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]

Mountains on Titan

Titan is by far the largest moon in orbit around Saturn and the 2nd largest moon in the Solar System. It has a diameter of 5,152 km, making it nearly 1.5 times the size of Earth's Moon. Titan has a thick atmosphere and opaque haze layers obscure its entire surface. From inside out, the bulk of Titan is believed to be comprised of a partially differentiated interior of rock and ice, a high pressure ice layer (consisting of ice III, V, and VI), a subsurface ocean of liquid water and an outer ice I shell. Ice III, V, and VI are high pressure phases of ice which do not occur naturally on Earth. Ice I is basically normal ice and all naturally occurring ice on Earth is ice I.

Figure 1: Saturn’s fourth-largest moon, Dione, can be seen through the haze of the planet’s largest moon, Titan, in this view of the two posing before the planet and its rings from an image taken by the Cassini spacecraft. Credit: NASA/JPL-Caltech/Space Science Institute.

Figure 2: Artist’s concept showing a possible scenario for the internal structure of Titan, as suggested by data from the Cassini spacecraft.

A thermal model of the interior of Titan developed by Mitri et al. (2010) show that the long term cooling of Titan can cause a global volume contraction of ~0.01. As Titan cools, the base of its subsurface ocean would freeze onto the top of its high pressure ice layer while the top of its subsurface ocean would freeze onto the underside of its outer ice I shell. Because high pressure ice is a lot denser than liquid water (~10 to 30 percent denser) and ice I is only marginally less dense than liquid water (<10 percent less dense), the gradual freezing of Titan's subsurface ocean into high pressure ice and ice I would cause an overall reduction in the volume of Titan.

It seems that the presence or absence of a high pressure ice layer in the interior of an object can determine whether or not it will undergo global contraction or expansion during cooling. For example, the interior of Jupiter's moon Europa is comprised of a rocky interior, an overlying subsurface ocean and an outer ice I shell. Unlike Titan, Europa does not have a high pressure ice layer. Since its outer ice I shell has a lower density than the underlying subsurface liquid water ocean (i.e. water is less dense than ice), the long term cooling of Europa will cause the outer ice I shell to thicken and result in overall global volume expansion.

Figure 3: A model of the topography produced by the contractional deformation of Titan's icy lithosphere. (Mitri et al., 2010)

Figure 4: Cassini radar imagery showing three elongated radar bright features that may be fold ridges formed from the contractional deformation of Titan's icy lithosphere. A topographic profile across one of the ridges (black rectangle) show that it has a height of 1,930 m. (Mitri et al., 2010)

The global volume contraction of Titan leads to contractional deformation of Titan's icy lithosphere, producing fold features (i.e. mountains). These fold features can reach topographic heights of up to several kilometres, especially so if Titan underwent more rapid cooling in the early Solar System and thereby experienced more contraction. The radar instrument on the Cassini spacecraft has imaged mountainous topography on Titan that is consistent with fold features produced by the contractional deformation of Titan's icy lithosphere. Perhaps, such a contractional deformation process may have formed most of Titan's mountains.

Reference:

Mitri et al., “Mountains on Titan: Modelling and Observations”, Journal of Geophysical Research: Planets, Volume 115, Issue E10, October 2010

Shockwaves from the Rheasilvia Impact

Vesta is one of the largest asteroids in the Solar System, measuring 573 km by 557 km by 446 km in size. It is a member of the main asteroid belt and it circles the Sun between the orbits of Mars and Jupiter. From July 2011 to September 2012, NASA’s Dawn spacecraft was in orbit around Vesta and the spacecraft conducted numerous observations of this large asteroid. Centred on the south pole of Vesta is a large impact feature known as the Rheasilvia basin. The basin has a depth of ~20 km and a diameter of ~500 km, nearly as large as Vesta itself.

 Figure 1: This topographic map from NASA’s Dawn spacecraft shows the two large impact basins in the southern hemisphere of Vesta. The map is colour-coded by elevation, with red showing the higher areas and blue showing the lower areas. Rheasilvia, the largest impact basin on Vesta, is ~500 km in diameter. It is estimated to have formed no more than ~1 billion years ago by counting the number of smaller craters that have formed on top of it. The other basin, Veneneia, is ~400 km in diameter and it lies partially beneath Rheasilvia. Veneneia is estimated to be at least ~2 billion years old. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI.

