoplanet Database. There, they can be archived long-term and used by professionals and other amateurs to build scientific knowledge of interesting planets. Credit: NASA/JPL-Caltech/AMES/Univ. of Birmingham.

I never followed up with larger home telescopes of my own, but I admire the dedicated amateurs who have plunged into this work. The synergy between amateur and professional can be productive. With TESS (Transiting Exoplanet Survey Satellite) now in operation, we’re reminded that thousands of exoplanet candidates are going to turn up in the next two years. Follow-up observations on the TESS candidates are to be submitted to NASA. The TESS Follow-Up Observing Program is coordinating its efforts with the AAVSO and has adopted its guidelines for best practices, as noted in this AAVSO news release.

“This emphasizes the value that nonprofessionals bring to the field of science,” says Stella Kafka, Ph.D., AAVSO Executive Officer. “People with moderate means can contribute from the ground to the knowledge base of the community. In principle, one can see the AAVSO as an international collaboration between professional and non-professional astronomers, working together to understand some of the most exciting phenomena in the universe.”

Image: The light curve shown here records the dimming of the exoplanet WASP-12b, taken Jan. 5, 2016, by Dennis Conti, Ph.D., founder and chair of the AAVSO’s Exoplanet Section. Conti used equipment available to amateur astronomers and compared his results to published data to show that he was able to successfully and accurately detect the exoplanet. The AAVSO’s Exoplanet Database will provide a place for amateur astronomers following established procedures to make their exoplanet transit observations available to the broader community of researchers, and to have their data archived long-term. Credit: Dennis Conti.

Many eyes on target with a wide range of instruments are better than a few, and bear in mind that observing an exoplanet transit from different locations and times can help astronomers assemble data on a complete transit that might otherwise be lacking. I’ll also remind non-astronomers with a passion to contribute of the Planet Hunters site, where participants can look at exoplanet data and sort through lightcurves from the Kepler mission. The ways for amateurs to make a contribution to exoplanet science are multiplying.


Crater Beneath the Greenland Ice

A crater roughly the size of the area inside Washington DC’s beltway has been found beneath the Greenland ice. On this, some thoughts, but first, a reminiscence. If you’ve ever driven the Capital Beltway at rush hour, you’ll have some sense of the crater’s size. My own experiences of it have been few, but the most memorable was the afternoon I spent at NASA Goddard Space Flight Center, where Greg Benford was speaking. We had agreed that after his talk, Greg and I would head out for dinner at a local restaurant, the exact venue to be determined later.

It was about 5:00 PM when we were in the GSFC parking lot ready to go, now joined by Gloria Lubkin, editor emerita at Physics Today. With the help of Greg’s nephew Dominic, we had chosen a French restaurant about 10 miles away. The problem: Greg and Gloria were in one car, I was in another, and it was rush hour. An out-of-towner who rarely got to DC, I was not remotely prepared for the beltway under these conditions.

I had no smartphone then, no GPS, and the only recourse was to follow the bumper of Gloria’s car. If I lost Gloria and Greg, I wouldn’t have a clue where to go. I leave it as an exercise for the reader’s imagination what it was like to be in packed lanes of high-speed traffic as night fell trying to stay close enough to the bumper ahead so as not to lose it, while simultaneously ensuring enough distance to avoid a collision. Success seemed doubtful, but we reached the restaurant together, and the meal was a gastronomic and conversational delight.

Image: Ah the Beltway. Now put all this into high-speed motion. Credit: Craig F. Walker/Boston Globe.

Greenland seems a much tamer place. The crater identified here is about 300 meters (1000 feet) deep and over 30 kilometers (19 miles) in diameter. That puts it among the 25 largest impact craters on our planet. Described in the journal Science Advances, the work was led by scientists from the University of Copenhagen’s Centre for GeoGenetics at the Natural History Museum of Denmark. The data feeding this three-year effort came from NASA.

