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  • richardmitnick 11:43 am on November 4, 2021 Permalink | Reply
    Tags: "Researchers Revise Recipe for Building a Rocky Planet Like Earth", , , , Protoplanetary disks, , Seeing infant disks of dust and gas surrounding young stars.   

    From Quanta Magazine (US) : “Researchers Revise Recipe for Building a Rocky Planet Like Earth” 

    From Quanta Magazine (US)

    November 3, 2021
    Jonathan O’Callaghan


    Pebble accretion may explain where Earth and its water came from.Credit: Ana Kova/Quanta Magazine

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    Credit: Ana Kova/Quanta Magazine

    Bob O’Dell wasn’t quite sure what he was looking at. It was 1992, and he had just got his hands on new images from the Hubble Space Telescope that zoomed in on young stars in the Orion Nebula. O’Dell had been hoping to study the nebula itself, an interesting region of star formation relatively close to Earth. Yet something else caught his attention. Several of the stars didn’t look like stars at all, but were instead enveloped by a dim shroud. They seemed to form a “silhouette against the nebula,” said O’Dell.

    At first O’Dell and his colleagues thought they might be seeing an image artifact resulting from Hubble’s warped primary mirror, which had been molded ever so slightly into the wrong shape and would be fixed by a space shuttle mission in 1993. “We really wondered if this was a residual effect of the flawed primary mirror,” said O’Dell, who had been Hubble’s project scientist. Soon, however, they saw more and more of the phenomena in the images, even after the mirror was fixed, and realized it wasn’t a flaw at all. They were actually seeing infant disks of dust and gas surrounding young stars. They were, for the first time, witnessing the birth of planets.

    O’Dell’s discovery of protoplanetary disks sparked a transformation in our understanding of planet formation. In the following decades, astronomers would realize that our classical idea of how planets form — small rocks clump into bigger rocks, which then clump further — might not be correct. For the gas giants, such as Jupiter and Saturn, a model called pebble accretion, where a dominant object gobbles up smaller rocks — would come to replace the old views of how such monstrous worlds come to be.

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    These Hubble Space Telescope images provided the first direct evidence of protoplanetary disks around distant stars. C.R. O’Dell (Rice University (US), and The National Aeronautics and Space Agency (US).

    The rocky worlds of the inner solar system are trickier. Planetary scientists have intensely debated whether pebble accretion can explain how Earth and its neighbors arose, or whether the older view is still most likely. The clash has played out over the past few years in journal articles [Science Advances] and even, more recently, in a castle in the Bavarian Alps.

    The debate doesn’t only affect great mysteries such as the origin of Earth — and its water. The answer will also help reveal just how prevalent Earth-like worlds are across the universe. Are such worlds a cosmic fluke, merely a combination of chance events that make the prospects of life elsewhere in the universe slim? Or are habitable planets a certainty in solar systems with just the right ingredients, making us but one of many?

    “It’s part of the human experience to ask how the world around us formed,” said Konstantin Batygin, a planetary scientist at The California Institute of Technology (US). If it formed from pebbles, it will have huge consequences for how many more worlds like ours are out there.

    Invasion of the Pebbles

    In 2012, Anders Johansen and Michiel Lambrechts, astronomers at Lund University [Lunds universitet] (SE), made a bold prediction [Astronomy & Astrophysics]. For much of the preceding few decades, astronomers had believed that planets such as Earth and Jupiter grew from the gradual accumulation of asteroid-like objects, planetesimals, that collided with each other in young solar systems. This process, known as planetesimal accretion, would be slow — perhaps taking up to 100 million years to form a planet. But it made sense. We could see lots of asteroids in our solar system, and it seemed reasonable to assume there were many more when it formed 4.5 billion years ago, enough to form all the worlds we see today.

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    The ALMA telescope array can create exceptionally high resolution images, allowing researchers to examine planet-forming disks around other stars. Credit: Sergio Otarola (The European Southern Observatory [Observatoire européen austral][Europäische Südsternwarte](EU)(CL)/The National Astronomical Observatory of Japan [国立天文台](JP)/The National Radio Astronomy Observatory (US))

    But there were problems. No one was quite sure how planetesimals themselves formed — how they made the jump from tiny dust grains to city-size rocks, a problem known as the meter-size barrier. The presence of liquid water on Earth was confusing, as it relied on the chance arrival of water-bearing asteroids. And most troubling, planetesimal accretion would take far too long to build Saturn, Uranus and Neptune. By the time their solid cores formed — after tens of millions of years — it would be too late for them to accumulate enough gas from the protoplanetary disk to become gas giants, as “most disks go away in a few million years,” said ‪André Izidoro, a planetary scientist at Rice University.

    Johansen and Lambrechts proposed a new model. Instead of multiple planetesimals colliding together, they instead suggested that a single dominant planetesimal could grow to a huge size in a short amount of time — just a few million years — by sweeping up material inside a protoplanetary disk “like a vacuum cleaner,” said Johansen. This material would consist of tiny seedlike rocks that surrounded young stars. They called the idea pebble accretion.

    Pebbles are extremely small, just a few millimeters to centimeters in size, whereas planetesimals are much larger, up to hundreds of kilometers wide, like many of the asteroids we see in the solar system today. Both would be found in a star’s protoplanetary disk, with the latter occasionally smashing into one another.

