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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).

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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.

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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.

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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).

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A Super-Earth Orbiting Barnard’s Star

The detection of a planet around Barnard’s Star really hits home for me. No, this isn’t a habitable world, but the whole topic of planets around this star has resonance for those of us who remember the earliest days of exoplanet study, which could be extended back to Peter van de Kamp’s work at Swarthmore’s Sproul Observatory in Pennsylvania. The astronomer thought he had found evidence for a 1.6 Jupiter mass planet in a 4.4 AU orbit there, based on what he interpreted as telltale wobbles in photographic plates of the star taken between 1916 and 1962.

This work, ending in the early 1970s, turned out to be the result of errors in the instrument van de Kamp was using, but the buzz about possible planets around Barnard’s Star had been sufficient to create a small crest of enthusiasm for exoplanet studies in general. The British Interplanetary Society saw in Barnard’s Star a target worth investigating, and designed their Daedalus star probe around a mission there. In any case, van de Kamp’s assumption that planetary systems were common has been proven out, and now we do have a planet around this star, though not the one that the Swarthmore researcher thought he had found.

What Guillem Anglada-Escudé (Queen Mary University, London) and Ignasi Ribas (Institute of Space Studies of Catalonia and the Institute of Space Sciences, CSIC in Spain) have uncovered is a super-Earth in an orbit near the star’s snowline. From the paper in Nature:

Here we report that the combination of numerous measurements from high-precision radial velocity instruments reveals the presence of a low-amplitude but significant periodic signal at 233 days. Independent photometric and spectroscopic monitoring, as well as the analysis of instrumental systematic effects, show that this signal is best explained as arising from a planetary companion. The candidate planet around Barnard’s star is a cold super-Earth with a minimum mass of 3.2 Earth masses orbiting near its snow-line.

Image: An artist’s impression of the surface of Barnard’s Star b. Credit: ESO – M. Kornmesser. Licence: Creative Commons with Attribution, https://creativecommons.org/licenses/by/4.0/

Barnard’s Star b thus becomes the second closest known exoplanet to Earth (although bear in mind that the search for planets around Centauri A and B continues, as does examination of Proxima Centauri for other planets beyond Proxima b). The snow-line at Barnard’s Star, where volatiles like water can condense into solids, is close to the planet’s orbit at 0.4 AU, a reminder that the host is a red dwarf that provides but 2% of the energy the Earth receives from the Sun. The scientists believe the temperature of Barnard’s Star b would be in the range of -150 ℃.

The new planet’s position in relation to the snow-line is itself interesting, as the paper notes:

The candidate planet Barnard’s star b lies almost exactly at the expected position of the snow-line of the system, located at about 0.4 au. It has long been suggested that this region might provide a favourable location for forming planets, with super-Earths being the most common planets formed around low-mass stars. Recent models incorporating dust coagulation, radial drift, and planetesimal formation via the streaming instability support this idea . Although this has yet to be shown to be part of a general trend, observational evidence would significantly constrain theories of planetary migration.

The authors point out that the Barnard’s Star work “…pushes the limits of the radial velocity technique into a new regime of parameter space,” comprising our ability to detect super-Earths in much wider orbits than ever before. Until now, such a detection would have been a matter for gravitational microlensing, with the attendant problem that such studies are inherently one-off affairs, depending on chance celestial alignments. Now we learn of a serious leap forward for radial velocity studies, one that highlights what a tricky catch this was.

Consider the range of instruments involved, among them the European Southern Observatory’s HARPS (High Accuracy Radial velocity Planet Searcher) and UVES (Ultraviolet and Visual Echelle Spectrograph) spectrographs. Says Anglada Escudé:

“HARPS played a vital part in this project. We combined archival data from other teams with new, overlapping, measurements of Barnard’s star from different facilities. The combination of instruments was key to allowing us to cross-check our result.”

