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.
How does this fit with the “Nice” and the “Jumping grand tack” models ? Could the Earth be a Habitable Evaporated Core . What the HEC!?
If we continue observing and studying a particular protoplanetay disc for lets say 1 million years, we may have observed a clumping if disc material that would support a range of current theories of planetary formation.
This is not to say that speculation isn’t useful. But, it should be made CLEAR that all of our current theories about planetary formation are not verifiable within the next 10M years using the SCIENTIFIC METHOD. We just haven’t been observing long enough to say with “certainty” how planets, or for that matter solar systems, form and evolve. That ability to verify even extends to galactic formation, motion and evolution despite the “certainty” afforded the Big Bang Theory.
My hope is that before I die (I’m 61) humanity will have at the very least sent more robotic missions to the moons of Jupiter and Saturn, land and perform useful experiments that will advance our knowledge regarding the certainty of LIFE beyond our planet.
Might be a silly question but if the heated gas giants can affect the accretion disc how come the moons around Jupiter and Saturn which are made mostly of volatile ices are there, surly they are close enough to have been affected by this heat as well.
Well said. The limitation is finance most frustratingly . Nasa are still paying off JWST and will be some time to come. They got lucky with the “free” mirror for WFIRST. Trouble is they tend to operate on a feast or famine basis with nothing in between. Europa clipper, very much of Cassini and Galileo heritage is projected to cost $2 billion for a big science return 3 year primary mission. At present the “flagship” programme is suspended. I would reintroduce it but capped at $2 billion. Budgetary constraints on smaller Discovery and New Frontiers have produced some great missions ( Kepler for one ) through innovation. ( necessity is the mother of invention ?) . Yet looking at the next New Frontiers round , one concept has a Saturn atmospheric probe . Great idea , but an 8 year journey and just 50 minutes of science for $1 billion ! You can see where the cut off for Saturn or Jupiter missions lies ! The arrival of the cheap Falcon Heavy will help as it will reduce transit times to the outer solar system thus reducing mission “operations” and engineering costs on top of its already cheap price. Nano sats/cube sats will help keep costs down too as will new lightweight materials . The biggest obstacle at present is power. Not practical for solar power that far out and limited stocks of the plutonium 238 necessary for Radioisotope Thermal Generators necessary to operate any spacecraft in the outer solar system environment.
It is fascinating to read about all these new models of planetary formation. To follow up Black Sci-Fi’s point, how are we going to test/discriminate between these models and theories. How would we test them in our own solar system for example?
Black Sci-Fi, you are setting an absurdly high bar for the “scientific method” to verify a process (continuous observation over the process time). By that logic we can’t verify anything in the universe, since humans have only been observing it scientifically for a few hundred years. The fact is that we observe similar objects at different stages of development and can form many testable theories about processes over long time scales.
No need to be too downhearted. A Lot is being done already. We have come a long way in the last twenty to thirty years and progress is picking up speed. I think more will be achieved in the next fifteen years than the last thirty which to be fair haven’t been at all bad !
. Studying ancient solar system bodies like Ceres.Vesta and Pluto/Charon and other KBOs afterwards as well as Rosettas comet mission and others to come . The 2021 asteroid capture mission too isn’t just about Mars prep it will allow analysis of returned samples . They tell the story of where and when they were formed. The structure of exoplanet systems helps too by way of comparison.
The main thing missing is an ice giant mission to Uranus or Neptune. These planets type gas been shown in different sizes to be the commonest in the known Universe so constraining their nature will add a great deal to our knowledge of planets and the dynamics between them. The Nature article Paul cites above is all about the interaction between planets and the protoplanetary disk which helps build a picture of solar system architecture and the nature of the planets themselves , especially the outer planets, tells us where they formed.this in conjunction with exoplanet science allows a picture of interplanetary dynamics to be built .
Juno’s mission to Jupiter will tell us a lot about Gas giants as will the various icy moon probes of the next decade as well as more info on icy bodies. Meanwhile larger , more sophisticated telescopes like the ELTs,JWST ,WFIRST and Gaia ( armed with uber sensitive spectroscopes ) will reveal immense amounts the protoplanetary disks of young systems .
