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]