Figure 1. The Sun-like star
WASP-47 hosts a transiting Hot Jupiter flanked by two low-mass planets, and all
three have periods shorter than 10 days. This was the first and so far the only
detection of such a configuration. Above, an artist’s rendering shows a Hot Jupiter
in transit across the face of its star. Credit: European Southern Observatory
------------------------------
A few months
back I wrote about WASP-47b, the first Hot Jupiter ever found with
low-mass planets on nearby orbits. I noted my eagerness to see a theoretical
study of the possible formation mechanisms behind this unusual system, while
venturing that the orbital architecture of WASP-47 could not have resulted from
in situ accretion.
Then last month
I took a
close look at a study by Konstantin Batygin & Greg Laughlin (2015), who
offered a novel explanation for the formation of the inner planets of the Solar
System. Their approach folds the evolutionary history of the four terrestrial
planets into the Nice
Model and the Grand
Tack. I observed in passing that, although Laughlin was one of the earliest
proponents of in situ theory (which underlies
the scenario that he and Batygin proposed), he hadn’t written anything purely
theoretical on this topic for quite a while.
Two new
preprints respond serendipitously to those conjectures and observations. One is
a theoretical study by Batygin, Bodenheimer & Laughlin (2015) on the
potential for Hot Jupiters – Hot Jupiters, no less! – to form in situ. The authors develop a new
formation scenario for star-hugging gas giants and use numerical simulations to
test it. Then they appeal to the WASP-47 system as possible evidence that this
mechanism really works.
The second
preprint is also highly theoretical. Nathan Kaib and John Chambers (2015) tersely
present the results of a large ensemble of numerical simulations designed to
test the Nice Model of Solar System evolution. Their findings question the utility
of this approach as an explanation for the Late
Heavy Bombardment. They can also be interpreted as evidence that our four
terrestrial planets formed after, not before, the dynamical instability addressed
by the Nice model. Although Kaib & Chambers don’t mention the recent
articles by Batygin & Laughlin or Volk
& Gladman on the inner Solar System, their conclusions call for rethinking
both the Nice model and the Grand Tack.
a burning cradle for baby giants?
The new study by
Batygin & colleagues (hereafter B15) is called “In situ formation and dynamical evolution of Hot Jupiter systems.” The
title is a misnomer, however, as they don’t actually model the in situ accretion of solids on orbits
where Hot Jupiters are observed. Instead, as often happens in simulation
studies, they use simplified initial conditions to examine a narrowly defined
question: Can a dense planet of sufficient mass, orbiting at or near the inner
edge of a protoplanetary nebula, accrete enough ambient hydrogen to balloon
into a gas giant?
B15 offer four
representative evolutionary sequences in which solid planetary cores at a range
of masses follow circular orbits around a Sun-like star at a semimajor axis of
0.05 AU. They “purposely adopted an agnostic viewpoint” regarding formation
mechanisms, explaining that their results “are largely independent of how the
solid core arises.” The simulations seem equally friendly to x) strict in situ formation involving the
accretion of local mass only; y) migration of solids from cooler orbits into
the hot inner nebula and subsequent accretion at the new location; or z) migration
of fully-formed cores from cooler orbits.
In units of
Earth mass (Mea), the core masses they select are 4 Mea, 10 Mea, and 15 Mea.
The object of 15 Mea is used in two different set-ups, one with low ambient gas
density and the other with higher density. As a reality check, I note that
atmosphere-free planets of 10 Mea or more are not attested (Jontof-Hutter et
al. 2015), while realistic models of planet formation have great difficulty
forming an object of 4 Mea or more at 0.05 AU, where silicate dust sublimates
(Dullemond & Monnier 2010).
Regarding gas
accretion, B15 concede that insufficient mass in ambient gas would be available
in the local feeding zone to build a gas giant at 0.05 AU. Therefore, they
assume that the local supply of hydrogen/helium is constantly replenished by
the regular flow of gas through the nebula onto the infant star. Their model seems
to hinge on the capacity of the short-period planet to capture this inrushing
gas.
