Katherine M. Deck

Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1

One aim of modern astronomy is to detect temperate, Earth-like exoplanets that are well suited for atmospheric characterization. Recently, three Earth-sized planets were detected that transit (that is, pass in front of) a star with a mass just eight per cent that of the Sun, located 12 parsecs away. The transiting configuration of these planets, combined with the Jupiter-like size of their host star—named TRAPPIST-1—makes possible in-depth studies of their atmospheric properties with present-day and future astronomical facilities. Here we report the results of a photometric monitoring campaign of that star from the ground and space. Our observations reveal that at least seven planets with sizes and masses similar to those of Earth revolve around TRAPPIST-1. The six inner planets form a near-resonant chain, such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.1 and 12.35 days) are near-ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inwards. Moreover, the seven planets have equilibrium temperatures low enough to make possible the presence of liquid water on their surfaces.

Kepler-36: A Pair of Planets with Neighboring Orbits and Dissimilar Densities

So Close and So Different In our solar system, the rocky planets have very distinct orbits from those of the gas giants. Carter et al. (p. 556, published online 21 June) report on a planetary system where this pattern does not apply, posing a challenge to theories of planet formation. Data from the Kepler space telescope reveal two planets with radically different densities orbiting the same star with very similar orbital periods. One planet has a rocky Earth-like composition and the other is akin to Neptune. The Kepler spacecraft detected a super-Earth and a Neptune-like planet in very tightly spaced orbits around the same star. In the solar system, the planets’ compositions vary with orbital distance, with rocky planets in close orbits and lower-density gas giants in wider orbits. The detection of close-in giant planets around other stars was the first clue that this pattern is not universal and that planets’ orbits can change substantially after their formation. Here, we report another violation of the orbit-composition pattern: two planets orbiting the same star with orbital distances differing by only 10% and densities differing by a factor of 8. One planet is likely a rocky “super-Earth,” whereas the other is more akin to Neptune. These planets are 20 times more closely spaced and have a larger density contrast than any adjacent pair of planets in the solar system.

Stellar Spin-Orbit Misalignment in a Multiplanet System

Misaligned Planets Stars with multiple coplanar planets have not been seen to show misalignments between the equatorial plane of the star and the orbital plane of the planets—a diagnostic of the dynamical history of planetary systems. Huber et al. (p. 331) analyzed the Kepler 56 planetary system, which contains a giant-sized and an intermediate-sized planet. The planets have orbits that are close to coplanar, but the planetary orbits are misaligned with the stellar equator. A third companion in a wide orbit, which could be another star or a planet, could explain the misaligned configuration. Kepler observations show that stellar spin-orbit misalignments are not confined to planetary systems with hot Jupiters. Stars hosting hot Jupiters are often observed to have high obliquities, whereas stars with multiple coplanar planets have been seen to have low obliquities. This has been interpreted as evidence that hot-Jupiter formation is linked to dynamical disruption, as opposed to planet migration through a protoplanetary disk. We used asteroseismology to measure a large obliquity for Kepler-56, a red giant star hosting two transiting coplanar planets. These observations show that spin-orbit misalignments are not confined to hot-Jupiter systems. Misalignments in a broader class of systems had been predicted as a consequence of torques from wide-orbiting companions, and indeed radial velocity measurements revealed a third companion in a wide orbit in the Kepler-56 system.

All Six Planets Known to Orbit Kepler-11 Have Low Densities

United States. National Aeronautics and Space Administration (Kepler Participating Scientist Program, NASA grant NNX12AD23G)

FIRST-ORDER RESONANCE OVERLAP AND THE STABILITY OF CLOSE TWO-PLANET SYSTEMS

Motivated by the population of observed multi-planet systems with orbital period ratios 1 < P 2/P 1 2, we study the long-term stability of packed two-planet systems. The Hamiltonian for two massive planets on nearly circular and nearly coplanar orbits near a first-order mean motion resonance can be reduced to a one-degree-of-freedom problem. Using this analytically tractable Hamiltonian, we apply the resonance overlap criterion to predict the onset of large-scale chaotic motion in close two-planet systems. The reduced Hamiltonian has only a weak dependence on the planetary mass ratio m 1/m 2, and hence the overlap criterion is independent of the planetary mass ratio at lowest order. Numerical integrations confirm that the planetary mass ratio has little effect on the structure of the chaotic phase space for close orbits in the low-eccentricity (e 0.1) regime. We show numerically that orbits in the chaotic web produced primarily by first-order resonance overlap eventually experience large-scale erratic variation in semimajor axes and are therefore Lagrange unstable. This is also true of the orbits in this overlap region which satisfy the Hill criterion. As a result, we can use the first-order resonance overlap criterion as an effective stability criterion for pairs of observed planets. We show that for low-mass ( 10 M ?) planetary systems with initially circular orbits the period ratio at which complete overlap occurs and widespread chaos results lies in a region of parameter space which is Hill stable. Our work indicates that a resonance overlap criterion which would apply for initially eccentric orbits likely needs to take into account second-order resonances. Finally, we address the connection found in previous work between the Hill stability criterion and numerically determined Lagrange instability boundaries in the context of resonance overlap.

TWO TRANSITING EARTH-SIZE PLANETS NEAR RESONANCE ORBITING A NEARBY COOL STAR

Discoveries from the prime Kepler mission demonstrated that small planets (< 3 Earth-radii) are common outcomes of planet formation. While Kepler detected many such planets, all but a handful orbit faint, distant stars and are not amenable to precise follow up measurements. Here, we report the discovery of two small planets transiting K2-21, a bright (K = 9.4) M0 dwarf located 65

TTVFast: AN EFFICIENT AND ACCURATE CODE FOR TRANSIT TIMING INVERSION PROBLEMS

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MIGRATION OF TWO MASSIVE PLANETS INTO (AND OUT OF) FIRST ORDER MEAN MOTION RESONANCES

6 pc from Earth. We detected the transiting planets in photometry collected during Campaign 3 of NASAs K2 mission. Analysis of transit light curves reveals that the planets have small radii compared to their host star, 2.60

Planet Hunters. VII. Discovery of a New Low-mass, Low-density Planet (PH3 C) Orbiting Kepler-289 with Mass Measurements of Two Additional Planets (PH3 B and D)

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An empirically derived three-dimensional Laplace resonance in the Gliese 876 planetary system

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