The use of infrared as the wavelength of observation choice is because it isn’t absorbed easily over large distances and allows pictures of new planets forming inside large optically opaque dust clouds. This is why JWST operates predominantly in the IR as do other large space telescopes like Herschel and Spitzer previously and SPICA to come. IR telescopes revolutionised astronomy in conjunction with hugely improved processing power that also allows increasingly accurate simulations.( more than I would like and I concede there isn’t enough big telescopes anywhere)
Gaia particularly will take the crucial science of astrometry to new levels , for stars predominantly ( but see below) but also gas and ice Giants . Future missions as soon even as WFIRST will do sensitive astrometry for nearby Earth sized planets too . WFIRST will also visualise planets thanks to its state of the art coronagraph that allows contrast imaging unimaginable even 5 years ago and on a steep upward curve already . It will visualise their systems and with the help of JWST will spectroscopically analyse both planets and the protoplanetary disks from which they form. It’s targets will be provided initially by the new generation of advanced ultra sensitive laser comb spectroscopes like ESPRESSO ,as will TESS in just two years time.
The ALMA sub millimetre and Square kilometres arrays will image protoplanetary disks with unprecedented accuracy at molecular wavelengths showing their chemical content ( organic in particular like Poly aromatic hydrocarbons and amino acids) all in under ten years too,.
Meantime the often forgot science of asteroseismology is developing at a phenomenal rate and reveals the nature of stars in increasingly precise detail. The one thing that determines planetary formation more than anything is Stars themselves . Learn about them and you learn about their planets . Thats how “transit” missions work .That’s what missions like Kepler ,Corot and PLATO have or will do, though this gets forgotten amid the excitement of a further increase the planetary discovery totometer.
TESS doesn’t do astrometry but with JWST can analyse the atmospheres of Super Earths perhaps with a touch of luck even finding biosignstures ( ever the optimist but MIT are too ) which will immediately attract more funding .Given the financial constraints meantime I don’t think that’s bad. By 2030 when hopefully a large space ELT is launched we will know of tens of thousands of planets , planetary systems and protoplanetary disks . Several hundred or more will have been analysed in detail in and positioned with unprecedented accuracy , and all the same for their stars too. I too would like to see more though so I’ve got my fingers crossed like mad for TESS and JWST short term.
Ashley, I grew up with diner table discussions about NASA and the robotic planetary missions of the 60’s and 70’s because my dad worked for NASA on mechanical designs (cosmic ray detectors, etc) for every planetary robotic mission up to and including Voyager (1-2) . So, my lack of patience is highly personal with regard to the cosmic timescales of factual discovery and the human lifetime timescale of practical applications and explainations that are the foundation of absolute truth.
I’ve been on a journey through the theoritical to the point of fatigue with BBT, CDM, CBR and even as far out as Plasma Cosmology and The Electric Universe. For every cosmological origin theory that is promising, the chinks in the the theories are just as demoralizing. But, what has become clear, at least to me, is that theories are circular in nature. They are announced, peer reviewed, expanded upon and then, more often than not, discarded. Then, someone dusts off an old theory (Velikovsky?) and find useful ideas therein to expand upon their “discovery” (see above planetary migration theory) that turns out to be “very” useful in explaining what was once peer reviewed (St. Sagen) into oblivion.
I will continue to use ALL AVAILABLE INFORMATION to find links to understanding the cosmos and I hope that our great government research labs and university think tanks will do the same. After all, a robotic mission to settle the theoretical concepts of Plasma Cosmology shouldn’t be as expensive as building and manning a lab in a mine in the North Pole designed to discover dark matter. Seriously, that research grant application must have been something else.
Thanks for your detailed reply. I will use that information as I continue my quest for understanding.
I have to agree on some of the esoteric projects that get approval. The ESA are worse. Post JUICE there is a decade of such obscure projects with no wide appeal which is surely what tax payers money warrants. No one will like everything and some will like nothing but publicly funded science needs to justify itself and appeal to a majority.
That’s why I like exoplanets so much . It’s a genuinely new and exciting field where new progress is made towards a very exciting endpoint and everyone I share my interest with has an opinion on or to whom , and I paraphrase the Starship Troopers Heinlein movie spoof ,” do you want to know more?” applies. Critically progress that everyone can understand and relate to also.
I’m just glad that NASA decided to go the extra mile in “truthiness” and now uses the term “earthlike” when talking about the goals of exoplanet exploration and the exoplanets discovered thus far. Now, if every article they publish had a realistic distance/time scale attached we’d go from truthiness to truth with regard to “habitability. You can’t inhabit what you can’t reach.
