Figure 1. Architecture of the mixed-mass planetary system around
HD 219134, a nearby K dwarf, as characterized by Gillon et al. 2017 and Johnson
et al. 2016. All five planets are shown at their approximate relative sizes. Planets
b and c are observed in transit, so their radii are known. The radii of
the other planets are based on those of planets with similar masses and measured
radii (e.g., planets d and e are assigned the same radii as Neptune
and Saturn, respectively). See Table 1.
----------
Fresh from their discovery of four new Earth-size planets transiting
TRAPPIST-1 (a minuscule M dwarf in the Sun’s back yard), Michaël Gillon &
colleagues have contributed exciting new data to our developing picture of
another nearby exoplanetary system: HD 219134. The host star is a metal-rich K3
dwarf located only 6.55 parsecs away (21 light years) in the direction of
Cassiopeia. Gillon & colleagues confirm a previous report by Motalebi &
colleagues (blogged here) that the
innermost planet (b) is visible in transit,
while announcing that the second planet (c)
also transits. In addition, they present radial velocity masses for both transiting
planets (hereafter tranets), as well
as a refined radius for planet b.
For both objects, these values are consistent with a purely rocky composition.
After the 2015 announcement by Motalebi & colleagues
(hereafter M15), two subsequent studies offered additional data and analyses of
the HD 219134 system: Vogt & colleagues (hereafter V15, blogged here) and Johnson &
colleagues (hereafter J16, blogged here). Many findings from these
studies overlap, but others are mutually inconsistent, including results on the
precise number of planets and their approximate masses and periods.
Nonetheless, all three studies agreed that HD 219134 hosts two or more low-mass
planets on hot and warm orbits, plus one gas giant less massive than Saturn outside
the system ice line.
In their new study, Gillon & colleagues confine their
attention to the inner system, where they identify a total of four low-mass
planets. This result contrasts with the three small planets proposed by M15 and
the five preferred by V15. Along with their transit findings, Gillon’s group also
report the latest radial velocity data on all four planets from continuing observations
with the HARPS-N spectrograph. In passing, they affirm the presence of the
giant planet but do not comment on the conflicting results of the earlier
studies. Table 1 summarizes their
findings on planets b through f, supplemented by the findings of J16
on planet e (which J16 called planet
h, even though no one has proposed
as many as seven planets – i.e., b
through h – for this system).
Table 1. Revised
system parameters of HD 219134
Tags: Period = orbital period
in days; a = semimajor axis in astronomical units (AU); Radius = radius in
Earth units; Mass = mass in Earth units; Teq = equilibrium temperature in
Kelvin. Data on planet e are based
on those for planet h in Johnson et
al. 2016. All other data are based on Gillon et al. 2017.
-----------------------------
conflicts
resolved?
We’ve now seen four successive studies of HD 219134
conducted by three different scientific teams. (I regard Gillon et al. 2017 as
the same team as M15, since the two author groups overlap substantially and
both report HARPS-N data.) Our picture of planet b is largely unchanged from the results of M15 and V15, except that
we have a firmer understanding of its radius. With the new transit data and
improved radial velocity data on planet c,
we can also be confident of that planet’s mass, radius, and density, with no
change in its estimated period. Even planet d retains virtually the same period reported by M15 and V15, although
its mass falls between the values they provided.
Although planet f
was not included in the analysis of M15, and a potential planet with a similar
periodicity was rejected by J16, the object reported by Gillon & colleagues
looks a lot like the planet f
proposed by V15: its period is virtually the same, while its mass is a bit
lower – a minimum of 7.3 Earth masses (7.3 Mea) instead of 8.9 Mea. Notably,
the newly published period of planet f
appears to place it just outside a 2:1 mean motion resonance with planet d. Similar period ratios have appeared
in several compact low-mass systems discovered by the Kepler Telescope.
Consistent with M15 and J16, Gillon & colleagues
implicitly reject V15’s proposed planet g
at 94 days. They remain silent on the parameters of the system’s gas giant, to
which J16 assign a period of almost 6 years and a semimajor axis of about 3 AU.
mass
distribution
The new data suggest that HD 219134 harbors a substantial
mass in refractory elements within a space much smaller than the region bounded
by Mercury’s orbit in our system (see Table
1). Gillon & colleagues argue that the two innermost planets, b and c, have minimal volatile constituents, with radial velocity data
indicating a combined refractory mass of about 9 Mea for this pair. Although we
remain ignorant of the radii of the next two planets, f and d, most exoplanets
with comparable masses (in the approximate range of 5 to 20 Mea) and comparable
thermal environments (cooler than Venus) support substantial envelopes of
hydrogen and helium. Existing studies suggest that the bulk mass composition of
such planets is generally between 1% and 30% hydrogen/helium (see Lopez &
Fortney 2014). Therefore, given their combined radial velocity masses of 23.5
Mea, we can estimate an aggregate mass of 16-22 Mea in refractory elements for
this cooler, fatter pair, with hydrogen and other volatiles accounting for the
remaining 1-7 Mea. That brings the total refractory mass of the four inner planets
into the range of 25-31 Mea.
