How planets form is not an issue that will be settled any time soon, but two models have emerged that continue to energize research. We saw yesterday in a review of Alan Boss’ new paper that gravitational instability is one way to create a gas giant. But I spent most of yesterday’s post talking about UV radiation and its effects on the atmospheres of planets around M stars, a key part of Boss’ explanation of so-called ‘super-Earths’ in these environments.
So let’s back up and talk about gravitational instability itself. As early as 1997, the astrophysicist had proposed that planet-sized clumps could form relatively quickly due to instabilities in the disk of dust and gas surrounding a young star. Boss believed these clumps could be massive enough to form a gas envelope, but the model was hard to use in any predictive sense and demanded more intensive computer simulations than were then available.
Later work by Thomas Quinn, however, bears Boss out. Quinn (University of Washington) set up simulations of a protoplanetary disk, or proplyd, using powerful new software developed by his team and 30,000 processor hours on LeMieux, the Pittsburgh Supercomputing Center’s terascale system. The result? Well, let Quinn tell it: “We used a new model of planet formation that couldn’t adequately be tested without this kind of computing power, and we found that these giant planets can form in hundreds of years, rather than the millions that the standard model predicts.”
Image: Simulations by Quinn and colleagues shows how a protoplanetary disk surrounding a young star begins, in a relatively short time, to fragment and form gas giant planets with stable orbits. Credit: Pittsburgh Supercomputing Center.
Got that? Hundreds of years. Which is quite a change from the older core-accretion model. The latter is a much lengthier process that relies on clumps of solids forming within the young protoplanetary disk; these merge gradually into larger and larger pieces due to the force of gravity. In a million years or so, you get a planet, and some of these objects will grow, over perhaps ten million years, into Jupiter-class gas giants. The catch has always been that a gaseous planetary disk like this doesn’t seem to last long enough to allow the model to function over these time scales. And that’s one reason we have a battle between differing models of planetary formation.
Greg Laughlin (UC-Santa Cruz) sees serious problems with Boss’ model, though he agrees that the process can be involved in some giant planet formation, and he pays particular attention to the Gl 876 system we talked about yesterday. “Another important point to stress is that Alan’s simulations certainly aren’t in error in the sense of being computationally wrong,” says Laughlin. “It’s just that I don’t agree with the generic validity of the initial conditions.” Read all of Laughlin’s comments on gravitational instability here.
The key paper by Boss is probably “Giant Planet Formation by Gravitational Instability,” Science 276, 1836-39 (2002). And here’s a quick take from Scientific American on the same subject. Quinn’s work is found in Mayer, Quinn, Wadsley et al., “Formation of Giant Planets by Fragmentation of Protoplanetary Disks,” Science 298, 1756-59 (2002).
Hi Paul
There is another way to trigger the gravitational instability – gravitational interactions between stars in their common birth nebula. The density of such nebula can reach 100,000 stars per cubic parsec, making near misses between the stars a near certainty. Other researchers have modelled this process quite extensively and it seems to produce planets and brown dwarfs pretty efficiently.
One version of the theory that I have studied for the past few years is Michael Woolfson’s Capture Theory, which is a version of the above that tries to solve the angular momentum problem. Instead of the planets forming out of the disk leftovers of the Sun’s formation, they’re captured from the extended disk of a lower mass protostar. The orbits of the protoplanets are initially large and highly eccentric, lasting hundreds of years and allowing them to condense sufficiently to survive the tides at perihelion, but they encounter drag as they pass through the Sun’s own disk and thus circularise in a few million years. This process involves a lot of precession causing the planets to undergo a few near misses (i.e. Uranus’ axial tilt) and a collision.
The inner two protoplanets collided to form Earth and Venus from their remnant cores. Mercury, Mars and the Moon (or Theia) formed as moons of the two planets. Mercury massed similar to Mars, but was abraded by the collision’s debris. Similarly Mars suffered some abrasion which gave it its hemispheric assymmetry. The rest of the collisional debris was eventually spread far and wide as the Belt and comet clouds.
One aspect of the collision is the formation of the anomalous isotopes in meteoroids by a brief period of nuclear fusion in the interface between the two colliding planets.
That’s Woolfson’s theory in summary, but I suspect it’s not the only possible scenario. However Disk-disk interactions should be pretty common in the early stages of star formation and I think the ‘quiescent’ models of planet formation assume stars that are too isolated. Even Greg’s ideas, AFAIK, require large O stars to ‘burn off’ the nebula gases to leave behind the Ice Giants. Tidal forces are probably as influential as UV light.
One aspect of the collision is the formation of the anomalous isotopes in meteoroids by a brief period of nuclear fusion in the interface between the two colliding planets.
I’m sorry, but that sounds completely crackpot. The temperature in a planetary collision will not reach anywhere near the level needed to fuse any significant amount of material, particularly of heavy elements.
Hi pfdietz
Firstly, his papers are mostly available online via NASA’s ADS so check them out for yourself.