Figure 2: This image obtained by the framing camera on NASA’s Dawn spacecraft shows the south pole of the giant asteroid Vesta. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

Observations by NASA’s Dawn spacecraft suggests that the impact which excavated the Rheasilvia basin may have been sufficiently large to create disrupted terrains at the impact antipode, which is the area on Vesta opposite to the point of impact. Compared to the age of the Solar System, the Rheasilvia basin is relatively young, estimated to be no more than ~1 billion years old. Modelling work performed by Bowling et al. (2013) show that the degree of antipodal deformation is very sensitive to the mantle porosity and core strength of Vesta.

In the “control” model with zero mantle porosity and a strong rock-like core, the shockwaves from the Rheasilvia impact passes through the mantle and core of Vesta with little attenuation. The shockwaves eventually converge around the antipode with sufficient energy to uplift enough material to create a ~6 km tall antipodal peak. More realistically, the presence of mantle porosity and/or a weaker core would result in a smaller degree of antipodal deformation. In fact, the models show that unless the mantle porosity is relatively low and the core is relatively strong, no antipodal deformation would occur.

Figure 3: Modelled antipodal topography 1500 seconds after the Rheasilvia impact. All simulations in this series were run with a strong, rock-like core. (T. J. Bowling et al., 2013)

Topographic maps of Vesta’s north pole, acquired by NASA’s Dawn spacecraft, show an area near the impact antipode that is ~5 to 10 km higher than the surrounding plains. However, the antipodal point itself lies within a ~63 km diameter crater named Pomponia. Pomponia is believed to have formed more recently than the Rheasilvia basin and its formation would have obliterated much of the predicted antipodal topographic uplift from the Rheasilvia impact. Additionally, a ~90 km diameter crater named Albana lies right next to Pomponia.

Figure 4: Topography at the north pole of Vesta. The white dot represents the approximate location of the impact antipode corresponding to the Rheasilvia basin. The region marked ‘2’ indicates the area in which a crater size frequency distribution was produced. (T. J. Bowling et al., 2013)

If the ~5 to 10 km elevated area near the impact antipode is a product of the Rheasilvia impact, it would suggest that Vesta has a low mantle porosity and a core of considerable strength. Unfortunately, the presence of the craters Pomponia and Albana make it difficult to determine what portion of the elevated area is a product of topographic uplift from the Rheasilvia impact and what portion is due to more recent impacts.

Nevertheless, a study of the crater size frequency distribution in an area near the impact antipode shows a deficiency of smaller craters with diameters between 3 km to 9 km. In comparison, craters with diameters larger than ~10 km are as common around the impact antipode as elsewhere on Vesta. The deficiency of smaller craters is evidence that some degree of antipodal deformation from the Rheasilvia impact did occur. This is because the powerful converging shockwaves from the Rheasilvia impact around the antipodal point would have erased small craters more effectively than larger ones.

The very presence of antipodal deformations from the Rheasilvia impact indicates that Vesta must have relatively low mantle porosity and a relatively strong core. This study by Bowling et al. (2013) show that the features observed at the antipode of the Rheasilvia impact can serve as a crude method of constraining the internal structural properties of Vesta. Finally, this method may also be used to constrain the internal properties of some other objects in the Solar System that have craters large enough to have perhaps produced deformation features at their antipodes. An example of one such object is Saturn’s icy moon Mimas with its relatively large crater named Herschel.

Figure 5: An image of Saturn’s moon Mimas taken by the Cassini spacecraft on 13 February 2010. The large crater on the left is Herschel. In the background is the enormous bulk of the planet Saturn. With a diameter of 396 km, Mimas is thought to be about the smallest an object can be and still crunch itself into a near-spherical shape. Credit: NASA/JPL/Space Science Institute.

