Centauri Dreams
Imagining and Planning Interstellar Exploration
Looking Ahead
Centauri Dreams posts will unfortunately be sporadic over the next couple of weeks as I attend to some unrelated matters. But I do have several excellent upcoming articles already in the pipeline, including Al Jackson on Apollo 8 at the end of this week. Al, you’ll recall, was involved in Apollo as astronaut trainer on the Lunar Module Simulator, so his thoughts on the program’s extraordinary successes are always a high point.
Image credit: Manchu.
Ashley Baldwin, who knows the ins and outs of space-based astronomy better than anyone I know, will be looking at the key issues involved, with specific reference not only to WFIRST and HabEX but also a mission called EXCEDE, not currently approved but very likely the progenitor of something like it to come.
In early January, Jim Benford will be talking about beamed propulsion in a two-part article that looks to resolve key particle beam issues, with methods worked out by himself and the ingenious Alan Mole. There are all kinds of advantages to particle beaming but doing it without serious beam divergence is a problem we’ve addressed before. A possible solution emerges.
And, of course, we do have Ultima Thule coming up for New Horizons, on New Year’s Eve, no less. Data return including imagery will take some time, so we’ll be talking about the results throughout January. Emily Lakdawalla’s breakdown of the likely schedule gives an overview of the process.
Let me wish you all the best for the holidays. Here’s hoping for spectacular success for New Horizons along the way. Champagne and a working mission in the Kuiper Belt. What a night!
Climate Change and Mass Extinctions: Implications for Exoplanet Life
The right kind of atmosphere may keep a planet habitable even if it crowds the inner region of the habitable zone. But atmospheric evolution involves many things, including the kind of geological activity our own planet has experienced, leading to sudden, deep extinctions. Centauri Dreams regular Alex Tolley today takes a look at a new paper that examines the terrestrial extinction of marine species in the Permian event some 252 million years ago. As we examine exoplanet habitability, it will be good to keep the factors driving such extinctions in mind. Tolley is a lecturer in biology at the University of California and author, with Brian McConnell, of A Design for a Reusable Water-Based Spacecraft Known as the Spacecoach (Springer, 2016). A key question in his essay today: Is our definition of the habitable zone simply too broad?
by Alex Tolley
In the search for life on exoplanets, questions about whether the planet is within the HZ given a plausible atmosphere is based on timescales as a fraction of stellar lifetime on the main sequence. With water may come the emergence of life as we know it, and then the long, slow evolution to multicellular life and possible technological civilization. Planets may initially form too close to a pre-main sequence star to be in the HZ, then enter the HZ, only to leave it again as the star increases in luminosity with age. Earth has experienced about a 30% increase in solar luminosity over its lifetime. The CO2 level needed to maintain a constant surface temperature via the greenhouse effect has had to decline to offset the increased insolation. In 1 to 2 billion years, the further increase in solar luminosity will require the CO2 levels to decline below that needed for photosynthesis, or the Earth’s surface will heat up beyond that sustainable for life.
Yet when considering the environment on a world in the HZ, we should be cognizant that climatic instability may create shocks in the short term that have major impacts on life. Earth has experienced 5 major extinctions based on our reading of the fossil record. The most famous being the dinosaur-killing KT event that ended the Cretaceous and allowed mammals to evolve into the newly vacated ecological niches. However, the largest extinction is known as the Permian extinction, or “Great Dying” when over 95% of marine species became extinct about 252 mya. Unlike the KT event, which was a cosmic throw of the dice, the Permian extinction is believed to be due to massive volcanism of the Siberian Traps that released vast quantities of CO2 into the atmosphere that increased its concentration at least several fold. This caused a rapid temperature rise of 10s of degrees Fahrenheit and was accompanied by ocean acidification.
A new paper by Julian Penn et al suggests that this global temperature change caused the extinction of marine species primarily by metabolic stress and hypoxia.
