Wolf 1061 Unlikely to Host Habitable Worlds

by Paul Gilster on January 26, 2017

A key way to learn more about a given exoplanet is to home in on the properties of its star. So argue Stephen Kane (San Francisco State University) and colleagues in a new paper slated for the Astrophysical Journal. The star in question is Wolf 1061 (V2306 Ophiuchi), an M-class red dwarf some 13.8 light years away in the constellation Ophiuchus. In December of 2015, Australian astronomers announced the discovery of three planets around the star.

Drawn out of data from the HARPS spectrograph at La Silla, the planets are all super-Earths, their radial velocity data supplemented with eight years of photometry from the All Sky Automated Survey. All three seem likely to be rocky planets, but firming this up would take transits, which the discovery team at the University of New South Wales estimated might occur, with a likelihood of about 14 percent for the inner world, dropping to 3% for the outer.

Kane and team investigate the transit question in light of the fact that that two recent papers have produced sharply different orbital periods for the outermost planet here, though both find planet c near or within the habitable zone, which itself depends on the star’s luminosity and effective temperature. Both the recent papers see reasonably high transit possibilities, but Kane’s work rules out transits of the two inner worlds, leaving open a possibility for the outer.

The researchers have used observations from the Center for High Angular Resolution Astronomy (CHARA) interferometric array at Mount Wilson Observatory near Los Angeles. From this emerges a precise stellar radius measurement (0.3207±0.0088 R), from which the team has calculated the star’s effective temperature and luminosity. The photometry data reveal a stellar rotation period of 89.3±1.8 days. The work has useful implications for upcoming space-based exoplanet studies. As the paper notes:

The assessment of host star properties is a critical component of exoplanetary studies, at least for the realm of indirect detections through which exoplanet discoveries thus far have predominantly occurred. This situation will remain true for the coming years during which the transit method will primarily be used from space missions such as the Transiting Exoplanet Survey Satellite (TESS), the CHaracterising ExOPlanet Satellite (CHEOPS), and the PLAnetary Transits and Oscillations of stars (PLATO) mission. Of particular interest are the radius and effective temperature of the stars since the radius impacts the interpretation of observed transit events and the combination of radius and temperature is used to calculate the extent of the HZ.

And indeed, it is through this painstaking analysis of stellar properties that the researchers have been able to calculate habitable zone boundaries for Wolf 1061 of 0.09–0.23 AU. Have a look at the paper’s Figure 8, which shows the Wolf 1061 system as diagrammed from above.

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Image: Figure 8 from the paper shows the orbits of the planets overlaid on the habitable zone. The scale here is 1.0 AU to the side, with the ‘conservative’ habitable zone shown as light gray, and the more optimistic extension shown in dark gray. Credit: Kane et al.

Notice the outer planet (d), which passes briefly through the habitable zone at closest approach to the star before widening further out in its eccentric orbit. Indeed, only 6 percent of the orbital period takes place within the habitable zone. Planet c spends 61 percent of its orbit within the habitable zone, but only within the optimistic assumptions for the HZ. Taking into account the recent orbital solutions for this system, both inner worlds are problematic:

…planet c is quite similar to the case of Kepler-69 c, which was proposed to be a strong super-Venus candidate by Kane et al. (2013). Indeed, both of the inner two planets, terrestrial in nature according to the results of both Wright et al. (2016) and Astudillo-Defru et al. (2016b), lie within the Venus Zone of the host star (Kane et al. 2014) and are thus possible runaway greenhouse candidates.

Measuring stellar parameters unlocks the boundaries of the habitable zone, allowing the researchers to study the planetary orbits and weigh the chances for liquid water on the surface. The results hardly favor the Wolf 1061 system as a promising candidate for life.

We find that, although the eccentric solution for planet c allows it to enter the optimistic HZ, the two inner planets are consistent with possible super-Venus planets (Kane et al. 2013, 2014). Long-term stability analysis shows that the system is stable in the current configuration, and that the eccentricity of the two inner planets frequently reduces to zero, at which times the orbit of planet c is entirely interior to the optimistic HZ. We thus conclude that the system is unlikely to host planets with surface liquid water.

The paper is Kane et al., “Characterization of the Wolf 1061 Planetary System,” accepted for publication at the Astrophysical Journal (preprint).

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PROCYON: An Overview of Cometary Water

by Paul Gilster on January 25, 2017

The Japanese PROCYON spacecraft (Proximate Object Close flyby with Optical Navigation) has just given us an interesting case of repurposing a scientific instrument, not to mention drawing value out of a mission whose initial plans had gone awry. Launched together with JAXA’s Hayabusa 2 probe in late December of 2014, PROCYON was to have flown by asteroid 2000 DP107 in 2016, but a malfunctioning ion thruster put an end to that plan.

Fortunately, PROCYON carried LAICA, a telescope that was put to use to study the Earth’s geocorona (the outermost layer of the atmosphere). Developed at Japan’s Rikkyo University, LAICA observes emissions from hydrogen atoms, a useful capability when turned to comet studies, as a team of researchers has now done with comet 67P/Churyumov-Gerasimenko. Water being the most abundant cometary ice, its release rate helps map activity on the comet and offers clues to how water was incorporated into comets in the early Solar System.

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Image: The PROCYON spacecraft and comet 67P/Churumov-Gerasimenko (Conceptual Image). Credit: NAOJ/ESA/Go Miyazaki.