Reference:

T. J. Bowling et al., “Antipodal terrains created by the Rheasilvia basin forming impact on asteroid 4 Vesta”, Journal of Geophysical Research: Planets, Volume 118, Issue 9, pages 1821-1834, September 2013

Heat Redistribution on WASP-18b

WASP-18b is a massive extrasolar planet with a mass equal to 10 Jupiter masses and it is in a close-in 0.94-day orbit around an F6V parent star - a star that is somewhat hotter and larger than the Sun. The planet is believed to be tidally-locked with the same side permanently facing its parent star. As a result, temperatures on the day-side of WASP-18b can get as high as ~3000 K. Like Jupiter, WASP-18b is a gas giant planet comprised primarily of hydrogen and helium.

Figure 1: Artist’s impression of HAT-P-7b. Like WASP-18b, HAT-P-7b is gas giant planet in a close-in orbit around its parent star. Credit: NASA, ESA, and G. Bacon (STScI).

Observations of WASP-18b together with atmospheric models show nearly no day-side to night-side redistribution of heat. Any winds transporting heat away from the planet’s day-side is expected to be very weak. In an atmospheric model of WASP-18b that is consistent with almost no heat redistribution, the planet’s day-to-night temperature difference is around 2000 K to 3000 K (Figure 2). In another atmospheric model of WASP-18b, this time with 6.5 km/s superrotating winds racing around the planet from day-side to night-side, the day-to-night temperature difference drops to ~800 K (Figure 3). The effect of the superrotating winds is more efficient heat redistribution, resulting in a lower day-to-night temperature contrast.

Figure 2: Modelled atmospheric thermal profiles for selected longitudes for the case with almost no redistribution of heat. (N. Iro, P.F.L. Maxted, 2013)

Figure 3: Modelled atmospheric thermal profiles for selected longitudes for the case with 6.5 km/s superrotating winds redistributing heat from day-side to night-side. (N. Iro, P.F.L. Maxted, 2013)

Using the Spitzer space telescope, observations of the secondary eclipses of WASP-18b (i.e. when the planet passes behind its parent star and is blocked) show a relatively high planet-to-star flux ratio that is consistent with a very hot day-side, indicating nearly no redistribution of heat (Figure 4). This is because, with efficient heat redistribution, like in the atmospheric model with superrotating winds, the planet’s day-side would not be as hot, resulting in a lower planet-to-star flux ratio (Figure 5).

Figure 4: Modelled planet-to-star flux ratio as a function of wavelength for the case with almost no redistribution of heat. The red circles with error bars represent actual data points from observations by the Spitzer space telescope. (N. Iro, P.F.L. Maxted, 2013)

Figure 5: Modelled planet-to-star flux ratio as a function of wavelength for the case with 6.5 km/s superrotating winds redistributing heat from day-side to night-side. The red circles with error bars represent actual data points from observations by the Spitzer space telescope. (N. Iro, P.F.L. Maxted, 2013)

References:

- N. Iro, P.F.L. Maxted, “On the heat redistribution of the hot transiting exoplanet WASP-18b”, Icarus 226 (2013) 1719-1723

- Nymeyer et al. (2011), “Spitzer Secondary Eclipses of WASP-18b”, arXiv:1005.1017 [astro-ph.EP]

Giant Planet in the Habitable Zone

Gravitational microlensing has led to the detection of planets with masses ranging from more than Jupiter to a few times the mass of Earth. It involves measuring the magnification of light from a distant background star due to the lensing effect by the gravitational field of a foreground star. During a microlensing event, a lightcurve of the background star is produced as the foreground star crosses in front of it. The presence of planets around the foreground star can produce sharp deviations in the otherwise smooth and symmetric lightcurve of the background star.

Figure 1: Artist’s impression of a habitable Earth-like moon around a gas giant planet.

Figure 2: An illustration of a microlensing event of a distant background star by a foreground star with and without a planet.

