Figure 1. Configuration of the
Kepler-11 system, based on new data. Planetary radii are represented at the
same scale, with darker & bluer colors indicating higher density and
lighter & greener colors indicating lower density. (Planet g
is represented by an open circle because its radius is known but its density is not.)
Semimajor axes are measured in astronomical units (AU).
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Metaphors such as “Rosetta stone,” “natural laboratory,” and
“testbed” are overused in scientific discourse. For the Kepler-11 planetary system,
however, they are completely appropriate. Six planets are known, all with radii
measured by transit observations, and five with masses determined by transit
timing variations. Their discovery was one of the highlights of 2011 (Lissauer
et al. 2011), and their value for the study of planetary composition and system
architecture remains unsurpassed. A new study by Kepler mission scientists, based
on the much larger dataset resulting from continued observations, provides a
revised view of this prototypical system (Lissauer et al. 2013).
Early findings demonstrated that Kepler-11 is a G-type star,
similar to our Sun in mass, temperature, radius, and metallicity. The latest
data point to a slightly closer resemblance. While the star’s mass and radius were
previously reported as 0.95 and 1.1 Solar, respectively, cumulative
observations have revised these values to 0.96 Solar masses (Msol) and 1.05 Solar
radii (Rsol). Age remains the most striking difference between the two stars: Kepler-11
is somewhere in the range of 7 to 10 billion years, compared to 4.6 billion for
our Sun. Nevertheless, Kepler-11 still looks youthful, without the enlargement
in radius that develops as stars evolve off the main sequence.
The similarity between this star and our Sun makes the differences
in their system architectures all the more striking. Five planets between the
masses of Earth and Uranus orbit Kepler-11 inside a radius smaller than the
semimajor axis of Mercury, while a sixth planet in the same mass range orbits
just beyond. Initial data demonstrated that at least the inner five planets are
much lower in density than Earth and Venus.
The revised data are presented in Table 1. Values for
orbital period are unchanged, while values for semimajor axis differ only
slightly. Orbital eccentricities, formerly unknown, can now be estimated. All
are small, as expected: comparable to those of the Solar planets.
Table 1. Parameters of the
planets around Kepler-11, based on the latest data from Lissauer et al. (2013).
Mea = planet mass in Earth units; Rea = planet radius in Earth units; a =
semimajor axis in astronomical units; Period = orbital period in days; e =
orbital eccentricity.
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The biggest changes appear in the values for mass and
radius, which are critical for understanding planet structure and formation. With
one exception (Kepler-11d), values for these parameters have been revised
downward. In two cases the reductions in mass are substantial – the inner
planet, Kepler-11b, is less than half as massive as previously reported, while
the second planet, Kepler-11c, is only about 20% as massive.
The five well-constrained planets (b-f) now seem to fall
into two sub-populations: smaller planets with masses between 1.9 and 2.9 Earth
masses (Mea) and radii between 1.8 and 2.9 Earth radii (Rea), and larger
planets with masses above 6 Mea and radii above 3 Rea. Following a taxonomy
that I outlined last year, I might be tempted to call the first group the true
Super Earths (scaled-up versions of home) whereas I would call the second group
gas dwarfs (planets less massive than ~40 Mea with hydrogen atmospheres, such
as Uranus, Neptune, and GJ 436 b).
But that would be a mistake. It has always been clear that planets
c through g have hydrogen envelopes, and the newly reduced mass of planet b
raises the likelihood that it too supports such an atmosphere. (The alternative
is a planet less than 50% rock and more than 50% steam.) Therefore, all the
well-constrained companions of Kepler-11 now meet my definition of gas dwarfs, even though the
heaviest is less than half as massive as Neptune, while the two lightest are
only about twice the mass of Earth.
The revised parameters of Kepler-11 are persuasive evidence
that the term Super Earth is a misnomer for all but a tiny fraction of the confirmed
exoplanets with minimum or measured masses between 2 and 10 Mea.
Remarkably, the distribution of the Kepler-11 planets
reveals few regularities in terms of mass, radius, or density. As Lissauer and
colleagues observe, planet radii now appear to increase along with planet masses,
a pattern that was obscured by the initial data and analyses (Lissauer et al. 2013).
Nevertheless, masses and densities seem to be distributed at random: Kepler-11c
orbits between 11b and 11d, but it is less dense (i.e., it has retained a
larger fraction of hydrogen/helium) than either of its companions, while fluffy
Kepler-11e is similarly flanked by denser 11d and 11f. Moreover, although the
innermost planet in the system (b) is also the least massive – a typical
architectural feature of multiplanet systems – it is almost identical in mass
to the fifth planet (f).
Kepler-11 now looks a little more like Kepler-20,
the only other system that hosts a comparable number of transiting planets with
well-constrained masses. In both systems, low-mass planets are found adjacent
to higher-mass planets, on exterior as well as interior orbits, and low-density
planets are similarly intermixed with high-density planets. By contrast, our bizarre
Solar System segregates its planet populations, with a handful of dense rocky
planets in the inner system, a pair of gas giants in the middle system, and a
pair of gas dwarfs in the outer system.
REFERENCES
Lissauer JJ, Fabrycky DC, Ford EB, Borucki WJ,
Fressin F, Marcy GW, et al. (2011) A closely packed system of low-mass,
low-density planets transiting Kepler-11. Nature
470, 53-58. Abstract: http://adsabs.harvard.edu/abs/2011Natur.470...53L
Lissauer JJ, Jontof-Hutter D, Rowe JF, Fabrycky DC,
Lopez ED, Agol E, et al. (2012) All six planets known to orbit Kepler-11 have
low densities. In press: http://adsabs.harvard.edu/abs/2013arXiv1303.0227L
Lopez ED, Fortney J, Miller N. (2012) How thermal evolution and
mass loss sculpt populations of super-Earths and sub-Neptunes: Application to
the Kepler-11 system and beyond. Astrophysical
Journal 761, 59. Abstract: http://adsabs.harvard.edu/abs/2012ApJ...761...59L
Migaszewski C, Slonina M, Gozdziewski K. (2012) A
dynamical analysis of the Kepler-11 planetary system. Monthly Notices of the Royal Astronomical Society 427, 770-789. http://adsabs.harvard.edu/abs/2012MNRAS.427..770M
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