    In 2014, just two years after Johansen and Lambrechts published their pebble model, observations revealed that disks were indeed full of pebbles. A network of 66 telescopes called ALMA (the Atacama Large Millimeter/submillimeter Array) revealed up to 100 Earth masses’ worth of pebbles inside a protoplanetary disk surrounding a young star [The Astrophysical Journal], including wide gaps created by growing planets carving out their orbits. Inside these disks, pebbles were everywhere. ALMA “showed that protoplanetary disks are born with enormous mass reservoirs of small pebbles, not planetesimals,” said Lambrechts.

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    ALMA observations of protoplanetary disk around HL Tauri in 2014 revealed hidden structures, including the presence of pebbles in the disk. Credit: ALMA (ESO/NAOJ/NRAO)

    Before long, most scientists came to agree that pebble accretion formed the giant planets. It just seemed to be the only way for them to grow fast enough. “For the cores of the giant planets there is no doubt pebble accretion is the solution,” said Alessandro Morbidelli, a planetary scientist at the Côte d’Azur Observatory in France.

    Yet, while it seemingly explained the formation of Jupiter, Saturn, Uranus and Neptune, pebble accretion raised considerable questions about the formation of the terrestrial planets: Mercury, Venus, Earth and Mars. “In principle one could form the terrestrial planets with planetesimal accretion,” said Lambrechts. “But now there’s this invasion of the pebbles.”

    In the pebble accretion model, you begin with a protoplanetary disk around a young star, just like in the planetesimal accretion model. Both models then require planetesimals to form via a phenomenon called streaming instability. Essentially, dust and pebbles experience drag as they encounter the gas surrounding the star. This causes the pebbles to clump together, until some clumps “are so massive that they become gravitationally bound, and they collapse into planetesimals” up to hundreds of kilometers wide, said Joanna Drążkowska, an astrophysicist at The Ludwig Maximilians University of Munich [Ludwig-Maximilians-Universität München](DE). The clumps may then rotate as they form, which gives them two lobes. “This is exactly what we see” in outer solar system objects such as Arrokoth, said Drążkowska. The process is expected to be incredibly quick, perhaps taking only 100 years.

    From here, the two models diverge. Under planetesimal accretion, these planetesimals form everywhere in the disk, leaving few pebbles left. Over tens of millions of years, the large planetesimals collide and merge, eventually giving rise to the terrestrial planets we see today.

    In pebble accretion, just a few planetesimals become dominant. These planetesimals begin to sweep up pebbles in the protoplanetary disk, which stream down onto the surface of the planetesimal in long riverlike filaments. It is an extremely energetic process, with hot magma oceans glowing on the surface as pebbles rain down. “These planets would shine,” said Lambrechts. The process is very efficient; Earth would grow to its full size in just a few million years, compared to perhaps 100 million years in planetesimal accretion.

    One of the most interesting outcomes of pebble accretion is that it gives a direct prediction of how habitable planets form. Rather than relying on water-rich asteroids to haphazardly collide with protoplanets, the model suggests that incoming icy pebbles from the outer solar system could provide a steady supply of water to a planet like Earth, an idea known as pebble snow. “The nice thing about pebble snow is that it becomes predictable,” said Johansen. “The amount of water and carbon and nitrogen that comes down to Earth is something that can be calculated.”

    Thus, if the pebble accretion model for terrestrial planet formation is correct, it may bode well for the prospects of other life in the universe. Whereas under planetesimal accretion the existence of water on Earth was a chance event, in pebble accretion it might be expected in a planetary system like our own. Take a proto-Earth and put it around a similar star in a similar position, and the amount of water it collects could be the same. Habitable worlds would not be chance events; their existence would be a calculable outcome if a planetary system has the right ingredients. “One can use this as a starting point for understanding prebiotic chemistry and the origin of life,” says Johansen.

    The Great Architect

    Pebble accretion seems like an attractive idea. It solves the problem of rapid planet growth, it explains the presence of water on Earth, and we can even observe pebbles in developing exoplanetary systems. “With ALMA we know now pebbles are concentrated in particular regions that lead to planetesimal formation and potentially planets,” said Paola Pinilla, a planetary formation scientist at The University College London (UK).

    Yet, while it provides a good explanation of giant planet growth, pebble accretion has some notable issues when it comes to terrestrial planets.

    First, where did the pebbles in the inner solar system come from? In recent years, planetary scientists have come to believe that Jupiter, the largest planet in our solar system, was the primary force shaping the destiny of the planets. “The emergent picture is that Jupiter was the great architect of the solar system,” said Batygin.

    Soon after Jupiter’s rapid formation, it created a barrier between the inner and outer solar system, preventing material from the “mass-rich” outer regions from flowing to the “mass-starved” inner terrestrial planets, said Batygin. “The giant planets blocked the flux of dust and pebbles,” said Morbidelli. Pebbles in the inner disk may have dissipated before the terrestrial planets could form, and without more material coming in from the outer solar system, there simply would not have been enough material to make Earth.


    Journey to the Birth of the Solar System 360 VR
    Join David Kaplan on a virtual-reality tour showing how the sun, the Earth and the other planets came to be. Quanta Magazine and Chorus Films.