The researchers, from the National Astronomical Observatory of Japan, University of Michigan, Kyoto Sangyo University, Rikkyo University and the University of Tokyo, set out to map the entire hydrogen coma of comet 67P, the same comet studied in such spectacular detail by the European Space Agency’s Rosetta mission. The challenge to Rosetta, however, was that being inside the cometary coma, it could not observe the entire coma structure.

None of this was in the original PROCYON mission, but it became clear that the spacecraft could be valuable in supplementing the Rosetta observations. Rosetta had studied specific areas on the comet, but the researchers wanted an estimate of the total amount of water released by the comet per second, which in turn demanded a model for the coma itself. If Rosetta could not provide the answer, PROCYON could study the coma in its entirety.

Adapting PROCYON/LAICA for a comet proved workable. The hydrogen atoms in a cometary coma come from water molecules ejected from the nucleus which are then broken apart by ultraviolet radiation from the Sun. Coma models based on these processes allowed the team to estimate the water release rate based on a brightness map of the hydrogen atoms.

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Image: Processed and cropped hydrogen-Lyα image of the comet 67P/C-G in Rayleigh units (upper panel) taken by the LAICA telescope on September 13, 2015 UT and the hydrogen coma appearance predicted by a two-dimensional axi-symmetric model of the atomic hydrogen coma (lower panel). The yellow dotted arrow in the lower panel indicates the direction to the Sun at the time of observation. Credit: NAOJ.

Working with observations of comet 67P’s entire hydrogen coma, the researchers were able to derive its absolute water production rates near the 2015 perihelion. This allowed them to test their developing models for the coma, which could be combined with the Rosetta results to estimate the total ejected mass of the comet during this period. Credit the high spatial resolution of the LAICA instrument and the pointing control of PROCYON for the result.

These are useful results, but let’s also look at what they imply about how we design our missions. PROCYON is a small cube about 60 cm to the side weighing 65 kg. The low-cost mission has been able to support key elements of a much larger mission with additional observations in a way that JAXA believes to be a model for small spacecraft in the future.

As we continue to explore what we can do with CubeSats and other micro-designs, we can think in terms of ‘clusters’ of small spacecraft networking together to reduce overall risk and provide widely dispersed observing platforms for a variety of targets. Swarms of micro-sailcraft to the outer planets are just one scenario that may grow out of spacecraft interactions like these.

The paper is Shinnaka et al. 2017 “Imaging observations of the hydrogen coma of comet 67P/Churyumov-Gerasimenko in September 2015 by the PROCYON/LAICA,” Astronomical Journal, Volume 153, Issue 2, Article number 76 (24 January 2017). Abstract.

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Probing the Surface of Ceres

by Paul Gilster on January 24, 2017

It doesn’t stretch credulity to hypothesize that the early Earth benefited from an influx of comet and asteroid material that contributed water and organic compounds to its composition. The surface of a world can clearly be affected by materials from other bodies in the Solar System. Now we’re learning that the dwarf planet Ceres may have a surface dusted by material from asteroid impacts. The findings come from a team of astronomers investigating Ceres with SOFIA, the airborne Stratospheric Observatory for Infrared Astronomy. The observatory is a highly modified 747SP aircraft carrying a 2.5m reflecting telescope.

The study shows that not just Ceres but other asteroids and dwarf planets may be coated with asteroid fragments, a result that adjusts our view of Ceres’ surface composition. After all, what we’re looking at may simply be the result of asteroid impacts in the early days of the Solar System’s formation. Three quarters of all asteroids, including Ceres, have been classified as type C (carbonaceous) on the basis of their colors, but the SOFIA infrared data show a substantial difference between the dwarf planet and C-type asteroids in nearby orbits.

Carbonaceous asteroids are dark (albedo in the range of 0.03-0.09, on a scale where a white, perfectly reflecting surface has an albedo of 1.0), with a composition depleted in hydrogen, helium and other volatiles. What SOFIA shows us is that Ceres doesn’t fit this model. Pierre Vernazza is a research scientist at the Laboratoire d’Astrophysique de Marseille:

“By analyzing the spectral properties of Ceres we have detected a layer of fine particles of a dry silicate called pyroxene. Models of Ceres based on data collected by NASA’s Dawn as well as ground-based telescopes indicated substantial amounts of water-bearing minerals such as clays and carbonates. Only the mid-infrared observations made using SOFIA were able to show that both types of material are present on the surface of Ceres.”

Observations of ceres

Image: Ceres’ surface is contaminated by a significant amount of dry material while the area below the crust contains essentially water-bearing materials. The mid-infrared observations revealed the presence of dry pyroxene on the surface probably coming from interplanetary dust particles. The Internal structure of the Dwarf Planet Ceres was derived from NASA Dawn spacecraft data. Credit: SETI Institute.

Interplanetary dust particles, according to this SETI Institute news release, are the most likely source for the pyroxene, and are also implicated as having accumulated on other asteroid surfaces. Ceres thus takes on the coloration of some of its drier neighbors, while actually housing more substantial resources of water below. The larger picture is that infrared observations may help us better understand an asteroid’s true composition. Vernazza even speculates that ammoniated clays mixing with watery clay on Ceres may point to an origin in the outer parts of the Solar System, with migration occurring later in the dwarf planet’s life.

“The bottom line is that seeing is not believing when it comes to asteroids,” says Franck Marchis, senior planetary astronomer at the SETI Institute, a researcher who collaborated in this project. “We shouldn’t judge these objects by their covers, as it were.”