Altogether, the radial velocity data indicate a total
mass of about 33 Mea for the inner system of HD 219134, placing it in roughly
the same ballpark as the aggregate mass of the six-planet systems around
Kepler-11 (about 30 Mea) and Kepler-20 (about 55 Mea). All these endowments are
much richer than the aggregate mass of the four terrestrial planets in our
Solar System, which collectively sum to 1.98 Mea.
stingy
spitzer scheduling
The late, lamented Kepler Mission demonstrated the need
to collect continuous photometric data over a period of years in order to
characterize the inner reaches of any planetary system. The longer the sequence
of light curves, the more robust the resulting analysis of potential transits. In
light of that history, I was surprised to learn that Gillon’s group was able to
observe only two transits each for planets b
and c. Their observing sessions with
the Spitzer Space Telescope were confined to periods of 6.5 to 7.5 hours
centered on the transit window predicted for each planet. As a result, we have
no idea whether any of the other planets orbiting HD 219134 can be observed in
transit. Definitive findings one way or the other would dramatically improve
our understanding of this system’s distinctive architecture and constrain the
degree of coplanarity among the orbits of the inner planets
Notably, Spitzer had to monitor TRAPPIST-1 for 20 consecutive
days in order to untangle the orbits of its seven known planets, and even that
allotment was too brief to obtain a clear picture of the outermost planet,
which has a period just shy of 19 days. To illuminate the inner system of HD
219134 would require an observing run in excess of 200 days – evidently an
impossibility at present. What we humans need is a whole array of space-based
observatories staring year after year at all the most interesting stars in our
neighborhood.
architecture
and habitability
As I discussed in an earlier post on HD
219134, this system can be characterized as a “rich mixed-mass system,” defined
as a planetary system with at least two low-mass planets plus at least one gas
giant on a wider orbit. Including our Solar System, about a dozen such configurations
are known. They share important similarities. In most of them – including HD
219134 – two or more low-mass planets are observed in transit, while the gas
giant tends to be the outermost of the known planets. (The exceptions to the
latter generalization are Kepler-87, Kepler-89, and the Solar System, each of
which includes a gas giant with a low-mass planet on an exterior orbit.)
In more than half of the known systems –Kepler-48,
Kepler-68, Kepler-87, Kepler-167, HD 219134, HD 10180, and our Solar System –
we note a gap between the inner system of low-mass planets and the outer gas
giant. Except for our own system, we can’t be sure whether any of these apparent
gaps is truly empty. Some might be occupied by one or more planets that are
misaligned with the others, and thus not visible in transit, or hiding a planet
or planets that are too lightweight for radial velocity observations to detect.
In any case, such gaps are significant, because in HD
219134 and several other systems, they correspond to the classical habitable
zone. That’s why I’d like to see lots of follow-up observations of HD 219134,
accompanied by analyses of the orbital stability of potential Earth-mass
planets occupying the space between 0.5 and 1 AU. Three such analyses have
already been conducted for the habitable zone of HD 10180, with conflicting
results (as blogged here).
beware
of outsiders
Equally significant is the mechanism responsible for
creating and maintaining these orbital gaps. The difficulty of identifying such
a mechanism is underscored by the case of our Solar System, where we still have
no widely endorsed scenario to explain why mass is severely depleted between
the orbits of Earth and Jupiter, and altogether absent between the orbits of
Mars and the Main Belt asteroids. Jupiter is probably involved, but the details
are elusive (see Batygin & Laughlin 2015, Raymond et al. 2016).
The question of orbital gaps and missing mass in this
subset of system architectures is subsumed by a more general concern regarding the
role of longer-period gas giants in systems with multiple low-mass planets on
hot or warm orbits. Several recent studies have addressed the stability of
compact multiplanet systems in the presence of an outer giant, whether seen or
unseen (Hands & Alexander 2016, Hansen 2017, Becker & Adams 2017, Huang
et al. 2017, Jontof-Hutter et al. 2017, Read et al. 2017). The results suggest
that “friendly” giants, meaning those permitting the survival of several small,
closely spaced planets, need to be cool in more ways than one. Not only must
they follow orbits well-separated from the inner planets, in regions where
insolation and equilibrium temperatures are lower; they must also be
dynamically cool, with minimal eccentricities (deviations from circularity) and
inclinations (deviations from coplanarity).