To the general public “almost earth sized or habitable zone” can cover a lot of unfriendly space.
ATC, I’m wondering why NASA never tried to put a space telescope on an earth crossing asteroid with an excentric orbit. Can you imagine the pictures and data such a probe would deliver well into the next century. Considering the benifit, this type of project could be funded with a combination of public (NASA, ESA, etc) and private (Space-X, SETI, etc) funding and put a JWST type of instrument on an asteroid.
If the taxpayers don’t fund “obscure projects with no wide appeal”, who will? Private industry at best will fund projects that have some obvious profit, and at worst will fund nothing. Who’s left? It may well be that only (say) 5% of science will ever have any practical application or wide appeal, but it’s impossible to say in advance which 5% that will be. If so then (if science is to be done at all) we taxpayers just have to bite the bullet and fund a lot of obscure (but scientifically important) things that in ways we can’t imagine now may someday change the world.
Point conceded. Always the case of what is considered valuable and by who though especially trying to extrapolate. I’m sure there are many examples of projects that nearly ended and went on to be major successes. The important thing is obviously to be able to defend any project when its value is questioned by those who know less about it. Sadly astronomic knowledge is less in the general population than we would all like , which is why sites like Centauri Dreams are so important to spread the word and inform a wider audience . Challenges to specific projects are always possible with government funding when times are hard as they are perceived as easy targets .We all know about Cassini and the suggestion it might be shut down early .The UKIRT on Mauna Kea is a classic and sad example of this , a great loss to UK astronomy but still working under different ownership atleast. Jodrell Bank came within a whisker of closing 5 years ago before winning the contract to run the SKA- the director had had the foresight to anticipate the coming storm . There are lots of examples because there is so little money to go round .It’s the same in medicine with rare illnesses. Often rich philanthropists help as with Bill Gates with the LSST. Ironically , some non mainstream research ends up costing less because of its narrow focus and short term requirements, which need justification and accurate business cases to gain their funding. It’s always important to be ready to justify expenditure .
NS, the case for funding beautiful science rather than practical science is the same as that for art. The practical answer is to just burn the great masterpieces, and apply our wasted efforts into infrastructure improvements.
I bags that the true problem for science is too little effort to make it accessible to the common man.
The “beautiful science” and the “practical science” are part of the same process, and it’s impossible to know in advance which is which. They sink or swim together. If we fund only the beautiful science of today, we won’t acquire the practical science knowledge we need to create the beautiful science of tomorrow.
How Alien Planets Can Change Each Other’s Habitability Over Eons
by Adam Hadhazy, Astrobiology Magazine | April 07, 2015 07:01 am ET
A new study sheds light on how exoplanets in tightly-packed solar systems interact with each other gravitationally by affecting one another’s climates and their abilities to support alien life.
Because the exoplanets are so close to one another in these compact solar systems, they have tidal influence, much like the Earth and the moon have on each other. The tides modify the spin rates, axial tilts and orbits of these planets over time, and therefore alter their climates.
The study examines two exo-solar systems — Kepler-62 and Kepler-186 — that have made headlines for hosting worlds orbiting in the “habitable zone,” the potentially life-friendly band where water can remain liquid on a planetary surface. The findings show that tidal evolution can profoundly impact a world’s climate.
Full article here:
http://www.space.com/29027-alien-planets-habitability-climate-change.html
Visual inspection of the diagrams in the New York Times tally of Kepler planets supports Batygin and Laughlin’s hypothesis (http://www.nytimes.com/interactive/science/space/keplers-tally-of-planets.html?_r=0 ) .
In most of the extrasolar systems where Jupiter-sized worlds exist, we see a single gas giant in a tight orbit around its star (top two rows of the page). This suggests inward migration proceeded uninterrupted in these systems, sweeping away the smaller inner planets. Where families of super-Earths are observed, there are no large Jupiter-sized gas giants orbiting beyond the super-Earths, suggesting these planets were preserved because no Jupiters formed in these systems.
This still raises the questions:
1) Why do Jupiters-sized gas giants form in some systems but not in others, and
2) Why were we so lucky to have a second large gas giant (Saturn) form beyond Jupiter in our solar system?
Windbots to explore Jupiter and other worlds with hefty atmospheres?
http://www.jpl.nasa.gov/news/news.php?feature=4662