Specifically, NASA’s Operation IceBridge was in play, an airborne mission to study polar ice using ice-penetrating radar, complemented by earlier NASA airborne missions in Greenland. Located at the edge of the ice sheet in northwestern Greenland, the circular depression under Hiawatha Glacier had never been examined, but clear evidence of its existence could be found in satellite imagery from NASA’s Terra and Aqua satellites, which showed a circular pattern.

Image: Radar data from an intensive aerial survey of the Hiawatha crater in May 2016 is shown here in aqua-colored curtains. A blue arrow points to the central peak of the crater. Credit: NASA/Cindy Starr.

Subsequent radar maps made the crater’s dimensions clear. Another Beltway reference is the fact that Joe MacGregor, a NASA glaciologist at Goddard Space Flight Center, designed the later airborne mapping survey, using ice-penetrating radar from the University of Kansas. Says MacGregor:

“Previous radar measurements of Hiawatha Glacier were part of a long-term NASA effort to map Greenland’s changing ice cover. What we really needed to test our hypothesis was a dense and focused radar survey there. The survey exceeded all expectations and imaged the depression in stunning detail: a distinctly circular rim, central uplift, disturbed and undisturbed ice layering, and basal debris — it’s all there.”

You would think that glacial ice would quickly remove all trace of a crater, which is why the crater’s preservation after perhaps three million years is considered so unusual. The impactor was evidently an iron meteorite more than 0,8 kilometers (half a mile) wide. Kurt Kjær (Center for GeoGenetics at the Natural History Museum of Denmark), who is lead author on the study, believes the crater may be even younger, perhaps a remnant of an event that occurred toward the end of the last ice age. That would make it among the youngest craters on Earth.

Image: The Hiawatha impact crater is covered by the Greenland Ice Sheet, which flows just beyond the crater rim, forming a semi-circular edge. Part of this edge (top of photo) and a tongue of ice that breaches the crater’s rim are shown in this photo taken during a NASA Operation IceBridge flight on April 17. Credit: NASA/John Sonntag.

The paper goes into considerable detail on the issue of the crater’s age, which remains approximate. Consider this:

The sum of these tentative age constraints suggests that the Hiawatha impact crater formed during the Pleistocene, as this age is most consistent with inferences from presently available data. An impact before the Pleistocene cannot clearly explain the combination of the relative freshness of the crater’s morphology and the ice sheet’s apparently ongoing equilibration with the presence of the crater. We emphasize that even this broad age estimate remains uncertain and that further investigation of the age of the Hiawatha impact crater is necessary. Regardless of its exact age, based on the size of the Hiawatha impact crater, this impact very likely had significant environmental consequences in the Northern Hemisphere and possibly globally.

In 2016 and 2017, researchers returned to Hiawatha Glacier to map tectonic structures and collect samples of sediments emerging from below through a meltwater channel. Here is Nicolaj Larsen (Aarhus University, Denmark), one of the authors of the study:

“Some of the quartz sand coming from the crater had planar deformation features indicative of a violent impact; this is conclusive evidence that the depression beneath the Hiawatha Glacier is a meteorite crater.”

It’s interesting to speculate on other still undiscovered impact craters under ice. They’re a reminder that the Solar System was once a violent place indeed, as the surface of our Moon indicates. There, of course, the processes of wind and water erosion could not take place, so we see stark evidence of ancient impacts. Our planet likewise had its share even if a cursory glance at the globe shows only a few, and the continuing cataloging of near-Earth objects reminds us that a defense against collisions like these is a good insurance policy for our species.

The paper is Kjær et al, “A large impact crater beneath Hiawatha Glacier in northwest Greenland,” Science Advances Vol. 4, No. 11 (14 Nov. 2018). Full text. NASA has produced a helpful video available here.


