Figure
1. “Jupiter
rules all things in heaven with his brilliant light, and with his warmth he caresses
all creatures. With his mighty hand that God scatters cruel missiles, flashing
fire as he hurls thunderbolts from his high citadel.” Image credit: The Warburg
Institute
----------
As the
extrasolar census approaches 2000 confirmed planets located in more than 1200 different
systems, it’s no longer possible to ignore the oddity of our Solar System. About
half of all stars like our Sun host planets a few times the mass of Earth orbiting
inside circumstellar radii of 0.5 astronomical units (AU). A smaller fraction
support one or more gas giant planets on eccentric orbits substantially shorter
than Jupiter’s period of 12 years. One percent or less have gas giants (“Hot
Jupiters”) roasting inside 0.1 AU on orbits of just a few days.
Yet here at home, the Sun’s closest offspring, Mercury, maintains an average separation of about 36 million miles (0.39 AU) from our parent star, and masses less than quintuple our Moon. The bulk of the Solar System’s mass is located quite far from the Sun, outside a radius of 5 AU. In that cool region our two giants, Jupiter and Saturn, glide along slow, circular orbits with few parallels in the extrasolar catalog.
Why are the innermost regions of the Solar System empty of planets, when so many other stars, near and far, support either tightly packed families of low-mass planets or smaller collections of gas giants in the same orbital space? Two studies published earlier this year offer clever explanations. For different reasons, both propose that our system once had additional short-period planets that were destroyed by an inner-system catastrophe shortly after they formed. Both also assume that the formation of low-mass planets on hot orbits is robust even in the presence of gas giants. Subsequent evolution can then produce a variety of system architectures.
Yet here at home, the Sun’s closest offspring, Mercury, maintains an average separation of about 36 million miles (0.39 AU) from our parent star, and masses less than quintuple our Moon. The bulk of the Solar System’s mass is located quite far from the Sun, outside a radius of 5 AU. In that cool region our two giants, Jupiter and Saturn, glide along slow, circular orbits with few parallels in the extrasolar catalog.
Why are the innermost regions of the Solar System empty of planets, when so many other stars, near and far, support either tightly packed families of low-mass planets or smaller collections of gas giants in the same orbital space? Two studies published earlier this year offer clever explanations. For different reasons, both propose that our system once had additional short-period planets that were destroyed by an inner-system catastrophe shortly after they formed. Both also assume that the formation of low-mass planets on hot orbits is robust even in the presence of gas giants. Subsequent evolution can then produce a variety of system architectures.
a grand attack
From Konstantin Batygin and Greg Laughlin, via Publications of the National Academy of Science, came a scenario based on recent models of the evolution of our system’s four inner planets. In the traditional picture, these objects condensed near their present locations out of a broad field of planetesimals extending outward from 0.4 AU. Recent theoretical work, however, demonstrates that the terrestrial planets must have accreted within a narrow ring between 0.7 and 1 AU (Hansen 2009). During their formative years, Venus and Earth scattered Mercury inward and Mars outward. Although the widely endorsed Grand Tack scenario explains the outer edge of the birth ring by evoking perturbations by migrating Jupiter, Batygin & Laughlin noted that no rationale had yet been presented for its inner edge. Hence their study.
Figure
2.
The Grand Tack as Grand Attack
----------
They begin by
proposing that proto-Jupiter originally assembled somewhere between 3 and 10 AU
(Figure 2a). Once it reached a
threshold mass, tidal interactions with the primordial gas nebula caused its
orbit to shrink in a process known as Type
II migration. Simultaneously, proto-Saturn began accreting in
proto-Jupiter’s wake, following its big nursery-mate on an inbound voyage.
Jupiter’s passage into the inner Solar System scattered a substantial existing population
of planetesimals and protoplanets even farther inward, pitching most of them
into the Sun.
