A Snapshot of a Vigorously Active Field of Inquiry

by Dr. Tahir Yaqoob on June 9, 2013

As the impairment of the Kepler mission is mourned, the latest number of Kepler planet candidates from existing data increases to 3,216, as reported on the Kepler website on 7 June 2013. Kepler’s European “sister mission,” CoRoT, has also been sick since November, and is not currently collecting data due to an electronics malfunction. Experts are still trying to figure out if data collection can be resumed.

As a general testament to how the field of exoplanet research is so vigorously active in all areas covering ground-based and space-based observations (by both Kepler and CoRoT), as well as theory, the 3rd of June saw 12 papers submitted to the preprint archive (arXiv), a colossal 20.7% of the total number of submissions on that day. The papers cover a variety of topics and I will briefly summarize each one here in just a few sentences to give a broad-brush snapshot.

Two of the papers were on planet formation, one of the most interesting and controversial areas of exoplanet science. The first, entitled The Formation of Systems with Tightly-packed Inner Planets (STIPs) via Aerodynamic Drift (by A. C. Boley and E. B. Ford) comes to an interesting conclusion about the so-called STIPs. The authors point out that whereas, according to the current paradigm, planetesimals (embryonic planets) are expected to spiral into the host star and be destroyed, aerodynamic drift acting on an ensemble of bodies can concentrate mass at small orbital radii. The authors state that Kepler data support evidence for STIPs being found near silicate sublimation zones (basically the region where rock would be zapped and evaporated into the gaseous phase). The authors propose that the low mass of Mars is indeed due to the Solar System being a proto-STIP some time in the past.

The second planet formation paper has a nice snappy title: Inside-out Planet Formation (by S. Chatterjee et al.) This paper also directly address the failure of current planet-formation theories to account for the multi-planet systems found by Kepler. Actually there is somewhat of a similarity with the idea in the previous paper, although the two works are independent. Basically, the authors propose a continuous stream of what they call pebbles (ranging from cm to meter size) drifting inward in the disk around the host star. The pebbles accumulate to form a ring, which then eventually becomes unstable, and then self-gravitating to form a planet with a mass somewhere in the range of 1 to 10 Earth masses. After that the process repeats with a new set of pebbles.

There were four very different papers on hot Jupiters. In Evidence for the Tidal Destruction of Hot Jupiters by Subgiant Stars by K. C. Schlaufman and N. W. Joshua, the authors address the observational fact that hot Jupiters are commonly found around stars of similar mass (and type) as the Sun as opposed to more massive stars that are at later stages of nuclear burning (subgiants). Either planet formation is relatively suppressed around the subgiant stars, or the hot Jupiters are ripped apart and destroyed as the host star ages. The study concludes that the latter is more likely to be the correct interpretation of the data.

A paper entitled HATS-3b: An inflated hot Jupiter transiting an F-type star by D. Bayliss et al. reported on the discovery observation of a new hot Jupiter using the transit method.

A third paper, Characterizing the Orbital and Dynamical State of the HD 82943 Planetary System With Keck Radial Velocity Data by T. Xianyu reports on updated an analysis of a multi-planet system around the star named in the title of the paper. Previous studies left some ambiguity about the actual number of planets in the system and their associated orbital period resonances. The new analysis rules out a 3-planet system and deduces the presence of two hot Jupiters.

A fourth paper, on yet another aspect of hot Jupiter systems, was entitled A Nearly Polar Orbit for the Extrasolar Hot Jupiter WASP-79b, by B. C. Addison et al. The paper concerns the very intriguing problem of so-called spin-orbit misalignment in a growing number of exoplanet systems. Essentially, the problem that these present is that if the disk from which planets form is “spun out” from the host star, you would expect the plane of that disk to lie perpendicular to the rotation axis of the star (this is spin-orbit alignment). Yet, a growing number of systems are showing planets with an orbital plane that is highly skewed with respect to the stellar spin axis. The paper on the misaligned planet WASP-79b reports that the misalignment is so severe that the planetary orbit is nearly passes through the pole of the spin axis of the star (i.e. it couldn’t be much more misaligned than it is). Although ideas have been put forward to try to explain away spin-orbit misalignment, no satisfactory explanation has yet been found.

On the subject of celestial mechanics, the prize for the most theoretical paper of the bunch on June 3, 2013 goes to Resonant planetary dynamics: Periodic orbits and long-term stability by G. Voyatzis et al. In this paper the authors point to the lack of study of three-body systems in 3D as opposed to 2D and their work explores the regions of stable and unstable orbits in two-planet systems in 3D, where the plane of the orbit of a planet can be very different to that of another.

Still on the subject of stability in planetary systems, a paper The short-lived production of exozodiacal dust in the aftermath of a dynamical instability in planetary systems, by A. Bonsor et al., reported on a theoretical study that aimed to investigate a particular scenario for the origin of so-called exozodiacal dust. Signatures of these dust belts have been observed in about a third of nearby stars and a common suggested scenario is that the belts originate in the aftermath of a catastrophic dynamical instability, resulting in an event similar to the late bombardment period in our own solar system’s history. The study finds that the dust belts produced by such a scenario are too short-lived to account for the observations.

Two papers concerned direct imaging. The exoplanet known as beta Pictoris b is one of the few directly imaged exoplanets. The paper A Combined VLT and Gemini Study of the Atmosphere of the Directly-Imaged Planet, beta Pictoris b by T. Currie et al. reports on a study of the atmosphere of this planet, and in a particular scenario derives an age of the planet of only about 7 million years, implying formation very late in the life of its host star. In the paper The SEEDS Direct Imaging Survey for Planets and Scattered Dust Emission in Debris Disk Systems by M. Janson et al., the authors study the origin of the large gaps (around 27 Earth-Sun distances) found in debris disks around stars. The gaps are thought to be due to clearing by planets in the disk. However, the study finds that planets near gap edges are not very common (only 15-30% of the systems studied), and that these planets (if they are the cause of the gap) would be less massive than beta Pictoris b.

On a more touchy subject, the paper entitled, The effect of stellar spots on the high precision transit light curve, by M. Oshagh et al. addresses the thorny issue of how exoplanet transit data can be contaminated by phenomena that are not actually due to a planet transit across the host star, thus leading to erroneous conclusions. Specifically, the conincidence of the equivalent of a sun spot on the host star with a planet can lead to an inaccurate radius for the planet. The study shows that the minimum size of a stellar spot that can affect the precision typical for the Kepler telescope is about 3% of the size of the star. The study warns of anamalous results particularly for very active stars.

Finally, we come to the paper, CoRoT: harvest of the exoplanet program, by Moutou et al. The 25-page paper provides a comprehensive summary of the achievements of the mission that was launched at the end of 2006, including the detection of about 500 exoplanet candidates, of which about 5% are confirmed exoplanets. The paper ends with a rather sad note about the November 2012 failure, but it appears that the paper was completed before Kepler‘s problem since the conclusion section says that the latter mission continues to collect data as originally intended.

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