Figure
1.
In the early stages of system evolution, young planets may be engulfed by their
host stars.
Credit: ESA / L.
Calcada
To understand the potential architectures of a broad range
of planetary systems – not to mention the likelihood that a given architecture
might support habitable planets – we’ve found that mass
matters. Low-mass exoplanets resembling Earth and Neptune are often found
in the company of similar objects, whereas gas giants like Jupiter typically
have no near neighbors (Steffen et al. 2012,
Johansen et al. 2012).
A new study by Soko Matsumura and colleagues (2012) investigates
the relationship between the orbital dynamics of extrasolar gas giants and the
possibility of Earth-mass planets in the system habitable zone (the orbital
space where temperatures permit surface bodies of water on rocky planets).
Matsumura’s group builds on a pair of studies by Dimitri Veras and Philip
Armitage (2005, 2006), whose models predict that systems with eccentric gas
giants, even on relatively wide orbits, will be deficient in rocky planets on
smaller orbits (2006).
battle of the giants
Matsumura and colleagues ran 40 simulations of young
planetary systems containing 3 Jupiter-mass gas giants (1 Mjup each) orbiting
between 3.8 and 6.5 AU. Each system also had 11 Earth-mass planets (1 Mea each)
between 0.1 and 3 AU. All planets initially traveled on circular orbits.
In all runs, the gas giants rapidly experienced dynamical
instabilities that reduced their number from three to two. Giant subtraction followed
three possible pathways, in order of likelihood: the merger of two giants (43%),
the ejection of at least one giant (37%), or the engulfment of at least one
giant by the host star (20%). More than one pathway might be active in a single
system. Except for runs resulting in mergers, the surviving giants had
eccentric orbits. In two-thirds of systems, two giants remained, but in one-third
there was only one.
collateral damage
Violent episodes of planet scattering typically wreak havoc on
smaller, warmer planets, despite the latter’s status as innocent bystanders. Most
destructive is a giant sweeping through the inner system to collide with the host
star; in this case, all inner planets are likely to be annihilated. An ejected
giant is almost as harmful. Even if the inner planets are initially untouched,
long-term perturbations by surviving giants on eccentric orbits eventually
drive most inner planets into the star or out of the system. Planets with the
smallest semimajor axes (~0.1 AU) are the most likely to survive.
If the Solar System had suffered one of these catastrophes,
we would have lost Mars, Venus, Earth, and maybe even Mercury.
reality check
Matsumura’s group compared the results of their simulations
with current data on multiplanet systems that have retained both high- and
low-mass planets. According to their models, these systems either never
experienced violent instabilities or, if they did, managed to survive an epoch
of planet scattering. They found that mixed-mass systems tend to have low
eccentricities, consistent with peaceful dynamic histories.
They also note that this configuration is the exception
rather than the rule among exoplanetary systems, since they calculate that just
5% to 10% of multiplanet systems (considering both HARPS and Kepler results) contain
both gas giants and low-mass planets.
three-way slice
The findings of Matsumura’s group encouraged me to look at
mixed-mass systems as a potential architectural category, specifically in
contrast to multiplanet systems containing either low-mass planets only or gas
giants only. Table 1 compares
parameters for each of these three categories, using data on all confirmed,
well-constrained multiplanet systems listed in the Extrasolar Planets Encyclopaedia at the end of
October. For gas giants, the lower mass limit is 0.17 Mjup (55 Mea) and the
lower size limit is 7 Earth radii (7 Rea). For low-mass planets, mass values derive
from radial velocity or transit timing data, as available. For Kepler planets
with neither type of data, maximum masses are estimated from planet radii.
*
Values for orbital eccentricity and stellar metallicity are unavailable for
most low-mass systems.
---------------------------------------------
For many parameters, these three architectural categories
appear to define a continuum. The progression is most obvious for semimajor
axes, whose median values increase from one category to the next. Multiple
systems with low-mass planets are the most compact, while those with high-mass
planets are the most far-flung. Most low-mass and mixed-mass systems have one
or more planets orbiting within 0.1 AU, but only 16% of the high-mass systems
have such an object. Once again we see evidence that Hot Jupiters tend to be
solitary planets, shunning even their own kind.
An analogous progression from least to most is visible in
star mass, even if the contrasts are less striking. Low-mass stars tend to host
low-mass planets, Solar-mass stars may host both low- and higher-mass planets, and
high-mass stars typically host especially massive gas giants. No similar trend
is apparent for star metallicity. Instead of a steadily rising enhancement in
metals, we see a sharp cutoff between systems with and without gas giants, such
that the former have super-Solar metallicities and the latter sub-Solar metallicities.
High metallicity and large planet mass do not appear to be correlated.