Using the Keck telescope near the summit of Mauna Kea in Hawaii, a team of astronomers observed a microlensing event and reported the discovery of a gas giant planet that is probably within the habitable zone of its parent star. This planet is identified as MOA-2011-BLG-293Lb. It has a mass of 4.8 ± 0.3 Jupiter mass and orbits its parent star at a distance of 1.1 ± 0.1 AU, where one AU is the average Earth-Sun separation distance. The planet’s parent star is a Sun-like star with a mass of 0.86 ± 0.06 solar mass. MOA-2011-BLG-293Lb and its parent star are located at an estimated distance of 25,000 light years away, near the centre of the Milky Way galaxy.

MOA-2011-BLG-293Lb is interesting because a hypothetical terrestrial-size moon in orbit around it can support Earth-like conditions and may be potentially habitable. Since MOA-2011-BLG-293Lb circles its parent star near the outer edge of the habitable zone, a terrestrial-size moon around this giant planet would require some sort of greenhouse warming effect to keep its surface warm enough to sustain liquid water. MOA-2011-BLG-293Lb is also one of the furthest planets discovered to date.

Reference:

V. Batista et al. (14 October 2013), “MOA-2011-BLG-293Lb: First Microlensing Planet possibly in the Habitable Zone”, arXiv:1310.3706 [astro-ph.EP]

Io’s Global Magma Ocean

Figure 1: Io and Jupiter. Io has a diameter of 3,642 km, making it slightly larger than Earth’s Moon.

Io, a moon of Jupiter, is the most volcanically active object in the Solar System. This is due to the large amount of tidal heating being generated within the moon’s interior as it is pulled between Jupiter and the other Galilean satellites - Europa, Ganymede and Callisto. The extensive volcanism and high temperature lavas on Io suggest the presence of a global layer of magma beneath its crust.

The existence of such a subsurface global ocean of magma was determined by using Jupiter’s powerful magnetic field as a probe. At Io, induction caused by Jupiter’s magnetic field can be used to infer the conductivities and hence the properties of its subsurface layers. This is because the conductivity of rock material depends on its temperature and melt state. For instance, in comparison to solid rocks, fully or partially molten rocks have dramatically higher conductivities.

Figure 2: This cross-sectional visualization shows the internal structure of Jupiter’s moon Io as revealed by data from NASA’s Galileo spacecraft. A global magma ocean that is believed to be more than 50 km thick (shown in red-brown) underlies a low-density crust about 30 to 50 km thick (shown in gray). Io’s core, measuring about 1200 to 1800 km in diameter, is composed of iron and iron sulphide (shown in a metallic silver hue). Credit: NASA/JPL/University of Michigan/UCLA.

Data from magnetic observations carried out by the Galileo spacecraft during its flybys near Io in October 1999 and February 2000 show the presence of electromagnetic induction from a highly conductive global layer beneath the surface of Io. This global conducting layer is consistent with a subsurface magma ocean with a thickness of over 50 km and a rock melt fraction of 20 percent or more. The global magma ocean has an estimated temperature exceeding 1200°C and it exists beneath a low density crust 30 to 50 km thick.

“The hot magma in Io’s ocean is millions of times better at conducting electricity than rocks typically found on the Earth’s surface” said the study’s lead author, Krishan Khurana, a former co-investigator on the Galileo magnetometer team and a research geophysicist with UCLA’s Institute of Geophysics and Planetary Physics. “Just like the waves beamed from an airport metal detector bounce off metallic coins in your pocket, betraying their presence to the detector, Jupiter’s rotating magnetic field continually bounces off the molten rocks in Io’s interior. The bounced signal can be detected by a magnetometer on a passing spacecraft.”

The high conductivity of Io’s global magma ocean shields the interior of the moon from Jupiter’s powerfully magnetic field. As a result, the magnetic field inside Io maintains a vertical orientation despite the varying orientations of the external magnetic field (i.e. Jupiter’s magnetic field). This study was published in 2011 in the journal Science and it explains why Io is the most volcanic object known in the solar system.