Eventually, tagalong Saturn was embraced by Jupiter in a 3:2 mean motion resonance, such that Jupiter completed three orbits for every two of Saturn. Their planetary hook-up occurred at Jupiter’s maximum incursion into the inner system (Figure 2b), at a proposed radius of 1.5 AU, the present orbit of Mars. The marriage of Jupiter and Saturn initiated a kind of honeymoon cruise that carried them back into the outer system on ebbing tides of nebular gas (or in more astronomical lingo, this pair of gap-opening planets underwent resonant migration reversal).
Meanwhile, in the inner system, a nascent family of Super Earths and assorted rocky planetesimals were being overwhelmed by hordes of planetesimals displaced inward by Jupiter’s aborted invasion. Batygin & Laughlin suggest that the mass scattered by the marauding giant exceeded 10 Earth masses (10 Mea) and might have been as much as 20 Mea. As soon as Jupiter began migrating outward again, these scattered objects spiraled rapidly into the Sun. Unfortunately for the existing Super Earths, the wave of annihilation swept them along with it. As the authors argue, “Provided that the cumulative mass of the resonant planetesimal population is not negligible compared with the mass of the close-in planets, the planetesimals will gravitationally shepherd the close-in planets into the Sun.” All that remained after their engulfment was about 2 Mea of debris, left like a cosmic bathtub ring around a radius of 1 AU (Figure 2c). These leftovers were the substrate of the four terrestrial planets (Figure 2d).
Eventually, tagalong Saturn was embraced by Jupiter in a 3:2 mean motion resonance, such that Jupiter completed three orbits for every two of Saturn. Their planetary hook-up occurred at Jupiter’s maximum incursion into the inner system (Figure 2b), at a proposed radius of 1.5 AU, the present orbit of Mars. The marriage of Jupiter and Saturn initiated a kind of honeymoon cruise that carried them back into the outer system on ebbing tides of nebular gas (or in more astronomical lingo, this pair of gap-opening planets underwent resonant migration reversal).
Meanwhile, in the inner system, a nascent family of Super Earths and assorted rocky planetesimals were being overwhelmed by hordes of planetesimals displaced inward by Jupiter’s aborted invasion. Batygin & Laughlin suggest that the mass scattered by the marauding giant exceeded 10 Earth masses (10 Mea) and might have been as much as 20 Mea. As soon as Jupiter began migrating outward again, these scattered objects spiraled rapidly into the Sun. Unfortunately for the existing Super Earths, the wave of annihilation swept them along with it. As the authors argue, “Provided that the cumulative mass of the resonant planetesimal population is not negligible compared with the mass of the close-in planets, the planetesimals will gravitationally shepherd the close-in planets into the Sun.” All that remained after their engulfment was about 2 Mea of debris, left like a cosmic bathtub ring around a radius of 1 AU (Figure 2c). These leftovers were the substrate of the four terrestrial planets (Figure 2d).
The scenario of
Batygin & Laughlin blends smoothly into the Grand Tack and provides an
apparently self-consistent model of Solar System evolution. Its mechanism is
the forward scattering of a massive population of planetesimals by Jupiter’s
inward-then-outward migration, a process that swept the inner system clean
within a radius of about 0.7 AU. The authors emphasize that their results “imply
a strong anti-correlation between the existence of multiple close-in planets
and giant planets at orbital periods exceeding ~100 days within the same system,”
an anti-correlation supported by much – but hardly all – existing exoplanetary
data. (Current exceptions are HD
219134, HD 10180, Kepler-48, Kepler-68, Kepler-87,
Kepler-90,
and WASP-47.)
