by Paul Gilster | Apr 9, 2015 | Deep Sky Astronomy & Telescopes |
I’ve used the term ‘interstellar archaeology’ enough for readers to know that I’m talking about new forms of SETI that look for technological civilizations through their artifacts, as perhaps discoverable in astronomical data. But there is another kind of star-based archaeology that is specifically invoked by the scientists behind GALAH, as becomes visible when you unpack the acronym — Galactic Archaeology with HERMES. A new $13 million instrument on the Anglo-Australian Telescope at Siding Spring Observatory, HERMES is a high resolution spectrograph that is about to be put to work.
Image: I can’t resist running this beautiful 1899 photograph of M31, then known as the Great Andromeda Nebula, when talking about our evolving conception of how galaxies form. Credit: Isaac Roberts (d. 1904), A Selection of Photographs of Stars, Star-clusters and Nebulae, Volume II, The Universal Press, London, 1899. Via Wikimedia Commons.
And what an instrument HERMES is, capable of providing spectra in four passbands for 392 stars simultaneously over a two degree field of view. What the project’s leaders intend is to survey one million stars by way of exploring how the Milky Way formed and evolved. The idea is to uncover stellar histories through the study of their chemistry, as Joss Bland-Hawthorn (University of Sydney) explains:
“Stars formed very early in our galaxy only have a small amount of heavy elements such as iron, titanium and nickel. Stars formed more recently have a greater proportion because they have recycled elements from other stars. We reach back to capture this chemical state – by analysing the mixture of gases from which the star formed. You could think of it as its chemical fingerprint – or a type of stellar DNA from which we can unravel the construction of the Milky Way and other galaxies.”
Determining the histories of these stars with reference to 29 chemical signatures as well as stellar temperatures, mass and velocity should help the researchers create a map of their movements over time. This should be a fascinating process, for views of galaxy formation have changed fundamentally since the days when Allan Sandage and colleagues proposed (in 1962) that a protogalactic gas cloud that settled into a disk could explain galaxies like the Milky Way.
That concept suggested that the oldest stars in the galaxy were formed from gas that was being drawn toward the galactic center, collapsing from the halo to the plane, and in Sandage’s view, this collapse was relatively rapid (on the order of 100 million years), with the initial contraction beginning roughly ten billion years ago. Later we begin to see a different model developing, one in which the galaxy formed through the agglomeration of smaller elements like satellite galaxies. Both these processes are now believed to play a role, with infalling satellite systems affecting not just the galactic halo but also the disk and bulge.
Image: Structure of the Milky Way, showing the inner and outer halo. Credit: NASA, ESA, and A. Feild (STScI).
Galactic archaeology is all about detecting the debris of these components, making it possible to reconstruct a plausible view of the proto-galaxy. How the galactic disk and bulge were built up is the focus, determined by using what the researchers call ‘the stellar relics of ancient in situ star formation and accretion events…’ The authors explain the challenges they face:
… unraveling the history of disc formation is likely to be challenging as much of the dynamical information such as the integrals of motion is lost due to heating. We need to examine the detailed chemical abundance patterns in the disc components to reconstruct [the] substructure of the protogalactic disc. Pioneering studies on the chemodynamical evolution of the Galactic disc by Edvardsson et al. (1993) followed by many other such works (e.g. Reddy et al. 2003; Bensby, Feltzing & Oey 2014), show how trends in various chemical elements can be used to resolve disc structure and obtain information on the formation and evolution of the Galactic disc, e.g. the abundances of thick disc stars relative to the thin disc. The effort to detect relics of ancient star formation and the progenitors of accretion events will require gathering kinematic and chemical composition information for vast numbers of Galactic field stars.
Two days ago we looked briefly at globular clusters and speculated on what the view from a planetary surface deep inside one of these clusters might look like. The globular clusters, part of the galaxy’s halo, contain some of its oldest stars, and the entire halo is poor in metals. Going into the GALAH survey, the researchers believe that a large fraction of the halo stars are remnants of early satellite galaxies that evolved independently before being acquired by the Milky Way, a process that seems to be continuing as we discover more dwarf satellites and so-called ‘stellar streams,’ associations of stars that have been disrupted by tidal forces.
