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)

Reference:

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

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.

Reference:

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

Low Density Planets of Kepler-51

Kepler-51 is a fairly young star with an estimated age of ~300 million years and it is also slightly more luminous than the Sun. Observations of Kepler-51 by NASA’s Kepler space telescope found that it hosts three transiting planet candidates - Kepler-51 b, Kepler-51 c and KOI-620.02. The three planets have orbital periods of 45.2 days (Kepler-51 b), 85.3 days (Kepler-51 c) and 130.2 days (KOI-620.02), placing them close to a 1:2:3 resonance. By measuring the amount of light each planet blocks as it transits its host star, the size of each planet is found to be 7.1 (Kepler-51 b), 9.0 (Kepler-51 c) and 9.7 (KOI-620.02) times the Earth’s diameter.

Figure 1: Artist’s impression of a gaseous planet.

Figure 2: Phase-folded transit light curves of Kepler-51 b (top), Kepler-51 c (middle) and KOI-620.02 (bottom). Black dots are the observed fluxes and coloured solid lines show the best-fit models.

As the three planets circle their host star, they gravitationally perturb one another. This leads to transit timing variations (TTVs) where each planet transits the host star at slightly earlier or later timings, deviating somewhat from strictly periodic transit intervals. By studying the TTVs, Masuda (2014) derived the mass for each of the three planets to be 2.1 (Kepler-51 b), 4.0 (Kepler-51 c) and 7.6 (KOI-620.02) times the Earth’s mass. With the size and mass of each planet known, all three planets were found to have remarkably low densities of less than 5 percent the density of water, possibly the lowest densities yet determined for exoplanets. In comparison, the Earth has a mean bulk density of 5.52 times the density of water. With this finding, the Kepler-51 system serves as yet another example of a very low-density compact multi-transiting planetary system.

The planets around Kepler-51 have mean densities that are much lower than any of the planets in the solar system. To explain their “puffiness”, each planet probably possesses an extended outer hydrogen-helium envelop surrounding a denser core. Assuming the planetary system has an age of ~300 million years; calculations show that the observed radii of the Kepler-51 planets can be explained if they have about 10 percent (Kepler-51b), 30 percent (Kepler-51c) and 40 percent (KOI-620.02) of their masses in their hydrogen-helium envelopes. All three planets are unlikely to be habitable, at least for the type of life found on Earth, given that the planets have thick gaseous envelopes and equilibrium temperatures that exceed 100°C.

Reference:

Masuda (2014), “Very Low-Density Planets around Kepler-51 Revealed with Transit Timing Variations and an Anomaly Similar to a Planet-Planet Eclipse Event”, arXiv:1401.2885 [astro-ph.EP]

At the Edge of Destruction

M. Gillon et al. (2014) report the discovery of WASP-103 b, an ultra-short-period planet at the edge of tidal disruption. WASP-103 b orbits an F-type star at a distance of just ~2 stellar radii from the star's surface, taking a mere 22.2 hours to complete an orbit. The WASP transit survey is sensitive to detecting ultra-short-period giant planets when these planets happen to cross in front of their host stars. WASP-103 b has 1.49 times the mass and 1.53 times the diameter of Jupiter. This newfound planet joins a small group of gas giants that are known to be at the verge of being tidally disrupted by their host stars. The group include planets such as WASP-12 b and WASP-19 b.

Artist’s impression of a gas giant. Credit: Daniel Mallia.

WASP-103 b is significantly inflated and has a bulk density that is only 55 percent the density of water. The low density of WASP-103 b is not just because of the intense irradiation it receives due to its extreme closeness to its host star. Tidal heating is also expected to contribute significantly to the planet's "bloatedness" since the planet's orbit is only 15 to 20 percent away the Roche Limit. Any closer, the planet is expected to be tidally destructed by the gravity of its host star.