Secondly, we’re talking temporary temperatures reached under extreme shock-driven pressures in planets that are mostly light elements like the other giant planets. Woolfson and colleagues did the maths on the reactions and simulated the collision – those are the temperatures and conditions they got. Very rapidly the high temperatures forced the two apart explosively.
Adam
I still don’t buy it. Let’s look at the numbers.
The escape speed of Jupiter is around 60 km/s. If infalling material has a neutron/proton ratio of about 1 (generous; hydrogen would be lower) and reaches a temperature where it is fully ionized, then completely converting the kinetic energy at this speed into thermal kinetic energy of the electrons and ion will give a mean thermal energy of 19 eV/particle. The easiest fusion reactions won’t be detectable until the temperature is nearly two orders of magnitude higher than this, and fusion involving all but the lightest nuclei will not be significant for several orders of magnitude beyond that.
(Indeed, the temperature required for fusion reactions involving many heavy nuclei is so high that it’s impossible to achieve in uncompressed or lightly compressed material in thermal equilibrium — the energy lost to the bath of blackbody radiation that coexists with the material in equilibrium goes as T^4 and will be impossibly large, exceeding the rest energy of the material! Massive stars get around this by doing fusion of heavy elements at very low rates and/or in high density degenerate matter.)
Perhaps you could save this idea if there is some reaction involving fusion of light nuclei that would produce neutrons that would be thermalized, then absorbed on the heavier nuclei. The obvious candidate would be DD fusion, but the D is diluted with lots of ordinary hydrogen, and I understand there are severe constraints on the exposure of protosolar materials to neutrons (look at variations in isotopic composition of the most neutron-sensitive elements, like samarium and gadolinium.)
Ok, I thought about this some more, and I see there is a way to heat some of the material to a temperature greater than would be produced by its own kinetic energy.
When the planets first collide, shock heating will cause the lower density material at the contact point to become moderately warm, as indicated before. But this gas is still at rather low pressure. If it cannot get out of the way, it will then be subject to increasing pressure, adiabatically compressing it and increasing its temperature still further. This could very well increase its temperature to the keV level.
I presume the nuclear reactions that would be of interest would be the (p,alpha) reactions on 15N and 17O, which would affect the abundance of those isotopes.
Astrophysics, abstract
astro-ph/0702549
From: Jeffrey Oishi [view email]
Date: Tue, 20 Feb 2007 23:14:05 GMT (707kb)
Turbulent Torques on Protoplanets in a Dead Zone
Authors: Jeffrey S. Oishi, Mordecai-Mark Mac Low, Kristen Menou
Comments: 34 pages, 12 figures. submitted to ApJ
Migration of protoplanets in their gaseous host disks may be largely responsible for the observed orbital distribution of extrasolar planets. Recent simulations have shown that the magnetorotational turbulence thought to drive accretion in protoplanetary disks can affect migration by turning it into an orbital random walk. However, these simulations neglected the disk’s ionization structure. Low ionization fraction near the midplane of the disk can decouple the magnetic field from the gas, forming a dead zone with reduced or no turbulence. Here, to understand the effect of dead zones on protoplanetary migration, we perform numerical simulations of a small region of a stratified disk with magnetorotational turbulence confined to thin active layers above and below the midplane. Turbulence in the active layers exerts decreased, but still measurable, gravitational torques on a protoplanet located at the disk midplane. We find a decrease of two orders of magnitude in the diffusion coefficient for dead zones with dead-to-active surface density ratios approaching realistic values in protoplanetary disks. This torque arises primarily from density fluctuations within a distance of one scale height of the protoplanet. Turbulent torques have correlation times of only $\sim 0.3$ orbital periods and apparently time-stationary distributions. These properties are encouraging signs that stochastic methods can be used to determine the orbital evolution of populations of protoplanets under turbulent migration. Our results indicate that dead zones may be dynamically distinct regions for protoplanetary migration.
http://arxiv.org/abs/astro-ph/0702549
Jupiter’s Core Bigger And Icier Than Thought
Red Orbit
http://www.redorbit.com/news/space/1602717/jupiters_core_bigger_and_icier_than_thought/
November 25, 2008
Jupiter has a rocky core that is more than twice as large as previously thought, according to computer calculations by a UNIVERSITY OF CALIFORNIA, BERKELEY, GEOPHYSICIST who simulated conditions inside the planet on the scale of individual hydrogen and helium atoms….
The simulation predict the properties of hydrogen-helium mixtures at the extreme pressures and temperatures that occur in Jupiter’s interior, which cannot yet be studied with laboratory experiments.
Applying techniques originally developed to study semiconductors, UC BERKELEY’S BURKHARD MILITZER, AN ASSISTANT PROFESSOR OF EARTH AND PLANETARY SCIENCE AND ASTRONOMY, calculated the properties of hydrogen and helium for temperature, density and pressure at the surface all the way to the planet’s center…. The research was supported by the National Aeronautics and Space Administration and the National Science Foundation.