Batygin &
Laughlin also draw a second and grimmer inference from their work: “the majority of
Earth-mass planets are strongly enriched in volatile elements and are
uninhabitable.” Although they don’t pause to detail the rationale behind that
conclusion, I believe they mean something like this: Absent dynamic
instabilities caused by migrating or scattered gas giants, inner system
evolution is likely to produce several planets of a few Mea, all of which can
accrete hydrogen atmospheres as long as they assemble before the gas nebula
dissipates Although stellar irradiation can strip this hydrogen layer from
small planets on hot orbits, planets of Earth mass or more in the system
habitable zone will potentially retain their primordial envelopes indefinitely, causing an
intense greenhouse effect that rules out surface water.
consolidation &
crushing
Just two months
after the publication of this widely praised work came a preprint by Kathryn
Volk and Brett Gladman addressing the same problem from a different angle. Their
study has since been published in the Astrophysical
Journal Letters. The title itself is a great hook: “Consolidating and crushing
exoplanets: Did it happen here?” Like Batygin & Laughlin (hereafter BL15), Volk
& Gladman argue that most stars originally form compact systems of low-mass
planets like Kepler-11
and Kepler-62,
but then lose those planets in later stages of system evolution. Our Solar
System is just one member of the dispossessed majority. Unlike BL15, Volk &
Gladman do not make their scenario contingent on the Grand Tack. Without
explicitly saying so, they offer an alternative model whose outcome would pre-empt
the Nice
model as an explanation for the Late
Heavy Bombardment and replace the Grant Tack as an account of the sculpting
of the inner Solar System.
Volk & Gladman introduce their study
by referring to the anomaly of our system’s mass cut-off inward of 0.4 AU. But
their explanation is unrelated to a putative incursion by Jupiter and Saturn.
Instead, they argue that all compact low-mass systems persist on the brink of
chaos, and that the sample observed today represents only the survivors of an
endogenous wave of orbital disaster that destroys most such systems. As Volk
& Gladman describe it, their scenario “fits our solar system into a
framework where dynamical instability mercilessly consolidates or degrades
close-in planets.”
They begin with the construct of the minimum mass Solar nebula, which is
based on the present-day masses of the eight surviving planets. As long as we
extend that construct beyond its traditional but unmotivated inner limit, “several
Earth masses of material [would be] available” between 0.05 AU and 0.7 AU, the
present semimajor axis of Venus.
Volk &
Gladman propose that, during primordial times, at least three rocky planets with
a collective mass of 4 Mea coalesced inside Venus’ orbit. These worlds
coexisted with Venus, Earth, and Mars for many millions of orbits, making the
ancient Solar System an analog of those crowded Kepler systems. Then, at some
point between system ages of 50 million and 500 million years or more, the
innermost planets experienced a dynamical upset. The result was a cascade of
orbit crossings and high-speed, shattering collisions that disintegrated 90% of
their collective mass. Much of that mass was ground into fine dust that simply
blew away, leaving battered Mercury as “the last remaining relic” of our inner
system apocalypse.
To test their
hypothesis of a primordial inner system of packed planets, Volk & Gladman
numerically simulated analogs of 13 Kepler systems with 5 or more low-mass planets,
eventually generating about 600 synthetic systems that they integrated for at
least 100 million years. In half of the simulated systems, two planets eventually
collided. On this evidence Volk & Gladman concluded that 90% to 95% of all
tightly packed low-mass systems experience similar dynamic instabilities. The
result is either “consolidation,” where smaller planets collide and coalesce to
form larger planets, or “destruction,” where shattering destroys colliding
planets. In their view, the low-mass, high-multiplicity systems observed by
Kepler represent the 5% to 10% that avoided such an instability, whereas our
Solar System is one of the more common cases in which destruction outpaced
consolidation.
In their preferred
model, four Earth-mass planets formed inside 0.5 AU. The model appears to be
agnostic regarding the origin of Venus, Earth, and Mars, since it doesn’t
address their accretion. Volk & Gladman conducted test integrations indicating
that those three planets could coexist for at least half a billion years with a
hypothetical packed inner system, and would remain unaffected by the furious
collisions that eventually consumed their inner companions.
Volk &
Gladman also suggest that the destruction of the inner planets could have caused
the storm of asteroidal and cometary collisions variously known as the Lunar
Cataclysm and the Late Heavy Bombardment. This hypothesis appears to contradict
the Nice Model, which proposes that Neptune’s migration into the Solar System’s
outer debris disk (now known as the Kuiper Belt) scattered comets inward among
the planets and their moons, leaving the cratered landscapes we see today.
more different than not
The models of
BL15 and Volk & Gladman 2015 (hereafter VG15) share important similarities.
