Figure 1. All
announced planets of Kepler-20 at their relative sizes, with colors
corresponding to the densities provided by Buchhave et al. 2016 (see Figure 2
for the color key; redder hues indicate higher densities). Since planet g does not transit, the radius and
composition shown here are informed guesses, as indicated by the striped fill. Planets
e and f are about the same size as Earth, but none of the planets in this
system are cool enough to support Earth-like conditions.
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Lars Buchhave and colleagues just reported new radial
velocity data on Kepler-20, one of
the best-known systems revealed by the Kepler Mission. The new results include
more precise mass estimates for three of the system’s five known planets (Kepler-20b, c, d) and robust evidence
for a previously unknown sixth candidate (Kepler-20g), whose orbit is apparently misaligned with the others.
The new planet is notably more massive than its five companions, but despite its
short orbital period (35 days), it was not observed in transit by the Kepler
Telescope.
When preliminary Kepler data started circulating in 2011,
we learned that a highly specific orbital architecture is more common than
anyone ever dreamed: compact systems with three or more planets visible in
transit (Lissauer et al. 2011b). Lissauer & colleagues reported 55 in 2011.
By May 2016, the Extrasolar Planets Encyclopaedia listed
154.
Two systems have always stood out from the pack. One is
Kepler-20 (Gautier et al. 2012), which harbors five transiting planets inside a
semimajor axis of 0.4 astronomical units (AU). The other is Kepler-11 (Lissauer
et al. 2011a), with six transiting planets inside a semimajor axis of 0.5 AU.
Both systems center on mature G-type stars like our Sun, yet each one sustains a
rich multiplanet architecture confined within a radius similar to the semimajor
axis of Mercury (0.39 AU). The corresponding region of our Solar System, of
course, is empty.
From the beginning, Kepler-11 has received the lion’s
share of attention, for one simple reason. Five of its six planets exhibit transit timing variations (TTVs), such
that their orbital motion periodically speeds up or slows down to avoid awkward
encounters with neighboring planets. Although this behavior had previously been
theorized, Kepler-11 and another early discovery, Kepler-9, were the first cases
ever actually confirmed (Ford et al. 2011).
For Kepler-11, analysis of TTVs enabled estimates of the masses
of the five inner planets, which presented yet another surprise (Lissauer et
al. 2011a). Although their radii were originally estimated in the
range of 2 to 4.5 Earth units (Rea), all five turned out to be substantially
less massive than Neptune, whose radius of 3.9 Rea contains a mass of 17.2 Earth
units (Mea). Like Neptune, Lissauer & colleagues concluded, the Kepler-11 planets
must be enveloped in extended atmospheres of hydrogen and helium (H/He), even
though their individual masses are smaller than 10 Mea. With that inference began
the modern era of planetology.
The planets of Kepler-20 are packed almost as tightly as
those of Kepler-11, but no TTVs are available to provide mass estimates.
Fortunately, Kepler-20 is substantially closer to our Solar System than is Kepler-11.
Its distance is estimated at 290 parsecs (945 light years), versus 613 parsecs
(1998 light years) for Kepler-11. This proximity enabled the collection of ground-based
radial velocity observations in 2009-2011 by the Keck/HIRES spectrograph, which
placed rough constraints on the masses of Kepler-20b, c, and d (Gautier et al.
2012). These constraints indicated that, despite the broad similarities between
Kepler-20 and Kepler-11, the planets of the former star are both denser and
more massive than those of the latter.
Figure 2. Kepler-20 and Kepler-11 Compared
Planets are rendered at their relative sizes on the same orbital scale, with semimajor axes in astronomical units (AU). Numbers in red indicate approximate planet masses in Earth units, rounded to the nearest integer. Planet colors indicate approximate densities, with values taken from Buchhave et al. (2016) for Kepler-20 and from Lissauer et al. (2013) for Kepler-11. Buchhave et al. propose that Kepler-20e and -20f have Earth-like compositions, given their similarity in size to Venus and Earth. The radius and composition of Kepler-20g are informed guesses, as are the mass and composition of Kepler-11g.
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Observations of these two benchmark systems have
continued since their discovery. A follow-up study using additional quarters of
Kepler data revised the masses and radii of the Kepler-11 planets, mostly
downward, resulting in even puffier planets (Lissauer et al. 2013). Another follow-up
study reported radial velocity data on Kepler-11 obtained by Keck/HIRES in
2014 (Weiss et al. 2015), placing an upper limit of twice the mass values derived
by Lissauer & colleagues.
Now Buchhave & colleagues (2016) have refined the
masses of the known planets around Kepler-20 and validated a sixth planet by
analyzing archival HIRES data and new HARPS-N data. Figure 2, above, is a graphic comparison of the two systems based
on current findings; Table 1, below,
provides comparative numbers.
