Figure 1. Les Grands Zig-zags. The assembly and migration of Jupiter and Saturn through the primordial Solar nebula, according to a scenario presented by Sean Raymond & colleagues (2016).Numbers alongside the concentric semicircles indicate radii in astronomical units (AU), where 1 AU equals the radius of Earth’s present orbit and 5.2 AU equals the radius of Jupiter’s present orbit.
This one slipped past me during the excitement of the spring music festivals: a new contribution by Sean Raymond & colleagues to the ongoing debate over the history of the inner Solar System. Here’s the background:
Our Solar System is weird. Among its most notable oddities is the dearth of mass inside a radius of one astronomical unit (AU, where 1 AU equals the average separation between Earth and Sun). Outside our system, the nearest G-type star known to harbor planets is 82 Eridani, at a distance of 20 light years. This metal-poor G8 star supports three planets with an aggregate mass at least 10 times Earth (10 Mea) inside an area equivalent to the orbit of Mercury. The next-nearest G-type host is 61 Virginis, located about 28 light years away. This G5 star hosts three planets with an aggregate mass in excess of 46 Mea inside the equivalent of the orbit of Venus. Both of these architectures are typical of multiplanet systems discovered by radial velocity and transit searches.
By contrast, the three planets closest to our Sun – Mercury, Venus, and Earth – have an aggregate mass smaller than 2 Mea.
Last year, two studies tried to find an explanation for the missing mass. Batygin & Laughlin (2015), hereafter BL15, proposed a solution that extends the popular scenario of the Grand Tack, which has been advocated by Pierens & Raymond (2011) and is discussed here and here. BL15 hypothesized that several Super Earth-type planets formed inside the present orbit of Mercury during the first few million years after the birth of our Sun, when it was still surrounded by an extensive nebula. (In this context, Super Earths should be understood as rocky or gassy planets of a few Earth masses.) No sooner had these objects assembled than the growing core of Jupiter began migrating from the outer Solar nebula to the vicinity of the present orbit of Mars. Young Jupiter’s inward passage swept a huge swarm of planetesimals into the inner system, causing perturbations that destabilized the orbits of the Super Earths. As a result, both the planets and the planetesimals were engulfed by the Sun. At this point, Jupiter executed a dramatic course change (the Grand Tack itself) which carried it back into the outer system on the nebular tides, courtesy of an intimate connection with its smaller and chillier sidekick, Saturn. Meanwhile, the inner system catastrophe left behind a ring of debris that coalesced into the four terrestrial planets we know today.
Volk & Gladman (2015), hereafter VG15, also proposed that our system originally harbored a cluster of planets inside the present orbit of Mercury (“intra-Mercurians”). In all other ways, however, their model differs markedly from BL15. To start with, their intra-Mercurian objects are similar in mass to Earth and Venus, not to Super Earths like 82 Eridani b or 61 Virginis b. A more striking difference is that VG15 implicitly reject both the Nice Model and the Grand Tack, since their scenario does not involve the outer Solar System at all. Instead, VG15 suggest that the four terrestrial planets assembled exactly where we see them today, outside the orbits of the intra-Mercurian planets, with which they peacefully coexisted for tens of millions of years. Then, one bright millennium, an abrupt dynamical instability upset the apple cart. The intra-Mercurians devolved into a donnybrook of orbit crossings and collisions that rapidly ground them into dust. The dust itself was then engulfed by our Sun, while the four terrestrial planets continued circling the scene of the catastrophe like horrified onlookers. The only remaining evidence of those disintegrated inner planets can be found in the cratered surfaces of Mercury, the Moon, and Mars, which were pelted by fragments during the period of annihilation. In sharp contrast, the Nice Model argues that these rocky worlds were etched by a storm of asteroids that originated in a dynamic instability among the four outer planets – Jupiter, Saturn, Uranus, and Neptune.
Figure 2. Inside-out planet formation
This image appears as Figure 1 of Chatterjee & Tan 2014. (i) The magnetic field of the young star creates a cavity in the center of the gaseous protoplanetary disk. Immediately outside the cavity is a “dead zone” through which pebbles drift. (ii) Pebbles accumulate in a ring around the edge of the cavity, where the gas pressure is at its maximum. (iii) The pebble ring coalesces into an Earth-size planet. (iv) The dead zone retreats from the star, creating a new pressure maximum at a larger radius where new pebbles accumulate and potentially form a new planet. Abbreviations: MRI = magnetorotational instability; P max = pressure maximum.
pebble pile-up with outward migration
Raymond & colleagues (hereafter R16) take a completely different approach to the problem, one that does not involve either a primordial clutch of small planets or an inner system catastrophe. Although R16 note that their model is consistent with the Grand Tack, they emphasize its theoretical independence. Their starting point is actually the scenario of “Inside-Out Planet Formation,” as presented in an article of the same name by Sourav Chatterjee and Jonathan Tan (2014). Figure 2 provides a high-level summary.