Both assume that the infant Solar System contained a compact family of
short-period planets, and both argue that this ensemble was annihilated, leaving
a void inside Mercury’s present orbit. But they differ on significant points, including
the amount of mass originally available for planet formation inside 0.5 AU, the
mechanism of the dynamical instability, and the timing of the resulting
cataclysm, which has important implications for the terrestrial planets we know
today.
Regarding mass,
VG15 suggest about 4 Mea, while BL15 go an order of magnitude higher. BL15’s
assumptions depend critically on recent models of in situ formation, which are discussed below, whereas VG15 use the
traditional model of the minimum mass Solar nebula. They simply extend the
nebula’s inner edge, as in many other recent studies.
Regarding
mechanism, VG15 propose an internal trigger, while BL15 appeal to gravitational
perturbations by wandering Jupiter. For VG15, the original planets simply
self-destruct in an epoch of shattering collisions. For BL15, they are
shepherded into the Sun by the swarm of planetesimals kicked into motion by
Jupiter’s tacking maneuver.
As for timing, BL15’s
scenario is constrained to play out within the first 10 million years of system
evolution, possibly within the first 2 million. This hard limit derives from
the observed lifetimes of protoplanetary nebulae around Sun-like stars, which
have a median of 2 to 3 million years (Williams & Cieza 2011). The proposed
tacking of Jupiter and Saturn from narrower to wider orbits could be sustained only
by their interaction with the nebular gas, whose dissipation closed the window
on such activity. None of this contradicts the established Grand Tack/Nice
model, which has been developed over more than a decade of theoretical testing.
Thereafter, the formation of the terrestrial planets proceeds along accepted
lines, by giant impacts among protoplanets.
VG15 suggest a
more leisurely schedule for their catastrophe. After the initial
clutch of hot planets formed, they survived for hundreds of millions years
before an instability developed. In fact, VL15 propose that the catastrophe
coincided with the Lunar Cataclysm. Considerable evidence dates this event (or
series of events) to an approximate system age of 800 million years. In other
words, the Lunar Cataclysm is the same system-wide upheaval that the Nice model
was developed to explain. Thus, while BL15 explicitly fold their scenario into
the Nice/Grand Tack framework, VG15 appear to ignore or even oppose it. Their
results, therefore, seem to conflict with what we know about the assembly of
the existing terrestrial planets.
questions for volk & gladman
I find it hard
to accept that the massive catastrophe described by VG15 would leave Venus,
Earth, and Mars largely untouched. Maybe my instincts have been shaped more by
science fiction than by astrophysics, but wouldn’t an instability on the scale they
envision propagate throughout the system, destroying the three cool terrestrials
along with the hotter hypothetical planets?
In addition,
VG15 appear to neglect the evidence for the formation history of Earth and Mars. Radiogenic isotopes indicate that Mars formed about 10 million years
after the Sun’s ignition, while the Earth-Moon system required 30 to 50 million
years. Since VG15 suggest that all three terrestrials had already formed when the innermost
planets disintegrated, something other than the birth ring presented by Hansen
(2009) is needed to explain their assembly. VG15 never mention the well-known mass
deficit in the space between present-day Earth and Jupiter – i.e., the “small
Mars” problem and the presence of an attenuated debris field instead of a Super
Earth inside Jupiter’s orbit. To my knowledge, the Grand Tack is the only
plausible explanation offered to date for these architectural features (Walsh
et al. 2011, Pierens & Raymond 2011). Since VG15 implicitly reject both the
Nice model and the Grand Tack, accepting their scenario means we would need to
find a new explanation for the architecture of the Solar System between 2 and 5
AU.