Seventy astronomers from seventeen institutions in eight countries are involved in GALAH, which is led by Bland-Hawthorn, Gayandhi De Silva (University of Sydney) and Ken Freeman (Australian National University). Their work should give us much new information not just about the halo and globular clusters but the interactions of stars throughout the disk and central bulge. The paper on the project is De Silva et al., “The GALAH survey: scientific motivation,” Monthly Notices of the Royal Astronomical Society Vol. 449, Issue 3 (2015), pp. 2604-2617 (abstract). A University of Sydney news release is also available.
And in case you’re interested, the classic paper by Sandage et al. is “Evidence from the motions of old stars that the Galaxy collapsed,” Astrophysical Journal, Vol. 136 (1962), p. 748 (abstract).
by Paul Gilster | Apr 8, 2015 | Outer Solar System |
What to make of the latest news about Ganymede, which seems to have a bulge of considerable size on its equator? William McKinnon (Washington University, St. Louis) and Paul Schenk (Lunar and Planetary Institute) have been examining old images of the Jovian moon taken by the Voyager spacecraft back in the 1970s, along with later imagery from the Galileo mission, in the process of global mapping. The duo discovered the striking feature that Schenk described on March 20 at the 46th Lunar and Planetary Science Conference in Texas. Says McKinnon:
“We were basically very surprised. It’s like looking at old art or an old sculpture. We looked at old images of Ganymede taken by the Voyager spacecraft in the 1970s that had been completely overlooked, an enormous ice plateau, hundreds of miles across and a couple miles high… It’s like somebody came to you and said, ‘I have found a thousand mile wide plateau in Australia that was six miles high.’ You’d probably think they were out of their minds or spent too much time in the Outback.”
The bulge is about 600 kilometers across and 3 kilometers tall, and the researchers believe that it may be an indication of the moon’s sub-surface ocean. The going theory is that the bulge emerged at one of Ganymede’s poles and slid along the top of the ocean in a motion called true polar wander (TPW). The find sets us up for future mapping of Ganymede, for the polar wander theory leads McKinnon and Schenk to believe that a similar bulge should exist opposite this one. If current mission planning holds, we may learn the answer early in the 2030s.
Image: Jupiter’s moon Ganymede probably has a sub-surface ocean, as recent work suggests. Credit: NASA/JPL.
If it takes a global sub-surface ocean to produce true polar wander, then we should expect the same thing on Europa, and indeed, the evidence points to the phenomenon, though here the signs of TPW are much clearer presumably because the ice crust is thinner. Strain produced by the shell’s rotation forces concentric grooves — the researchers call them ‘crop circles’ — to emerge. McKinnon and Schenk found that the ‘two incomplete sets of concentric arcuate trough-like depressions’ previously identified on Europa are offset in a pattern that fits the stresses of true polar wander. Moreover, additional features show evidence of TPW, including fissure-like fractures and smaller subsidiary fractures that seem to be associated with the concentric features and their patterning on the surface. “The TPW deformation pattern on Europa,” the authors add, “is thus more complex than the original features reported.” Here again it will take future mapping to provide the higher resolution needed to explore these issues.
The bulge now identified on Ganymede indicates that at some point in the past, the moon’s surface ice rotated, with what had been thicker polar shell material now being found at the equator. McKinnon’s surprise at the finding is understandable given that there is no other surface sign of true polar wander on Ganymede, as the LPSC proceedings paper makes clear:
Extensive search of the entire Voyager and Galileo image library, including all terminator image sequences and all high resolution images reveals no trace of any of the features currently associated with TPW on Europa. No arcuate troughs, no irregular depressions, no raised plateaus, no crosscutting en-echelon fractures. This may be consistent with a thicker ice shell on Ganymede.