Ultra-short-period gas giants that are right at the edge of being tidally disrupted might experience mass loss and significant tidal distortion. One such planet, WASP-12 b, is known to be surrounded by planetary material that has escaped it. In the case of WASP-103 b, the extreme irradiation it receives, the planet's inflated size and the brightness of its host star makes it favourable for atmospheric characterisation with existing ground-based and space-based telescopes. Observing signs of mass loss and tidal distortion for such extreme planets can shed light on the final stages in the lives of hot-Jupiters.

Reference:

M. Gillon et al. (2014), "WASP-103b: a new planet at the edge of tidal disruption", arXiv:1401.2784 [astro-ph.EP]

Birth of a Brown Dwarf

Brown dwarfs are sub-stellar objects that are not massive enough to fuse hydrogen in their interiors and shine as full-fledged stars. Nevertheless, brown dwarfs are thought to form in the same way as stars do - from collapsing clouds of gas and dust. A study by Lee et al. (2013) of an isolated dense molecular cloud core, L328, shows that it contains three sub-cores. One of which, identified as L328-IRS, is a Very Low Luminosity Object (VeLLO) that is believed to be in the process of collapsing to form a brown dwarf.

Artist’s impression of a young brown dwarf that is in the process of accreting matter. A pair of bipolar jets can be seen stemming from it. Credit: ESO.

Observations of carbon monoxide as a tracer for the motion of matter reveal a bipolar outflow stemming from L328-IRS. By analysing the outflow, the accretion rate of the proto-brown dwarf is found to be an order of magnitude less than the accretion rate for standard star formation, consistant with the formation of a brown dwarf. Based on the accretion rate, L328-IRS is expected to grow to no more than ~0.05 solar mass. However, the accretion rate may be uncertain due to several unknown factors of the outflow itself.

Nonetheless, L328-IRS has a small total envelop mass of ~0.09 solar mass and ~100 percent star formation efficiency is also unlikely. As a result, L328-IRS is expected to be a proto-brown dwarf since it is unlikely to accrete more than ~0.08 solar mass, which is the minimum mass necessary to become a full-fledged star. The three sub-cores in L328 are though to have formed concurrently in a gravitational fragmentation process. In one of the sub-cores, global contraction of the gaseous envelop is underway to form the proto-brown dwarf L328-IRS. All these indicate that the formation of L328-IRS is consistant with the idea that brown dwarfs form like normal stars.

Reference:

Chang Won Lee et al., “Early Star-forming Processes in Dense Molecular Cloud L328; Identification of L328-IRS as a Proto-brown Dwarf”, ApJ, 777:50 (15pp), 2013 November 1

CoRoT-27b: A Massive and Dense Planet

Parviainen H. et al. (2014) report the discovery of a massive high-density planet on a close-in 3.58 day orbit around a 4.2 billion year old Sun-like star. The planet is identified as CoRoT-27b. Like Jupiter, CoRoT-27b is a gas-giant planet. Its presence was detected by the CoRoT space telescope as the planet periodically transits its parent star and blocks a small fraction of the star’s light. CoRoT-27b weighs in at 10.39 ± 0.55 Jupiter-masses and has 1.01 ± 0.04 times the radius of Jupiter. This gives CoRoT-27b a mean density of 12.6 times the density of water, which is more than twice the mean density of Earth and almost 10 times the mean density of Jupiter.

Figure 1: Artist’s impression of a gas-giant planet.

Like Jupiter, CoRoT-27b is a gaseous planet comprised primarily of hydrogen and helium. The structure and composition of CoRoT-27b can be inferred from two models. For the first model, the planet is assumed to be made of a central rocky core surrounded by an extensive hydrogen-helium envelop. The 1st model is consistant with a heavy element mass fraction of 0.11, representing a core mass of 366 Earth-masses. For the second model, a central rocky core is absent and the heavy elements are present throughout the hydrogen-helium envelop. The 2nd model is consistant with a heavy element mass fraction of 0.07, representing a heavy element mass of 219 Earth-masses.