All told, data from seven different instruments were put into play, a total of 771 measurements covering 18 years in a collaboration organized by the Red Dots project, which searches for terrestrial planets in warm orbits around the red-dwarf stars closest to the Sun. It was the Red Dots collaboration that was responsible for the discovery of Proxima Centauri b in 2016.

In addition to HARPS and UVES, the work included data from HIRES (High Resolution Echelle Spectrograph) at the Keck 10-meter telescope; PFS (Planet Finder Spectrograph) at the Carnegie’s Magellan 6.5-m telescope; APF (Automated Planet Finder) at the 2.4-m telescope at Lick Observatory; and CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs) at the Calar Alto Observatory. Further observations were made with the 90-cm telescope at Sierra Nevada Observatory, the 40-cm robotic telescope at SPACEOBS (San Pedro de Atacama Celestial Explorations), and the 80-cm Joan Oró Telescope of the Montsec Astronomical Observatory (OAdM).

I don’t usually list every last instrument used in a particular detection, but I did so here because it’s a measure of what it took to find a planet of this size in a relatively cool orbit. Consider this: The data show that Barnard’s star is approaching and moving away from us at about 1.2 m/s, which is roughly the speed I make when out for my morning walk. This tiny motion detected through Doppler methods is at the heart of the new planet’s detection. The researchers note that further observations are still necessary and are underway:

“After a very careful analysis, we are over 99% confident that the planet is there, since this is the model that best fits our observations,” says Ignasi Ribas. “However, we must remain cautious and collect more data to nail the case in the future, because natural variations of the stellar brightness resulting from starspots can produce similar effects to the ones detected.”

Image: Graphic representation of the relative distances to the nearest stars from the Sun. Barnard’s star is the second closest star system, and the nearest single star to us. Credit: IEEC/Science-Wave – Guillem Ramisa Licence: Creative Commons with Attribution, https://creativecommons.org/licenses/by/4.0/

Barnard’s Star is surely in our future if we reach the stage of sending instrumented probes to other stellar systems. At 6 light years, it is our closest target after the Alpha Centauri stars, and has been famous since its 1916 discovery for its remarkable proper motion of 10.3 arcseconds per year relative to the Sun. What that means is that from our perspective on Earth, this star moves faster against the background stars than any other, covering a distance equivalent to the Moon’s diameter across the sky every 180 years. In comparison to younger M-dwarfs, it is relatively quiet in terms of flare activity. Thought to be about twice the age of our Sun, it’s a reminder of our own star’s position among many stars much older than our Solar System.

The paper is Ribas, Anglada Escudé et al., “A super-Earth planet candidate orbiting at the snow-line of Barnard’s star,” Nature 15 November 2018.

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Low Metallicity in Compact Multi-Planet Systems

When astronomers talk about metals, they’re using the term in a specific sense. A metal in stellar terms is any element heavier than helium. Thus iron, silicon, magnesium and carbon qualify, all elements that are components of small, rocky planets. It was iron that John Michael Brewer (Yale University), Debra Fischer and colleagues singled out as a proxy in their recent work on the metal content of exoplanet systems. The work focuses specifically on compact, multi-planet systems as one of several system architectures found in close orbit of a host star.

What’s interesting here is that these domains seem mutually exclusive, or almost so. Unlike our Solar System, a system with multiple planets on tight orbits can squeeze its worlds into a region as close as Mercury. Likewise near the host star, we sometimes find massive planets in close orbits, known as ‘hot Jupiters.’ Few of these have close planetary neighbors, and few compact multi-planet systems have massive planets.

And there is another distinguishing factor. Where the relatively uncommon hot Jupiters appear, they are most numerous around stars with high metallicity, whereas small worlds appear around stars with a wide range of metallicity. Do systems lighter in metals have problems creating larger planets? The jury is still out, but some studies have made an association between low metallicity and small planet formation that Brewer and Fischer now strengthen in this new work.

Image: Artist’s conception of a compact multi-planet system. Credit: Michael S. Helfenbein.