    Even if there was enough material, pebble accretion runs into another problem: It is extremely efficient, but perhaps too much so. If Earth and the other terrestrial planets did form by pebble accretion, it is not clear why they did not grow larger and larger, eventually becoming super-Earths — worlds somewhere between Earth and Neptune in size, which seem to be relatively common in other planetary systems. “The difficulty with pebble accretion is it’s either not very efficient or it’s very efficient,” said Sean Raymond, an astronomer at the Astrophysics Laboratory of Bordeaux in France. “It’s rarely in between. And to work for the terrestrial planets, you need to have just the right amount of stuff.” Too little material and planets simply never grow. Too much and the planets grow too large too quickly “and the solar system would have super-Earths rather than terrestrial planets,” said Raymond.

    These issues have caused considerable debate among planetary scientists in the last few years, with much ongoing research from both sides of the argument. In September, Morbidelli and his colleagues published an article in Nature Astronomy based on studies of the protoplanet Vesta that suggested how planetesimals would explain the current configuration of the solar system. The study suggests that a ring of planetesimals once orbited the sun at the current location of Earth. In time, this ring formed two large planets — Earth and Venus — toward the middle of the ring, with two smaller worlds — Mars and Mercury — on the flanks.

    Others, however, continue to investigate ways in which pebbles may birth terrestrial planets. In February, Johansen and colleagues described how our own solar system could have formed in this way. Then last month, Drążkowska and colleagues used pebble accretion to explain why super-Earths are relatively uncommon around other sunlike stars.

    At a workshop at Ringberg Castle in Germany last month, the debate flowed freely. Some, like Johansen and Lambrechts, remain very much in favor of a pebble accretion model for terrestrial planets. “There’s very strong evidence that this is the dominant process,” said Johansen. Others are less convinced. “I think pebble accretion is a very important process to understand planet formation, but I don’t think it’s the process that built the terrestrial planets in our solar system,” said Thorsten Kleine, a planetary scientist at The University of Münster [Westfälische Wilhelms-Universität Münster](DE). The two processes could also have worked in concert, with pebble accretion creating planetesimals that then merged after Jupiter cut off the flow of incoming pebbles.

    Some hope that turning to cosmochemistry, the study of the compositions of cosmic objects, might reveal the answer. Johansen pointed out that if the planetesimal accretion model were correct, we would expect to find asteroids of a similar composition to Earth, given that they were likely the building blocks of our world. Yet this is not the case. “I think that’s a limitation of the classical model, because they haven’t been able to find one,” he said. “There are really no meteorites that look like Earth at all.”

    If Earth formed via pebble accretion, however, we might have expected to see a “much higher abundance” of volatile elements like nitrogen and carbon on Earth, delivered by pebbles coming from the outer solar system, said Conel Alexander, a cosmochemist at The Carnegie Institution for Science (US). “We just don’t see that,” he said. Scientists like Alexander hope that combining new ideas of cosmochemistry with modeling of the early solar system could provide some useful answers. “Both the modelers and the cosmochemists have a bit of work to do,” he said.

    Elsewhere, continued studies of exoplanetary systems could reveal more information. Already more than 5,000 protoplanetary disks have been observed by ALMA, said Pinilla, from very young disks of less than 1 million years to disks up to 30 million years in age. On one occasion, we have even seen a giant planet, a world called PDS 70b, being born inside such a disk, with more sightings hoped for in the future. Some disks show the glow of dust, indicating the possible presence of colliding planetesimals — although how many isn’t clear. Upcoming observations from the James Webb Space Telescope (JWST), set to launch in December, alongside work from ALMA, could provide invaluable additional clues.

    National Aeronautics Space Agency(USA)/European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU)/ Canadian Space Agency [Agence Spatiale Canadienne](CA) Webb Infrared Space Telescope(US) James Webb Space Telescope annotated. Scheduled for launch in October 2021 delayed to December 2021.

    Much remains uncertain. The major question now is: Was our planet the result of repeated collisions between huge asteroid-like bodies, or are we standing on top of a world made of trillions upon trillions of tiny, perhaps ice-rich cosmic pebbles? Solving that fundamental question will provide a window not only into our own past, but into Earth-like worlds everywhere.

    “If we bring chemical traces from JWST, and the pebble distribution from ALMA, we can have some hints of what types of planets can form in the inner parts of the disks,” said Pinilla.

    See the full article here .


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    Formerly known as Simons Science News, Quanta Magazine (US) is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 9:18 am on August 21, 2020 Permalink | Reply
    Tags: "Blanet: A new class of planet that could form around black holes", Active Galactic Nucleus, , , , , , Protoplanetary disks,   

    From Astronomy Magazine: “Blanet: A new class of planet that could form around black holes” 

    From Astronomy Magazine

    August 12, 2020

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    Supermassive black holes are typically surrounded by vast disks of gas and dust, as seen in this artist’s concept. And now, researcher think planets might form in these wild environments. Credit: Jurik Peter/Shutterstock.

    Supermassive black holes are among the most exciting and puzzling objects in the universe. These are the giant, massive bodies that sit at the heart of most, perhaps all, galaxies. Indeed, they may be the seeds from which all galaxies grow.

    Messier 87*, The first image of a black hole. This is the supermassive black hole at the center of the galaxy Messier 87. Image via JPL/ Event Horizon Telescope Collaboration.

    Supermassive black holes are at least a hundred thousand times the mass of our Sun. They are often surrounded by thick clouds of gas that radiate vast amounts of energy. When this happens, they are called active galactic nuclei. Discovering the properties of these clouds, and their curious central residents, is an ongoing exercise for astrophysicists.