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Jupiter in the Public Eye

by Paul Gilster on January 23, 2017

Have a look at Jupiter as seen by the Juno spacecraft on its third close pass. A view as complex as the one below reminds us how images can be manipulated to bring out detail. This happens so frequently in astronomical images that it’s easy to forget this view is not necessarily what the human eye would see, and we always have to check to find out how a given image was processed. In this case, we’re looking at the work of a ‘citizen scientist,’ one Eric Jorgensen, who enhanced a JunoCam image to highlight the cloud movement.

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Image: This amateur-processed image was taken on Dec. 11, 2016, at 1227 EST (1727 UTC), as NASA’s Juno spacecraft performed its third close flyby of Jupiter. At the time the image was taken, the spacecraft was about 24,400 kilometers from the gas giant planet. Credit: NASA/JPL-Caltech/SwRI/MSSS/Eric Jorgensen.

The image shows a region of Jupiter southeast of what is known as the ‘pearl,’ one of eight rotating storms at 40 degrees south latitude on the planet, a region of vast and roiling turbulence. Citizen science efforts like Planet Hunters, SETI@Home and Galaxy Zoo have brought private individuals into contact with scientific data and fostered interest in a wide range of sciences, with Planet Hunters rising to particular visibility thanks to its work with Boyajian’s Star and the still mysterious light curves observed there.

The Juno mission is delving into this realm with the announcement that on the spacecraft’s February 2 pass of Jupiter, the public will have had a voice in the selection of targets for the imaging team. As JPL notes in this news release, JunoCam will begin taking pictures as Juno approaches Jupiter’s north pole. Scientists have to keep an eye on onboard storage limitations as they consider which images to collect with JunoCam. Each close pass (‘perijove’) happens in a 2-hour window as the spacecraft goes from the north pole of the giant planet to the south pole, with JunoCam imaging a circumscribed strip of territory.

The voting for the February 2 flyby is still open, but the process repeats: Each orbit will have a voting page, and each perijove on Juno’s 53-day orbit will have space for two polar images within which the public can participate in prioritizing particular points of interest, in accordance with the science goals the mission is trying to meet. Several pages at the voting site will be devoted to unique points of interest that will be within range of JunoCam’s field of view during the next close approach. Raw images will then be made available for processing.

“The pictures JunoCam can take depict a narrow swath of territory the spacecraft flies over, so the points of interest imaged can provide a great amount of detail,” said Juno co-investigator Candy Hansen, (Planetary Science Institute). “They play a vital role in helping the Juno science team establish what is going on in Jupiter’s atmosphere at any moment. We are looking forward to seeing what people from outside the science team think is important.”

Bear in mind that JunoCam was included on the mission because, working in color and visible light, it could offer a wide field of view that would, among other things, spur public interest and involvement. So it’s not surprising to see this citizen science angle being brought forward, offering engagement not just from amateur scientists but students worldwide. Building public support is also a key component in keeping up the pressure for better space funding.

The February 2 flyby makes its closest approach to Jupiter at 0758 EST (1258 UTC), with the spacecraft about 4300 kilometers above the cloud tops. We’ll see Jupiter up close once again through a spacecraft’s lens, translated for us into images that mimic what we would see with our own eyes before we get to work processing them. If you’re interested in having a say on future JunoCam targets, click here for information on how to get involved.

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Image: Jupiter’s south pole as seen during perijove 3, in an image processed by Julien Potier (Planetario Silvia Torres Castilleja, Ags, Mexico), rotated, cropped to get rid of yellowish band, processed with RGB levels, brightness, contrast and HDR Toning.

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A Vision to Bootstrap the Solar System Economy

by Paul Gilster on January 20, 2017

Early probes are one thing, but can we build a continuing presence among the stars, human or robotic? An evolutionary treatment of starflight sees it growing from a steadily expanding presence right here in our Solar System, the kind of infrastructure Alex Tolley examines in the essay below. How we get to a system-wide infrastructure is the challenge, one analyzed by a paper that sees artificial intelligence and 3D printing as key drivers leading to a rapidly expanding space economy. The subject is a natural for Tolley, who is co-author (with Brian McConnell) of A Design for a Reusable Water-Based Spacecraft Known as the Spacecoach (Springer, 2016). An ingenious solution to cheap transportation among the planets, the Spacecoach could readily be part of the equation as we bring assets available off-planet into our economy and deploy them for even deeper explorations. Alex is a lecturer in biology at the University of California, and has been a Centauri Dreams regular for as long as I can remember, one whose insights are often a touchstone for my own thinking.

by Alex Tolley

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Crewed starflight is going to be expensive, really expensive. All the various proposed methods from slow world ships to faster fusion vessels require huge resources to build and fuel. Even at Apollo levels of funding in the 1960’s, an economy growing at a fast clip of 3% per year is estimated to need about half a millennium of sustained growth to afford the first flights to the stars. It is unlikely that planet Earth can sustain such a sizable economy that is millions of times larger than today’s. The energy use alone would be impossible to manage. The implication is that such a large economy will likely be solar system wide, exploiting the material and energy resources of the system with extensive industrialization.

Economies grow by both productivity improvements and population increases. We are fairly confident that Earth is likely nearing its carrying capacity and certainly cannot increase its population even 10-fold. This implies that such a solar system wide economy will need huge human populations living in space. The vision has been illustrated by countless SciFi stories and perhaps popularized by Gerry O’Neill who suggested that space colonies were the natural home of a space faring species. John Lewis showed that the solar system has immense resources to exploit that could sustain human populations in the trillions.