The researchers compared systems of a particular architecture to all known planet hosts as a function of metallicity, adding in a third architecture, systems with cool Jupiters (orbiting further than 0.3 AU from the host star). They identified in their sample 104 hot Jupiter systems, 87 cool Jupiter systems and 105 compact multi-planet systems. The latter were defined as systems with three or more planets orbiting closer than 1 AU. Note that only one hot Jupiter system is found in a compact multi-planet system, while nine cool Jupiters are found in compact systems.

The abundance of compact, multi-planet systems around stars of low metallicity that emerges is clear and points in interesting directions. Many more of these systems may exist than we have assumed. Bear in mind that such worlds are a tricky catch for radial velocity methods, but because they tend to be co-planar, they can be spotted in transit searches, which can observe transits of multiple worlds. We seem to be in a very early phase of compact system detection.

It was Fischer who demonstrated over a decade ago that higher metallicity stars were the most likely to form large gas giants, in work that supported the core accretion model of gas giant planet formation. But smaller systems have been more difficult.

“Our surprising result, that compact systems of multiple, small planets are more likely around lower metallicity stars suggests a new, important clue in understanding the most common type of planetary system in our galaxy,” said co-author Songhu Wang, a 51 Pegasi b Fellow at Yale.

Standing out in this work is the apparent connection between iron and silicon. The researchers found a high silicon-to-iron ratio in stars with lower metallicity. Moreover, low-metallicity stars are long-lived, offering older, potentially habitable worlds whose longevity may make them prime targets for astrobiology. We are likely to learn that such systems are common. From the paper:

Stars of lower metallicity and higher Si/Fe [silicon to iron] ratios are generally older or members of the galactic thick-disk population (Kordopatis et al. 2015). This could point to a changing mix of planet architectures based on formation time and location. In fact, one of the oldest verified and low-metallicity planet hosts, Kepler-444, is home to a compact multi-planet system (Campante et al. 2015). New high-precision radial velocity surveys looking for Earth-massed planets (Jurgenson et al. 2016; González Hernández et al. 2017) may find a much larger population of small planets around these lower-metallicity stars.

Fischer, in this Yale University news release, likens the ratio of silicon to iron to a “thermostat for planet formation. As the ratio increases, nature is dialing up the formation of small, rocky planets.”

Image: This is Figure 2 from the paper. Caption: Stars with low metallicity or a high ratio of Si/Fe do not seem to form hot Jupiters, and are increasingly likely to host compact multi-planet systems. Plotted here are planet-hosting main sequence stars from our sample (green points), comparing the Si/Fe ratio to the log solar relative iron abundance, [Fe/H], with hot Jupiters (orange stars) confined to the lower right-hand side of the plot and compact multi-planet systems (blue triangles) making up a large fraction of the upper-left region. The Sun (yellow circle) is plotted for reference and the dashed lines are drawn to highlight the different populations. Stars with both high Si/Fe and high [Fe/H] are thought to be from the galactic thick-disk and are poorly represented in most planet search samples due largely to their greater distance. Credit: Brewer et al.

Exciting stuff, because we are moving into an era when new instruments like the Extreme Precision Spectrometer (EXPRES) developed by Fischer’s team at Yale, not to mention next-generation ground- and space-based resources, will make it more likely that we can find compact multi-planet systems where no transits occur.

The paper is Brewer et al., “Compact Multi-planet Systems are more Common around Metal-poor Hosts,” The Astrophysical Journal Letters Vol. 867, No. 1 (24 October 2018). Full text. Also note Zhu, “Influence of Stellar Metallicity on Occurrence Rates of Planets and Planetary Systems,” submitted to The Astrophysical Journal Letters, which likewise finds multi-planet systems more common around lower-metallicity stars (preprint).

