Tuesday, February 4, 2014

Kepler-413b: A Circumbinary “Warm-Neptune”

Two years ago, the first transiting circumbinary planet, Kepler-16b, was discovered. Since then, several more circumbinary planets have been found. As the term suggests, a circumbinary planet is a planet that orbits two stars instead of one. Recently, Kostov et al. (2014) report the discovery of Kepler-413b - a Neptune-sized transiting circumbinary planet. Kepler-413b orbits a pair of stars, both of which are less massive and less luminous than the Sun. The two stars, one a spectral class K-type star and the other an M-type star, circle around each other every 10.1 days. Both stars form a K + M eclipsing binary system. Further out, Kepler-413b circles the pair every ~66 days on a somewhat eccentric orbit.

Figure 1: Artist’s impression of a possible view from the surface of a circumbinary planet. Credit: NASA/JPL-Caltech.

Kepler-413b was discovered using data from NASA’s Kepler space telescope. The Kepler light curve data for Kepler-413b show a set of 3 transits separated by ~66 days, followed by ~800 days with no transits, followed by another set of 5 transits with each transit again ~66 days apart. There is a small misalignment of ~2.5° between the orbital planet of the K + M binary and the orbital plane of Kepler-413b around the binary. As a consequence, the orbit of Kepler-413b precesses, causing long periods with no transits. In fact, the next transit is not expected to occur until 2020. Due to orbital precession from the influence of the central binary and the planet’s own eccentric orbit, Kepler-413b is likely to experience complex seasonal cycles with interesting climate patterns.

The combined incident stellar flux from the K + M binary at the orbital location of Kepler-413b varies from a minimum of ~1.64 to a maximum of ~3.86 times the average flux Earth receives from the Sun. This places Kepler-413b within the inner edge of the habitable zone around its host stars, suggesting temperatures that are probably too warm for life. Nevertheless, if a planet is a dry desert planet, it can remain habitable at even closer distances in a region known as the dry desert habitable zone. Interestingly, Kepler-413b is in this zone for most of its orbit (Figure 4) and a dry terrestrial-sized moon around Kepler-413b would be on the verge of habitability.

Figure 2: Photodynamical fits (red) to the 8 observed (and a possible 9th, labelled as “A” near time 188.35) transits. Kostov et al. (2014)

Figure 3: Orbital configuration of Kepler-413b over the course of 1/8 precession period (1/8 of ~11 years). Kostov et al. (2014)

Figure 4: Orbital location of Kepler-413b (black line) as a function of the orbital phase of the planet and equilibrium temperature, assuming a planetary Bond albedo of 0.34. The inner (red line) and outer (blue line) edges of the habitable zone are indicated, and the dashed line indicates the inner edge of the dry desert habitable zone. The planet is in the dry desert habitable zone for most of its orbit. Kostov et al. (2014)

Kostov et al. (2014), “Kepler-413b: a slightly misaligned, Neptune-size transiting circumbinary planet”, arXiv:1401.7275 [astro-ph.EP]

Saturday, February 1, 2014

Raining Molten Iron on Luhman 16B

Brown dwarfs are substellar objects that are more massive than planets, but not massive enough to sustain hydrogen fusion and shine as full-fledged stars. These objects start out hot, and cool gradually as they age. When cooled below a temperature of ~2300 K, it is believed that silicate minerals and molten iron begin to condense to form patchy cloud systems in the atmosphere. At cooler temperatures of below ~1300 K, these clouds disappear, probably sinking into the warmer and unobservable deeper layers of the atmosphere.

Figure 1: Artist’s impression of weather on a brown dwarf. Credit: NASA/JPL-Caltech/T. Pyle (IPAC).

Using the European Southern Observatory’s Very Large Telescope (VLT) in Chile, I. Crossfield et al. (2014) have created the first ever global weather map of a brown dwarf named Luhman 16B. This object is a member of a pair of brown dwarfs known together as Luhman 16AB. As the name suggests, Luhman 16AB was discovered by Kevin Luhman, an astronomer at Pennslyvania State University in March 2013 using data from NASA’s Wide-field Infrared Survey Explorer (WISE). At a distance of just 6.5 light-years away, Luhman 16AB are not only the two closest known brown dwarfs, they are also the third nearest system - only the Alpha Centauri system and Barnard’s star are closer.

Brown dwarfs are notoriously difficult to study due to their faintness and relatively small size. However, the close proximity of Luhman 16AB puts them within easy reach of VLT’s gaze. Although both brown dwarfs were observed in the same fashion, only Luhman 16B exhibits strong temporal variability of its thermal radiation. The observed variability is consistant with Luhman 16B’s rotation period of 4.9 hours. As the brown dwarf rotates, brighter and darker areas of its surface come in and out of view to produce the observed variability. The brighter regions are believed to represent upper cloud layers that obscure the deeper and hotter parts of the atmosphere. In contrast, the brighter regions are believed to be gaps in the upper cloud layers that allow the deeper and hotter layers of the atmosphere to be seen.

Figure 2: Surface map of brown dwarf Luhman 16B. The lightest and darkest regions shown correspond to brightness variations of roughly 10%. Credit: ESO/I. Crossfield.

Luhman 16B has an estimated temperature of ~1400 K, extremely inclement by any standard. The global cloud map of Luhman 16B hints at the complexity of weather patterns on brown dwarfs. Clouds on Luhman 16B are probably comprised of iron and silicate minerals in a largely hydrogen-helium atmosphere. Here, iron can precipitate from the clouds in showers comprising droplets of molten iron. Weather on Luhman 16B can truly be exotic.

“Previous observations have inferred that brown dwarfs have mottled surfaces, but now we can start to directly map them,” said Ian Crossfield of the Max Planck Institute for Astronomy, lead author of the study. “What we see is presumably patchy cloud cover, somewhat like we see on Jupiter. In the future, we will be able to watch cloud patterns form, evolve and dissipate - eventually, maybe exo-meteorologists will be able to predict whether a visitor to Luhman 16B can expect clear or cloudy skies,” added Crossfield.

I. Crossfield et al., “A global cloud map of the nearest known brown dwarf”, Nature 505, 654-656 (30 January 2014)