CoRoT-27b falls within a sparsely populated overlapping mass regime between the most massive planets and brown dwarfs. Given its high mass, gravity on the “surface” of CoRoT-27b is 27 times the surface gravity on Earth. Technically, CoRoT-27b does not have a surface since it is gaseous through, right down to a central rocky core, if one is present. Being so near to its parent star, the equilibrium temperature on CoRoT-27b is estimated to be 1500 ± 130 K. The discovery of CoRoT-27b is an important addition to a scarcely populated class of massive close-in planets.

Figure 2: Radial velocity curve showing how much CoRoT-27b gravitationally tugs at its parent star. This information allows the planet’s mass to be estimated. Parviainen H. et al. (2014).

Figure 3: Transit light curve showing the amount of dimming of the parent star when CoRoT-27b passes in front of it. This information allows the size of the planet to be measured. Parviainen H. et al. (2014).

Figure 4: CoRoT-27b mass, period and density compared with the population of confirmed transiting exoplanets. Parviainen H. et al. (2014).

Reference:

Parviainen H. et al. (2014), “Transiting exoplanets from the CoRoT space mission XXVII. CoRoT-27b: a massive and dense planet on a short-period orbit”, arXiv:1401.1122 [astro-ph.EP]

A Planet on the Verge of Engulfment

Figure 1: Artist’s impression of a hot-Jupiter transiting its host star. Credit: Mark A. Garlick.

The exoplanet Kepler-91 b orbits around an evolved K3 host star that is in the process of transforming into a red giant. Observations show that Kepler-91 b is a gas-giant planet measuring 0.88 times the mass and 1.38 times the radius of Jupiter. Its host star has 1.3 times the mass and 6.3 times the radius of the Sun. Kepler-91 b circles around its host star in a slightly eccentric, close-in orbit with a period of 6.25 days. Given the planetary mass and radius, the mean density of Kepler-91 b works out to be 0.33 times the density of Jupiter. This low density suggests that Kepler-91 b is somewhat inflated due to the strong stellar irradiation from its host star.

Although the orbit of Kepler-91 b is nowhere near the shortest for exoplanets, the sheer size of its host star means that Kepler-91 b is a mere 1.32 stellar radii from the surface of its host star at closest approach. As the host star continues to expand into a red giant, estimates show that Kepler-91 b is expected to be swallowed in less than 55 million years - a mere blink of the eye on astronomical scales. Even that is considered as an upper limit to the planet’s life. The equilibrium temperature of Kepler-91 b is estimated to be over 2000 K.

Figure 2: Best-fit solutions for the transit of Kepler-91 b in front of its host star. Source: Lillo-Box et al. (2013).

 Figure 3: Diagram illustrating the irradiation of Kepler-91 b by its host star. The red lines represent the boundaries of the stellar irradiation that hits the planet’s surface. The yellow part represents the dayside of the planet. The black part represents the night side and the red part is the extra region illuminated due to the close-in orbit and the large stellar radii of the host star. Source: Lillo-Box et al. (2013).

The close-in orbit of Kepler-91 b and the sheer size of its host star result in more than half of the planet being illuminated by the host star (Figure 3). In fact, around 70 percent of the planet is illuminated by the host star. When Kepler-91 b is at closest approach, its host star would appear to subtend a remarkable 48 degrees, covering around 10 percent of the sky as seen from the planet. In comparison, the Sun covers only 0.0005 percent of the sky as seen from Earth. Kepler-91 b is indeed on the verge of being swallowed by its host star.

Reference:

Lillo-Box et al. (2013), “Kepler-91b: a giant planet at the end of its life”, arXiv:1312.3943 [astro-ph.EP]

Deep Alien Biospheres

Life on Earth not only exists on the surface, but it also includes a subsurface biosphere extending several kilometres in depth. At such depths, the only reasonable source of energy to sustain life comes from the planet's own internal heat. Indeed, a planet that is located far from its host star, resulting in surface temperatures too cold to support life, can potentially harbour a thriving subsurface biosphere that is sustained solely by the planet's own internal heat.