That thicker ice shell would have made true polar wander less likely, but the phenomenon could have occurred in the past over a thinner surface crust. The paper goes on:
A thicker ice shell (and lithosphere) will not deform as easily, and will resist polar wander in the first place; a thinner icy shell, more plausible in Ganymede’s past, may have undergone polar wander, but the resultant stresses will be lower by a factor of 3 compared with those on Europa and may not have created such a distinctive tectonic signature. We are engaged in a global search for other manifestations of TPW on Ganymede and will report on our findings.
The question, then, is how a bulge the size of the one the researchers have identified on Ganymede can still be in place. In an article on this work in National Geographic (Bizarre Bulge Found on Ganymede, Solar System’s Largest Moon), McKinnon had this to say:
“Any ideas about how you support a three-kilometer-high [two-mile] ice bulge, hundreds of kilometers wide, over the long term on Ganymede are welcome… We’ve never seen anything like it before; we don’t know what it is.”
As we’ve seen recently, Ganymede’s sub-surface ocean has been confirmed by Joachim Saur and colleagues (University of Cologne) through the study of auroral activity (see Evidence Mounts for Ganymede’s Ocean). Now we have further indication that crustal slippage has occurred on the moon, all but requiring an ocean separating surface materials from the deeper core. We can expect to learn much more, including whether or not there is a corresponding bulge opposite to this one, when the Jupiter Icy Moon Explorer mission arrives in 2030. If the proposed timeline is met, JUICE will begin orbital operations around Ganymede in 2033.
A St. Louis Public Radio news release on McKinnon and Schenk’s work is also available.
by Paul Gilster | Apr 7, 2015 | Deep Sky Astronomy & Telescopes |
How often do two stars collide? When you think about the odds here, the likelihood of stellar collisions seems remote. You can visualize the distance between the stars in our galaxy using a method that Rich Terrile came up with at the Jet Propulsion Laboratory. The average box of salt that you might buy at the grocery store holds on the order of five million grains of salt. Two hundred boxes of salt, then, make a billion grains, while 20,000 boxes give us 100 billion. That’s now considered a low estimate of the number of stars in our galaxy, which these days tends to be cited at about 200 billion, but let’s go with the low figure because it’s still mind-boggling.
So figure you have 20,000 boxes of salt and you spread the grains out to mimic the actual separation of stars in the part of the galaxy we live in. Each grain of salt would have to be eleven kilometers away from any of its neighbors. These are considerable distances, to say the least, but of course there are places in the galaxy where stars are far closer to each other than here.
I was reminded of this recently while reading Jack McDevitt’s novel Seeker (Ace, 2005), which Centauri Dreams reader Rob Flores had mentioned in our discussion of Scholz’s Star and its close pass by the Solar System some 70,000 years ago (see Scholz’s Star: A Close Flyby). Seeker is one of Jack’s tales about Alex Benedict, a far future dealer in antiquities, some of which are thousands of years in our own future. Here’s a bit of dialogue that brings up stellar collisions. On the distant world of the McDevitt universe, Benedict’s aide Chase Kolpath is discussing the matter with an astrophysicist and asks how frequent such collisions are:
“They happen all the time, Chase. We don’t see much of it around here because we’re pretty spread out. Thank God. Stars never get close to one another. But go out into some of the clusters—” She stopped and thought about it. “If you draw a sphere around the sun, with a radius of one parsec, you know how many other stars will fall within that space?”
“Zero,” I said. “Nothing’s close.” In fact the nearest star was Formega Ti, six light-years out.
“Right. But you go out to one of the clusters, like maybe the Colizoid, and you’d find a half million stars crowded into that same sphere.”
“You’re kidding.”
“I never kid, Chase. They bump into one another all the time.” I tried to imagine it. Wondered what the night sky would look like in such a place. Probably never got dark.