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Lucy in the Sky

Extended operations at multiple targets, as Dawn showed us, are possible with ion propulsion. But we still learn much from flybys, something New Horizons reminded us with its spectacular success at Pluto/Charon, and again reminds us as it closes on MU69. Likewise, a mission called Lucy will visit multiple objects, using traditional chemical propulsion with gravity assist to achieve flybys of seven different targets. The destination: Jupiter’s trojan asteroids. With launch scheduled for 2021, Lucy’s will study six Jupiter trojans and one asteroid in the Main Belt.

Image: Jupiter’s extensive trojan asteroids, divided into ‘Trojans’ and ‘Greeks’ in a nod to Homer, but all trojans nonetheless. Credit: “InnerSolarSystem-en” by Mdf at English Wikipedia – Transferred from en.wikipedia to Commons. Licensed under Public Domain via Commons.

The trojans are interesting bodies orbiting at the L4 and L5 Lagrange points 60° ahead and behind the gas giant. Jupiter’s trojans are the best known but the term is generic — Neptune has trojans, as does Mars, Uranus and even the Earth (2010 TK7). In fact, some Solar System moons themselves have trojans, as we saw recently when discussing Saturn’s moon Dione, which has the trojans Helene and Polydeuces. Saturn’s moon Tethys also has two trojans.

But as befits Jupiter’s massive size, it’s associated with over 6000 trojans already identified, and a larger population perhaps reaching as high as one million objects over a kilometer in diameter. 617 Patroclus is a particularly intriguing object, a D-type asteroid thought to have water ice in its interior. This object is actually a binary, with a moon named Menoetius slightly smaller than the primary. But we have C- and P- type asteroids in these Lagrange points as well, and Lucy will give us a view of each type as it makes its way into both clusters of Trojans.

The assumption is that the Jupiter trojans are remnants of primordial planet-building material, with clues to the Solar System’s formation and possibly the origins of organic material on Earth. While C-type asteroids are primarily found in the outer regions of the Main Belt, the darker P- and D-type objects have similarities to Kuiper Belt objects beyond the orbit of Neptune. Evidently abundant in dark carbon compounds, all are thought to be rich in volatiles.

Image: This diagram illustrates Lucy’s orbital path. The spacecraft’s path (green) is shown in a frame of reference where Jupiter remains stationary, giving the trajectory its pretzel-like shape. After launch in October 2021, Lucy has two close Earth flybys before encountering its Trojan targets. In the L4 cloud Lucy will fly by (3548) Eurybates (white), (15094) Polymele (pink), (11351) Leucus (red), and (21900) Orus (red) from 2027-2028. After diving past Earth again Lucy will visit the L5 cloud and encounter the (617) Patroclus-Menoetius binary (pink) in 2033. As a bonus, in 2025 on the way to the L4, Lucy flies by a small Main Belt asteroid, (52246) Donaldjohanson (white), named for the discoverer of the Lucy fossil. After flying by the Patroclus-Menoetius binary in 2033, Lucy will continue cycling between the two Trojan clouds every six years. Credits: Southwest Research Institute.

The Lucy mission has just passed the milestone known as Key Decision Point C, a confirmation review that authorizes continuation of the project into its development phase and sets its cost and schedule. This means as well that the confirmation review panel has approved the instrument suite, budget and risk factor analysis for the overall mission. Up next comes the Critical Design Review, which thoroughly vets all aspects of the system design.

Lucy, in other words, is well on its way, says principal investigator Hal Levison (SwRI):

“Up until now this mission has entirely been on paper. Now we have the go ahead to actually cut metal and start putting this spacecraft together.”

Emphasizing the connection with the origins of the Solar System and the possible delivery of organics to Earth, PI Levison named the mission after Lucy, the fossil remains of a three million year old hominid. But he’s enough of a Beatles fan to see a connection there as well, as noted in an older quote on the mission:

“These asteroids really are like diamonds in the sky in terms of their scientific value for understanding how the giant planets formed and the solar system evolved.”