The core idea is that multicellular, aerobic organisms require critical oxygen pressures to live, with their lowest levels of metabolism during resting, and higher levels for activities, such as swimming or feeding. Sessile organisms may have just a 1.5x increase in active metabolic rate over resting, while energetic organisms like fish may be 5x or more. As temperatures rise, so does the metabolic rate. This, in turn, requires adequate oxygen for respiration. But as the temperatures rise, the dissolved oxygen levels fall, placing additional stress on the animals to maintain their respiration rate. Penn integrated climate models to compute the temperature change and dissolved oxygen partial pressures, with the estimated metabolic rates for the activity of various modern animals to represent Permian species, to determine how ocean habitat temperatures impact the metabolisms of marine genera and probable extinction rates.
Figure 1 shows the relation between metabolic rate and temperature, and the temperature increased metabolic index of ocean habitat by latitude and depth. The polar latitudes and shallower depths show the highest changes in the metabolic index, indicating the most stressed habitats.
Figure 1. Physiological and ecological traits of the Metabolic Index (F) and its end-Permian distribution. (A) The critical O2 pressure (pO2 crit) needed to sustain resting metabolic rates in laboratory experiments (red circles, Cancer irroratus) vary with temperature with a slope proportional to Eo from a value of 1/Ao at a reference temperature (Tref), as estimated by linear regression when F = 1 (19). Energetic demands for ecological activity increase hypoxic thresholds by a factor Fcrit above the resting state, a value estimated from the Metabolic Index at a species’ observed habitat range limit. (B) Zonal mean distribution of F in the Permian simulation for ecophysiotypes with average 1/Ao and Eo (~4.5 kPa and 0.4 eV, respectively). (C and D) Variations in F for an ecophysiotype with weak (C) and strong (D) temperature sensitivities (Eo = 0 eV and 1.0 eV, respectively), both with 1/Ao ~ 4.5 kPa. Example values of Fcrit (black lines) outline different distributions of available aerobic habitat for a given combination of 1/Ao and Eo. Credit: Justin Penn and Curtis Deutsch, University of Washington.
Figure 2 shows the spatial changes in ocean temperature and oxygen concentrations. Oceanic temperatures rise, particularly towards the poles, and with it a reduction in dissolved oxygen. As expected the greatest rises in temperature are at the shallower depths, particularly with the highly productive continental shelves. Oxygen level declines are most widely seen at all depths at the poles, but far less so in the tropics.
Figure 2. Permian/Triassic ocean temperature and O2. (A) Map of near surface (0 to 70 m) ocean warming across the Permian/Triassic (P/Tr) transition simulated in the Community Earth System Model. The region in gray represents the supercontinent Pangaea. (B) Simulated near surface ocean temperatures (red circles) in the eastern Paleo-Tethys (5°S to 20°N) and reconstructed from conodont d18Oapatite measurements (black circles) (4). The time scale of the d18Oapatite data (circles) has been shifted by 700,000 years to align it with d18Oapatite calibrated by U-Pb zircon dates (open triangles) (1), which also define the extinction interval (gray band). Error bars are 1°C. (C) Simulated zonal mean ocean warming (°C) across the P/Tr transition. (D) Map of seafloor oxygen levels in the Triassic simulation. Hatching indicates anoxic regions (O2 < 5 mmol/m3). (E) Simulated seafloor anoxic fraction ƒanox (red circles). Simulated values are used to drive a published one-box ocean model of the ocean’s uranium cycle (8) and are compared to d238U isotope measurements of marine carbonates formed in the Paleo-Tethys (black circles). Error bars are 0.1‰. (F) Same as in (C) but for simulated changes in O2 concentrations (mmol/m3). Credit: Justin Penn and Curtis Deutsch, University of Washington.
The authors conclude:
The correspondence between the simulated and observed geographic patterns of selectivity strongly implicates aerobic habitat loss, driven by rapid warming, as a main proximate cause of the end-Permian extinction.
However, while the temperature is the proximate cause, the authors note that other factors are also involved.
“In our simulations, net primary productivity is reduced by ~40% globally, with strongest declines in the low latitudes, where essential nutrient supply to phytoplankton is most curtailed.”
Ocean acidification is also a potential factor, as we may be seeing today. Acidification will be higher at the poles, creating a habitat barrier for species that require more calcification.