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Image credit: John Frassanito & Associates

But now we run into a problem. Even with the most optimistic estimates of reduced launch costs, and assuming people want to go and live off planet probably permanently, the difficulties and resources needed to develop this economy will make the US colonization by Europeans seem like a walk in the park by comparison. No doubt it can be done, but our industrial civilization is little more than a quarter of a millennium old. Can we sustain the sort of growth we have had on Earth for another 500 years, especially when it means leaving behind our home world to achieve it? Does this mean that our hopes of vastly larger economies, richer lives for our descendents and an interstellar future for humans is just a pipe dream, or at best a slow grind that might get us there if we are lucky?

Well, there may be another path to that future. Philip Metzger and colleagues have suggested that such a large economy can be developed. More extraordinary, that such an economy can be built quickly and without huge Earth spending, starting and quickly ending with very modest space launched resources. Their suggestion is that the technologies of AI and 3D printing will drive a robotic economy that will bootstrap itself quickly to industrialize the solar system. Quickly means that in a few decades, the total mass of space industrial assets will be in the millions of tonnes and expanding at rates far in excess of our Earth-based economies.

The authors ask, can we solve the launch cost problem by using mostly self-replicating machines instead? This should remind you of the von Neumann replicating probe concept. Their idea is to launch seed factories of almost self-replicating robots to the Moon. The initial payload is a mere 8 tonnes. The robots will not need to be fully autonomous at this stage as they can be teleoperated from Earth due to the short 2.5 second communication delay. They are not fully self-replicating at this stage as need for microelectronics is best met with shipments from Earth. Almost complete self-replication has already been demonstrated with fabs, and 3D printing promises to extend the power of this approach.

The authors assume that initial replication will neither be fully complete, nor high fidelity. They foresee the need for Earth to ship the microelectronics to the Moon as the task of building fabs is too difficult. In addition, the materials for new robots will be much cruder than the technology earth can currently deliver, so that the next few generations of robots and machinery will be of poorer technology than the initial generation. However the quality of replication will improve with each generation and by generation 4, a mere 8 years after starting, the robot technology will be at the initial level of quality, and the industrial base on the Moon should be large enough to support microelectronics fabs. From then on, replication closure is complete and Earth need ship no further resources to the Moon.

GenHuman/Robotic InteractionArtificial IntelligenceScale of IndustryMaterials ManufacturedSource of Electronics
1.0Teleoperated and/or locally operated by a human outpostInsect-likeImported, small-scale, limited diversityGases, water, crude alloys, ceramics, solar cellsImport fully integrated machines
2.0TeleoperatedLizard-likeCrude fabrication, inefficient, but greater throughput than 1.0(Same)Import electronics boxes
2.5TeleoperatedLizard-likeDiversifying processes, especially volatiles and metalsPlastics, rubbers, some chemicalsFabricate crude components plus import electronics boxes
3.0Teleoperated with experiments in autonomyLizard-likeLarger, more complex processing plantsDiversify chemicals, simple fabrics, eventually polymersLocally build PC cards, chassis and simple components, but import the chips
4.0Closely supervised autonomyMouse-likeLarge plants for chemicals, fabrics, metalsSandwiched and other advanced material processesBuilding larger assets such as lithography machines
5.0Loosely supervised autonomyMouse-likeLabs and factories for electronics and robotics. Shipyards to support main belt.Large scale productionMake chips locally. Make bots in situ for export to asteroid belt.
6.0Nearly full autonomyMonkey-likeLarge-scale, self-supporting industry, exporting industry to asteroid main beltMakes all necessary materials, increasing sophisticationMakes everything locally, increasing sophistication
X.0Autonomous robotics pervasive throughout Solar System enabling human presenceHuman-likeRobust exports/imports through zones of solar systemMaterial factories specialized by zone of the Solar SystemElectronics factories in various locations

Table 1. The development path for robotic space industrialization. The type of robots and the products created are shown. Each generation takes about 2 years to complete. Within a decade, chip fabrication is initiated. By generation 6, full autonomy is achieved.

AssetQty. per setMass minus Electronics (kg)Mass of Electronics (kg)Power (kW)Feedstock Input (kg'hr)Product Output (kg/hr)
Power Distrib & Backup12000-----------------
Excavators (swarming)570190.3020----
Chem Plant 1 - Gases1733305.5841.8
Chem Plant 2 - Solids1733305.58101.0
Metals Refinery110191910.00203.15
Solar Cell Manufacturer1169190.500.3----
3D Printer 1 - Small Parts4169195.000.50.5
3D Printer 2 - Large Parts4300195.000.50.5
Robonaut assemblers3135150.40--------
Total per Set~7.7 MT
launched to Moon
64.36 kW20 kg
regolith/hr
4 kg
parts/hr

Table 2. The products and resources needed to bootstrap the industrialization of the Moon with robots. Note the low mass needed to start, a capability already achievable with existing technology. For context, the Apollo Lunar Module had a gross mass of over 15 tonnes on landing.

The authors test their basic model with a number of assumptions. However the conclusions seem robust. Assets double every year, more than an order of magnitude faster than Earth economic growth.

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Figure 13 of the Metzger paper shows that within 6 generations, about 12 years, the industrial base off planet could potentially be pushing towards 100K MT.

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Figure 14 of the paper shows that with various scenarios for robots, the needed launch masses from Earth every 2 years is far less than 100 tonnes and possibly below 10 tonnes. This is quite low and well within the launch capabilities of either government or private industry.