I’ve always had the same thought, and tried to imagine myself in a globular cluster like 47 Tucanae or the ancient Messier 5. Another place that (almost) never got dark was the planet in Isaac Asimov’s story “Nightfall” (Astounding Science Fiction, September 1941). Asimov took us to a world where six stars kept the sky continually illuminated except for a brief night every 2049 years. The story, so Asimov’s autobiography tells us, grew out of John Campbell’s asking Asimov to write something based on a famous quotation from Ralph Waldo Emerson:
If the stars should appear one night in a thousand years, how would men believe and adore, and preserve for many generations the remembrance of the city of God!
Of course, the Asimov tale involves a six-star system in which the inhabitants know nothing beyond the stars that keep them illuminated. In a galactic cluster, things get incredibly tight, with typical star distances in the range of one light year, but distances in the core much closer to the size of the Solar System. Planetary orbits in such tight regions would surely be unstable, but we can still try to imagine a sky spangled with stars this close in all directions, if only as an exercise for the imagination. Keep in mind, too, that clusters like Omega Centauri can have several million solar masses worth of stars. An environment like this one is surely ripe for stellar collisions.
Image: This sparkling jumble is Messier 5 — a globular cluster consisting of hundreds of thousands of stars bound together by their collective gravity. But Messier 5 is no normal globular cluster. At 13 billion years old it is incredibly old, dating back to close to the beginning of the Universe, which is some 13.8 billion years of age. It is also one of the biggest clusters known, and at only 24 500 light-years away, it is no wonder that Messier 5 is a popular site for astronomers to train their telescopes on. Messier 5 also presents a puzzle. Stars in globular clusters grow old and wise together. So Messier 5 should, by now, consist of old, low-mass red giants and other ancient stars. But it is actually teeming with young blue stars known as blue stragglers. These incongruous stars spring to life when stars collide, or rip material from one another. Credit: Cosmic fairy lights by ESA/Hubble & NASA. Via Wikimedia Commons.
17th Century Detection of a Collision?
Now we have word that astronomers have found evidence that the nova known as Nova Vulpeculae 1670 was actually the result of a stellar collision. Appearing in 1670 and recorded by both Giovanni Domenico Cassini and Johannes Hevelius, great figures in the astronomy of their day, the ‘new star’ was a naked eye object that varied in brightness over the course of two years. It vanished, reappeared, vanished, reappeared and finally disappeared for good.
We’ve known since the 1980s that a faint nebula in the suspected location of the event was probably all that was left of the star, but new work using APEX (Atacama Pathfinder Experiment telescope), the Submillimeter Array (SMA) and the Effelsberg radio telescope has allowed us to study the chemical composition of the nebula and measure the ratios of different isotopes in the gas. We learn that these ratios do not correspond to what we would expect from a nova.
So what was Nova Vul 1670? The mass of material was too great to be the product of a nova explosion. The paper on this work argues that we are looking at what is left after a collision between two stars, which leaves us with what is known as a red transient, in which material from the stellar interiors is blown into space, leaving only a cool, dusty remnant.
Only a few such objects, also known as luminous red novae, have been detected, with the first confirmed instance being the object M85 OT2006-1 in Messier 85. We also have the case of V1309 Scorpii, which appears to be an interesting instance of the merger of a contact binary, detected in 2008. With so few examples to work with, we have much to learn about the frequency and nature of these phenomena. But globular clusters do appear to be the best place to look for collisions. Back in 2000, Michael Shara (American Museum of Natural History) told a symposium that several hundred collisions per hour could be expected throughout the visible universe, almost all of which we will never detect. (see Two Stars Collide; A New Star Is Born).
Shara estimated as well that in the ten billion year lifetime of the Milky Way, about one million collisions have occurred within globular clusters, or about one every 10,000 years. So Jack McDevitt’s astrophysicist seems to have it about right. If the entire universe is your stage, then stars collide all the time. When Hevelius described Nova Vul 1670 as ‘nova sub capite Cygni’ — a new star below the head of the Swan — he could have no idea how rare his observation was in terms of a human lifetime, but how common on a cosmic time scale.