The most striking difference between the planetary
systems of Kepler-11 and Kepler-20 appears in the bulk compositions of their
planets. The estimated density of Kepler-20b substantially exceeds that of Earth,
the densest planet in our system. Its composition might be explained by an
enhancement in iron. Likewise, despite their similarity in mass to Uranus,
Kepler-20c and -20d are much denser, indicating a smaller bulk percentage of
H/He and an enrichment in metal, rock, and potentially ice. Conclusive data are
lacking for the other three planets of Kepler-20, but planet g is likely to be similar in
composition to planets c and d, while planets e and f might resemble
our system’s terrestrial planets.
All the planets of Kepler-11 are less massive than
Uranus, but two of them (b, d) have
densities comparable to Neptune and Uranus, respectively. The other three
planets with estimated densities (c, e,
f) are comparable to Saturn, the most rarefied planet in our system,
despite masses in the range of 2 to 8 Mea. For context, Saturn is 95 Mea, and
if our ringed planet fell into an ocean large enough, it would float. Half the
Kepler-11 planets could float along with it, bobbing like rubber ducklings after
a colossal rubber duck.
kepler-20: six-planet architecture
The new study by Buchhave & colleagues (hereafter B16)
begins by reexamining the properties of the host star. For Kepler-20, they
report a higher mass (0.948 Solar masses or Msol), a slightly higher
metallicity (+0.07 ±0.08), and an earlier spectral type (G2) than did earlier
sources. These new parameters imply an even closer resemblance among Kepler-20,
Kepler-11, and our own Sun than previously indicated. Kepler-20 and Kepler-11 are
now assigned virtually identical masses and metallicities, while both appear older
and slightly less massive than our Sun.
The most significant contribution from B16 is their discovery of the new planet, Kepler-20g. Notably, Hansen & Murray (2013) previously found that a stable orbit might be available between planets f and d, and the HARPS-N radial velocity data have confirmed their hypothesis. In addition, B16 report a more precise mass for Kepler-20d, which remains the outermost of the known planets. Its new mass value (10 Mea) is just half the upper limit reported in the discovery paper (Gautier et al. 2012), although the precision of that estimate is still inferior to those for planets b and c.
Other major results from B16 support the basic picture
unveiled by the discovery paper, including null results for TTVs. As before, we
see a distinctive mass distribution in which smaller, more lightweight planets
alternate with larger, more massive planets inside 0.25 AU. The two smallest
planets (e, f) are remarkably
similar in radius – and probably in mass and composition – to the terrestrial
planets of our Solar System. Their diminutive profiles are inconsistent with
H/He envelopes, whereas such envelopes are essential to explain the radii of
the two largest planets (c, d). We
are also safe in assuming an H/He atmosphere for the most massive planet (g).
The innermost planet (b), however, appears to be rocky, without any contribution from water
or H/He. Kepler-20b is therefore the most massive rocky planet discovered to
date. All other objects with similar radii and well-constrained masses – with
the possible exception of 55 Cancri e
– require substantial volatile content to explain their profiles.
Unfortunately, B16 did not present stability limits for
any additional planets that might orbit outside 0.4 AU, in the cooler region where
the system’s habitable zone is located. As a result, the potential of Kepler-20
to support habitable planets remains unexplored.
Table 1. Kepler-20 and Kepler-11 System Parameters
Masses and radii are expressed in Earth
units. Mass is rounded to a single
decimal place, with uncertainties omitted. Radius
is rounded to two decimal places. a
= semimajor axis, expressed in astronomical units (AU), where 1 AU = separation
between Earth and Sun. Period =
days. Teq = equilibrium temperature,
expressed in Kelvin (K); for context, the Teq of Earth is 255 K. Density = grams/cc. Data on Kepler-20
are from Buchhave et al. 2016, except for Teq values in parentheses, which are
older estimates from the Kepler Table based on a lower stellar effective
temperature. Data on Kepler-11 are from Lissauer et al. 2013.
----------
kepler-11 and kepler-20: formation scenarios
Although they do not discuss any specific formation
models for the planets of Kepler-20, B16 opine that all six planets assembled shortly
before the dissipation of the primordial circumstellar nebula. This timing
enabled all six to accrete small amounts of H/He from the nebula without
initiating a runaway process that would turn them into gas giants. After the nebula
dispersed, stellar flux likely ablated the H/He envelopes from planets b, e, and f – the former because of its star-hugging orbit, the latter two
because of their small masses. The cooler and heavier planets (c, d, g) were able to retain their
lightweight atmospheres, consistent with current models of atmospheric
evolution (Lopez & Fortney 2014, Erkaev et al. 2016).
The release of B16 was almost simultaneous with the
publication of a new study of possible evolutionary scenarios for Kepler-11
(D’Angelo & Bodenheimer 2016). Given the similarities between the two host
stars, a scenario that can explain one system architecture should also tell us
something about the other.
