A Closely-Packed System of Low-Mass, Low-Density Planets Transiting Kepler-11 (1102.0291v1)

Abstract: When an extrasolar planet passes in front of its star (transits), its radius can be measured from the decrease in starlight and its orbital period from the time between transits. Multiple planets transiting the same star reveal more: period ratios determine stability and dynamics, mutual gravitational interactions reflect planet masses and orbital shapes, and the fraction of transiting planets observed as multiples has implications for the planarity of planetary systems. But few stars have more than one known transiting planet, and none has more than three. Here we report Kepler spacecraft observations of a single Sun-like star that reveal six transiting planets, five with orbital periods between 10 and 47 days plus a sixth one with a longer period. The five inner planets are among the smallest whose masses and sizes have both been measured, and these measurements imply substantial envelopes of light gases. The degree of coplanarity and proximity of the planetary orbits imply energy dissipation near the end of planet formation.

• The paper reveals that all six detected planets are low-mass with low densities, suggesting they possess extensive light gas envelopes.
• The paper employs transit photometry and transit timing variations (TTV) to precisely determine planetary masses and analyze orbital dynamics.
• The paper demonstrates that the compact, coplanar configuration of Kepler-11 challenges existing models of planetary formation and evolution.

Overview of a Closely-Packed System of Low-Mass, Low-Density Planets Transiting Kepler-11

This paper delineates the discovery and subsequent analysis of a densely-packed system of six transiting planets orbiting a Sun-like star named Kepler-11. Utilizing data from the Kepler space telescope, the authors provide an investigation into one of the most intricate planetary systems discovered, with particular focus on the dynamics, stability, and compositional characteristics of these celestial bodies.

The uniqueness of the Kepler-11 system lies in its configuration of six planets, where five have relatively short orbital periods ranging between 10 to 47 days, with the sixth planet having a longer orbital period of 118 days. This system challenges existing paradigms of planetary formation and dynamics, primarily due to the close proximity and low densities of the planets, which suggest the incorporation of substantial envelopes of light gases.

Methodology

The Kepler spacecraft, with its 0.95 m aperture, employs transit photometry to discern minute dips in starlight indicative of planetary transits. The Kepler-11 system was identified through the detection of periodic starlight reductions, signifying multiple transiting planets. Through meticulous analysis of Kepler's light curves, the team discerned six unique signatures corresponding to individual transits of planets, now designated Kepler-11b through Kepler-11g.

The paper employs both spacecraft photometry and ground-based spectroscopic techniques to constrain stellar properties. By utilizing Transit Timing Variations (TTVs), the paper meticulously estimates the masses of the planets and confirms the planetary nature of all six candidates through mutual gravitational interactions. Specifically, the TTV analysis underscores significant deviations amenable to precise mass evaluation for the inner planets, while employing Bayesian analysis to validate the more distant Kepler-11g against astrophysical false positives.

Main Findings

1. Planetary Characterization: All six planets were found to be of relatively low mass and density. The inner five planets have both size and mass measured, placing them among the smallest planets with these dual measurements. Notably, the measurement implies these planets have light gas envelopes.
2. Dynamics and Stability: The report explores the dynamical properties, eccentricities, and relative inclinations of the system. The planets exhibit significant coplanarity, indicative of energy dissipation during the late stages of formation. Stability analysis projects long-term stability for the system, despite the inner pair being exceptionally close in proximity.
3. Mass and Composition: The dynamical modeling through TTV leads to mass predictions for the planets, with estimations highlighting a considerable proportion of gaseous components over rocky constitution. The comparative lack of high bulk densities, particularly for planets Kepler-11d, e, and f, implies these bodies likely formed during the gaseous phase of the protoplanetary disk.

Theoretical and Practical Implications

The Kepler-11 discovery provides indispensable insights into the formation of compact planetary systems, especially in explaining the dynamics of planets with low densities and significant gas envelopes. The compositions derived—indicating substantial contributions from hydrogen and other volatiles—offer significant parallels to our understanding of planetary formation and atmospheric evolution in extrasolar systems. As such, these findings impact models of planetary system evolution, particularly regarding rapid accretion timescales and migration phenomena in nascent star systems.

Future Directions

The paper heralds further investigation on long-term evolutionary processes of such compact systems, particularly with ongoing Kepler data that promise more comprehensive characterizations. These insights could refine existing planet formation models, especially those applicable to systems with proximally-lying low-mass planets. The advancements in observational techniques—both spectroscopic and computational modeling—could refine mass estimations and investigate atmospheric compositions further, potentially bridging the gap between observed planetary attributes and theoretical expectations in exoplanet studies.

In summary, the paper presents a thorough exploration of the Kepler-11 system's planetary characteristics, dynamics, and stability, contributing valuable data to the paper of exoplanetary systems. The findings not only advance our understanding of planetary compositions and system evolution but also set a precedent for analyzing similar systems using advanced photometric and spectroscopic methods.