Once robots become sophisticated enough, with sufficient AI and full self-replication, they can leave the Moon and start industrializing the asteroid belt. This could happen a decade after initiation of the project.

With the huge resources that we know to exist, robot industrialization would rapidly, within decades not centuries, create more manufactures by many orders of magnitude than Earth has. Putting this growth in context, after just 50 years of such growth, the assets in space would require 1% of the mass of the asteroid belt, with complete use within the following decade. Most importantly, those manufactures, outside of Earth’s gravity well, require no further costly launches to transmute into useful products in space. O’Neill colonies popped out like automobiles? Trivial. The authors suggest that one piece could be the manufacture of solar power satellites able to supply Earth with cheap, non-polluting power, in quantities suitable for environmental remediation and achieving a high standard of living for Earth’s population.

With such growth, seed factories travel to the stars and continue their operation there, just as von Neumann would predict with his self-replicating probes. Following behind will be humans in starships, with habitats already prepared by their robot emissaries. All this within a century, possibly within the lifetime of a Centauri Dreams reader today.

Is it viable? The authors believe the technology is available today. The use of telerobotics staves off autonomous robots for a decade. In the 4 years since the article was written, AI research has shown remarkable capabilities that might well increase the viability of this aspect of the project. It will certainly need to be ready once the robots leave the Moon to start extracting resources in the asteroid belt and beyond.

The vision of machines doing the work is probably comfortable. It is the fast exponential growth that is perhaps new. From a small factory launched from Earth, we end up with robots exploiting resources that dwarf the current human economy within a lifetime of the reader.

The logic of the model implies something the authors do not explore. Large human populations in space to use the industrial output of the robots in situ will need to be launched from Earth initially. This will remain expensive unless we are envisaging the birthing of humans in space, much as conceived for some approaches to colonizing the stars. Alternatively an emigrant population will need to be highly reproductive to fill the cities the robots have built. How long will that take? Probably far longer, centuries, rather than the decades of robotic expansion.

Another issue is that the authors envisage the robots migrating to the stars and continuing their industrialization there. Will humans have the technology to follow, and if so, will they continue to fall behind the rate at which robots expand? Will the local star systems be full of machines, industriously creating manufactures with only themselves to use them? And what of the development of AI towards AGI, or Artificial General Intelligence? Will that mean that our robots become the inevitable dominant form of agency in the galaxy?

The paper is Metzger, Muscatello, Mueller & Mantovani, “Affordable, Rapid Bootstrapping of the Space Industry and Solar System Civilization,” Journal of Aerospace Engineering Volume 26 Issue 1 (January 2013). Abstract / Preprint.

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A Possible Planet Hidden in the Data

by Paul Gilster on January 19, 2017

One of the great joys of science is taking something that seems beyond reach and figuring out a way to do it. We can use a coronagraph, for example, to screen out much of the light of a star to see planets around it, but coronagraphs can only do so much, as planets too near the star are still hidden from view. Now scientists have used an unusual observation to deduce information about one such hidden planet and its interactions with a circumstellar disk.

Announced at the recent meeting of the American Astronomical Society, the work involves 18 years of archival observations with the Hubble Space Telescope, which have yielded an intriguing shadow sweeping across the disk of the TW Hydrae system. We’re evidently looking at a young planetary system in formation, as the star — slightly less massive than the Sun and about 192 light years away in the constellation Hydra — is only about 8 million years old. Helpfully for our work, the TW Hydrae disk is seen face-on from our perspective.

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Image: These images, taken a year apart by NASA’s Hubble Space Telescope, reveal a shadow moving counterclockwise around a gas-and-dust disk encircling the young star TW Hydrae. The two images at the top, taken by the Space Telescope Imaging Spectrograph, show an uneven brightness across the disk. Through enhanced image processing (images at bottom), the darkening becomes even more apparent. These enhanced images allowed astronomers to determine the reason for the changes in brightness. The dimmer areas of the disk, at top left, are caused by a shadow spreading across the outer disk. The dotted lines approximate the shadow’s coverage. The long arrows show how far the shadow has moved in a year (from 2015-2016), which is roughly 20 degrees. Credit: NASA, ESA, and J. Debes (STScI).

We have no other circumstellar disk with an archival dataset this rich, allowing the effect to be studied in depth. John Debes (Space Telescope Science Institute, Baltimore) was able to put observations from Hubble’s Space Telescope Imaging Spectrograph (STIS) together with images from different observing runs, some of which included the Hubble Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

The STIS observations use the instrument’s coronagraph to look close to the star, but the coronagraph still can’t reveal an image of the planet itself. The shadow, however, is another matter. It has swept around the disk counter-clockwise until returning in 2016 to the same position it was in in 2000. The disk’s slow rotation ruled out a feature that was itself a part of the disk, implicating a shadow caused by a tilt to the inner disk relative to the outer. Observations from the Atacama Large Millimeter Array (ALMA) at submillimeter wavelengths backed the idea.

Debes believes a hitherto unseen planet is the most likely cause of the twisted inner disk, pulling material out of the disk plane to block light from the star, thus producing the shadow sweeping across the outer disk. If this is the case, we are talking about a planet roughly 160 million kilometers from the star, too close to observe even with STIS, which can penetrate as close to the star as the orbit of Saturn (roughly 1.5 billion kilometers).

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Image: This diagram reveals the proposed structure of a gas-and-dust disk surrounding the nearby, young star TW Hydrae. The illustration shows an inner disk that is tilted due to the gravitational influence of an unseen companion, which is orbiting just outside the disk. Credit: NASA, ESA, and A. Feild (STScI).