Across
simulations, objects of 15 Mea were the only ones that managed to accrete gas
envelopes equal in mass to their solid cores and thereby trigger runaway gas
accretion. Thus, they find that the minimum core mass capable of growing into a
gas giant in the vicinity of a Sun-like star is 15 Mea.
Figure 2. Light curves of the three inner planets
of WASP-47
Phase-folded light
curves of the three transiting planets detected by the K2 mission: Becker et
al. 2015
---------------------------------------
B15 conclude
that their scenario will regularly produce gas giants, so that in situ formation might “represent the
dominant channel for hot Jupiter generation.” They proceed to a discussion of
WASP-47b as an example of what this channel might bring.
But I’m not
convinced. The likeliest origin for an object of 15 Mea with a semimajor axis
around 0.05 AU is migration from a wider orbit where accretion can more readily
produce such massive objects. As far as I know, a gas giant can migrate into a
hot orbit just as easily as a Neptune-mass object can. So why resort to a
two-step process – first migrate the core, then capture the atmosphere – when a
well-theorized one-step process (Type II migration) works even better? B15
appear to be offering a solution in search of a problem.
They cite
WASP-47 as a rare system in which a compact family of Super Earths somehow
managed to promote one of their number to giant status without changing any
semimajor axes or otherwise disrupting the rest of the family. But it seems
that considerable acrobatics would be needed to achieve this remarkable outcome.
For example, how did only one planet get pumped up? What kept the others down?
I suggest that,
along with exploring novel in situ
explanations for the evolution of WASP-47, the exoplanet community should test
whether migration scenarios could explain this
architecture with fewer contortions.
Figure 3. The wreck of the Kronan
In
1676, the Swedish warship Kronan capsized while
trying to execute a sharp turn under too much sail, causing an explosion in the
gunpowder magazine. Few survived. Illustrated is an imaginative (and historically
inaccurate) rendering of the disaster by Claus Moinichen in 1686.
---------------------------------------
does the sailing master have no clothes?
The study by
Kaib & Chambers is conducted on a grander scale than B15. They ran almost
300 high-resolution simulations of Solar System evolution to test three
different versions of the Nice model and explore the effects of the resulting dynamical
instability on the four terrestrial planets. A simulation was considered
successful if the final system met three criteria after the instability concluded:
A) all four outer planets from Jupiter to Neptune survived, with the orbits of
Saturn and Jupiter lying between a 2:1 and 3:1 mean motion resonance; B) all
four inner planets also survived on stable orbits; and C) the angular motion
deficit of the four inner planets was less than or equal to its present value
after their orbits were integrated for an additional billion years.
In all three
versions of the initial set-up, the four terrestrial planets were assigned
their present masses and orbits. In two versions, the outer system contained
Jupiter, Saturn, and three Neptune-like planets. In the third, it contained
Jupiter, Saturn, two Neptune-like planets, and two “Super Earths” of 8 Mea
each. Among all iterations of these set-ups, the success rate according to
Criterion A was 13% to 16%. However, the set-up with six outer planets failed
both of the other criteria, while the other two models produced just 1 and 2
systems, respectively, that met Criterion B. Only one iteration of one of these
models also met Criterion C. All other iterations lost at least one terrestrial
planet (usually Mercury), and many lost two (usually Mercury and Mars).
These results are
very likely to inform future discussions of the Nice Model. As Kaib &
Chambers conclude:
[W]e find a
probability of 1% or less that the orbital architectures of the inner and outer
planets are simultaneously reproduced in the same system. These small
probabilities raise the prospect that the giant planet instability occurred
before the terrestrial planets had formed. This scenario implies that the giant
planet instability is not the source of the Late Heavy Bombardment and that
terrestrial planet formation finished with the giant planets in their modern
configuration.
Notably, the
recent study by Volk & Gladman (2015) proposed an approach to the evolution
of the four terrestrial planets that dispensed with the Nice Model altogether,
suggesting an alternative explanation for the Late Heavy Bombardment. Although
I don’t find their alternative persuasive, I hope these efforts inspire further
evolutionary studies to address the problem of our denuded inner system.