    Now researchers have a new phenomenon to consider — the idea that planets can form in the massive clouds of dust and gas around supermassive black holes. Last year, Keichi Wada at Kagoshima University in Japan, and a couple of colleagues showed that under certain conditions planets ought to form in these clouds. These black hole planets, or blanets as the team call them, would be quite unlike any conventional planet and raise the possibility of an entirely new class of objects for astronomers to dream about.

    Protoplanetary Disk

    The generally agreed theory of planet formation is that it occurs in the protoplanetary disk of gas and dust around young stars. When dust particles collide, they stick together to form larger clumps that sweep up more dust as they orbit the star. Eventually, these clumps grow large enough to become planets.

    Wada and Co say a similar process should occur around supermassive black holes. These are surrounded by huge clouds of dust and gas that bear some similarities to the protoplanetary disks around young stars. As the cloud orbits the black hole, dust particles should collide and stick together forming larger clumps that eventually become blanets.

    The scale of this process is vast compared to conventional planet formation. Supermassive black holes are huge, at least a hundred thousand times the mass of our Sun. But ice particles can only form where it is cool enough for volatile compounds to condense.

    This turns out to be around 100 trillion kilometers from the black hole itself, in an orbit that takes about a million years to complete. Birthdays on blanets would be few and far between!

    Next the team considered how large these bodies might grow. An important limitation is the relative velocity of the dust particles in the cloud. Slow moving particles can collide and stick together, but fast-moving ones would constantly break apart in high-speed collisions. Wada and Co calculated that this critical velocity must be less than about 80 meters per second.

    At the same time, the rate of collisions must be high enough for blanets to form during the lifetime of an active galactic nuclei, thought to be perhaps a hundred million years. That leaves just a small parameter of space in which blanets can form, unless there is another factor that promotes blanet formation.

    The focus of the team’s current work is on just such a factor: the impact of radiation on the dust cloud. The radiation from an active galactic nucleus would tend to drive dust particles away from the black hole, creating a constant “wind” of fresh material for blanet formation.

    Active Galactic Nucleus

    That has a significant impact, say Wada and Co. Under these conditions, blanets grow faster and can reach sizes up to 3,000 times the mass of Earth (beyond which they would be massive enough to form brown dwarfs). Without this dust wind, blanets would grow to no more than six times the mass of Earth. “Our results suggest that blanets could be formed around relatively low-luminosity active galactic nuclei during their lifetime,” say Wada and co.

    Just what these bodies would be like is an open question. Wada and Co say they cannot be gaseous giants like Jupiter or Neptune. “The gaseous envelope of a blanet should be negligibly small compared with the blanet mass,” they say. And neither would they be much like Earth. “Blanets are extraordinarily different from the standard Earth-type planets,” add the team.

    For the moment, the work is entirely theoretical, and the prospect of observing a blanet does not seem high. The closest active galactic nucleus, Centaurus A, is 11 million light-years from Earth, well beyond the scope of current exoplanet searches, which stretch just a few thousand light years.

    But if blanets do exist, the next question is whether they might support life. Exactly this question arose following the release of the movie Interstellar, which included a potentially habitable planet orbiting a black hole. The answer: probably not, although that is no reason for astronomers to stop looking. Happy blanet hunting!

    See the full article here .


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    Please help promote STEM in your local schools.

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    Astronomy is a magazine about the science and hobby of astronomy. Based near Milwaukee in Waukesha, Wisconsin, it is produced by Kalmbach Publishing. Astronomy’s readers include those interested in astronomy and those who want to know about sky events, observing techniques, astrophotography, and amateur astronomy in general.

    Astronomy was founded in 1973 by Stephen A. Walther, a graduate of the University of Wisconsin–Stevens Point and amateur astronomer. The first issue, August 1973, consisted of 48 pages with five feature articles and information about what to see in the sky that month. Issues contained astrophotos and illustrations created by astronomical artists. Walther had worked part time as a planetarium lecturer at the University of Wisconsin–Milwaukee and developed an interest in photographing constellations at an early age. Although even in childhood he was interested to obsession in Astronomy, he did so poorly in mathematics that his mother despaired that he would ever be able to earn a living. However he graduated in Journalism from the University of Wisconsin Stevens Point, and as a senior class project he created a business plan for a magazine for amateur astronomers. With the help of his brother David, he was able to bring the magazine to fruition. He died in 1977.

     
  • richardmitnick 9:30 pm on October 2, 2018 Permalink | Reply
    Tags: , , , , , , , Protoplanetary disks,   

    From Science: “Cosmic conundrum: The disks of gas and dust that supposedly form planets don’t seem to have the goods” 

    AAAS
    From Science Magazine

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    Artist’s illustration of the protoplanetary disk surrounding a young star. JPL-Caltech/NASA

    Astronomers have a problem on their hands: How can you make planets if you don’t have enough of the building blocks? A new study finds that protoplanetary disks—the envelopes of dust and gas around young stars that give rise to planets—seem to contain orders of magnitude too little material to produce the planets.

    “This work is telling us that we really have to rethink our planetary formation theories,” says astronomer Gijs Mulders of the University of Chicago in Illinois, who was not involved in the research.

    Stars are born from colossal clouds of gas and dust and, in their earliest stages, are surrounded by a thin disk of material. Dust grains within this halo collide, sometimes sticking together. The clumps build up into planetary cores, which are big enough to gravitationally attract additional dust and gas, eventually forming planets.

