It’s no surprise that Titan’s north polar region is covered with vast lakes and seas of liquid methane — these have been imaged many times by Cassini during its ten years in orbit around Saturn. What is surprising though is just how incredibly smooth the surfaces of these lakes have been found to be.
One would think that such large expanses of surface liquid — some of Titan’s seas are as big the Great Lakes — would exhibit at least a little surface action on a world with an atmosphere as dense as Titan’s. But repeated radar imaging has shown their surfaces to be “as smooth as the paint on a car.” Over the past several years scientists have puzzled over this anomaly but now they may have truly seen the light — that is, reflected light from what could actually be waves on Titan!
Using data acquired during flybys of Titan in 2012 and 2013, planetary scientist Jason Barnes from the University of Idaho and a team of researchers from several other institutions including JPL, Cornell, and MIT, have identified what might be waves in the surface of Punga Mare, one of Titan’s biggest lakes.
The Hunt for Exomoons with Kepler (HEK): IV. A Search for Moons around Eight M-Dwarfs
Authors:
Kipping et al
Abstract:
With their smaller radii and high cosmic abundance, transiting planets around cool stars hold a unique appeal. As part of our on-going project to measure the occurrence rate of extrasolar moons, we here present results from a survey focussing on eight Kepler planetary candidates associated with M-dwarfs. Using photodynamical modeling and Bayesian multimodal nested sampling, we find no compelling evidence for an exomoon in these eight systems. Upper limits on the presence of such bodies probe down to ∼0.4M⊕ in the best case. For KOI-314, we are able to confirm the planetary nature of two out of the three known transiting candidates using transit timing variations. Of particular interest is KOI-314c, which is found to have a mass of 1.0+0.4−0.3M⊕, making it the lowest mass transiting planet discovered to date. With a radius of 1.61+0.16−0.15R⊕, this Earth-mass world is likely enveloped by a significant gaseous envelope comprising ≥17+12−13% of the planet by radius. We find evidence to support the planetary nature of KOI-784 too via transit timing, but we advocate further observations to verify the signals. In both systems, we infer that the inner planet has a higher density than the outer world, which may be indicative of photo-evaporation. These results highlight both the ability of Kepler to search for sub-Earth mass moons and the exciting ancillary science which often results from such efforts.
A Sub-Earth-Mass Moon Orbiting a Gas Giant Primary or a High Velocity Planetary System in the Galactic Bulge
Authors:
Bennett et al (super al!)
Abstract:
We present the first microlensing candidate for a free-floating exoplanet-exomoon system, MOA-2011-BLG-262, with a primary lens mass of M_host ~ 4 Jupiter masses hosting a sub-Earth mass moon. The data are well fit by this exomoon model, but an alternate star+planet model fits the data almost as well. Nevertheless, these results indicate the potential of microlensing to detect exomoons, albeit ones that are different from the giant planet moons in our solar system. The argument for an exomoon hinges on the system being relatively close to the Sun. The data constrain the product M pi_rel, where M is the lens system mass and pi_rel is the lens-source relative parallax. If the lens system is nearby (large pi_rel), then M is small (a few Jupiter masses) and the companion is a sub-Earth-mass exomoon. The best-fit solution has a large lens-source relative proper motion, mu_rel = 19.6 +- 1.6 mas/yr, which would rule out a distant lens system unless the source star has an unusually high proper motion. However, data from the OGLE collaboration nearly rule out a high source proper motion, so the exoplanet+exomoon model is the favored interpretation for the best fit model. However, the alternate solution has a lower proper motion, which is compatible with a distant (so stellar) host. A Bayesian analysis does not favor the exoplanet+exomoon interpretation, so Occam's razor favors a lens system in the bulge with host and companion masses of M_host = 0.12 (+0.19 -0.06) M_solar and m_comp = 18 (+28 -100 M_earth, at a projected separation of a_perp ~ 0.84 AU. The existence of this degeneracy is an unlucky accident, so current microlensing experiments are in principle sensitive to exomoons. In some circumstances, it will be possible to definitively establish the low mass of such lens systems through the microlensing parallax effect. Future experiments will be sensitive to less extreme exomoons.
THE HUNT FOR EXOMOONS WITH KEPLER (HEK). III. THE FIRST SEARCH FOR AN EXOMOON AROUND A HABITABLE-ZONE PLANET
Authors:
Kipping et al
Abstract:
Kepler-22b is the first transiting planet to have been detected in the habitable zone of its host star. At 2.4 R ⊕, Kepler-22b is too large to be considered an Earth analog, but should the planet host a moon large enough to maintain an atmosphere, then the Kepler-22 system may yet possess a telluric world. Aside from being within the habitable zone, the target is attractive due to the availability of previously measured precise radial velocities and low intrinsic photometric noise, which has also enabled asteroseismology studies of the star. For these reasons, Kepler-22b was selected as a target-of-opportunity by the "Hunt for Exomoons with Kepler" (HEK) project. In this work, we conduct a photodynamical search for an exomoon around Kepler-22b leveraging the transits, radial velocities, and asteroseismology plus several new tools developed by the HEK project to improve exomoon searches. We find no evidence for an exomoon around the planet and exclude moons of mass MS greater than 0.5 M ⊕ to 95% confidence. By signal injection and blind retrieval, we demonstrate that an Earth-like moon is easily detected for this planet even when the time-correlated noise of the data set is taken into account. We provide updated parameters for the planet Kepler-22b, including a revised mass of MP less than 53 M ⊕ to 95% confidence and an eccentricity of $0.13_{-0.13}^{+0.36}$ by exploiting Single-body Asterodensity Profiling. Finally, we show that Kepler-22b has a greater than 95% probability of being within the empirical habitable zone but a less than 5% probability of being within the conservative habitable zone.