Including imaging and mapping instruments — a color imaging and infrared mapping spectrometer, a high-resolution visible imager, and a thermal infrared spectrometer — the science instrument package is similar to what flew on New Horizons and OSIRIS-REx. Lucy should reach its first targets, the L4 trojans, in 2025, followed by a return to Earth and gravity assist there to move on to the L5 trojan cluster in 2033, The craft will also make a flyby of Main Belt asteroid 52246 Donaldjohanson, which was named for the discoverer of the Lucy fossil.

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Parker Solar Probe: Already a Record Setter

Over the sound system in the grocery store yesterday, a local radio station was recapping events of the day as I shopped. The newsreader came to an item about the Parker Solar Probe, then misread the text and came out with “The probe skimmed just 15 miles from the Sun’s surface.” Yipes!

I was in the vegetable section but you could hear him all over the store, so I glanced around to see how people had reacted. Nobody as much as raised an eyebrow, which either says people tune out background noise as they shop or they have little knowledge of our star.

The correct number is 15 million miles (24,1 million kilometers), and it’s still a hugely impressive feat, but I hope the station got the story right later on. I go easy on this kind of thing because it’s easy enough to make a mistake when reading radio copy (I’ve done this myself). Anyway, there is always some listener who calls it in, which I should have but didn’t. I was pushed for time that morning, making choices about squash and rutabagas and thinking about close approaches.

Image: Artist’s concept of the Parker Solar Probe spacecraft approaching the sun. The spacecraft will provide new data on solar activity and make critical contributions to our ability to forecast major space-weather events that impact life on Earth. Credit: NASA/JHU/APL.

After the Parker Solar Probe’s close pass, the spacecraft has gone nearer the Sun than any other craft. The Helios B probe was the previous recorder holder, setting the mark back in 1976. Helios B reached perihelion in April of 1976 at a distance of 43.4 million kilometers (26.9 million miles), inside the orbit of Mercury. A record the Parker probe surpassed with ease.

And the good news about Parker, reflected in the faces in the image below, is that the craft handled the heat and solar radiation without damage. Four status beacon signals are available, the best being the A signal that was received by mission controllers at JHU/APL on the late afternoon of November 7. Our latest mission to the Sun is live and collecting data.

Image: Members of the Parker Solar Probe mission team celebrate on Nov. 7, 2018, after receiving a beacon indicating the spacecraft is in good health following its first perihelion. Credit: NASA/Johns Hopkins APL/Ed Whitman.

Parker is also setting speed records. Again we turn to Helios-B as the previous record-holder, at 70.2 km/s (157,078 mph). At perihelion on November 7, the Parker spacecraft reached 213,200 miles per hour, or 95.3 kilometers per second. By way of comparison, Voyager 1 moves at approximately 17 kilometers per second as it continues to push into interstellar space. Still in the heliosheath, sister spacecraft Voyager 2 is at a slightly more sedate 15.4 km/sec.

But back to the Parker Solar Probe, whose Sun-facing Thermal Protection System, an 11-centimeter thick carbon-carbon composite shield, reached about 820 degrees Fahrenheit, or 437 degrees Celsius. This is just the beginning, for the spacecraft will continue making closer and closer approaches in the course of its 7-year mission. 24 passes by the Sun are anticipated. The spacecraft will eventually close to a scorching 6.2 million kilometers from our star.

Deep space implications? The Parker Solar Probe’s findings will teach us much about the plasma flow leaving the Sun, a solar ‘wind’ that may offer future magnetic sails (magsails) one option for reaching high velocities within the Solar System (though we first must determine whether this highly variable flow can be efficiently exploited by future magsail designs).

The other implication is using a close solar pass in a ‘sundiver’ mission, accelerating a large payload that would be flung outbound in a spectacular gravitational assist, reaching velocities in the hundreds of kilometers per second. We’ll gain a great deal of knowledge about operations close to the Sun through the performance of Parker’s heat shield, all of which should be helpful if we do decide to explore sundiver options for reaching into the Kuiper Belt and beyond.