Reference:

KK Khurana et al., “Evidence of a Global Magma Ocean in Io’s Interior”, Science 3 June 1011: Vol. 332 pp. 1186-1189

Smashing onto a Magnetar

Neutron stars are very compact objects that form from the gravitational collapse of massive stars. A typical neutron star packs as much mass as half-million Earths within a diameter of only ~20 km. Magnetars are part of a very rare group of neutron stars that have extremely powerful magnetic fields. Occasionally, magnetars exhibit ‘glitches’ that are observed as sudden spin-ups of these compact objects. Glitches are believed to be caused by the sudden transfer of angular momentum from the faster rotating superfluid interior to the slower rotating solid outer crust of a magnetar.

Figure 1: Artist’s impression of a neutron star shown to scale with Manhattan Island. Credit: NASA.

In the May 30 issue of the journal Nature, R. F. Archibald et al. (2013) report the discovery of an ‘anti-glitch’ (i.e. a sudden spin-down) of the magnetar 1E 2259+586. This unexpected sudden spin-down is contrary to the spin-ups caused by glitches. Ordinarily, the magnetar has a spin period of 7 seconds, but the anti-glitch slowed its spin by 2 millionths of a second. In another paper by Huang and Geng (2013), the authors suggest that the sudden spin-down of 1E 2259+586 is caused by the collision of a solid object with the magnetar. The solid object came in from a direction that is opposite to the spin of the magnetar, collided with the magnetar and led to the sudden spin-up.

Observations of 1E 2259+586 reveal a decaying X-ray afterglow that is associated with the anti-glitch. Based on the energy released, the mass of the colliding solid object is estimated to be ~1/5,000,000th the mass of Earth. If the solid object is a dense iron-nickel body, it would have a diameter of 64 km. The impact of the solid object onto the surface of the magnetar is an incredibly violent process. As the solid object approaches, the immense gravity of the magnetar would stretch the object into an elongated shape before breaking it up entirely. Material from the destructed object is then compressed to ultra-high densities before slamming onto the surface of the magnetar. This process releases a huge amount of energy in a very short period of time. In fact, an intense burst of hard X-rays lasting 36 milliseconds was detected by the Fermi Gamma-ray Burst Monitor on 21 Aril 2012, consistant with the timing of the anti-glitch.

Figure 2: Artist’s impression of a magnetar. Credit: ESA.

Observations have already confirmed the existence of planetary systems around neutron stars. As a result, there are a number of ways in which a solid object, like an asteroid, can be placed on a collision trajectory with a neutron star. Firstly, the presence of planets can gravitationally perturb and scatter asteroids towards the central neutron star. Secondly, like for the Solar System, an extended cloud of small objects might also exist around the neutron star and some of them could fall towards the neutron star due to disturbances from nearby stars.

Thirdly, in a system with multiple planets, planets may collide, throwing off chunks of material where some would eventually impact the central neutron star. Lastly, the neutron star could be speeding on an escape trajectory out from its own planetary system due to the large velocity ‘kick’ it received at birth from asymmetric gravitational collapse. As the neutron star speeds through its planetary system, it can capture and ram into small objects that happen to lie in its path, resulting in the anti-glitch and X-ray burst observed for the magnetar 1E 2259+586.

References:

- R. F. Archibald et al., “An anti-glitch in a magnetar”, Nature 497, 591-593 (30 May 2013)

- Y. F. Huang and J. J. Geng (12 October 2013), “Anti-glitch induced by collision of a solid body with the magnetar 1E 2259+586”, arXiv:1310.3324 [astro-ph.HE]

Habitability vs. Colonizability

“Do there exist many worlds, or is there but a single world? This is one of the most noble and exalted questions in the study of Nature.”

- St. Albertus Magnus (13th century)

In the article “A Tale of Two Worlds” by novelist Karl Schroeder, the author states that in the detection and characterization of planets around other stars, habitability and colonizability are not the same thing. NASA’s Kepler space telescope has shown that Earth-size planets that are neither too hot nor too cold to support life are surprisingly common. These potentially habitable planets may at first seem to be where humans and their machines could one day settle. However, Schroeder mentions that the current definition of whether a planet is habitable has nearly nothing to do with its colonizability.