Figure 1: Artist’s impression of a terrestrial planet.

 Figure 2: Artist’s impression of a terrestrial planet. Credit: Kevin Sherman.

A study by S. McMahon et al (2013) show that subsurface liquid water maintained by the internal heat of a planet can support an underground biosphere even if the planet is too far from its host star to support life on the planet's surface. The authors introduce a term known as the “subsurface-habitability zone” (SSHZ) to denote the range of distances from a star where a terrestrial planet (i.e. a rocky planet like the Earth) can sustain a subsurface biosphere at any depth below the surface down to a certain maximum habitable depth. This maximum depth depends on numerous factors, but in general, it is the depth where the enormous pressure starts to make the material too compact for life to infiltrate.

Based on the premises that the global average temperature of a terrestrial planet (1) decreases with increasing distance from its host star and (2) increases with depth beneath the planet's surface, the inner (i.e. closer to the host star) and outer (i.e. further from the host star) boundaries of the SSHZ can be determined. The outer edge of the SSHZ is where temperatures are below the freezing point of water at all depths down to the maximum habitable depth. The inner edge of the SSHZ is where the average surface temperature reaches the boiling point of water.

Figure 3: If the maximum habitable depth for an Earth-analogue planet is 5 km, the outer edge of the SSHZ would be at 3.2 AU. For a maximum habitable depth of 10 km, the outer edge of the SSHZ would be at 12.6 AU. At a maximum habitable depth of 15.4 km, the outer edge of the SSHZ tends towards infinity. Credit: S. McMahon et al (2013).

 Figure 4: The relationship between subsurface habitability and surface albedo (i.e. surface reflectivity of the planet). Two extremes of planetary albedo are shown: a = 0.9 (high reflectivity) and a = 0 (zero reflectivity). Other than surface albedo, the calculations assume a planet with the Earth’s current size, bulk density, heat production per unit mass and emissivity. Credit: S. McMahon et al (2013).

Figure 5: Subsurface habitability for three planetary masses of 0.1, 1.0 and 10 Earth-masses. Other than planet mass, the calculations assume a planet with the Earth’s current bulk density, heat production per unit mass, albedo and emissivity. Credit: S. McMahon et al (2013).

Results from the study show that for a planet with high albedo (high reflectivity), the SSHZ is narrower and closer to the star than for a planet with low albedo (low reflectivity) (Figure 4). Furthermore, planets with larger mass have subsurface biospheres that are thinner, shallower and less sensitive to the heat flux from the host star (Figure 5). This is because a more massive planet is expected to have a steeper geothermal gradient whereby the temperature rises more rapidly with increasing depth as compared to a less massive planet. In fact, a 10 Earth-mass planet can support a ~1.5 km thick subsurface biosphere less than ~6 km below its surface even if the planet is at an arbitrarily large distance from its host star.

The possibilities for subsurface biospheres mean that a planet whose surface is too cold for life can still support a deep biosphere that derives its energy and warmth from the planet's own internal heat. An advantage that life in a subsurface biosphere has is that it is well protected from ionizing stellar and cosmic radiation by the overlying rock layers. Since the SSHZ is vastly greater in extent than the traditional habitable zone, cold planets with subsurface biospheres may turn out to be much more common than planets with surface biospheres. Nevertheless, detecting the biosignature of a subsurface biosphere from remote sensing will be more challenging than for a surface biosphere.

Reference:

McMahon et al., “Circumstellar habitable zones for deep terrestrial biospheres”, Planetary and Space Science 85 (2013) 312-318

Habitability of Large Exomoons

Large exomoons around giant planets in the habitable zone of their host stars could serve as habitats for extraterrestrial life. Such an exomoon would need to have at least twice the mass of Mars or so (i.e. ~0.2 Earth masses) for it to be habitable. For comparison, Ganymede, the largest moon in the Solar System, is roughly 1/40 the mass of Earth. In addition, habitability requires a surface temperature that cannot be too high or too low. This is governed not just by stellar radiation from the host star, but also by stellar light reflected from the giant planet, thermal radiation from the giant planet itself and tidal heating.