    But many details about this process remain unknown, such as just how quickly planets arise from the disk, and how efficient they are in capturing material. The disks, surrounded by an obscuring haze of gas and dust, are difficult to observe. But radio telescopes can penetrate the haze and investigate young stars. The brightness of radio waves emitted by dust in the disk can be used to give a reasonable estimate of its overall mass.

    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

    The Atacama Large Millimeter Array (ALMA), a radio observatory in the Atacama Desert in Chile, has made it far easier to study protoplanetary disks. In the new study, astronomers led by Carlo Manara of the European Southern Observatory in Munich, Germany, used ALMA to compare the masses of protoplanetary disks around young stars between 1 million and 3 million years old to the masses of confirmed exoplanets and exoplanetary systems around older stars of equivalent size. The disk masses were often much less than the total exoplanet mass—sometimes 10 or 100 times lower, the team will report in an upcoming paper in Astronomy & Astrophysics.

    Although such findings have been reported before for a few star systems, the study is the first to point out the mismatch over several hundred different systems. “I think what this work does is really set this as a fact,” Manara says.

    It is possible that astronomers are simply looking at the disks too late. Perhaps some planets form in the first million years, sucking up much of the gas and dust, Manara says. ALMA has found that some extremely young stars, such as the approximately 100,000-year-old HL Tauri, already have ringlike gaps in their disks, potentially indicating that protoplanets are sweeping up material inside of them.

    “But if you solve one problem, you end up with another,” says astronomer Jonathan Williams of the University of Hawaii Institute for Astronomy in Honolulu, who was also not involved in the work. If planetary cores form early, when so much material remains in the disk, nothing would stop them from ballooning into Jupiter-size behemoths. Yet the emerging census of exoplanets shows that most are Earth- or Neptune-size worlds.

    Williams favors the idea that current telescopes are simply missing some of the material. ALMA’s wavelengths are tuned to best see the smallest bits of dust. But a great deal of mass, perhaps as much as 10 times what’s been observed, could be hidden in the form of pebbles, which are slightly too big to show up in such investigations. A proposed upgrade to the Very Large Array, a radio telescope in New Mexico, should be able to spot such hidden debris, perhaps accounting for some of the missing material.

    NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

    One final possibility is that protoplanetary disks are somehow sucking in additional material from the surrounding interstellar medium. Manara says some recent simulations show young stars drawing in fresh material for much longer periods of time than previously believed. He hopes that observations of the earliest stages of star formation from the upcoming Square Kilometer Array or James Webb Space Telescope will help researchers decide between these different hypotheses.

    NASA/ESA/CSA Webb Telescope annotated

    See the full article here .


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  • richardmitnick 6:30 am on June 17, 2017 Permalink | Reply
    Tags: , , , , , , Protoplanetary disks, Watermelon Dust is the Best Dust: Forming Planetesimals Near the Snow Line   

    From astrobites: “Watermelon Dust is the Best Dust: Forming Planetesimals Near the Snow Line” 

    Astrobites bloc

    Astrobites

    June 16, 2017
    Michael Hammer

    Title: Planetesimal Formation near the snow line: in or out?
    Authors: Djoeke Schoonenberg and Chris Ormel
    First Author’s Institution: Anton Pannekoek Institute for Astronomy, University of Amsterdam
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    Status: Published in A&A [open access]

    How is it possible for planets to exist? Even though we know planets must have formed from planetesimals that are tens of kilometers in size, the most basic models of protoplanetary disks have trouble forming planetesimals from the micron to centimeter-sized dust that populates these disks. For dust particles to grow into planetesimals, they need to be able to clump together enough to reach roughly the same level of concentration as the gas in the disk – which can be difficult since there is 100 times more gas than dust.

    A few months ago, I wrote an Astrobite describing a simple model that naturally achieves the conditions needed to form planetesimals in the inner disk, thereby offering a way to form planets in the inner solar system like Earth and Mars. However, this model leaves out the gas giant planets in the outer solar system like Jupiter!

    Fortunately, beyond the snow line at about 2 AU, the disk will get cold enough for water vapor to condense into solid ice. It is already widely accepted that this extra ice will enhance the concentration of solids enough to form planetesimals and in turn, planets in the outer disk. Yet, the most detailed models of this scenario leave out enough relevant effects that we cannot reliably determine the impact of this extra ice.

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    Rough sketch which shows the sharp increase of solid surface density at the snow line, by a factor of ~3-4. [https://ay201b.wordpress.com/the-snow-line-in-protoplanetary-disks/]

    Djoeke Schoonenberg and Chris Ormel, the authors of today’s paper, set out to improve our understanding of whether the snow line can trigger the formation of planetesimals by creating a more rigorous model that better captures the dynamic structure of the disk and of the dust grains themselves.

    Model Upgrades with a Side of Fruit

    Schoonenberg and Ormel develop a steady-state model of how the ice beyond the snow line evaporates as it moves inwards and how some of the water vapor inside the snow line condenses as it moves outwards.

    My favorite part of their model is that they factor in the structure of individual “icy dust” grains. Many studies of protoplanetary disks often leave out what the mass in the disk is made of – instead, only tracking a distribution of density across the disk or nondescript particles. However, not only do Schoonenberg and Ormel describe each icy dust grain as 50% silicates (the typical composition of a regular dust grain) and 50% ice; but they also establish how the silicate “seeds” in each grain are divided up. As shown in Figure 1, a dust grain can either have a single silicate core like an avocado (their “single-seed” model), or it can have many smaller seeds evenly distributed through the grain like a watermelon (their “many-seeds” model).