Figure 3 is a schematic of the model, fitting the probable extinction rates to the fossil record. Their model predicts a latitudinal impact of warming that is also suggested by the fossil record. Their explanation for this spatial pattern is that tropical organisms are preadapted to warmer temperatures and lower O2 levels. As the oceans warm, these organisms migrate polewards to cooler waters. However, polar species have nowhere to migrate to, and therefore have a higher rate of extinction.
Figure 3. An illustration depicting the percentage of marine animals that went extinct at the end of the Permian era by latitude, from the model (black line) and from the fossil record (blue dots). The color of the water shows the temperature change, with red representing the most severe warming and yellow less warming. At the top is the supercontinent Pangaea, with massive volcanic eruptions emitting carbon dioxide. The images below the line represent some of the 96 percent of marine species that died during the event. Credit: Justin Penn and Curtis Deutsch, University of Washington.
As our current analog of the Permian climate change impacts the oceans, we are already seeing warm water species appearing in the cold North Atlantic, far north of their historic ranges. We can also expect species like the Arctic ice fish that has no red blood cells due to the high O2 concentrations in polar waters to become extinct as polar waters continue to warm.
What about the extinction of terrestrial life? 70% of terrestrial faunal species went extinct. The attractiveness of this theory is that it also applies to terrestrial life, although the oxygen depletion was not a factor. What is clear as well is that the CO2 increase heated the planet, overwhelming any cooling from dust blown up into the atmosphere, as experienced with the 2 year global cooling after Mt. Pinatubo erupted.
Had the Earth been closer to our sun, or temperatures risen further due to greater volcanic activity, the extinctions might conceivably have been 100% for all multicellular genera. Earth life might have been pushed back to primarily archaea and bacteria. The atmosphere might have reverted back to its Archaean state. If photosynthesizers were still present, how long would it take for aerobic multicellular life to evolve again?
The major extinctions have implications for life on exoplanets. Worlds closer to the inner edge of the HZ may be supportive of life if the atmosphere stays stable. However, as we have seen with the example of the Permian extinction, geologic processes can upset that balance, potentially making a world uninhabitable for a period, forcing any life to be restricted to simpler forms. How frequently could such events cause mass, even total extinctions, on other worlds, despite long-term conditions being favorable for life? It is perhaps worth considering whether the inner edge HZ limits should be made more conservative to allow for such events.
The paper is Penn et al., “Temperature-dependent hypoxia explains biogeography and severity of end-Permian marine mass extinction” Science Vol. 362, Issue 6419 (7 December 2018). Abstract (Full Text behind paywall).
Early Returns from Bennu
The science return from OSIRIS-REx has been surprisingly swift as the spacecraft returns data on near-Earth asteroid 101955 Bennu. We’re aided here by the timing, as early results are being discussed at the ongoing conference of the American Geophysical Union (AGU) in Washington, DC. The imagery we’ve received of Bennu’s surface has scientists buzzing. Thus Humberto Campins (University of Central Florida) a member of the OSIRIS-REx Science Team, who notes the comparison between what we see now and the Arecibo radar imagery in the late 1990s:
“The images are spectacular and spot on, what we expected thanks to predictions made with the instrumentation at the Arecibo Observatory in the late 90s and early 2000s. We will spend a year and a half mapping Bennu and have to wait until mid 2020 [when] we collect the sample, but it is pretty amazing to actually see it now. Christmas came early.”
The Arecibo work began shortly after the asteroid’s discovery in 1999, when both the Puerto Rico observatory and the Goldstone planetary radar system were used to examine Bennu. A second Arecibo investigation led to a shape model and pole orientation study published in 2013 by the OSIRIS-REx science team chief, Michael Nolan (University of Arizona). Arecibo’s radar data also firmed up Bennu’s size and rotation period, while even detecting a boulder on the asteroid’s surface. The 2013 model predicted Bennu’s shape, rotation rate, inclination and diameter, all of which have been confirmed by the OSIRIS-REx OCAMS camera suite.
“Radar observations don’t give us any information about colors or brightness of the object, so it is really interesting to see the asteroid up close through the eyes of OSIRIS-REx,” Nolan said. “As we are getting more details, we are figuring out where the craters and boulders are, and we were very pleasantly surprised that virtually every little bump we saw in our radar image back then is actually really there.”