The paper on Nova Vul 1670 is Kami?ski et al., “Nuclear ashes and outflow in the oldest known eruptive star Nova Vul 1670,” published online in Nature 23 March 2015. The link to the abstract is broken as of this morning, but I’ll post it when it’s functional.
by Paul Gilster | Apr 6, 2015 | Astrobiology and SETI |
Recently we looked at Fast Radio Bursts (FRBs) and the ongoing effort to identify their source (see Fast Radio Bursts: SETI Implications?) Publication of that piece brought a call from my friend James Benford, a plasma physicist who is CEO of Microwave Sciences. Jim noticed that the article also talked about a different kind of signal dubbed ‘perytons,’ analyzed in a 2011 paper by Burke-Spolaor and colleagues. Detected at the Parkes radio telescope, as were all but one of the FRBs, perytons remain a mystery. As described in the essay below, Jim’s recent trip to Australia gave him the opportunity to discuss the peryton question with key players in the radio astronomy community there. He has a theory about what causes these odd signals that is a bit closer to home than some of our speculations on the separate Fast Radio Burst question, and as he explains, we’ll soon know one way or another if he’s right.
by James Benford
A few weeks ago I visited Swinburne University in Melbourne Australia. I was invited there to give a public address about the controversy surrounding METI (Messaging to Extraterrestrial Intelligence). I also visited the radio astronomy group and discussed how to search for an explanation for the Perytons, dispersed swept-frequency signals. My host was Ian Morrison. I also spoke for several hours with Emily Petroff, Willem van Straten and Matthew Bailes, the head of the group.
Ian had sent me the Burke-Spolaor paper before I arrived, so I knew what the basic observations were. In our discussions, I learned that there have been other observations of Perytons and that they have the same general features: They occur from one portion of the sky (which is a clue), happen around midday, and peak in the southern hemisphere’s winter around July (another clue). The shape of the frequency versus time curve is not quite the same as true dispersion measure (DM) signals. There are kinks and dropouts in the frequency-time graph. And the shape is a bit off of the standard DM scaling, 1/f2. And they always occur at the same frequency, about 1.4 GHz.
Although they had concluded in the 2011 Burke-Spolaor paper that signals came from the horizon and were not local to the Parkes radio telescope site, they are now beginning to think that it might be emission from some local electronics. Since they knew of my knowledge of microwave sources, they asked me whether or not whether microwave ovens could be an explanation.
I had already concluded that it was a likely explanation because microwave ovens are highly nonlinear devices and can produce several frequencies. They are designed to stay on a single frequency, 2.45 GHz, but can oscillate at a variety of frequencies. If the magnetron voltage changes, other oscillation modes, with lower frequencies, will occur.
Although microwave ovens when they leave the factory have Faraday shields around them and do not radiate into the environment, over time these precautions can fail due to wear on the equipment. The primary means of preventing radiation from an oven into kitchens is redundant safety interlocks, which remove power from the magnetron if the door is opened. Microwaves generated in microwave ovens cease once the electrical power is turned off.
Image: The Parkes Visitor Centre with the 65-m dish in the background. Credit: Jim Benford.
After describing magnetrons in some detail I offered a hypothesis that leakage of microwaves was occurring from the magnetron, which generates them, and then leak from the enclosing metallic cabinet. They can easily develop separations between the metal case that the microwave magnetron is in and also in the outer case, of which the door is a part. The door is the weakest part of the shield.
Microwave ovens operate at a single frequency and are not dispersed, as their Peryton signals are. But magnetrons sometimes fail to produce a single frequency, due to mechanical disturbances or changing electrical characteristics.
Conditions change in the magnetron when it is turning on or turning off. The voltage on the cathode rises at the turn on and falls at the turn off. That changes the resonance condition and thus excites different oscillation modes, with lower frequencies. This ‘mode hopping’ may explain the observed Perytons fall in frequency.