It looks
like VG15 raise more questions than they answer. Among them is the very plausibility
of consolidation and crushing as a mechanism for annihilation. A similar hypothesis
was explored by Anders Johansen and colleagues in 2012 to explain the observed
“Kepler dichotomy” between systems with a single transiting planet and those
with several. Like VG15, Johansen and colleagues (hereafter J12) tested dynamical
instability as an explanation for planetary depletion, and they did so by integrating
a set of synthetic planetary systems based on real Kepler systems. Despite
these similarities, the results of J13 directly contradict those of VG15. Most salient
is their finding that the synthetic systems, containing planets on circular
orbits with small mutual inclinations, were extremely stable over billions of years.
Instances of planetary collision and consolidation were too rare to explain the
observed system architectures. To assess this question in more detail, J12
integrated successively mass-boosted versions of their sample until collisions
became significant. They found that smash-ups tended to happen only when the
synthetic planets were boosted into the gas giant range, and even then,
instabilities required more than a billion years to develop. On this and
related evidence, J12 concluded that dynamical instabilities were not a
significant factor in the evolution of compact Kepler systems. [Note: another
recent simulation study by Hansen & Murray (2014) also found that, in
compact systems of planets less massive than 10 Mea, “significant dynamical
instability may not occur because, in most cases, orbit crossing cannot be
achieved.”]
As a
fan instead of an astronomer, I can’t authoritatively measure VG15 and J12 against each other.
I can only note some of the ways in which they differ. One is their choice of
simulated system architectures. J12 restricted their selection to systems of
exactly three planets orbiting inside 0.5 AU, observing that the Kepler sample
of higher-order multiples available in 2012 was too small to provide a
statistically significant template for constructing synthetic systems. VG15 limited
their simulations to systems with at least five planets – a surprising choice,
since they also argued that a maximum of four hot planets formed in the
primordial Solar System. In addition, one of their 13 systems (Kepler-90)
appears to include a gas giant. Still worse, all 13 systems support planets substantially
more massive than Earth, whereas VG15 propose only planets of approximately Earth-mass
for their hypothetical Solar System ensemble. I have to wonder whether VG15 chose
the most appropriate sample to investigate their study question.
Another
difference between the two is that J12 addressed only ejections and collisional
consolidations, without considering collisions that cause shattering. Yet the
latter type was the explicit focus of VG15. Of course, without collisions, nothing shatters, so J12 cannot be faulted for omitting any mention of this outcome. In this regard it seems significant
that VG15 kept their investigation of shattering collisions separate from their simulation study. They tested shattering only in analytic terms, and noted that these results were independent of the findings from the simulations. Yet without a high frequency of shattering, those findings would have no bearing on the emptiness of the inner Solar System.
Remarkably, VG15
never comment on the major findings of J12, even though they cite that study to
support a statement about orbital inclinations. Why didn't they discuss its conclusions?
questions for batygin & laughlin
So I have to favor BL15 over VG15. Nevertheless, despite my great respect for the work of Batygin and Laughlin, both separately and in collaboration, I find several nits to pick. These fall under three headings (with some overlap): their handling of Jupiter’s original birthplace, their choice of Kepler-11 as a model for the hypothetical inner Solar System, and their adherence to the theory of in situ formation.
For the Grand Tack, BL15 state that the exact starting point of Jupiter’s inward migration is unimportant. I believe that’s debatable, and I note that they actually model it as 6 AU. That semimajor axis is not only wider than Jupiter’s present orbit, but also wider than previous proposals for the Grand Tack, which placed the starting point between 2 and 5 AU (Walsh et al. 2011, Pierens & Raymond 2011). I can’t argue that 6 AU is implausible (in fact, it would make sense as long as the snow region commenced at a wider semimajor axis than it does today, as implied by theories of viscous heating), but I have to ask why BL15 chose that location in particular.
questions for batygin & laughlin
So I have to favor BL15 over VG15. Nevertheless, despite my great respect for the work of Batygin and Laughlin, both separately and in collaboration, I find several nits to pick. These fall under three headings (with some overlap): their handling of Jupiter’s original birthplace, their choice of Kepler-11 as a model for the hypothetical inner Solar System, and their adherence to the theory of in situ formation.