Image: This mosaic image of asteroid Bennu is composed of 12 PolyCam images collected on Dec. 2 by the OSIRIS-REx spacecraft from a range of 24 km (15 miles). Credit: NASA/Goddard/University of Arizona.
But science observations were, of course, occurring long before the arrival at Bennu. During the approach phase, which began in August, OSIRIS-REx turned its two spectrometers, the OSIRIS-REx Visible and Infrared Spectrometer (OVIRS) and the OSIRIS-REx Thermal Emission Spectrometer (OTES), on the target. We learn that the resulting data show the presence of oxygen and hydrogen atoms bonded together as hydroxyls, which researchers believe exist across the entire asteroid in water-bearing clay minerals.
To be sure, Bennu is too small an object to have had water on its surface at any point in its evolution, but at some point, its rocky components must have interacted with water. The implication is that liquid water was present on Bennu’s parent body, a much larger asteroid. What a find for the OSIRIS-REx team, given that the mission was designed to study the volatiles and organics found in the early Solar System through the lens of this asteroid. Rolling the dice on a sample mission here is paying off, as by 2023 we’ll have surface materials in a lab right here on Earth..
“This finding may provide an important link between what we think happened in space with asteroids like Bennu and what we see in the meteorites that scientists study in the lab,” said Ellen Howell, senior research scientist at the UA’s Lunar and Planetary Laboratory, or LPL, and a member of the mission’s spectral analysis group. “It is very exciting to see these hydrated minerals distributed across Bennu’s surface, because it suggests they are an intrinsic part of Bennu’s composition, not just sprinkled on its surface by an impactor.”
The number and variety of boulders on the surface is said to be unexpected, as is the size of the large boulder near the asteroid’s south pole. Observations from the ground pegged its height at 10 meters, but OSIRIS-REx is showing us through OCAMS imagery that it is more like 50 meters tall, with a width of approximately 55 meters. Given the scarcity of smooth surfaces, the search for a suitable sample site may be a complicated one.
With orbital insertion planned for December 31, we have an interesting conjunction for deep space aficionados, as New Horizons will be making its flyby of Kuiper Belt object Ultima Thule later that evening. After the OSIRIS-REx orbit is established, the spacecraft will remain in orbit until February, when it will begin another series of survey flybys. The upcoming orbit, by the way, is interesting in its own right. It will take the spacecraft between 1.4 and 2 kilometers from Bennu’s center, making this the tightest orbit of a space object by any spacecraft.
But before the orbit can be established, mission controllers are working on Bennu’s mass, a vital issue given its effects on the gravitational field of the object. The current preliminary survey passes within 7 kilometers of the north pole, equator and south pole, retrieving data that will also be useful in understanding the internal structure and composition of the asteroid. This mission has plenty of work ahead, but the early results could not be more interesting.
Voyager 2 Makes It Through
Voyager 2 has now gone interstellar, making it not only NASA’s single longest-running mission but one of only two spacecraft that have crossed over from the heliosphere to true interstellar space, what scientists call the Local Interstellar Medium (LISM). On that note, it’s interesting to put the Solar System in context. Depending on how you define the term, the Solar System takes in a great deal of interstellar space. Many astronomers put its outer edge at the outer Oort Cloud, perhaps 100,000 AU away, and both Voyagers have yet to reach the inner Oort.
At an estimated 1,000 AU, the inner boundary of the Oort Cloud is where the vast cometary cloud around our star becomes apparent, housing in its entirety trillions of comets and extending about 40 percent of the way to the Alpha Centauri stars. The Voyagers will keep going, of course, and will reach the inner Oort in perhaps 300 years, though without working instrumentation. The steady diminishment of power from the crafts’ radioisotope thermal generators (about 4 watts per year) means we have ten years or less to power instruments.
What a splendid run this has been, and we’re still performing good science.
Image: Artist’s concept of Voyager 2 with 9 facts listed around it. Credit: NASA.