People simply opening the door, interrupting operation, could well cause this odd radiation. Yanking the door open shuts down the voltage on the cathode of the magnetron, but the electron cloud in the resonator takes a short time to collapse because the cathode is still hot, and still can emit electrons as the voltage falls. Therefore the magnetron will continue to resonate until both the voltage goes to zero and the cathode cools down.
A microwave oven doesn’t cease to radiate instantly when the electricity drops off. The timescale for the cessation is not well documented. It’s a contest between the L/R timescale of the electrical circuit and the cooling of the cathode. The Peryton signals last a few tenths of a second, which could be consistent with the fall time of the voltage and the time for the electron cloud to collapse. The frequency shifts as the voltage falls because a resonant condition, which depends on the ratio of applied voltage to insulating magnetic field (V/B), is changing. (The magnetic field doesn’t change because it’s produced by a permanent magnet.) V/B is proportional to the circulation speed of electrons in the cavity of the magnetron, which relates to the resonant frequency of the device. (For more on magnetron operation, see High Power Microwaves, Second Edition, Benford, Swegle & Schamiloglu, Taylor & Francis, 2007).
Image: Microwave magnetron from a microwave oven. Credit: Jim Benford.
I was thinking that the radio telescope at Parkes looks at a small part of the sky. But it also has side lobes through which the telescope is less sensitive to signals at other angles. The most important of these is the back lobe exactly opposite to the direction in which the telescope is pointing. This occurs because any source directly behind the dish radiating signals will diffract around the edge of the dish. This diffracted signal will arrive coherently at the receiver at the focal point of the dish.
So I inquired as to what was directly behind the dish when it was pointed at the Peryton location. They said it was the Visitor Center.
I realized at once that the Visitor Center was the microwave oven location that would have the most use and fitted all the clues. That use would occur primarily in the midday. And that in the southern hemisphere winter, more people would visit Parkes in the outback.
So I made a prediction: That they would find that the Perytons were coming from the microwave oven in the Visitor Center. I suggested the Swinburne researchers could check on that by several tests:
1) The simplest thing would be to simply remove the old oven and replace it with a new one. The Perytons would cease. But that would require taking a lot of data over time to see if they had really disappeared since Perytons are infrequent phenomena.
2) They could replace all ovens with a non-microwave cooker. That’s also a slow approach.
3) A more aggressive approach would be to rewire the oven, to defeat the safety interlocks and turn the oven on, allowing it to radiate directly into the Visitor Center. Then the signal should be quite evident and they would see a lot of Perytons. Turning the oven off and on would prove it to be the source. (Of course one would evacuate people and whoever turns the oven on would need to be behind a conducting radiation shield.)
I hear the Swinburne team is going to conduct such experiments. I hope they get a clear result. If my hypothesis is proved true, it may call into question whether the famous Lorimer Burst of 2001 was in fact a Peryton. If so, it was not extragalactic, as its large DM was taken to mean.
Perhaps we shall soon know the origin of these mysterious signals.
The Burke-Spolaor paper on perytons is “Radio Bursts with Extragalactic Spectral Characteristics Show Terrestrial Origins,” Astrophysical Journal Vol. 727, No. 1 (2011), 18 (abstract). The paper on the ‘Lorimer Burst’ is Lorimer et al., “A Bright Millisecond Radio Burst of Extragalactic Origin,” Science Vol. 318 no. 5851 (2 November 2007), pp. 777-780 (abstract).
by Paul Gilster | Apr 3, 2015 | Outer Solar System |
An interesting model of planetary formation suggests that the architecture of our Solar System owes much to the effects of the giant planets as they migrated through the protoplanetary disk. Frédéric Masset (Universidad Nacional Autónoma de México) and colleagues go so far as to speculate that planetary embryos in orbits near Mars and the asteroid belt may have migrated outwards, depleting the region of materials that would become the cores of Jupiter and Saturn. The key is the heat an embryonic planet generates in the protoplanetary disk.