To produce the effects Hubble has noted, the planet would need to be about the size of Jupiter. Recent work on TW Hydrae with ALMA has confirmed a gap in the disk about 14.5 million kilometers from the star, perhaps the signature of another planet clearing the inner disk. Thus we are working with a phenomenon that allows us to study early planet formation in an inner system that is otherwise inaccessible, simply by measuring these broad effects.

“What is surprising is that we can learn something about an unseen part of the disk by studying the disk’s outer region and by measuring the motion, location, and behavior of a shadow,” Debes said. “This study shows us that even these large disks, whose inner regions are unobservable, are still dynamic, or changing in detectable ways which we didn’t imagine.”

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A New Context for Complex Life

by Paul Gilster on January 18, 2017

We normally think of the appearance of oxygen on Earth in terms of a ‘great oxygenation event,’ sometimes referred to as the ‘oxygen catastrophe’ or ‘great oxidation.’ Here oxygen begins to emerge in the atmosphere about 2.3 billion years ago as oceanic cyanobacteria produce oxygen by photosynthesis. The actual oxygenation event would be the point when oxygen is not all chemically captured but becomes free to escape into the atmosphere.

It’s a straightforward picture — we move from a lack of oxygen to gradual production through photosynthesis and then a concentration strong enough to destroy many anaerobic organisms, an early and huge extinction event as life on our planet adjusted to the new balance. But a team of researchers led by Michael Kipp (University of Washington) has produced a paper showing a much more complicated emergence of oxygen, one that produced a surge in oxygenation that lasted a quarter of a billion years before easing.

Kipp and team studied oxygen in the Earth’s atmosphere between 2 and 2.4 billion years ago. Their work focuses on the element selenium and its isotopic ratios in sedimentary shale, using mass spectrometry techniques at the University of Washington Isotope Geochemistry Lab. The question: How have isotopic ratios been changed by the presence of oxygen? The reduction of oxidized selenium compounds causes a shift in these ratios which can be measured, and the abundance of selenium itself increases as oxygen levels climb.

What the team found is that oxygen levels were higher far earlier than we’ve believed. Indeed, these levels may have supported complex life, at least for a time. For instead of a gradual and continuing rise, these levels then drop. Roger Buick (UW Astrobiology Program) explains:

“There is fossil evidence of complex cells that go back maybe 1 ¾ billion years. But the oldest fossil is not necessarily the oldest one that ever lived – because the chances of getting preserved as a fossil are pretty low. This research shows that there was enough oxygen in the environment to have allowed complex cells to have evolved, and to have become ecologically important, before there was fossil evidence. That doesn’t mean that they did — but they could have.”

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Image: This is a 1.9-billion-year-old stromatolite — or mound made by microbes that lived in shallow water — called the Gunflint Formation in northern Minnesota. The environment of the oxygen “overshoot” described in research by Michael Kipp, Eva Stüeken and Roger Buick may have included this sort of oxygen-rich setting that is suitable for complex life. Credit: Eva Stüeken.

Thus shallow coastal waters may have held the oxygen needed for complex life hundreds of millions of years earlier than thought. The researchers call this event an ‘oxygen overshoot,’ a significant increase in atmospheric oxygen and in the surface ocean, but one that did not affect the deep ocean. Oxygen levels would have risen for a quarter of a billion years before sinking back. That makes the so-called ‘great oxygenation event’ a more complex process than we realized, with a sharp peak in oxygen before a drop to a lower, more stable level.

About this complex process there remain plenty of questions. What caused the elevation of oxygen levels in the first place, and what precipitated its decline? The researchers have no answer, but can only point to a selenium isotope record that clearly sets this period apart. The selenium technique is a potent way to analyze our own planet’s past, but it also reminds us of the need to be cautious in evaluating exoplanet habitability. Says lead author Kipp:

“The recognition of an interval in Earth’s distant past that may have had near-modern oxygen levels, but far different biological inhabitants, could mean that the remote detection of an oxygen-rich world is not necessarily proof of a complex biosphere.”

The paper is Kipp et al., “Selenium isotopes record extensive marine suboxia during the Great Oxidation Event,” published online by Proceedings of the National Academy of Sciences 18 January 2017 (abstract). This UW news release is also helpful.

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Galactic Interaction: Rivers of Stars

by Paul Gilster on January 16, 2017

Discovered as recently as 1994, the Sagittarius dwarf spheroidal galaxy is a satellite of the Milky Way, and one with an interesting history. One of the nearest of the dwarf galaxies, the Sagittarius dwarf lies 25 kiloparsecs (roughly 82,000 light years) from the center of the Milky Way, and has passed through the disk of the parent galaxy more than once. The result: We see what a new paper on this object calls a ‘stream of tidally stripped stars’ that wraps completely around the celestial sphere. Our own Sun, in fact, is close enough to the Sgr galaxy’s orbital plane that it lies within the width of what can be called the debris tail.

What astronomers would like to do is to reconstruct the orbital history of this interesting dwarf galaxy, something Marion Dierickx (Harvard-Smithsonian Center for Astrophysics), working with her PhD advisor Avi Loeb (Harvard) have now managed through computer simulations. Dierickx and Loeb simulated the movements of the Sgr dwarf for the past 8 billion years, varying factors like the initial velocity and angle of approach to the Milky Way. As expected, these variables have a major effect on the orbit and the resulting stellar stream. Each passage around the Milky Way has pulled the Sgr dwarf apart, costing it component stars.