For the Grand Tack, BL15 state that the exact starting point of Jupiter’s inward migration is unimportant. I believe that’s debatable, and I note that they actually model it as 6 AU. That semimajor axis is not only wider than Jupiter’s present orbit, but also wider than previous proposals for the Grand Tack, which placed the starting point between 2 and 5 AU (Walsh et al. 2011, Pierens & Raymond 2011). I can’t argue that 6 AU is implausible (in fact, it would make sense as long as the snow region commenced at a wider semimajor axis than it does today, as implied by theories of viscous heating), but I have to ask why BL15 chose that location in particular.
Since the
mechanism underlying the inner system cataclysm in BL15’s model is a horde of
scattered planetesimals, the horde’s aggregate mass is a critical factor.
The wider Jupiter’s original orbit, the larger the available mass. BL15 note
that 20 Mea in planetesimals would be available inside 6 AU, while stating vaguely
that the mass in planetesimals must be “not negligible” in comparison with the
aggregate mass of the inner planets. But what does that mean? At least 10%, more
than 50%, even 100% of the total mass? It would be helpful to know both the
approximate fraction as well as the approximate mass against which it was
estimated. For example, VG15 set a low bar by positing an aggregate mass of 4
Mea for their hypothetical planets, whereas 20 Mea would be another story
entirely.
Another issue
raised by Jupiter’s birthplace is the nature of the region through which
proto-Jupiter and Saturn migrated on their way to sunnier orbits. If the early
Solar System had several planets inside 0.5 AU and several more outside 6 AU,
what was going on in the vast region between those endpoints? Core accretion
theory says that coagulation of solids progressed throughout the Solar nebula.
I would expect more planets to have formed in the space between, and that one
or more of them would have complicated Jupiter’s inward passage. This doesn’t
seem to be a fatal flaw in the theory, but it’s worth looking into.
Regarding the primordial family of
planets doomed by Jupiter, BL15 mention the likelihood that they were
“multi-Earth-mass” objects, as in so many Kepler systems. Three paragraphs
later, they offer an illustration of the potential mass involved by referring
to their simulation of the dynamical evolution of the Kepler-11 planets. In so
doing they express appropriate caution: “We are not suggesting that a primordial
population of the Solar System’s close-in planets would have necessarily
borne any similarity to the Kepler-11 system.” Nevertheless, their previous mention of “multi-Earth-mass”
planets implies a considerable aggregate mass. For the Kepler-11 system in
particular, their opening paragraph cites Lissauer et al. 2011 to define a
total in excess of 40 Mea. (That total was later revised downward to about 30
Mea by Lissauer et al. 2013.) In context, then, their “not negligible” mass in
planetesimals would be 50% of the aggregated planetary masses (20 Mea of
planetesimals/40 Mea of planets).
But how could our Solar System get 30 to
40 Mea worth of planets inside 0.5 AU?
the problem with in situ formation
While conceding
that the theory is controversial, BL15 assume that their hypothetical family of
Super Earths arose by in situ
formation. In this model, planets of several Earth masses readily form in place
on short-period orbits, tracing the primordial distribution of solids in the
protoplanetary disk. Apart from a few earlier inquiries of limited scope (Raymond
et al. 2008, Montgomery & Laughlin 2009), this theory emerged in 2012-2013 as
a fully-fledged paradigm with various instantiations, some strict and some less
so.
Chiang & Laughlin (2013) presented
the “strict” version (their word). They argued that “disk-driven migration
seems too poorly understood to connect meaningfully with observations,” and deprecated
models that require this mechanism as both “premature” and “naïve.” In their
place they proposed a minimum mass
extrasolar nebula (MMEN). They obtained this construct by plotting the
masses and orbits of all small Kepler planets then known, an exercise that highlights
the pile-up of mass inside 0.25 AU. They explained this pile-up by arguing that
the MMEN supports a much larger concentration of solids at small semimajor axes
than previous models. Such a large mass, they argued, readily congeals into
several Super Earths, as they illustrated with reference to the Kepler-11
system.