Writing in Nature, the authors describe computations that model what happens to the rocky cores that will become gas giants. Tidal forces affecting planets in the protoplanetary disk have been thought to cause them to lose angular momentum, making their orbits gradually decay. The migration in this case should be inwards toward the star. But the researchers’ model takes heat generated by material impacting onto the planetary embryos into account, a factor that may slow and can perhaps reverse migration.
Image: An artist’s impression showing the formation of a gas giant planet in the ring of dust around a young star. The protoplanet is surrounded by a thick cloud of material so that, seen from this position, its star is almost invisible and red in colour because of the scattering of light from the dust. Credit: ESO/L. Calçada.
Remember that a gas giant is composed of a small rocky core surrounded by a huge envelope of gas. The ‘heating torque’ the authors describe works at high efficiency when the mass of the embryo — which will become the core — is between 0.5 and 3 Earth masses, a useful number because this is the mass range needed for such a core to develop into a Jupiter-class world once it has migrated outwards. Studying mass and density of the protoplanetary disk near the forming planet, the authors show that the embryo heats the disk near it, creating regions that are hotter and less dense than surrounding material.
From the paper:
This situation favours the lobe that appears behind the planet: its material approaches closer to the planet, receives more heat and is consequently less dense than the other lobe, leading to a positive torque on the planet… The heating torque therefore constitutes a robust trap against inward migration in any realistic disk, when accretion rates are large enough.
As for the depletion of material and its possible signs in the asteroid belt, the authors note that the heating torque they describe should be less efficient on the warm side of the ‘snow line’ (the distance from the star allowing water ice to form) because the opacity of the disk drops there. But planetary embryos that formed beyond where the snow line will eventually be should have experienced strong heating torque and thus outward migration, factors that should cause a depletion of solid material in the region. Jupiter’s rocky core may thus have formed within the region where the main asteroid belt is today before migration set in.
Estimates of the snow line in our Solar System range from 2.7 AU to 3.1 AU, while the main asteroid belt lies between 1.8 and 4.5 AU. By this theory, we can expect to find a similar depleted region inside the orbit of the first giant planet in many planetary systems.
The authors suggest that migration can take two routes: Planetary embryos between 0.5 and 3 Earth masses avoid inward migration as long as accretion rates in the disk are high. When accretion rates are low, embryos undergo inward migration, though at a slower rate. So we have two types of behavior depending upon the accretion rate of the embryo within the disk. The researchers find that accretion rates that correspond to a mass-doubling time of less than roughly 60,000 years produce outward migration on objects in the size range specified.
These two behaviors may help to explain the correlation between giant planets and the heavy metal content (metallicity) of the host star. From the paper:
…since the heating torque scales with the accretion rate and the accretion rate, in turn, scales with the amount of solid content (a proxy of which is the metallicity), protoplanetary disks with larger metallicity will engender planets that can avoid inward migration and grow to become giant planets. In contrast, embryos born in lower-metallicity environments cannot avoid inward migration, leading to results as hitherto found in models of planetary population synthesis, with low yields of giant planets and ubiquitous super-Earths.
The authors note, though, that the relation of super-Earths to host star metallicity is still controversial, and add that they have not performed calculations on embryos forming at very small orbital distances, although heating torque in these regions would likely be high. To learn more about the consequences of heating torque will require more data on protoplanetary disks and the embryos moving within them. A mechanism that explains giant planet formation and associated migration would be a welcome addition to our toolkit for exoplanet research.
The paper is Benitez-Llambay et al., “Planet Heating Prevents Inward Migration of Planetary Cores,” Nature Vol. 520 (2 April 2015). Abstract available. For another take on our Solar System’s seemingly unusual architecture and what explains it, see Batygin and Laughlin, “Jupiter’s decisive role in the inner Solar System’s early evolution,” Proceedings of the National Academy of Sciences, published online February 11, 2015 (abstract). The latter is ably described by Lee Billings in Jupiter, Destroyer of Worlds, May Have Paved the Way for Earth.