    The authors run separate models for each type of dust grain. They then solve for the enhancement of ice and the solid-to-gas ratio to see if these values reach a high enough concentration to form planetesimals.

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    Figure 1. Structures of icy dust grains. In the avocado model, each grain has a single silicate core surrounded by a shell of ice. In the watermelon model, each grain has a bunch of smaller silicate seeds evenly distributed through the ice. When an avocado dust grain evaporates, it slowly loses only its ice before leaving its core behind. When a watermelon dust grain evaporates, it slowly loses both its ice and its silicate seeds together as it drifts inward. Adapted from Figure 1.

    Forming Planetesimals Early and with Watermelon

    Besides varying the structure of the dust grains, the authors also experiment with different disk viscosities, dust particle sizes, and a few other variables. In particular, they find that higher viscosities are best-suited for producing planetesimals. Since these higher viscosities are more likely to occur early on in a disk’s lifetime, this suggests that disks can form planetesimals right away!

    In the single-seed “avocado” model with the optimal higher viscosities, Schoonenberg and Ormel find that the concentration of ice can be enhanced by a factor of 3 to 5 just beyond the snow line (see Figure 2). Interestingly with the many-seeds “watermelon dust” model, they find that this enhancement can double! This occurs because in the single-seed model, only the extra ice from inside the snow line contributes to the enhancement. However, in the watermelon model, the small size of the watermelon seeds plays a key role. Since the seeds are so small, they get carried by the gas that makes up most of the disk. When some of the gas from inside the snow line moves outward, some of the watermelon seeds follow it outward. As a result, many of these watermelon seeds end up captured in icy dust grains and also contribute to the extra ice, which doubles the enhancement.

    Ultimately, the authors expect the ice enhancement in an actual disk to be in-between the results from the avocado and watermelon models due to the fact that both structures of dust grains are plausible, and also because other dust grains may be a “hybrid” of both fruits and have a large silicate core as well as additional smaller silicate seeds in the outer icy shell.

    4
    Figure 2. Solid-to-gas ratio near the ice line (blue vertical line). Solids are ice plus dust. The many-seeds model safely exceeds the threshold (orange horizontal line) to form planetesimals. Since the amount of solids is just an average at a given radius, the single-seeds model should also be able to form planetesimals. Adapted from Figure 5.

    Summary

    The ice enhancement of about ~7 that the authors find is ten times lower than the enhancement of 75 found with a simpler model, emphasizing the importance of considering the more intricate details of the problem! Thankfully, this enhancement is still high enough for the concentration of large dust grains to reach the concentration of gas and produce planetesimals (see Figure 2). More importantly, the authors find that the best conditions for forming planetesimals happen soon after a star and its disk are born, supporting observational evidence that giant planets in the outer disk can form quickly.

    Lastly, Schoonenberg and Ormel expect the cm-sized ice pile-up near the snow line to be detectable by radio (cm-wavelength) telescopes. Seeing this feature in a real disk would be the best test for finding out how well we understand planetesimal formation near the snow line.

    Featured Image Credit: Luis Calçada

    See the full article here .

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  • richardmitnick 11:45 am on January 27, 2016 Permalink | Reply
    Tags: , , , , Protoplanetary disks,   

    From ALMA: “ALMA confirms predictions on the interaction between protoplanetary disks and planets” 

    ESO ALMA Array
    ALMA

    27 January 2016
    Héctor Cánovas
    Universidad de Valparaíso Valparaíso, Chile
    Tel: +56 032 – 299 5555
    Tel: +56 02 84144232
    E-mail: hector.canovas@uv.cl

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    Observatory

    Tokyo, Japan

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    E-mail: hiramatsu.masaaki@nao.ac.jp

    Protoplanetary disc from ALMA
    Image taken by ALMA of the dust ring that surrounds the young star Sz 91. This ring is primarily made up of mm-sized dust particles. The interaction between several recently formed planets and the protoplanetary disk that still surrounds the star probably generate the dust ring observed by ALMA.

    New observations made with the Atacama Large Millimeter/submillimeter Array (ALMA) of the disk that surrounds a young star, less massive than the Sun, confirm theories about the interaction between recently formed planets and disks. A team of astronomers led by Héctor Cánovas from Universidad de Valparaíso and the Millennium ALMA Disk Nucleus (MAD) observed the dust ring possibly sculpted by planets in formation around the star Sz 91, at a distance roughly 650 light years from Earth.

    The results obtained show the first disk around a star that is less massive than ours – it has only half of the mass of our Sun – which simultaneously presents a migration of dust particles from the outermost zones and evident signs of interaction between young planets with the disk in the innermost zone.

    Planets are born in dust and gas disks that surround young stars and feed them with matter, leaving a “footprint” of this interaction in the structure of the disk. The theoretical models that study this interaction predict that the planets carve the protoplanetary disk, creating a “hole” in the innermost part of the disk, and preventing mm-sized dust particles (like grains of sand on a beach) from continuing their journey towards the central star. At the same time, dust particles in the outermost parts of the disk (the farthest from the star) are attracted by the gravitational force of the star.

    The combination of both effects should create dust structures in the form of a ring in disks that host recently formed giant planets on the inside.

    “The sharp image from ALMA shows a ring around the young star. And it is a surprisingly large ring, over three times the size of Neptune’s orbit (a radius of approximately 110 astronomical units (AU)” explains Héctor Cánovas.