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Fine-Tuning Mechanisms for Water Delivery

We’ve long been interested in how the Earth got its oceans, with possible purveyors being comets and asteroids. The idea trades on the numerous impacts that occurred particularly during the Late Heavy Bombardment some 4.1 to 3.8 billion years ago. Tuning up our understanding of water delivery is important not only for our view of our planet’s development but for its implications in exoplanet systems with a variety of different initial conditions.

Image: This view of Earth’s horizon was taken by an Expedition 7 crewmember onboard the International Space Station, using a wide-angle lens while the Station was over the Pacific Ocean. Credit: NASA.

But the picture becomes more complex when we compare regular hydrogen atoms (one proton, one electron) with ‘heavy hydrogen,’ or deuterium atoms. The latter have a neutron in addition to a proton in the nucleus. A recent paper in the Journal of Geophysical Research digs into isotope ratios, the ratio of deuterium to ordinary hydrogen atoms, commonly referred to as the D/H ratio. One reason asteroids are favored by some scientists as the likely source of the bulk of Earth’s water is that asteroidal water has a D/H in the neighborhood of 140 parts per million. Contrast that with cometary water, which runs from 150 ppm to as high as 300 ppm.

When we examine Earth’s oceans, we find a D/H ratio close to that found in asteroids. The new study, from Jun Wu and colleagues at the School of Molecular Sciences and School of Earth and Space Exploration at Arizona State University, takes aim at the asteroid explanation, not by way of discounting it but rather of finding other sources making a contribution to Earth’s water.

“It’s a bit of a blind spot in the community,” said ASU’s Steven Desch, a co-author of the new study. “When people measure the [deuterium-to-hydrogen] ratio in ocean water and they see that it is pretty close to what we see in asteroids, it was always easy to believe it all came from asteroids.”

We are learning, however, that too uncritical an acceptance of D/H ratios may oversimplify the issue. For the hydrogen in Earth’s oceans is not necessarily representative of hydrogen deeper inside the planet, where D/H ratios close to the boundary between the core and mantle show considerably less deuterium. This may indicate a non-asteroidal source for at least some of the hydrogen.

Another telling point is that helium and neon, showing isotopic signatures inherited from the original solar nebula, have also been found in Earth’s mantle. Contrasting hydrogen at the core-mantle boundary with what we see in Earth’s oceans and factoring in these noble gases may change our thinking. The Wu study considers the formation of the planets in the earliest days of the Solar System, when small, often colliding planetary embryos up to the size of Mars went through gradual planetary accretion.

The new model works like this: As larger embryos formed largely from water-laden asteroids, they began to develop into planets. On Earth, decaying radioactive elements melted iron within the emerging world, pulling in asteroidal hydrogen and sinking to form a core. Collisions among planetesimals would meanwhile have created enough energy to melt the surfaces of the larger embryos like the Earth into magma oceans.

Hydrogen and noble gases from the solar nebula would be drawn in to create an early atmosphere. The nebular hydrogen, lighter than asteroidal hydrogen, would have dissolved into the molten iron of the magma ocean, eventually being drawn into the mantle, along with hydrogen from other sources.

What the authors argue is that this process created a slight enrichment of hydrogen in the molten iron and left a higher ratio of deuterium behind in the magma (the process is called isotopic fractionation). Hydrogen is attracted to iron, while the heavier isotope, deuterium, less attracted to iron, would have remained in the magma which would eventually become Earth’s mantle. We would end up with lower D/H ratios in the core than in the mantle and oceans. The authors argue that while most of Earth’s water is asteroidal, some of it did in fact come from the solar nebula.