Brad Hansen & Norm Murray (2012)
presented a “less strict” variation on the model (according to Chiang &
Laughlin). They proposed that 30-100 Mea of solids could migrate from the outer
regions of the protoplanetary disk to clump within 1 AU, where the
planetesimals would stop migrating and rapidly condense into an ensemble of Super
Earths (which I would call gas dwarfs). In a subsequent publication (Hansen &
Murray 2013), the authors reduced the aggregate mass required to 20 Mea, while
arguing that migration might not be necessary after all to account for such a
concentration of solids. (Presumably their revision upped the strictness
factor.)
Critiques of in situ theory came pretty quickly. The principal objection was
that MMEN and similar models require protoplanetary disk masses much larger
than those actually observed (Raymond et al. 2014) – so large that they would
be gravitationally unstable (Schlichting 2014). Another objection was that MMEN implies a universal disk structure that is incompatible with the diversity
of known multiplanet system architectures (Raymond & Cossou 2014). Still
another was that “strict” in situ
models rule out disk-driven migration. Yet as Izidoro et al. (2014) argued, “Assuming
that no migration occurs essentially ignores 30 years of disk-planet studies
that show the inevitability of orbital migration.” Indeed, Schlaufman (2014) found that without migration,
the observed Kepler system architectures would be impossible. Additional
research (Inamdar & Schlichting 2015) argued that planets forming on
short-period orbits, even by condensation of solids originating in the outer
system, could not accrete the substantial hydrogen envelopes observed in the
Kepler sample.
According to the Astrophysical Data
System, Greg Laughlin hasn’t published anything theoretical on in situ formation since 2013. However, his
colleague Eugene Chiang has continued to expand on this approach. Chiang’s
recent work makes notable concessions to migration theory. He and his
collaborators acknowledge that strict in
situ approaches make the formation of gas giants at least as likely as
Super Earths, possibly requiring some fine-tuning in the time scale for nebular dissipation (Lee et al. 2014). A brand-new study (Lee & Chiang
2015) even concedes that in situ
models are insufficient to explain the diversity of the existing population of
low-mass planets. Dividing the sample into two groups – “Super-Earths,” which they define as planets between 1 and 4 Rea, and “Super-Puffs,” which
have radii between 4 and 10 Rea but remain in the mass range of gas dwarfs – they
offer this remarkable conclusion: “Unlike Super-Earths, which can form in situ, Super-Puffs probably migrated
in to their current orbits.”
Apparently the strict in situ paradigm is stalled for now,
leaving us with many fluid variations on the theme of disk-driven
migration.
still
looking pretty shiny
From my back alley
perspective, the essential feature of Batygin & Laughlin’s scenario is its
appeal to Jupiter’s migration as the source of the catastrophe. The giant’s inward
passage was the mechanism that scattered forward a huge mass in planetesimals,
which then efficiently herded all the baby planets in the inner nebula into the
Sun. Although Batygin & Laughlin assume the existence of a system of
compact Super Earths (which I would call gas
dwarfs) inside 0.7 AU, their scenario doesn’t seem to require such a
configuration, especially not one with an aggregate mass of 20 to 40 Mea. Any
collection of protoplanets would do, including the modest system of terrestrial objects proposed by Volk & Gladman.
By now the limitations
of the MMEN and similar in situ
models have been discussed by numerous peer-reviewed studies, while a major
proponent of the early model (Chiang) has since revised his approach. Simultaneously,
increasingly sophisticated theories based on migration continue to proliferate.
My hunch is that the latest migratory models are largely consistent with
Batygin & Laughlin’s scenario, especially since it depends so critically on
Type II migration. Kepler-11 aside, their comprehensive approach provides the
best explanation yet for the strange things that must have happened before our
world could be born.
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Super-Earths. Monthly Notices of the
Royal Astronomical Society 431, 3444-3455. Abstract: 2013MNRAS.431.3444C
Hansen B.
(2009) Formation of the terrestrial planets from a narrow annulus. Astrophysical Journal 703, 1131-1140.
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