Sunday, April 7, 2013

Kepler-11 Revisited

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).

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.


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.

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:
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:  
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:
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.


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