    The image from ALMA only shows the ring, as the radio telescope detects the cold dust particles that make it up, and not the planets and the star, as these are primarily made up of hot gas.

    “Based on the current paradigm of planet-disk interactions, only giant planets orbiting the innermost parts of the disk can explain the presence of a ring with such a large radius,” indicates Antonio Hales, ALMA astronomer and member of the research team.

    The accumulation of dust particles in a narrow annular structure, as is the case with Sz91, can favor the formation of more planets, because the high density of dust particles in the ring would provide the ideal conditions for the dust particles to agglutinate and grow in size until they form small planetary nuclei.

    “The results of this investigation show that Sz91 is a highly important protoplanetary disk for the study of planetary formation, planet-disk interactions, and the evolution of these disks around stars of lower mass, as Sz91 shows evidence of all these processes simultaneously,” concludes Matthias Schreiber, coauthor of the study.

    Additional information

    This investigation was presented in an article entitled A ring-like concentration of mm-sized particles in Sz 91, written by Héctor Cánovas and collaborators, which will soon be published in the specialized journal Monthly Notices of the Royal Astronomical Society (MNRAS).

    The research team is made up of Héctor Cánovas, Claudio Cáceres, Matthias Schreiber, Adam Hardy (all from Universidad de Valparaíso and from the Millennium ALMA Disk Nucleus (MAD), Chile), Lucas Cieza (Universidad Diego Portales and MAD, Chile), Francois Ménard (Universidad de Chile) and Antonio Hales (JAO-ALMA, Chile).

    Link

    A ring-like concentration of mm-sized particles in Sz 91

    See the full article here .

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    The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Organization for Astronomical Research in the Southern Hemisphere (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan.

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  • richardmitnick 3:58 pm on December 15, 2015 Permalink | Reply
    Tags: , , , Protoplanetary disks   

    From ESO: “Mind the Gap” 

    [I picked this up from 2008 because it relates to an ALMA release due out on 12.16.15.]


    European Southern Observatory

    8 September 2008
    Klaus Pontoppidan
    California Institute of Technology
    Pasadena, USA
    Tel: +1 626 395 4900
    Cell: +1 626 679 5793
    Email: pontoppi@gps.caltech.edu

    Ewine van Dishoeck
    Leiden University
    Leiden, Netherlands
    Tel: +31 71 527 58 14
    Email: ewine@strw.leidenuniv.nl

    VLT instrument hints at the presence of planets in young gas discs

    1

    Astronomers have been able to study planet-forming discs around young Sun-like stars in unsurpassed detail, clearly revealing the motion and distribution of the gas in the inner parts of the disc. This result, which possibly implies the presence of giant planets, was made possible by the combination of a very clever method enabled by ESO’s Very Large Telescope.

    Planets could be home to other forms of life, so the study of exoplanets ranks very high in contemporary astronomy. More than 300 planets are already known to orbit stars other than the Sun, and these new worlds show an amazing diversity in their characteristics. But astronomers don’t just look at systems where planets have already formed – they can also get great insights by studying the discs around young stars where planets may currently be forming. “This is like going 4.6 billion years back in time to watch how the planets of our own Solar System formed,” says Klaus Pontoppidan from Caltech, who led the research.

    Pontoppidan and colleagues have analysed three young analogues of our Sun that are each surrounded by a disc of gas and dust from which planets could form. These three discs are just a few million years old and were known to have gaps or holes in them, indicating regions where the dust has been cleared and the possible presence of young planets.

    The new results not only confirm that gas is present in the gaps in the dust, but also enable astronomers to measure how the gas is distributed in the disc and how the disc is oriented. In regions where the dust appears to have been cleared out, molecular gas is still highly abundant. This can either mean that the dust has clumped together to form planetary embryos, or that a planet has already formed and is in the process of clearing the gas in the disc.

    For one of the stars, SR 21, a likely explanation is the presence of a massive giant planet orbiting at less than 3.5 times the distance between the Earth and the Sun, while for the second star, HD 135344B, a possible planet could be orbiting at 10 to 20 times the Earth-Sun distance. The observations of the third star, TW Hydrae, may also require the presence of one or two planets.

    “Our observations with the CRIRES instrument on ESO’s Very Large Telescope clearly reveal that the discs around these three young, Sun-like stars are all very different and will most likely result in very different planetary systems,” concludes Pontoppidan.

    ESO CRIRES
    CRIRES

    “Nature certainly does not like to repeat herself” [1].

    “These kinds of observations complement the future work of the ALMA observatory, which will be imaging these discs in great detail and on a larger scale,” adds Ewine van Dishoeck, from Leiden Observatory, who works with Pontoppidan.

    To study the gaps in dust discs that are the size of the Solar System around stars that are located up to 400 light-years away is a daunting challenge that requires a clever solution and the best possible instruments [2].

    “Traditional imaging cannot hope to see details on the scale of planetary distances for objects located so far away,” explains van Dishoeck. “Interferometry can do better but won’t allow us to follow the motion of the gas.”

    Astronomers used a technique known as spectro-astrometric imaging to give them a window into the inner regions of the discs where Earth-like planets may be forming. They were able not only to measure distances as small as one-tenth the Earth-Sun distance, but to measure the velocity of the gas at the same time [3].

    “The particular configuration of the instrument and the use of adaptive optics allows astronomers to carry out observations with this technique in a very user-friendly way: as a consequence, spectro-astrometric imaging with CRIRES can now be routinely performed,” says team member Alain Smette, from ESO [4].