Take the exoplanets Kepler-62e and Kepler-78b as examples. Kepler-62e is a super-Earth in orbit around a star that is somewhat cooler than the Sun. It has 1.61 times the Earth’s diameter and is located at a comfortable distance from its parent star such that temperatures are just right to support life. Kepler-62e possesses the right properties for it to be a potentially Earth-like habitable planet. In contrast, Kepler-78b, formerly known as KIC 8435766 b, is an Earth-size planet in an extremely close-in 8.5-hour orbit around a Sun-like star. This planet is expected to be tidally-locked with one side permanently facing it parent star and experiencing hellish temperatures of 2300 K to 3100 K. Being so close to its parent star, any breathable atmosphere or liquid water is unlikely to be present on Kepler-78b. Nevertheless, the permanent night side of Kepler-78b is believed to be much cooler and temperatures there may even dip below freezing in the absence of any appreciable atmosphere to transport heat over from the scorching dayside.

Figure 1: Four potentially habitable exoplanets shown to scale alongside the Earth. Left to right: Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f, and Earth (except for Earth, these are all artists’ renditions). Credit: NASA Ames/JPL-Caltech.

 Figure 2: Artist’s depiction of Kepler-62e, a super-Earth in the habitable zone of a star that is smaller and cooler than the Sun. Credit: NASA Ames/JPL-Caltech.

Figure 3: Artist’s depiction of Kepler-20e - a planet with a smaller radius than Earth in a close-in orbit around a Sun-like star. Kepler-20e is believed to be tidally-locked with the same hemisphere always facing its parent star. The planet’s dayside temperature is estimated to be over 1000 K while its night side is much cooler. Kepler-78b is quite similar to Kepler-20e, just that it has a much hotter dayside. Credit: NASA/Ames/JPL-Caltech.

Kepler-62e is a potentially habitable planet while Kepler-78b is most certainly not. However, this may not imply that Kepler-62e is more colonisable than Kepler-78b. In fact, Kepler-78b may be more promising when it comes to colonizability. Assuming that Kepler-62e has the same density as Earth, its surface gravity will be 1.6 times of Earth’s. The stronger gravity will place an unavoidable permanent strain on humans and their machines. Even if the stronger gravity may be bearable, getting stuff off the surface of Kepler-62e into space will require exponentially more energy compared to Earth.

A study by L. Kaltenegger et al. (2013) suggests that planets in the size range of Kepler-62e are likely to be completely covered by ocean with no land in sight. The absence of land may yet again lower its potential for colonization even though the planet’s ocean may be a perfect environment for its local life. Actually, if life exists on a planet, it may immediately deem the planet unsuitable for colonization, regardless of the planet’s physical properties. This is because life on another world is likely to operate on a different biochemistry that is incompatible and possibly hostile to Earthly life. Furthermore, colonization also raises the problem of contaminating a pristine alien biosphere. Based on these considerations, an Earth-like habitable planet that is teeming with life (i.e. an Earth analogue) is almost certainly unsuitable for colonization.

Figure 4: Artist’s impression of an Earth-like planet.

Figure 5: Artist’s impression of a potentially habitable planet.

Compared to Kepler-62e, the planet Kepler-78b may appear inhospitable due to its superheated dayside. However, Kepler-78b is tidally-locked and the other half of the planet never faces its parent star. One can imagine conditions there being somewhat like within the cold permanently shaded craters at Mercury’s poles, but encompassing half a planet. An airtight habitat containing a breathable atmosphere could easily find its place on the cool night side of Kepler-78b. On a side note, the sight of its parent star from the stupendously hot dayside of Kepler-78b would certainly be terrifyingly spectacular. The huge temperature difference between the dayside and night side of Kepler-78b provides an enormous potential to move heat around, thereby generating power. Additionally, Kepler-78b is approximately the same size as Earth and this makes getting stuff off the planet’s surface into space no more difficult than it is for Earth, unless Kepler-78b is unusually dense.

Although Kepler-62e is undoubtedly well suited to support life as a habitable planet, the seemingly inhospitable Kepler-78b looks more promising with regard to its colonizability. In short, besides habitability, colonizability should also be used to judge the value of planets around other stars. Nevertheless, Kepler-62e and Kepler-78b are mere examples to distinguish between habitability and colonizability. Both planets are in no way prime interstellar destinations since they are located several hundred light years away. From here to there, there are innumerable stars with planets just like Kepler-62e and Kepler-78b.