In the meantime, the findings of Kaib
& Chambers might benefit from reframing. As discussed in earlier blog posts,
the phenomenon known as the Late Heavy Bombardment or Lunar Cataclysm can be
understood in different ways. In
one view, it was a self-contained period of heavy cometary impacts that were
preceded and followed by long stretches of relative calm; its peak came about
3.8 billion years ago (Gya), at a system age of ~800 million years (Gomes et
al. 2005). In another,
it was simply one episode in a long, saw-toothed series of bombardments that
began with Earth’s accretion and subsided only toward the end of the Archean
period, about 2.5 – 3 Gya (Morbidelli et al. 2012, Marchi et al. 2014). Instead
of triggering a late and relatively short-lived catastrophe, maybe the dynamic
instability happened in the first 100 million years of Solar System history and
inaugurated that long and violent epoch of bombardments. The Nice Model would
still explain how the massive outer planets nudged and jostled one another into
their mature (and usually wider) orbits.
Figure 4. Overview of Earth history
Again, maybe the failure of a single
model to explain the final architecture of the inner as well as the outer Solar
System is another clue that we need a new picture of inner system evolution.
What if the Grand Tack triggered an inner system catastrophe, as modeled by Batygin
& Laughlin (2015), but the resulting mess did not immediately resolve
into a four-planet system? Maybe the battle of oligarchs that followed
Jupiter’s retreat from the Sun’s inner territories was still raging while
Neptune was decimating the outer realms. Maybe lasting peace in the inner
system was contingent on resolving tensions among the outer worlds, a
resolution that evidently hinged on the exile of a Uranus-type planet. Maybe
the final detente between Jupiter and Saturn was what enabled Earth to dominate
its three smaller siblings.
-------------
REFERENCES
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
Batygin K, Bodenheimer P, Laughlin G. (2015) In situ formation and dynamical
evolution of Hot Jupiter systems. In press. Abstract: 2015arXiv151109157B
Becker
J, Vanderburg A, Adams F,
Rappaport S, Schwengler H. (2015) WASP-47: A Hot Jupiter system with two
additional planets discovered by K2. Astrophysical
Journal Letters 812, L18. Abstract: 2015ApJ...812L..18B
Dullemond CP, Monnier JD. (2010). The inner regions
of protoplanetary disks. Annual Review of
Astronomy & Astrophysics 48, 205-239.
Gomes R,
Levison HF, Tsiganis K, Morbidelli A. (2005) Origin of the cataclysmic Late
Heavy Bombardment period of the terrestrial planets. Nature, 435: 466-469. Abstract: 2005Natur.435..466G
Jontof-Hutter
D, Ford EB, Rowe JF, Lissauer JJ,
Fabrycky DC, Christa Van Laerhoven5, Agol E, Deck KM, Holczer T, Mazeh T. (2015) Robust TTV mass measurements: Ten
Kepler exoplanets between 3 and 8 Mearth with diverse densities and
incident fluxes. In press.
Kaib NA, Chambers JE. (2015) The fragility of the
terrestrial planets during a giant planet instability. Monthly Notices of the Royal Astronomical Society. In press.
Abstract: http://arxiv.org/abs/1510.08448
Marchi
S, Bottke WF,
Elkins-Tanton LT, Bierhaus M, Wuennemann K, Morbidelli A, Kring DA. (2014)
Widespread mixing and burial of Earth’s Hadean crust by asteroid impacts. Nature 511, 578-582. 2014Natur.511..578M
Morbidelli A, Marchi S,
Bottke WF, Kring DA. (2012) A sawtooth-like timeline for the first billion
years of lunar bombardment. Earth and
Planetary Science Letters 355, 144-151. Abstract: http://adsabs.harvard.edu/abs/2012E%26PSL.355..144M
Volk
K, Gladman B. (2015) Consolidating and crushing
exoplanets: Did it happen here? Astrophysical
Journal Letters, 806: L26. Abstract: 2015ApJ...806L..26V
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