The process is complex, and also takes in impacts from smaller embryos and other objects that continued to add water and mass until Earth reached its final size. The authors provide this synopsis as their caption for Figure 1 (above), which I’ll reproduce verbatim but break into sections for reasons of readability:

  • (a) Earth accreted from embryos with chondritic [asteroidal] levels of water concentrations and D/H ratios.
  • (b) These embryos differentiated and stored relatively light hydrogen in their cores, raising the D/H of hydrogen in their mantles.
  • (c) The largest embryo accreted a proto-atmosphere and sustained a magma ocean into which nebular hydrogen diffused.
  • (d) The largest embryo’s magma ocean crystallized and overturned, mixing light hydrogen into the mantle, but incompletely.
  • (e) As smaller embryos were accreted, their mantles joined the proto‐Earth’s mantle, and their cores merged with the proto‐Earth’s core.

The result is:

  • (f) Earth’s mantle today contains approximately three oceans of water in its mantle and surface, with average D/H ≈ 150 × 10−6, and ~4.8 oceans’ worth of hydrogen in its core, with D/H ≈ 130 × 10−6. Mantle plumes can sample low‐D/H material from the core‐mantle boundary.

Wu says this: “For every 100 molecules of Earth’s water, there are one or two coming from [the] solar nebula.” But we now see the potential for including gas left over from the formation of our Solar System in the question of water delivery and accumulation. In other stellar systems where water-bearing asteroids may not be as abundant, this implies that water could still be in place from the system’s own stellar nebula. Wu adds: “This model suggests that the inevitable formation of water would likely occur on any sufficiently large rocky exoplanets in extrasolar systems. I think this is very exciting.”

The paper concludes:

Our comprehensive model for the origin of Earth’s water considers, for the first time, the effects of isotopic fractionation as hydrogen dissolved into metal and was sequestered into the core. Based on the behaviors of proxies, we consider it likely that the D/H ratio of the core is ~10% lighter than the mantle. We hypothesize that Earth accreted a few to tens of oceans of water from chondrites, mostly carbonaceous chondrites. Drawing on the latest theories of planet formation, which argue for rapid (<2 Myr) formation of planetary embryos, we favor ingassing of a few tenths of an ocean of solar nebula hydrogen into the magma oceans of the embryos that formed Earth.

The paper is Wu et al., Journal of Geophysical Research: Planets 09 October 2018 (full text).

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On the Earliest Stars

If you’ve given some thought to the Fermi question lately — and reading Milan Ćirković’s The Great Silence, I’ve been thinking about it quite a bit — then today’s story about an ancient star is of particular note. Fermi, you’ll recall, famously wanted to know why we didn’t see other civilizations, given the apparent potential for our galaxy to produce life elsewhere. Now a paper in The Astrophysical Journal adds punch to the question by making the case that the part of the galaxy in which we reside may be older than we have thought.

Finding that our Sun is younger than many nearby stars, an issue that Charles Lineweaver (Australian National University), among others, has examined, would allow even more time for civilizations to have emerged in the galactic neighborhood. But let’s now leave Fermi behind to look at the tiny star that prompts these ruminations (and to be sure, the paper on this star makes no mention of Fermi, but does tell us something quite interesting about the early cosmos).

Discovered by Kevin Schlaufman (Johns Hopkins University), 2MASS J18082002–5104378 B is the smaller of a binary pair that orbit a common barycenter. While the primary had been previously discovered, it was up to Schlaufman and team to uncover the far more interesting companion.

Schlaufman used data from the Magellan Clay Telescope, Las Campanas Observatory and the Gemini Observatory in finding and characterizing this star. What distinguishes 2MASS J18082002–5104378 B is its size, metallicity and age. On the latter, Schlaufman believes it could be as little as a single generation removed from the Big Bang itself, and the paper pegs its age at approximately 13.5 billion years. We’ve discovered other ancient stars with low metal content, but this one is located in the Milky Way’s thin disk, that part of the galaxy in which we reside. Hence the issue of the age of local stars, as the paper recounts:

Given its thin disk orbit, the 13.535 ± 0.002 Gyr age of the 2MASS J18082002–5104378 system provides a lower limit on the age of the thin disk. Similarly old but not quite as metal-poor stars have also been seen on thin disk orbits (e.g., Casagrande et al. 2011; Bensby et al. 2014). This is somewhat older than the 8–10 Gyr age of the thin disk suggested by classical studies of field stars (Edvardsson et al. 1993; Liu & Chaboyer 2000; Sandage et al. 2003), the white dwarf luminosity function (e.g., Oswalt et al. 1996; Leggett et al. 1998; Knox et al. 1999; Kilic et al. 2017), and the ages of the oldest disk open clusters Berkeley 17 and NGC 6791 (e.g., Krusberg & Chaboyer 2006; Brogaard et al. 2012).

Image: This edge-on diagram of the Milky Way shows the thin disk in green. Credit: Wikimedia Commons (CC BY-SA 3.0).

We are talking about a star with a content of metals roughly the same as the planet Mercury. Contrast that with the Sun, whose heavy element content is equal to approximately 14 Jupiters.

2MASS J18082002–5104378 B (here’s hoping it gets a new moniker, perhaps ‘Schlaufman’s Star’) is the lowest-mass ultra metal-poor star currently known. Yet despite its extreme age and low metallicity, it is found in the thin disk, and in fact is the most metal-poor star yet found that is part of the thin disk. We would expect stars forming not long after the Big Bang to be low in metals, given that hydrogen, helium and trace lithium are all they had to work with. It would be later stellar generations that could form with the heavier elements these early stars produced in their cores, seeding the cosmos with metals through supernovae explosions.

Call that first generation Population III stars, which when first modeled by researchers produced stars far more massive than the Sun, giant objects that should form as single stars in isolation. Later models dropped the range of mass to as low as 10 solar masses but also extended it on the high end. Low-mass Population III stars only recently began to be considered when the issue of fragmentation began appearing in numerical simulations. The discovery of 2MASS J18082002–5104378 B makes the case for the emergence of such stars.

From the paper:

We use models of protostellar disks around both UMP [low-mass ultra metal-poor] and Pop III protostars plus scaling relations for the fragment mass and migration time to argue that the existence of the low-mass UMP star 2MASS J18082002–5104378 B and the extremely metal-poor (EMP) brown dwarf HE 1523–0901 B discovered by Hansen et al. (2015) implies the survival of solar-mass fragments around Pop III stars…

Thus we may be looking at a new observable that can take us back to conditions at the earliest era of star formation:

Whereas fragmentation at the molecular core scale will likely lead to massive binary stars, the emergence of gravitationally bound solar-mass clumps in protostellar disks via gravitational instability has the potential to produce low-mass Pop III stars that may be observable in the Milky Way.

Image: The new discovery is only 14% the size of the Sun and is the new record holder for the star with the smallest complement of heavy elements. It has about the same heavy element complement as Mercury, the smallest planet in our solar system. Credit: Kevin Schlaufman/JHU.

Thus far about 30 stars considered ultra metal-poor have been identified, all of roughly the Sun’s mass, but 2MASS J18082002–5104378 B is only 14 percent of the Sun’s mass. The mass, incidentally, was determined by radial velocity methods, examining the wobble of the primary star. Backing out to the wider picture, our view of the earliest stars as extremely massive objects, unobservable to us because they would have burned quickly and died, has to be modified to include low-mass stars that can, at least in some situations, emerge, as 2MASS J18082002–5104378 B did, burning for long lifetimes indeed. Says Schlaufman:

“If our inference is correct, then low-mass stars that have a composition exclusively the outcome of the Big Bang can exist. Even though we have not yet found an object like that in our galaxy, it can exist.”

The paper is Schlaufman et al., “An Ultra Metal-poor Star Near the Hydrogen-burning Limit,” Astrophysical Journal Vol. 867, No. 2 (5 November 2018). Abstract / preprint.

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