Speaking of instruments, it is the robust health of one in particular that has made the Voyager 2 crossover so apparent. Both Voyagers carry a Plasma Science Experiment (PLS), but the one on Voyager 1 stopped working in 1980. But Voyager 2’s PLS told the tale: Measuring the plasma outflow from the Sun — commonly called the solar wind — Voyager 2 could chart the speed, density, temperature, pressure and flux of the plasma. The steep decline in the speed of the solar wind particles on November 5, and the fact that since that time, the PLS has seen no solar wind flow around Voyager 2, leaves little doubt the craft has departed the heliosphere.
“Working on Voyager makes me feel like an explorer, because everything we’re seeing is new,” said John Richardson, principal investigator for the PLS instrument and a principal research scientist at the Massachusetts Institute of Technology in Cambridge. “Even though Voyager 1 crossed the heliopause in 2012, it did so at a different place and a different time, and without the PLS data. So we’re still seeing things that no one has seen before.”
Cameras aboard the Voyagers were turned off long ago to conserve power, but beside the PLS, three other instruments continue to function: The low-energy charged particle instrument, the magnetometer, and the cosmic ray subsystem. All of these show data consistent with Voyager 2’s having crossed the heliopause. We can now compare results from both Voyagers as we investigate the interstellar medium, learning how the heliosphere itself interacts with the plasma flow JPL calls the ‘interstellar wind.’
Image: Animated gif showing the plasma data. Credit: NASA/JPL-Caltech.
Bear in mind we also have the Interstellar Boundary Explorer (IBEX) in operation, making observations of the boundary from within the heliosphere. Also in the cards is the Interstellar Mapping and Acceleration Probe (IMAP), which will operate at the L1 Lagrange point about 1.5 million kilometers from Earth to monitor solar wind interactions at the edge of the heliopause by collecting and analyzing particles that make it through the boundary from the LISM. The latter mission is scheduled for launch in 2024, and I’ll have more to say about it soon.
Long-haul missions to deep space demand payloads that can function for decades and perhaps centuries, a fact that has concerned mission designers contemplating component lifetimes in this harsh environment. It’s heartening to think of the two Voyagers, then, for both were built to last five years, enough to make their flybys of Jupiter and Saturn. Uploaded programming helped with the Uranus and Neptune flybys, the latter occurring 12 years after launch. Who would have thought that 41 years into the mission we would still be taking data?
We’re learning numerous lessons about spacecraft longevity by their example, and can contemplate future missions specifically built for interstellar medium exploration. The challenges of getting to, say, the 550 AU gravity lens of the Sun are immense, but if spacecraft built so long ago can leave the heliosphere, next-generation missions are well within our capability. What kind of interstellar precursor will follow the Voyagers and New Horizons out toward the Oort?
The When and Where of Asteroid 101955 Bennu
You wouldn’t think the Yarkovsky effect would have any real significance on a half-kilometer wide pile of rubble like the asteroid 101955 Bennu. With a currently estimated mass somewhere between 60 and 80 billion kilograms, Bennu seems unlikely to receive much of a nudge from differences in heat on the object’s surface. But the people who specialize in these things say otherwise. Sunlight warms one side of the asteroid while the other experiences the cold of space. Rotation keeps the dark side radiating heat, accounting for a tiny thrust.
We call it the Yarkovsky effect after Ivan Osipovich Yarkovsky, a Polish engineer who came up with it in 1901, though if we want to give credit across the board, we might refer to the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect. Here we honor, in addition to Yarkovsky, an American scientist, a Russian astronomer and a NASA aerospace engineer, all of whom played a role in our understanding of the phenomenon as it relates to asteroids.
Image: Ivan Osipovich Yarkovsky (1844-1902). Credit: Wikimedia Commons.
The YORP effect turns up in interesting places, such as the near-Earth asteroid 2000 PH5, whose rotation rate has been spun up about as fast as any asteroid known, an effect traced over a four-year period by a team led by Stephen Lowry at Queens University Belfast (citation below). When it comes to Bennu, where we now have OSIRIS-REx in active investigation, researchers have calculated that the effect has shifted its orbit about 284 meters per year toward the Sun since 1999. Remember that Bennu originally came our way from the main asteroid belt, a movement inward that was presumably assisted by the same YORP effect.