R16 begin with the very earliest stages of accretion in the Solar nebula. They hypothesize that a large rocky planet assembled out of pebbles orbiting near the inner edge of the nebula, at an approximate semimajor axis of 0.1 AU. This object was proto-Jupiter (Figure 2.iii). Once it attained a few Earth masses, it was subject to torques exerted by the ambient gases, which caused it to migrate outward to a semimajor axis of about 5 AU. Along the way, the migrating core cleared all solid material from the nebula interior to 1 AU, shepherding some of it onto exterior orbits. This shepherding process likely resulted in the formation of a second core – proto-Saturn – which young Jupiter continued to herd during its journey into cooler regions of the nebula. Both objects accreted mass during this process, eventually initiating the runaway accretion of extensive envelopes of hydrogen and helium (H/He).
As R16 argue, this scenario explains why the area inside Mercury’s orbit is completely empty of mass. The rocky material that originally accumulated in this space was accreted by young Jupiter, which continued accreting and scattering planetesimals as it ascended beyond the radii that would later mark the orbits of Earth and Mars. Once Jupiter attracted sufficient H/He to become a gas giant planet, it opened a gap in the surrounding nebula, initiating the process of Type II migration. Then, having arrived in the cool zone, Jupiter immediately turned around and retraced its path.
By this stage in the narrative we’ve reached the threshold of the Grand Tack. Saturn was now following Jupiter instead of being herded ahead of it, and the gap between the two planets kept narrowing until their orbits entered a mean motion resonance. At that point, when Jupiter had reached a radial distance of about 2 AU (Brasser & al. 2016), the direction of their migration switched again and they sailed into the outer nebula for the last time.
Figure 3. Jupiter Ascending with Ganymede
A boy and his eagle: Ganymede & Jupiter. In Greek mythology, Ganymede was a Trojan prince of uncommon beauty. Spying him from on high, Zeus (Latin Jupiter) assumed the form of an eagle, swooped down to Earth, and carried the boy back up to Mount Olympus, where Ganymede became the cupbearer of the gods. The beverage he served in heaven was nectar, a delicious liquid that confers immortality. In 1906, the Anheuser-Busch Brewing Ass’n appropriated the myth of Ganymede to promote their own version of heavenly nectar: Budweiser beer, now one of the most popular alcoholic beverages on Earth. Credit: Wikimedia.
The full scenario involves a double zigzag that calls to mind the (Figure 1). Jupiter’s outward-inward-outward path also recalls a Greek myth in which Zeus (the Greek equivalent of Roman Jupiter) swooped down into the terrestrial zone, abducted a youth named Ganymede, and carried him aloft into the upper spheres of heaven (Figure 3). Indeed, it seems likely that Jupiter’s satellite system – of which Ganymede is the most prominent member – formed just as the Solar nebula was dissipating (Alibert & al. 2005). In the Grand Zigzag scenario proposed by R16, this process is constrained to occur when Jupiter arrived in the vicinity of its present orbit, potentially bearing a circumplanetary disk of solids (proto-Ganymede) enriched by the planet’s wanderings through the nebula.
Wisely, R16 ask whether this intricate sequence of events is “a generic process” or a relatively rare occurrence “confined to only a limited range of conditions.” While they lean toward the first option, they concede that planetary systems containing both an inner system of Super Earths and an outer system of gas giants would be challenging to explain in the context of the Grand Zigzag. In this regard I hasten to note that Chatterjee & Tan (2014) originally developed their model of inside-out planet formation to explain the architecture of tightly packed systems of low-mass planets such as Kepler-11 and Kepler-20, not mixed-mass systems resembling HD 219134, Kepler-167, or our Solar System. The adoption of this approach by R16 appears quite novel in the context of the prehistory of the Solar System, where its application - in my view – seems tortuous.
critique of competing models
My favorite section of R16 is their discussion of alternative explanations for the missing mass in the inner Solar System. The authors make quick work of two studies to which they themselves contributed: Morbidelli & al. 2016, which explains the void as a lingering effect of the condensation front for silicate dust in the primordial Solar nebula, and Izidoro & al. 2015, which argues that the formation of Jupiter blocked the migration of solids from the outer system, starving the inner system of the mass it needed to form short-period Super Earths. Neither approach holds up under their analysis. I find this skeptical attitude particularly impressive, since healthy self-criticism is essential in scholarly work.