Spitzer Size Constraints on ‘Oumuamua

The first interstellar object detected in our own Solar System, ‘Oumuamua has a pleasing name, translating from the Hawaiian as something like ‘far visitor first to arrive,’ or words to that effect. It’s also proven a frustrating catch ever since detected by the University of Hawaii’s Pan-STARRS 1 telescope on Haleakala, Hawaii during a search for near-Earth asteroids. We’ve put telescope resources on Earth and in space on the object, but our observing time is up.

For ‘Oumuamua is now well on its way out of the Solar System, so we’re left to massage the data we have in hopes of gaining new insights. Davide Farnocchia (Center for Near Earth Object Studies, JPL) encapsulates the issue:

“Usually, if we get a measurement from a comet that’s kind of weird, we go back and measure it again until we understand what we’re seeing. But this one is gone forever; we probably know as much about it as we’re ever going to know.”

Thus Avi Loeb’s recent paper with Shmuel Bialy discussing the object’s acceleration in terms of solar radiation pressure — could it be a technological artifact? — and continued work on the issue of cometary outgassing to explain its anomalous acceleration. See ‘Oumuamua, Thin Films and Lightsails for the former. On the latter, we have new work from the abovementioned Farnocchia, working with lead author David Trilling (Northern Arizona University) and colleagues in a paper published in The Astronomical Journal.

Trilling and team examine data from the Spitzer Space Telescope taken in November of 2017, an analysis that shows that the object was too faint for Spitzer to detect when it began observations two months after ‘Oumuamua’s closest approach to the Sun in September of that year. That fact is itself valuable, for it sets limits on the object’s total surface area. The size issue is important, because an earlier study led by ESA’s Marco Micheli (citation below) found the object’s acceleration to be the result of outgassing, which worked, according to the team’s calculations, by assuming that ‘Oumuamua was smaller than typical Solar System comets (see ‘Oumuamua: New Data Point to a Comet).

Image: Is this the shape of ‘Oumuamua? An artist’s concept of interstellar asteroid 1I/2017 U1 (‘Oumuamua) as it passed through the solar system after its discovery in October 2017. Observations of ‘Oumuamua indicate that it must be very elongated because of its dramatic variations in brightness as it tumbled through space. Credit: European Southern Observatory / M. Kornmesser.

So what constraints can we apply from the Spitzer data? Spitzer works in the infrared, adding valuable adjunct information to the variations in ‘Oumuamua’s brightness already detected by ground-based telescopes and the Hubble space instrument. These changes in brightness suggested an object less than 800 meters in its longest dimension. Unable to determine shape, the Spitzer data can only set a limit on the object’s total surface area. Thus the authors plug a spherical shape into their calculations and use three different models to reach their conclusions.

With different inputs for its composition, ‘Oumuamua’s non-detection in the infrared implies a ‘spherical diameter’ ranging from a high of 440 meters to a low of 100 meters. The results are consistent with Micheli and team’s findings on the object’s likely size assuming outgassing. Again, the range is the result of different assumptions about the object’s composition, which is unknown. This JPL news release adds that, weighing infrared findings against optical observations, ‘Oumuamua may be up to 10 times more reflective than comets in our Solar System.

A comet warms as it approaches perihelion, with ice vaporizing and cleansing the surface dust and dirt to expose more reflective ice beneath. As with comets we have observed before, outgassing can also produce a new coating of ice and snow, adding to the object’s albedo after close solar passage. This kind of outgassing could have occurred during perihelion for ‘Oumuamua about five weeks before its discovery. Did we subsequently see a dark surface with millions of years of accumulated dust being swept away by released gases and covered in new snow?

The paper is Trilling et al., “Spitzer Observations of Interstellar Object 1I/’Oumuamua,” The Astronomical Journal Vol. 156, No. 6 (14 November 2018). Abstract. The Micheli paper is “Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua),” Nature 559 (27 June 2018), 223-226. Abstract. The Bialy & Loeb paper is “Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?” (preprint).