With regard to habitability, the ‘habitable zone’ is generally defined as a region around a star where temperatures are neither too hot nor to cold for a planet to have liquid water on its surface and thus capable of supporting life. On the contrary, a ‘colonizable zone’ does not have the same limitations as a ‘habitable zone’ since it depends on a planet by planet basis and may not be required to be around a star at all. A study by Strigari et al. (2012) show that for ever star in the galaxy, there may be as many as ~10,000 unbound objects with masses ranging from Pluto to somewhat larger than Jupiter. These objects are sometimes termed free-floating planets or rogue planets. Such worlds may serve as colonizable “pit stops” in the vast distances between stars.

Figure 6: Artist’s impression of a Pluto-like object and its large moon, orbiting far from its parent star.

In the solar system, objects including Mercury, Earth’s Moon and Pluto may turn out to be excellent places for colonization in a novel method proposed by K. L. Roy et al. (2009). The authors propose creating habitable environments for humans by enclosing airless and otherwise sterile planets, moons, large asteroids, and even free-floating planets within engineered shells. Within such a shell, an environment could be created that is similar in almost all respects to that of Earth except for gravity. These “shell worlds” could be constructed just about anywhere with a suitable planet, moon or large asteroid. It allows humans and their machines to colonize any star system without interfering with or contaminating a planet that has already developed life (i.e. a habitable planet).

References:

-  W. J. Borucki et al. (2013), “Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone”, arXiv:1304.7387 [astro-ph.EP]

- Sanchis-Ojeda et al. (2013), “Transits and Occultations of an Earth-Sized Planet in an 8.5-Hour Orbit”, arXiv:1305.4180 [astro-ph.EP]

- L. Kaltenegger et al. (2013), “Water-Planets in the Habitable Zone: Atmospheric Chemistry, Observable Features, and the case of Kepler-62e and -62f”, arXiv:1304.5058 [astro-ph.EP]

- Strigari et al. (2012), “Nomads of the Galaxy”, arXiv:1201.2687 [astro-ph.GA]

- K. L. Roy et al. (2009), “Shell Worlds: An Approach to Terraforming Moons, Small Planets, and Plutoids”, JBIS Vol. 62, pp. 32-38

Nucleosynthesis of Gold in Neutron Star Collisions

Gold is rare on Earth and it is also rare in the Universe. Unlike elements like carbon, silicon or iron, gold cannot be created within the core of a star. Instead, the creation of gold requires a more energetically cataclysmic event. Short-duration gamma-ray bursts (SGRBs) are intense flashes of gamma-rays lasting less than ~2 seconds. They are believed to be produced following the merger of compact object binaries involving two neutron stars (NS-NS) or a neutron star and a black hole (NS-BH).

A compact object binary forms when both massive stars in a binary system separately explode as supernovae and leave behind their collapsed cores as a tightly bound NS-NS, NS-BH or BH-BH pair. As both compact objects circle each other, they radiate away gravitational waves and draw closer to each other until they eventually collide. Nevertheless, only collisions involving NS-NS and NS-BH pairs can produce SGRBs. “It’s a very fast, catastrophic, extremely energetic type of explosion,” says Edo Berger, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA).

NS-NS and NS-BH mergers are expected to create significant quantities of neutron-rich radioactive nuclei via the r-process, also known as the rapid neutron capture process, from the ejection of neutron-rich material. These radioactive nuclei will decay and produce a faint transient, known as a “kilonova”, in the days following the SGRB. It is believed that NS-NS and NS-BH mergers may be the dominant source for stable r-process elements in the Universe. All r-process elements are heavier than iron, a list that includes gold, mercury, platinum, uranium, thorium and more.

Berger et al. (2013) present the first detection of a kilonova following a SGRB. The SGRB is identified as GRB 130603B and it was initially detected by NASA’s Swift satellite on 3 June 2013 at 15:49:14 UTC. Although the burst event itself lasted for less than two-tenths of a second, GRB 130603B displayed a slowly fading afterglow dominated by infrared light. Over the next few days, telescopes in Chile and the Hubble Space Telescope (HST) performed optical and near-infrared observations of the afterglow.