Figure 1: Artist’s impression of a giant planet hosting a system of moons. Credit: Kevin Sherman.

Over time, a gaseous giant planet contracts and releases thermal energy as it converts gravitational potential energy into heat. In a paper by Heller & Barnes (2013), the authors investigate how thermal radiation from a shrinking gaseous giant planet could drive a runaway greenhouse effect for an Earth-like exomoon if it is in a close enough orbit around the giant planet. This effect is particularly significant for a young giant planet during the first few hundred million years or so. During this period, the young and hot giant planet is cooling at a more rapid rate, and consequently releases a greater deal of thermal radiation.

To illustrate the combined effects of stellar radiation, thermal radiation from the giant planet and tidal heating, Heller & Barnes (2013) introduced five possible states for an exomoon: (1) Tidal Venus, (2) Tidal-Illumination Venus, (3) Super-Io, (4) Tidal Earth and (5) Earth-like. For these states, a Tidal Venus and a Tidal-Illumination Venus are uninhabitable, while a Super-Io, a Tidal Earth, and an Earth-like moon could be habitable. In the study, a rocky Earth-type exomoon orbiting a giant planet with a mass 13 times that of Jupiter is considered. Besides an Earth-type exomoon, a Super-Ganymede (i.e. a large exomoon with composition similar to Ganymede) is also considered.

At a distance of 1 AU from a Sun-like star, the results from the study show that the combined stellar radiation and thermal radiation on an Earth-type exomoon orbiting at 10 Jupiter-radii around a 13 Jupiter-mass giant planet would keep the Earth-type exomoon above the runaway greenhouse limit and uninhabitable for about 500 million years (Figure 2). For the Super-Ganymede, it would be in a runaway greenhouse state for about 600 million years. In fact, even in the absence of stellar radiation, thermal radiation from the giant planet alone can trigger a runaway greenhouse effect for the first ~200 million years.

Figure 2: The total illumination absorbed by an exomoon (thick black line) is composed of stellar radiation (black dashed line) and thermal radiation from the giant planet (red dashed line). The critical values for an Earth-type exomoon and a Super-Ganymede to enter the runaway greenhouse effect are indicated by dotted lines. Credit: Heller & Barnes (2013).

With the inclusion of tidal heating, the danger for an exomoon to undergo a runaway greenhouse effect increases. Heller & Barnes (2013) illustrate how the distance and orbital eccentricity of an Earth-type exomoon around a 13 Jupiter-mass giant planet determines whether the exomoon is in a Tidal Venus (red), Tidal-Illumination Venus (orange), Super-Io (yellow), Tidal Earth (blue) or Earth-like (green) state (Figure 3). Here, the giant planet is assumed to have an age of 500 million years. Furthermore, stellar radiation, thermal radiation from the giant planet and tidal heating are all included.

There is a minimum distance around the giant planet in which an Earth-type exomoon would be in a Tidal Venus or Tidal-Illumination Venus state, and hence uninhabitable. This minimum distance is referred to as the “habitable edge”. For a 13 Jupiter-mass giant planet at 1 AU from a Sun-like star, the habitable edges for orbital eccentricities of 0.1 and 0.0001 are 20 and 12 Jupiter-radii respectively. For the same giant planet at 1.738 AU from a Sun-like star, the habitable edges for orbital eccentricities of 0.1 and 0.0001 are 15 and 8 Jupiter-radii respectively. The habitable edge for an older giant planet would be smaller since thermal radiation from a giant planet is expected to decrease over time. As a means of comparison, Io, Europa, Ganymede, and Callisto orbit Jupiter at approximately 6.1, 9.7, 15.5, and 27.2 Jupiter-radii.