R16 dispose of VG15 with similar ease, arguing that the furious impacts invoked to explain the disappearance of several primeval planets “would not have fallen in the ‘super-catastrophic’ regime” needed to achieve total annihilation. Instead, the debris from any erosive impacts would simply be swept up by the remaining planets in the inner Solar System. R16 thereby confirm my own doubts that a whole subsystem of planets could vanish without a trace, but they do so with a better-informed argument than I could hope to make. As they conclude, “we do not expect that a system of close-in terrestrial planets could self-destruct.”
R16 devote considerably more space to their takedown of BL15. First, they find that the “massive pulse of collisional debris” generated by Jupiter’s inward migration in that scenario would merely accelerate mass accretion by any planets forming in the inner system. Second, they argue that this swarm of debris would not be physically capable of shepherding a clutch of Super Earths into ever-shrinking orbits. Instead, they suggest that this catastrophic outcome was just an artifact of the simulation code used by BL15. In reality, they contend, a mass of planetesimals would “self-interact and grow” rather than push a planet to the brink of its host star’s gravity well. Finally, R16 challenge the notion that planets can simply be pushed into their stars, because all evolving protoplanetary disks develop a cavity interior to about 0.1 AU where gas dynamics cease. An object forced into this void, they say, would be more likely to stabilize on a new orbit than to set the controls for the heart of the Sun.
In parting, R16 note a striking irony in the model presented by BL15. Whereas Batygin & Laughlin “invoke the rapid inward drift of solids to destroy super-Earths,” many other models have proposed the same process to create them.
Figure 4. Jupiter Ascending (2015)
Twenty-first century superstars Channing Tatum and Eddie Redmayne appeared in this star-crossed science fiction epic. “Jupiter” herself was played by Mila Kunis. All three actors have seen much more success in other movies.
Sometimes I wonder if I’m getting mental whiplash from the barrage of new theories about the evolution of our own planetary system and others. Nevertheless, when it comes to activities of the mind, I’d prefer an embarrassment of riches to an empty cupboard!
REFERENCESAlibert Y, Mousis O, Benz W. (2005) Modeling the Jovian subnebula. I. Thermodynamic conditions and migration of proto-satellites. Astronomy & Astrophysics 439, 1205-1213.
Batygin K, Laughlin G. (2015) Jupiter’s decisive role in the inner Solar System’s early evolution. Proceedings of the National Academy of Sciences 112, 4214-4217. Abstract: 2015PNAS..112.4214B
Brasser R, Matsumura S, Ida S, Mojzsis SJ, Werner SC. (2016) Analysis of terrestrial planet formation by the Grand Tack model: System architecture and tack location. Astrophysical Journal 821, 75.
Chatterjee S, Tan JC. (2014) Inside-out planet formation Astrophysical Journal 780, 53.
Izidoro A, Raymond S, Morbidelli A, Hersant F, Pierens A. (2015) Gas giant planets as dynamical barriers to inward-migrating Super-Earths. Astrophysical Journal Letters 800, L22.
Morbidelli A, Bitsch B, Crida A, Gounelle M, Guillot T, Jacobson S, Johansen A, Lambrechts M, Lega E. (2016) Fossilized condensation lines in the Solar System protoplanetary disk. Icarus 267, 368-376.
Pierens A, Raymond SN. (2011) Two phase, inward-then-outward migration of Jupiter and Saturn in the gaseous solar nebula. Astronomy & Astrophysics 533, A131. Abstract: 2011A&A...533A.131P
Raymond SN, Izidoro A, Bitsch B, Jacobson SA. (2016) Did Jupiter’s core form in the innermost parts of the Sun’s protoplanetary disk? Monthly Notices of the Royal Astronomical Society 458, 2962-2972. Abstract: 2016MNRAS.458.2962R
Volk K, Gladman B. (2015) Consolidating and crushing exoplanets: Did it happen here? Astrophysical Journal Letters, 806: L26. Abstract: 2015ApJ...806L..26V