Matching their models with current observations, the researchers have learned that over time, the dwarf galaxy, which begins the simulations with 10 billion times the mass of the Sun (about one percent of the mass of the Milky Way), loses a third of its stars and nine-tenths of its dark matter. The stripping of this material from the Sgr dwarf produced three streams of stars reaching as far as one million light years from the center of the Milky Way. These streams have become one of the largest structures observable on the sky.

From the paper:

The simulation presented here includes stellar overdensities at distances of up to ∼ 250 − 300 kpc, extending beyond the MW virial radius [the radius within which the density perturbation that will become a galaxy is collapsing]. We provide their predicted positions on the sky, heliocentric distances and line of sight velocities for possible future observational searches. The most distant known stars of the MW coincide with our predicted streams in both position and small radial velocities. If verified observationally, the distant branches of the Sgr stream would be the farthest-ranging stellar stream in the MW halo known to date.

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Image: In this computer-generated image, a red oval marks the disk of our Milky Way galaxy and a red dot shows the location of the Sagittarius dwarf galaxy. The yellow circles represent stars that have been ripped from the Sagittarius dwarf and flung far across space. Five of the 11 farthest known stars in our galaxy were probably stolen this way. Marion Dierickx / CfA.

The cosmological landscape that can emerge from work like this is striking. As the paper notes, we should be able to use the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) datasets to drill deeper into the location of these streams, and future data from the Large Synoptic Survey Telescope (LSST) will offer mapping of the outer galactic halo at visible wavelengths. The Wide Field Infrared Survey Telescope (WFIRST) will produce useful new data in the infrared. With all of these tools, we can probe the outer edges of the Milky Way, in conjunction with existing missions like Gaia. As the paper notes:

The Gaia mission will accurately map a large volume of the MW at smaller Galactocentric radii. With a complete picture of the MW mass distribution from the solar neighborhood to the outskirts of the halo, we will be able to place our Galaxy and the Local Group in a cosmological context.

Note that the 11 farthest known stars in the Milky Way are about 300,000 light years out, well beyond the galaxy’s spiral disk. About half of these are evidently stars captured from the Sagittarius dwarf galaxy, while the rest may have been pulled from a different dwarf galaxy. Says Dierickx, “The star streams that have been mapped so far are like creeks compared to the giant river of stars we predict will be observed eventually.”

The paper is Dierickx & Loeb, “Predicted Extension of the Sagittarius Stream to the Milky Way Virial Radius,” accepted at the Astrophysical Journal (preprint).

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Inconstant Moons: A New Lunar Origin Scenario

by Paul Gilster on January 13, 2017

A recent snowfall followed by warming temperatures produced a foggy night recently, one in which I was out for my usual walk and noticed a beautiful Moon trying to break through the fog layers. The scene was silvery, almost surreal, the kind of thing my wife would write a poem about. For my part, I was thinking about the effect of the Moon on life, and the theory that a large single moon might have an effect on our planet’s habitability. Perhaps its presence helps to keep Earth’s obliquity within tolerable grounds, allowing for a more stable climate.

But that assumes we’ve had a single moon all along, or at least since the ‘big whack’ the Earth sustained from a Mars-sized protoplanet that may have caused the Moon’s formation. Is it possible the Earth has had more than one moon in its past? It’s an intriguing question, as witness a new paper in Nature Geoscience from researchers at the Technion-Israel Institute of Technology and the Weizmann Institute of Science. The paper suggests the Moon we see today is the last of a series of moons that once orbited the Earth.

“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth,” says co-author Assistant Prof. Perets (Technion). “It’s likely that such moonlets were later ejected, or collided with the Earth or with each other to form bigger moons.”

To explore alternatives to giant impact theories, the researchers have produced simulations of early Earth impacts, varying the values for the impactor’s velocity, mass, angle of impact and the initial rotation of the target. The process that emerges involves multiple impacts that would produce small moons, whose gravitational interactions would eventually cause collisions and mergers, to produce the Moon we see today. Here’s how the paper describes the process:

… we consider a multi-impact hypothesis for the Moon’s formation. In this scenario, the proto-Earth experiences a sequence of collisions by medium- to large-size bodies (0.01–0.1M). Small satellites form from the impact-generated disks and migrate outward controlled by tidal interactions, faster at first, and slower as the body retreats away from the proto-Earth. The slowing migration causes the satellites to enter their mutual Hill radii and eventually coalesce to form the final Moon. In this fashion, the Moon forms as a consequence of a variety of multiple impacts in contrast to a more precisely tuned single impact.

Here’s a graphic from the paper (listed as Figure 1) that shows the process at work:

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Image (click to enlarge): a,b, Moon- to Mars-sized bodies impact the proto-Earth (a) forming a debris disk (b). c, Due to tidal interaction, accreted moonlets migrate outward. d,e, Moonlets reach distant orbits before the next collision (d) and the subsequent debris disk generation (e). As the moonlet–proto-Earth distance grows, the tidal acceleration slows and moonlets enter their mutual Hill radii. f, The moonlet interactions can eventually lead to moonlet loss or merger. The timescale between these stages is estimated from previous works.