In their
new study, Gennaro D’Angelo and Peter Bodenheimer (hereafter DB16) test the two
most popular formation processes for close-in planets: 1) in situ accretion of solids locally available in the inner nebula,
and 2) accretion of solids over a broad radial distance by a planetary core
migrating from the outer nebula to the vicinity of the central star. For
brevity they call the latter process “ex
situ formation.” For each scenario they seek initial conditions that permit
the formation of the known planets within currently observed parameters, and in
both cases they achieve some degree of success. Accordingly, they conclude that
“it is not possible to distinguish between the two modes of formation from [the
planets’] final properties.”
However,
a review of their models suggests that ex
situ is more plausible than in situ,
given two major flaws in the in situ scenario.
First, DB16 find that a protoplanetary nebula massing 0.18 Msol inside 70 AU
would be required to achieve the concentration of solid mass needed for in situ formation of the Kepler-11 planets
inside 0.5 AU. This total is equivalent to the mass of a mid to late M dwarf
star. Yet observations of protoplanetary nebulae in nearby star-forming regions
indicate that most have masses in the range of 0.002 to 0.01 Msol (Williams
& Cieza 2011, Andrews et al. 2013). Such findings argue that the in situ nebula invoked by DB16 is
unrealistic.
In the second
place, even assuming the existence of a protoplanetary nebula more massive than
Proxima Centauri, the in situ model still could not produce a
good analog of Kepler-11b. Since the inner nebula is completely dry in this model,
the six planets originally form as rock/metal cores surrounded by H/He
atmospheres. No water or other volatile materials are available to enhance
their composition. Given the low core mass and tight semimajor axis of
Kepler-11b, DB16 find that the planet’s primordial atmosphere would be stripped
by stellar flux within 40 million years after the evaporation of the ambient
nebula. To achieve the puffy radius observed today, some 8 billion years later,
the planet would have to outgas very large quantities of H/He from its interior
over a period lasting a few hundred million years – first to replace its
original envelope, and later to replenish the outgassed atmosphere, which would
remain vulnerable to stripping during the time it takes for a G star to settle into
maturity (Erkaev et al. 2016). DB16 acknowledge the difficulties involved in
this outcome.
Their ex situ or migratory model avoids both pitfalls.
They begin with a protoplanetary nebula of 0.03 Msol inside a radius of 60 AU.
Although this value is larger than a typical nebular mass, it still falls within
an order of magnitude of observations, and is therefore substantially more
realistic than the in situ nebula.
Within this structure DB16 insert newly formed planetary cores of approximately
Martian mass (0.1 Mea) at orbital radii ranging from 2.1 AU to 5.35 AU, in the region
of the nebula where ices are abundant. All cores begin accreting mass and
migrating inward. The timing of their insertion in the nebula is just as
important as their radial placement, since cores that achieve smaller final
masses need to begin accreting later than those that achieve larger masses in
order to avoid orbit crossings during migration.
Because
they originate in a well-hydrated region, all six forming planets accrete
abundant water along with their H/He envelopes, and all are subject to loss of
atmosphere once they reach their final destinations and the nebula dissipates.
With this model, DB16 succeed in reproducing the masses and radii of all six
planets of Kepler-11. In particular, they find that Kepler-11b retains a steam
atmosphere after the loss of its lightweight H/He envelope, and that this
heavier atmosphere can survive until the present age of the system, consistent
with the planet’s current radius.
DB16 underscore the difference in planetary compositions produced by their two contrasting approaches to system evolution. Although they do not apply the term, the water-rich bodies resulting from their ex situ model are consistent with the Ocean Planets predicted by Leger et al. (2004).
The approach taken by DB16 is paralleled in some of the studies that accompanied the recent announcement of Proxima Centauri b (Barnes et al. 2016, Coleman et al. 2016), as discussed in my previous post. Like DB16, Coleman & colleagues explored several scenarios for planet formation, including in situ accretion in an implausibly massive protoplanetary disk, as well as long-distance migration in a more realistic disk. As with DB16, the migration scenario provided a better fit with observations than did in situ formation. Notably, Coleman’s group explicitly invoked Ocean Planets to describe the objects produced by migration.
I look
forward to a study that applies similar models to the dual evolutionary
histories of Kepler-20 and Kepler-11. I suspect that in situ scenarios will continue to face difficult challenges.
the
high-multiplicity sample
With the confirmation of Kepler-20g, Kepler-20 moves from
one exclusive club – systems with at least five known planets – to the even
smaller elite – systems with at least six planets. The current exoplanetary
census offers only four examples of this rare architecture: HD 10180,
Kepler-11, Kepler-90, and now Kepler-20. Their orbital arrangements provide
invaluable data that will continue to inform our understanding of planet
formation and the distribution of specific planetary types (including temperate
rocky planets) in our region of the Galaxy.
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