A new study on magnetic fields around extrasolar giant planets sheds first light on the magnetic environment of extrasolar moons.
The work, authored by René Heller of the Department of Physics and Astronomy at McMaster University (Canada) and Jorge I. Zuluaga of the FACom group in the Institute of Physics of the University of Antioquia (Colombia), is the first to explore the complex magnetic environment of exomoons and its impact on the habitability of these peculiar bodies.
Regrettably the results are not completely encouraging. Even the most massive moons that can be expected from a formation point of view will be small compared to Earth. Thus, the only possibility these moons can be magnetically protected from the stellar and cosmic high-energy radiation is that they are encased by their giant planet’s magnetosphere. Yet, in orbits close to the planet, these moons can be subject to enormous tidal heating, potentially making them uninhabitable. These results represent just the beginning of an interesting research branch, which introduces a new key factor for the habitability of those “Pandora”-like environments.
link to article.
link to paper.
HABITABILITY OF EXOMOONS AT THE HILL OR TIDAL LOCKING RADIUS
Authors:
1. Natalie R. Hinkel (a)
2. Stephen R. Kane (a)
Affiliations:
a. NASA Exoplanet Science Institute, Caltech, MS 100-22, 770 South Wilson Avenue, Pasadena, CA 91125, USA
Abstract:
Moons orbiting extrasolar planets are the next class of object to be observed and characterized for possible habitability. Like the host-planets to their host-star, exomoons have a limiting radius at which they may be gravitationally bound, or the Hill radius. In addition, they also have a distance at which they will become tidally locked and therefore in synchronous rotation with the planet. We have examined the flux phase profile of a simulated, hypothetical moon orbiting at a distant radius around the confirmed exoplanets μ Ara b, HD 28185 b, BD +14 4559 b, and HD 73534 b. The irradiated flux on a moon at its furthest, stable distance from the planet achieves its largest flux gradient, which places a limit on the flux ranges expected for subsequent (observed) moons closer in orbit to the planet. We have also analyzed the effect of planetary eccentricity on the flux on the moon, examining planets that traverse the habitable zone either fully or partially during their orbit. Looking solely at the stellar contributions, we find that moons around planets that are totally within the habitable zone experience thermal equilibrium temperatures above the runaway greenhouse limit, requiring a small heat redistribution efficiency. In contrast, exomoons orbiting planets that only spend a fraction of their time within the habitable zone require a heat redistribution efficiency near 100% in order to achieve temperatures suitable for habitability. This means that a planet does not need to spend its entire orbit within the habitable zone in order for the exomoon to be habitable. Because the applied systems comprise giant planets around bright stars, we believe that the transit detection method is most likely to yield an exomoon discovery.
Capture of Terrestrial-Sized Moons by Gas Giant Planets
Author:
1. Darren M. Williams (a)
Affiliation:
a. Penn State Erie, The Behrend College, School of Science, Erie, Pennsylvania
Abstract:
Terrestrial moons with masses greater than 0.1 M possibly exist around extrasolar giant planets, and here we consider the energetics of how they might form. Binary-exchange capture can occur if a binary-terrestrial object (BTO) is tidally disrupted during a close encounter with a giant planet and one of the binary members is ejected while the other remains as a moon. Tidal disruption occurs readily in the deep gravity wells of giant planets; however, the large encounter velocities in the wells make binary exchange more difficult than for planets of lesser mass. In addition, successful capture favors massive binaries with large rotational velocities and small component mass ratios. Also, since the interaction tends to leave the captured moons on highly elliptical orbits, permanent capture is only possible around planets with sizable Hill spheres that are well separated from their host stars.
Exomoon Habitability Constrained by Illumination and Tidal Heating
Authors:
1. Rene´ Heller (a)
2. Rory Barnes (b,c)
Affiliations:
a. Leibniz-Institute for Astrophysics Potsdam (AIP), Potsdam, Germany
b. Astronomy Department, University of Washington, Seattle, Washington, USA
c. NASA Astrobiology Institute—Virtual Planetary Laboratory Lead Team, USA
Abstract:
The detection of moons orbiting extrasolar planets (“exomoons”) has now become feasible. Once they are discovered in the circumstellar habitable zone, questions about their habitability will emerge. Exomoons are likely to be tidally locked to their planet and hence experience days much shorter than their orbital period around the star and have seasons, all of which works in favor of habitability. These satellites can receive more illumination per area than their host planets, as the planet reflects stellar light and emits thermal photons. On the contrary, eclipses can significantly alter local climates on exomoons by reducing stellar illumination. In addition to radiative heating, tidal heating can be very large on exomoons, possibly even large enough for sterilization. We identify combinations of physical and orbital parameters for which radiative and tidal heating are strong enough to trigger a runaway greenhouse. By analogy with the circumstellar habitable zone, these constraints define a circumplanetary “habitable edge.” We apply our model to hypothetical moons around the recently discovered exoplanet Kepler-22b and the giant planet candidate KOI211.01 and describe, for the first time, the orbits of habitable exomoons. If either planet hosted a satellite at a distance greater than 10 planetary radii, then this could indicate the presence of a habitable moon.