Another unexpected result is the similarity between
mixed-mass and high-mass systems in terms of orbital eccentricity. The
conclusions of Matsumura’s group might suggest a more striking divide. Even so,
the high-mass systems typically feature more eccentric orbits than the other
two types. Since high-mass systems also support planets at much wider semimajor
axes, their formation processes were probably most active in regions beyond 1
AU.
Table 1 was arbitrarily constructed in terms of planet mass,
so the rising trend for this variable may simply reflect the initial setup.
Nevertheless, the median planet mass for each architectural category suggests
that the trend is significant. For low-mass systems, the median is about 10
Mea, near the dividing line between primarily rocky planets and planets with
substantial fractions of hydrogen and volatiles. For mixed-mass systems, the
median is about 55 Mea, near the boundary between gas dwarfs like Neptune and gas
giants like Jupiter. Planets of Jupiter mass or more are rare in this type of
system. But for high-mass systems, the median mass is 1.82 Mjup, well above the
median for all known extrasolar gas giants (1.5 Mjup in a sample of 574 with
mass > 0.16 Mjup).
A key difference between mixed-mass and high-mass systems
may be that the former tend to have lightweight gas giants on smaller orbits,
while the latter generally have far more massive giants on wider, more
eccentric orbits.
A key similarity across all three architectural types is the
tendency for lower-mass planets to follow smaller orbits than higher-mass
planets. Thus, in all multiplanet systems, we see a strong trend for the
innermost planet to be the most lightweight and the outermost to be the most
massive. The Solar System also follows this trend within a semimajor axis of 6
AU (which incidentally exceeds the widest orbits in all but 5% of the systems
sampled in Table 1).
We might expect the mixed-mass systems to be progressively mass-boosted
versions of the low-mass systems, containing varying proportions of gas giant
and low-mass planets, but what we see is a clear domination of the dwarfs by
the giants. Fifteen out of 18 mixed-mass systems contain only one low-mass
planet, which always has the shortest period in the system. Two others (GJ 876,
Kepler-30) feature a low-mass planet orbiting inside one or two gas giants with
a second low-mass planet orbiting outside the giant or giants. Just one system
– HD 10180 – contains a procession of low-mass planets (at least 6 in all) climaxing
in a single small gas giant at a larger semimajor axis.
prospects
It will be interesting to see if the mass-based
architectonics implied by my rather arbitrary lump-and-split approach is borne
out by future theory and observation. Will we begin to find gas giants in the
outer regions of systems that now look like compact collections of telluric
and gas dwarf planets? Will we begin to detect short- or long-period Neptune-mass
planets in systems where we now see only eccentric gas giants? Will we find
reason to believe that truly Earth-like planets (rocky objects with surface
water) are possible only in the rare
subset of systems that support analogs of Jupiter and Saturn? If our governments keep funding space-based astronomical
missions, we may start to get answers soon.
Meanwhile, the divide between low-mass and high-mass planets
shows all signs of being a fundamental split, as transit data in particular
continue to confirm. Figure 2 below
updates the graph I first posted in April,
using data from the Extrasolar Planets
Encyclopaedia. The sample of well-constrained low-mass planets continues to
grow rapidly, and the tendency for these objects to support hydrogen
atmospheres is robust and unmistakable, given the preponderance of planets with
radii between 2 and 5 Rea.
Figure
2. All
transiting planets smaller than 15 Earth radii (1.34 Jupiter radii) as of October
16, 2012
Abbreviations: E =
Earth, U = Uranus, S = Saturn, J = Jupiter
Johansen A,
Davies MB, Church RP, Holmelin V. (2012) Can planetary instability explain the
Kepler dichotomy? Astrophysical Journal
758, 39. Abstract: http://arxiv.org/abs/1206.6898
Matsumura S,
Ida S, Nagasawa M. (2012) Effects of dynamical evolution of giant planets on survival
of terrestrial planets. In press; abstract: http://adsabs.harvard.edu/abs/2012arXiv1209.1320M
Steffen JH,
Ragozzine D, Fabrycky DC, Carter JA, Ford EB, Holman MJ, Jason F. Rowe JF,
Welsh WF, Borucki WJ, Boss AP, Ciardi DR, Quinn SN. (2012) Kepler constraints
on planets near hot Jupiters. Proceedings
of the National Academy of Sciences 109, 7982-7987. Abstract: http://arxiv.org/abs/1205.2309
Veras D,
Armitage PJ. (2005) The influence of massive planet scattering on nascent
terrestrial planets. Astrophysical
Journal 620, L111-L114. Abstract: http://adsabs.harvard.edu/abs/2005ApJ...620L.111V
Veras D,
Armitage PJ. (2006) Predictions for the correlation between giant and
terrestrial extrasolar planets in dynamically evolved systems. Astrophysical Journal 645, 1509-1515.
Abstract: http://adsabs.harvard.edu/abs/2006ApJ...645.1509V