On a scale of billions of years, then, YORP can create serious movement within the Solar System. But one reason for having OSIRIS-REx on the case is that we need to learn more about how such effects work so we can make better predictions about the future position of asteroids. Will a given asteroid present problems, with a potential trajectory that could intersect with the Earth? The calculation is by no means easy. With YORP alone, so much depends on the nature of the object, and how it absorbs and releases heat. We’d better learn as much as we can about such objects, a need that plays a role in asteroid missions that also investigate the evolution of the Solar System and the ancient debris that circulates among the planets.
Image: This artist’s concept shows the Origins Spectral Interpretation Resource Identification Security – Regolith Explorer (OSIRIS-REx) spacecraft contacting the asteroid Bennu with the Touch-And-Go Sample Arm Mechanism or TAGSAM. The mission aims to return a sample of Bennu’s surface coating to Earth for study as well as return detailed information about the asteroid and its trajectory. Credit: NASA’s Goddard Space Flight Center.
If the YORP effect makes our orbital calculations problematic, so too do the gravitational forces imparted by the Sun, nearby planets and other asteroids. As this JPL news release points out, astronomers can predict the exact dates of the next four passes Bennu will make near our planet (defined here as within 7.5 million kilometers, or .05 AU). The years in question are 2054, 2060, 2080 and 2135. But things get increasingly tricky as we look further out. For each time Bennu comes near the Earth, our planet gives its trajectory another slight twitch.
If you’re trying to figure out where Bennu will be in coming decades, then, you have to take into account the increasingly fuzzy effects that occur with each pass by the Earth, so that by 2060, when another such passage is predicted, we can only say that the asteroid will pass the Earth at about twice the distance from Earth to the Moon. But it could pass any point in a 30 kilometer window of space. Keep magnifying these numbers with future orbits and you can see why firm predictions become so difficult.
By 2080, according to calculations performed by Steven Chesley at the Center for Near-Earth Object Studies (CNEOS) at JPL, the best window we can derive for Bennu’s passage is 14,000 kilometers wide. Switch ahead to 2135, a time when Bennu’s orbit is thought to take it closer than the Moon, and the flyby window reaches 160,000 kilometers. This is, by the way, a projection for the single near-Earth asteroid for which we have the best orbital assessment in our database.
We’ve been studying Bennu through optical, infrared and radio telescopes every six years since its discovery in 1999 to measure factors like shape, rotation rate and trajectory. Given all that, CNEOS can say that looking ahead over the next century, the asteroid has a 99.963 percent change of missing the Earth. That’s heartening, but it’s clear that tightening up our numbers will help. And we can do a lot by way of studying how the YORP effect nudges the asteroid.
“There are a lot of factors that might affect the predictability of Bennu’s trajectory in the future, but most of them are relatively small,” says William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado, and a participating scientist on the OSIRIS-REx mission. “The one that’s most sizeable is Yarkvovsky.”
Optical images from OSIRIS-REx will help determine Bennu’s precise location and its exact orbital path as of now, giving us a read on how its trajectory is changing with time. With the spacecraft tracking Bennu over a two-year period, the variance from the projected trajectory will help to determine the size of the YORP effect’s changes. We’ll also learn a great deal about the amount of solar heat radiating from the asteroid from what type of surfaces, which will help us refine the YORP numbers, a huge help in tightening the trajectories of other asteroids.
OSIRIS-REx should eventually be able to tell us how craters and boulders change photon scattering and momentum transfer. Says Chesley:
“We know surface roughness is going to affect the Yarkovsky effect; we have models. But the models are speculative. No one has been able to test them.”
Refining models through on the spot observation is a major reason for doing OSIRIS-REx. When the mission is over, the team believes our projections of Bennu’s orbit will be 60 times better than what we now have. If only Ivan Osipovich Yarkovsky could be here to see this.
The paper on 2005 PH5 is Lowry et al., “Direct Detection of the Asteroidal YORP Effect,” Science Vol. 316, Issue 5822 (13 April 2007), pp. 272-274 (abstract).
Exoplanet Possibilities in 12 Protoplanetary Disks
Almost all the exoplanets we know have been detected in evolved stellar systems, places where the protoplanetary disk has dissipated and the planets around the star can be observed. Seeing inside a disk in formation is tricky business, though prominent studies at stars like Beta Pictoris have told us much about the evolution of these disks as planets do begin to emerge. But just how common are disks with ring and gap structures? Do all such disks produce planets?