The Hill radius mentioned above describes the gravitational sphere of influence of an object; in this case, meshing Hill radii can produce interactions that sometimes lead to mergers. The paper notes that in head-on impacts, the rotation of the planet is important because the disk needs angular momentum resulting from the rotation to stay stable. With increased rates of rotation, the angular momentum of the disks increases. Moons like ours emerge from many of the simulations:

We find that debris disks resulting from medium- to large-size impactors (0.01–0.1M) have sufficient angular momentum and mass to accrete a sub-lunar-size moonlet. We performed 1,000 Monte Carlo simulations of sequences of N = 10, 20 and 30 impacts each, to estimate the ability of multiple impacts to produce a Moon-like satellite. The impact parameters were drawn from distributions previously found in terrestrial formation dynamical studies. With perfect accretionary mergers, approximately half the simulations result in a moon mass that grows to its present value after ~20 impacts.

If the multi-moon hypothesis proves credible, how would it affect the larger astrobiology question? In Ward and Brownlee’s Rare Earth (Copernicus, 2000), after a discussion of obliquity and the Moon’s effect on the Earth’s early history, the authors say this:

If the Earth’s formation could be replayed 100 times, how many times would it have such a large moon? If the great impactor had resulted in a retrograde orbit, it would have decayed. It has been suggested that this may have happened for Venus and may explain that planet’s slow rotation and lack of any moon. If the great impact had occurred at a later stage in Earth’s formation, the higher mass and gravity of the planet would not have allowed enough mass to be ejected to form a large moon. If the impact had occurred earlier, much of the debris would have been lost to space, and the resulting moon would have been too small to stabilize the obliquity of Earth’s spin axis. If the giant impact had not occurred at all, the Earth might have retained a much higher inventory of water, carbon and nitrogen, perhaps leading to a Runaway Greenhouse atmosphere.

The idea of a series of impacts eventually leading to a larger moon significantly muddies the waters here. It is true that in our Solar System, the inner planets are nearly devoid of moons, but we have no way of extending this situation to exoplanets without collecting the necessary data, which will begin with our first exomoon detections. Certainly if numerous collisions in an early planetary system can produce a large moon, as this paper argues, then we can expect similar collisional scenarios in many systems, making such moons a frequent outcome.

The paper is Rufu, Oharonson & Perets, “A Multiple Impact Hypothesis for Moon Formation,” published online by Nature Geoscience 9 January 2017 (abstract).

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A New Look at ‘Exocomets’

by Paul Gilster on January 12, 2017

Moving groups are collections of stars that share a common origin, useful to us because we can study a group of stars that are all close to each other in age. Among these, the Beta Pictoris moving group is turning out to be quite productive for the study of planet formation. These are young stars, aged in the tens of millions of years (Beta Pictoris itself is between 20 and 26 million years old). Within the moving group, we’ve detected planets around 51 Eridani and Beta Pictoris, while infalling, star-grazing objects have been found around Beta Pictoris.

Evidence of comet activity around another of these stars was discussed at the American Astronomical Society meeting in Texas. The star HD 172555, 23 million years old and about 95 light years from Earth, shows the presence of the vaporized remnants of cometary nuclei, marking the third extrasolar system where such activity has been traced. All the stars involved are under 40 million years old, giving us a glimpse of the kind of activity that happens during the era when young terrestrial planets have begun to emerge in their systems.

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Image: This illustration shows several comets speeding across a vast protoplanetary disk of gas and dust and heading straight for the youthful, central star. The comets will eventually plunge into the star and vaporize. The comets are too small to photograph, but their gaseous spectral “fingerprints” on the star’s light were detected by NASA’s Hubble Space Telescope. The gravitational influence of a suspected Jupiter-sized planet in the foreground may have catapulted the comets into the star. This star, called HD 172555, represents the third extrasolar system where astronomers have detected doomed, wayward comets. The star resides 95 light-years from Earth. Credit: NASA, ESA, and A. Feild and G. Bacon (STScI).

Carol Grady (Eureka Scientific/NASA GSFC) led the study reported on at the AAS. Her thoughts:

“Seeing these sun-grazing comets in our solar system and in three extrasolar systems means that this activity may be common in young star systems. This activity at its peak represents a star’s active teenage years. Watching these events gives us insight into what probably went on in the early days of our solar system, when comets were pelting the inner solar system bodies, including Earth. In fact, these star-grazing comets may make life possible, because they carry water and other life-forming elements, such as carbon, to terrestrial planets.”

The deflection of comets by the gravitational influence of a massive gas giant in an emerging planetary system is a vivid picture, one clarified by Grady and team’s work with the Hubble Space Telescope Imaging Spectrograph (STIS) and the Cosmic Origins Spectrograph (COS) in 2015. The team’s spectrographic analysis, using Hubble data collected from two observing runs separated by six days, detected carbon gas and silicon in the light of HD 172555 moving across the face of the star at a speed of 160 kilometers per second.

This work follows up a French study that first found exocomets transiting the same star in archival data from the HARPS spectrograph. That work detected signs of calcium. Grady and team have extended the analysis with a spectrographic analysis in ultraviolet light. They believe they are seeing gaseous debris left behind as comets disintegrated, vaporized materials that contain large chunks of the original comet. Helpfully, the disk around HD 172555 is seen almost edge-on from Earth, offering Hubble a clear view of the highly dispersed activity.

“As transiting features go, this vaporized material is easy to see because it contains very large structures,” Grady said. “This is in marked contrast to trying to find a small, transiting exoplanet, where you’re looking for tiny dips in the star’s light.”

To confirm that they are seeing the disintegration of icy comets as opposed to rocky asteroids, Grady’s researchers hope to use the STIS again to search for oxygen and hydrogen, a composition that would add further weight to these conclusions.

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