A kilonova model with estimated ejecta mass ~10,000 to 30,000 Earth masses travelling at ~10 to 30 percent the speed of light is consistant with the observed properties of the afterglow from GRB 130603B. Assuming 10 parts per million of the ejecta mass is in the form of gold, that works out to ~10 times the mass of the Moon in gold alone. In comparison, the total amount of gold that have been mined in human history is roughly equivalent in terms of volume to a cube 21 metres on a side.

GRB 130603B is the first SGRB with evidence for r-process rich ejecta and it provides a clear signature for a compact object merger event involving either a NS-NS collision or a NS-BH collision. Based on the ejecta mass estimated for GRB 130603B and on the known frequency of SGRBs, compact object mergers are likely to be the dominant site for the nucleosynthesis of stable r-process elements in the Universe. “It’s possible that supernovae still produce a small contribution, but they do not appear to be the dominant process,” says Berger. After being created and ejected outward, these heavy elements eventually become incorporated into the formation of subsequent generations of stars and planets elsewhere in the galaxy. “To paraphrase Carl Sagan, we are all star stuff, and our jewellery is colliding-star stuff,” says Berger.

Reference:

Berger et al. (2013), “An r-Process Kilonova Associated with the Short-Hard GRB 130603B”, arXiv:1306.3960 [astro-ph.HE]

A System of Low Density Worlds

Using data collected by NASA’s Kepler space telescope, four transiting planetary candidates were found around the star KOI-152. The four planet candidates, identified as KOI-152 b, c, d and e, range in size from 3.5 to 7 times the size of Earth. They circle KOI-152 with orbital periods of 13.5, 27.4, 52.1 and 81.1 days - near a 1:2:4:6 chain of commensurability. All four planet candidates orbit their parent star in a region that is tighter than Mercury’s orbit around the Sun. The planet candidates gravitationally perturb one another and cause the transit timing of each planet candidate to exhibit variations. By analysing the transit timing variations, the masses of all four planet candidates were found to be rather small for their sizes. This means that all four planet candidates around KOI-152 have low densities (Figure 3).

Figure 1: Artist’s impression of a low density planet with a substantial gaseous envelope.

Figure 2: Artist’s impression of a low density planet with a substantial gaseous envelope.

Figure 3: The planet candidates of KOI-152, with masses and radii in Earth units. The final column shows the amount of stellar flux each planet candidate receives, where a value of 1 represents the amount of flux Earth receives from the Sun.

The planet candidates around KOI-152 have masses ranging from 4.1 to 10.9 times the mass of Earth. Planets with such masses have no known analogues in the Solar System because the Solar System has no object intermediate in mass between Earth and Uranus/Neptune, both of which are more than 14 times the mass of Earth. Of the four planet candidates, KOI-152 b has the highest bulk density even it is similar in size to KOI-152 c and e. This suggests that KOI-152 b is comprised of either a substantial mass fraction of water and/or a relatively thin hydrogen-helium envelope, with a denser rocky interior. The less dense KOI-152 c and e are likely to have more voluminous envelopes of water and/or hydrogen-helium.

Finally, the largest of the four planet candidates, KOI-152 d, has a remarkably low density of just 9 percent the density of liquid water. KOI-152 d is expected to have a very voluminous hydrogen-helium envelope comprising more than 10 percent, but less than 50 percent of its mass. Compared to dense rocky planets such as Earth with 5.5 times the density of liquid water and Kepler-10b with 8.8 times the density of liquid water, the low density planet candidates around KOI-152, especially the extremely low density KOI-152 d, show that there is a tremendous compositional variation amongst planets with ~1 to 10 times Earth’s mass.

Reference:

Daniel Jontof-Hutter, Jack J. Lissauer, Jason F. Rowe1 and Daniel C. Fabrycky (2013), “KOI-152’s Low Density Planets”, arXiv:1310.2642 [astro-ph.EP]