The most delicately balanced configuration – and thus the
one most easily upset – contains several planets continually observable in
transit. This balance can be maintained only a) if no gas giant is present or b)
if the giant is either precisely aligned with the inner ensemble or very widely
separated from it. Higher values of eccentricity, inclination, and mass, as
well as lower values of semimajor axis, will lead to perturbations of the inner
planets. In a perturbed regime, their inclinations might oscillate, such that transits
periodically cease for certain planets and then resume after an interval, or
become permanently misaligned, such that only one planet, or none at all, is observed
in transit. In cases of extreme excitation of inclinations and eccentricities,
some or all of the inner planets would be lost altogether.
For example, Becker & Adams found that, among 18
Kepler systems with at least 4 transiting planets each, potential gas giant
companions would have to maintain semimajor axes of 10 AU or more to avoid
perturbing the inner planets. They also made more specific predictions for several
interesting systems featured in previous blog posts: WASP-47, Kepler-11, Kepler-62, Kepler-90, and Kepler-20.
The inner system of WASP-47 consists of a gas giant
flanked by two low-mass planets inside a semimajor axis of 0.10 AU; the outer
system contains another gas giant at 1.36 AU. Becker & Adams concluded that
the orbit of the outer giant must be approximately coplanar with those of the
inner tranets, or else they would become mutually misaligned and no longer be observable
in transit. For Kepler-11, Kepler-62, and Kepler-20, their conclusions were
even more restrictive: none of these systems could harbor an additional planet
of 30 Mea or more between 1 and 30 AU without upsetting the clockwork orbits of
the inner tranets. (Notably, Jontof-Hutter & colleagues reached a less
restrictive conclusion for Kepler-11, ruling out any slightly inclined
Jupiter-mass planets within 3 AU.) For Kepler-20, a total of six low-mass
planets are known, but only five are observed in transit, given the misalignment
of planet g. Becker & Adams
propose that an undetected gas giant on a cool orbit might be responsible for
this configuration.
In an analogous study, Read & colleagues investigated
the stability of systems containing short-period tranets and non-transiting
giants at larger semimajor axes, including two rich mixed-mass systems,
Kepler-48 and Kepler-68. For Kepler-48, they concluded that the outer giant
must be closely aligned with the inner system, whereas for Kepler-68, available
data provided no strong constraints on inclination.
The upshot of this group of studies is that compact inner
systems of low-mass planets can typically tolerate cool giants only when the
latter are well separated and well aligned. Accordingly, Brad Hansen included an
evocative short title for his recent article on the stability problem: Beware of Outsiders. Once we have more
data on HD 219134, comparable analyses should be able to constrain the
alignment of planet e.
tranets in near space
HD 219134 now warrants a throng of superlatives: it’s the
nearest star with a tranet of any description, the nearest Sun-like star with a
tranet, the nearest with more than one tranet, the nearest with a terrestrial
tranet, and the nearest with more than one terrestrial tranet. In order of
increasing distance from the Sun, its closest rivals are Gliese 436 at 10.2 parsecs (one hot tranet more massive than
Neptune), Gliese 1132 at 12 parsecs
(one hot terrestrial tranet), TRAPPIST 1
at 12.1 parsecs (seven terrestrial tranets), 55 Cancri at 12.3 parsecs (one very hot, massive terrestrial
tranet), LHS 1140 at 12.47 parsecs (one temperate terrestrial tranet), Gliese 1214 at 13 parsecs
(one hot, puffy tranet about half the mass of Uranus), HD 189733 at 19.2 parsecs (one transiting Hot Jupiter), HD 97658 at 21 parsecs (one hot tranet
more massive than Neptune), Gliese 3470
at 29 parsecs (one hot Uranus-mass tranet), HAT-P-11 at 38 parsecs (one hot tranet more massive than Neptune),
and Kepler-42 at 39 parsecs (three warm
terrestrial tranets in the nearest Kepler system). All the more distant tranets
are either Hot Jupiters or low-mass planets discovered by space-based
telescopes (Kepler and CoRoT).
Among the 12 transiting systems located within 40 parsecs
(130 light years), only one includes a transiting gas giant on a hot orbit (HD
189733), a reminder that the well-known species of Hot Jupiters is actually quite
rare. More than half of these nearby transiting systems center on M dwarfs, while
only two systems with rocky tranets orbit Sun-like stars (HD 219134 and 55
Cancri). Coincidentally or not, these two are also the only mixed-mass systems
in the group.
Given so many superlatives and distinctions, it’s safe to
predict that we’ll be hearing more news from HD 219134 for years to come.
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