    Notes

    Pontoppidan, K. M. et. al. 2008, Spectro-Astrometric Imaging of Molecular Gas Within Protoplanetary Disk Gaps, Astrophysical Journal, 684, 1323, 10 September 2008. Team members are Klaus M. Pontoppidan, Geoffrey A. Blake, and Michael J. Ireland (California Institute of Technology, Pasadena, USA), Ewine F. van Dishoeck (Leiden Observatory, The Netherlands, and Max-Planck-Institute for Extraterrestrial Physics, Garching, Germany – MPE), Alain Smette (ESO, Chile), and Joanna Brown (MPE).

    [1] The discs are about an hundred astronomical units (AU – the mean distance between the Earth and the Sun, or 149.6 million kilometres) across, but the stars are more than 200 light-years away (one light-year is 200 000 AU). To resolve structures on 1 AU scales in these systems corresponds to reading the license plate on a car at a distance of 2000 km – roughly the distance from Stockholm to Lisbon.

    [2] CRIRES, the near-infrared spectrograph attached to ESO’s Very Large Telescope, is fed from the telescope through an adaptive optics module which corrects for the blurring effect of the atmosphere and so makes it possible to have a very narrow slit with a high spectral dispersion: the slit width is 0.2 arcsecond and the spectral resolution is 100 000. Using spectro-astrometry, an ultimate spatial resolution of better than 1 milli-arcsecond is achieved.

    [3] The core of the spectro-astrometry imaging technique relies on the ability of CRIRES to be positioned very precisely on the sky, while retaining the ability to spread the light into a spectrum so that wavelength differences of 1 part in 100 000 can be detected. More precisely, the astronomers measure the centroid in the spatial direction of a spectrally resolved emission line: effectively, astronomers take a sharp emission line – a clear fingerprint of a molecule in the gas – and use data from several slit positions to locate the sources of particular emission lines, and hence to map the distribution of the gas with much greater precision than can be achieved by straightforward imaging. The astronomers have obtained spectra of the discs centred at wavelengths of 4.715 microns at 6 different position angles.

    [4] Alain Smette is the CRIRES Instrument Scientist.

    See the full article here .

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    ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

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  • richardmitnick 3:39 pm on September 11, 2015 Permalink | Reply
    Tags: , , , Protoplanetary disks   

    From ASS NOVA: ” Water Vapor in an Unexpected Location” 

    AASNOVA

    Amercan Astronomical Society

    11 September 2015
    Susanna Kohler

    1
    Artist’s impression of a pre-transitional disk. A recent study has, for the first time, found water vapor in the inner regions of such a disk. [NASA/JPL-CALTECH]

    The protoplanetary disk around DoAr 44 is fairly ordinary in most ways. But a recent study has found that this disk contains water vapor in its inner regions — the first such discovery for a disk of its type.

    Drying Out Disks

    DoAr 44 is a “transitional disk”: a type of protoplanetary disk that has been at least partially cleared of small dust grains in the inner regions of the disk. This process is thought to happen as a result of dynamical interactions with a protoplanet embedded in the disk; the planet clears out a gap as it orbits.

    2
    A schematic of the differences between a full protoplanetary disk, a pre-transitional disk, and a transitional disk. [Catherine Espaillat]

    Classical protoplanetary disks surrounding young, low-mass stars often contain water vapor, but transitional disks are typically “dry” — no water vapor is detected from the disk inner regions. This is probably because water vapor is easily dissociated by far-UV radiation from the young, hot star. Once the dust is cleared out from the inner regions of the disk, the water vapor is no longer shielded from the UV radiation, so the disk dries out.

    Enter the exception: DoAr 44. The disk in this system doesn’t have a fully cleared inner region, which labels it “pre-transitional”. It’s composed of an inner ring out to 2 AU, a cleared gap between 2 and 36 AU, and then the outer disk. What makes DoAr 44 unusual, however, is that it’s the only disk with a large inner gap known to harbor detectable quantities of water vapor. The authors of this study ask a key question: where is this water vapor located?

    Unusual System

    Led by Colette Salyk (NOAO and Vassar College), the authors examined the system using the Texas Echelon Cross Echelle Spectrograph, a visiting instrument on the Gemini North telescope.

    Texas Echelon-Cross-Echelle Spectrograph
    Texas Echelon Cross Echelle Spectrograph

    Gemini North telescope
    Gemini North Interior
    NOAO/Gemini North

    They discovered that the water vapor emission originates from about 0.3 AU — the inner disk region, where terrestrial-type planets may well be forming.

    Both dust-shielding and water self-shielding seem to have protected this water vapor from the harsh radiation of the central star, and the authors model this shielding to place constraints on the composition of the disk’s inner regions. They conclude that DoAr 44 has maintained similar physical and chemical conditions to classical protoplanetary disks in its terrestrial-planet forming regions, in spite of having formed a large gap.

    Why has DoAr 44 succeeded at maintaining its water vapor, unlike other transition disks? The authors propose that gas might be migrating across the gap in the disk, replenishing the inner disk from the outer. Future observations are planned to help better understand the overall architecture of the gap, as well as the implications of these detections for any possible planets embedded in the disk.

    Citation
    Colette Salyk et al 2015 ApJ 810 L24. doi:10.1088/2041-8205/810/2/L24

    See the full article here .

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