We’re beginning to learn more as instruments like the Atacama Large Millimeter Array (ALMA) continue to be used to examine infant systems. Many of these show disks that are uniform in appearance, lacking discernible features like rings or gaps. Others are brighter, marked by concentric rings with separations that imply planet formation. It’s natural enough that early efforts have been devoted to brighter disks with their suggestion of planetary activity.
Image: Until recently, protoplanetary disks were believed to be smooth, pancake-like objects. The results from this study show that some disks are more like doughnuts with holes, but even more often appear as a series of rings. The rings are likely carved by planets that are otherwise invisible to us. Credit: Feng Long.
A new effort led by Feng Long (Kavli Institute for Astronomy and Astrophysics at Peking University in Beijing, China) has now appeared that gives us a valuable statistical look at possible planets in protoplanetary disks. Where this study stands out is in its choice of targets. Rather than looking at disks based on the brightness suggestive of ring structures, the researchers chose 32 stars with disks of varying brightness to get a sense of general disk properties.
This is an important point, so let me quote the paper on it:
…the small number of systems observed at high-spatial resolution (? 0.1”) to date limits our knowledge about the origins of disk substructures. Moreover, the set of disks imaged at high resolution is biased to brighter disks, many with near/mid-IR signatures of dust evolution, and collected from different star-forming regions and thus environments. These biases frustrate attempts to determine the frequency of different types of substructures, how these substructures depend on properties of the star and disk, and any evolution of substructures with time.
So dim disks as well as bright ones are in the mix here. Feng Long’s survey of young stars targeted disks in the Taurus star-forming region, a vast cloud of dust and gas some 450 light years from Earth. 12 of the stars with protoplanetary disks showed clear indications of rings and gaps within them suggestive of planet formation. Out of the analysis we learn that super-Earths and Neptunes are probably the most common kind of planets forming in these disks, a finding that reinforces exoplanet statistics gathered from fully formed planetary systems.
Of the 12 stars with protoplanetary disks surveyed, only two show disks consistent with the development of a gas giant like Jupiter. All these possible planets emerge out of the team’s calculations applied to the ALMA data, but have not yet been observed. From the paper:
The presence of wider gaps at larger radii hints for planet-disk interaction. The low intensity contrast in most ring and gap pairs suggests the possible link to low mass planets. We follow the diagnostic used in planet-disk interaction simulations (the separation of ring and gap normalized to gap location) to infer planet mass, and find that super-Earths and Neptunes are good candidates if disk turbulence is low (? = 10?4), in line with the most common type of planets discovered so far.
Image: The Taurus Molecular Cloud, pictured here by ESA’s Herschel Space Observatory, is a star-forming region about 450 light-years away. The image frame covers roughly 14 by 16 light-years and shows the glow of cosmic dust in the interstellar material that pervades the cloud, revealing an intricate pattern of filaments dotted with a few compact, bright cores — the seeds of future stars. Credit: ESA/Herschel/PACS, SPIRE/Gould Belt survey Key Programme/Palmeirim et al. 2013).
Key to this study is the need to determine what the rings and gaps in some young systems really indicate. Are they planets in formation, or are they structures formed through other mechanisms? An alternative explanation in the literature involves variations in the chemistry across the disk depending on distance from the star. These presumed variations in pressure are called ‘ice lines.’ They are condensation fronts that occur when disk temperatures drop at larger distances from the priimary, causing various volatiles to freeze out onto dust grains.
However, the new study finds no correlations between stellar properties and gap or ring structures in the surrounding disks, noting no concentration of gap radii around major ice line locations. The conclusion: The rings and gaps do indeed flag nascent planets as the most likely cause of their formation, although other processes may also contribute to the result. The researchers will now adjust the spacing of the ALMA antennae to increase the array’s resolution, while probing at other frequencies sensitive to different sizes of dust grains.
The paper is Long et al., “Gaps and Rings in an ALMA Survey of Disks in the Taurus Star-forming Region,” Astrophysical Journal Vol. 869, No. 1 (6 December 2018). Abstract / Preprint.
Follow with RSS or E-Mail