Figure 3: The four panels show the possible states for an Earth-type exomoon around a 13 Jupiter-mass host planet that has an age of 500 million years. Distances from the giant planet are shown on a logarithmic scale. In the left two panels, the giant planet orbits at a distance of 1 AU from a Sun-like star. In the right two panels, the giant planet orbits at a distance of 1.738 AU. In the upper two panels, the orbit of the exomoon around the giant planet has an eccentricity of 0.1. In the lower two panels, the eccentricity is 0.0001. Starting from the giant planet in the centre, the white circle visualizes the Roche radius (i.e. within this region, an Earth-type exomoon would be tidally disrupted), and the exomoon types correspond to Tidal Venus (red), Tidal-Illumination Venus (orange), Super-Io (yellow), Tidal Earth (blue) and Earth-like (green) states. Dark green depicts the extent of orbits for Earth-like exomoons in prograde orbits (i.e. orbits in the same direction as the giant planet’s spin) and light green depicts the extent of orbits for Earth-like exomoons in retrograde orbits (i.e. orbits in the opposite direction to the giant planet’s spin). Credit: Heller & Barnes (2013).

Reference:

Heller & Barnes (2013), “Runaway greenhouse effect on exomoons due to irradiation from hot, young giant planets”, arXiv:1311.0292 [astro-ph.EP]

Heat Redistribution on a Strongly Irradiated Brown Dwarf

KELT-1b, a brown dwarf with 27 times the mass of Jupiter, circles around an F-type star in a close-in 1.2-day orbit. The tight orbit places KELT-1b in a highly irradiated environment, where the incident radiation it receives from its parent star is 5,800 times more intense than what Earth gets from the Sun. Although the radiation environment of KELT-1b is similar to that for hot Jupiters, KELT-1b is different due to it large mass which places it in the brown dwarf regime. With several Jupiter masses packed into a volume that is only slightly larger than Jupiter’s, the surface gravity on KELT-1b is a whopping 115 times the surface gravity on Earth. In a way, KELT-1b can be perceived as a “hot Jupiter” with a very high surface gravity.

Artist’s Impression of a hot Jupiter. Credit: NASA.

Observations of KELT-1b using the Spitzer space telescope show that the amount of heat redistribution from its day side to its night side is very low. This is because KELT-1b quickly radiates the energy it receives from its parent star back into space before it is transported to the night side. As a consequence, KELT-1b has a very hot day night and a much cooler night side. The day side is estimated to have temperatures as high as 3,100 K. As a brown dwarf, KELT-1b is unusual due to the huge amount of insolation it receives from its parent star. If KELT-1b were an isolated brown dwarf, it would have a temperature of about 700 K.

The day side of KELT-1b is so hot that it is above the ~2,000 K condensation temperature of titanium oxide (TiO). This can cause a day-night cold trap for TiO since the night side of KELT-1b is cool enough for TiO to condense and settle out of the atmosphere. In fact, the lack of a strong TiO signal indicates that a day-night cold trap may exist in KELT-1b’s atmosphere. Because gaseous TiO is a strong absorber of optical radiation, its presence in an atmosphere can cause a temperature inversion (i.e. temperature increases with altitude). Therefore, the depletion of TiO due to a day-night cold trap inhibits the presence of a temperature inversion.

KELT-1b was discovered using the using the Kilodegree Extremely Little Telescope (KELT) in southern Arizona. KELT is a small telescope optimized for imaging bright stars. The telescope images of tens of thousands of stars every night in an attempt to detect planets that happen to pass in front of the star that they are orbiting. The discovery of KELT-1b was announced in a paper published in June 2012.

Reference:

- Beatty et al. (2013), “Spitzer and z' Secondary Eclipse Observations of the Highly Irradiated Transiting Brown Dwarf KELT-1b”, arXiv:1310.7585 [astro-ph.EP]

- Siverd et al. (2012), “KELT-1b: A Strongly Irradiated, Highly Inflated, Short Period, 27 Jupiter-mass Companion Transiting a mid-F Star”, arXiv:1206.1635 [astro-ph.EP]