Thursday, April 14, 2016

Daydream Destinations, Part 2

Figure 1. In this illustration for The Chessmen of Mars by Edgar Rice Burroughs (1922), artist J. Allen St. John sketched the two Martian moons as an atmospheric backdrop for the main action, which features Gahan of Gathol scaling a tower in the lost city of Manator. In Martian mythology, the two moons represent a cold husband (Cluros, the outer moon) and his mad and wayward wife (Thuria, the inner moon).

This posting continues the discussion of daydream destinations in extrasolar space begun in Part 1. Today’s reveries involve exomoons, tidally locked planets, and monoform worlds.


Six out of eight planets in our Solar System have moons, with Jupiter and Saturn between them hosting a total of 11 spherical and approximately 100 non-spherical moons. Most of the latter are defined as “irregular” satellites on the basis of their orbital elements. In addition, Uranus has five spherical moons (plus more than 20 irregulars) and Neptune and Pluto have one each – respectively Triton and Charon. The ubiquity and similarity of satellite systems in the Solar System argue that extrasolar planets will host analogous systems. On this assumption, the term “exomoon” has become the standard designation for a hypothetical satellite of an exoplanet.

An extra moon or three in the night sky might simply be decorative, like the seven moons of Beta Lyrae or the two Martian satellites in Figure 1. There seems to be no limit on the number of natural satellites an Earth-like exoplanet could host. Given so much freedom, it would be interesting to explore the effects of competing tides on the hypothetical oceans of a Super Earth of two Earth masses (2 Mea) with three or four moons.

More beguiling speculations are possible if we shift our focus from habitable exoplanets to habitable exomoons (Tinney et al. 2011, Heller et al. 2014). Writers in earlier eras, from Loukianos of Samosata in the second century A.D. to H.G. Wells in the twentieth, offered intriguing visions of life on our own Moon. Unfortunately, we now know that such scenarios are purely fantastic. To support life, a world must also maintain an atmosphere and stable bodies of water. No such environment has ever been available on the Moon.

In the current generation of astronomers, 0.3 Mea has been adopted as the minimum mass for an Earth-like planet (Raymond et al. 2006), and by extension a habitable exomoon. This is more than five times the mass of Mercury and almost triple the mass of Mars.

Is it possible for an exomoon to attain such a high mass? That would depend on its formation pathway. Planets in our Solar System evidently acquired their moons by three different mechanisms: 1) accretion in a circumplanetary disk during the first few million years of system evolution; 2) a glancing collision with another protoplanet followed by accretion in the resulting circumplanetary ring of debris; and 3) gravitational capture of a fully formed object after planet formation finished, often resulting in a retrograde orbit for the satellite. The first mechanism brought us the spherical moons of Jupiter and Saturn, as well as a few of the non-spherical ones; the second brought us our own Moon, Pluto’s moon Charon, and probably the spherical moons of Uranus (but see Boue & Laskar 2010); while the third brought us Neptune’s moon Triton, and probably all the irregular moons of the other planets (Jewitt & Haghighipour 2007).

Figure 2. View from Pandora, imagined as the moon of an extrasolar gas giant

Each satellite system that formed along the first pathway has an aggregate mass in a ratio of about 5,000:1 to the mass of its host planet (see Canup & Ward 2006). For example, the aggregate of the four Galilean moons of Jupiter is 0.066 Mea (a bit more than Mercury’s mass). Given Jupiter’s mass of 318 Mea, the mass ratio of primary to satellites is about 4800:1. Similarly, the aggregate of the seven spherical moons of Saturn is 0.0236 Mea, while Saturn itself is 95.2 Mea, yielding a ratio of about 4000:1. Although Jupiter’s four largest moons have approximately similar masses, Saturn’s moons have an extremely lopsided mass distribution, with Titan accounting for 95% of the total.

Under the right conditions, then, all three of these mechanisms might be able to produce an exomoon of 0.3 Mea. The first would require a gas giant primary with a mass of 5 Mjup or more. More than 100 exoplanets are currently known in this range, but only about half orbit Sun-like stars; the rest are companions of hotter primaries. The second mechanism would require a very specific kind of collision between two very specific kinds of objects, at least one of which would need to have a large iron/silicate content and be at least as massive as Uranus. The third would likely require a very special three-body encounter between a gas giant and a habitable-mass terrestrial planet with a gravitationally bound companion (Agnor & Hamilton 2006). All three scenarios seem possible, with the first apparently the most straightforward and potentially the most frequent (see Figure 2).

In addition, the final orbit of the exoplanet/exomoon system produced by each of these mechanism must lie in the local habitable zone. Unfortunately, among the 100+ Super Jupiters noted above, fewer than 10% are found in the habitable zone of a Sun-like star, and half of those have eccentricities higher than 0.3, indicating extreme climates for any potential satellites. If these numbers are any reflection of the true extrasolar population, the most likely formation pathway for habitable exomoons would yield few of them. The success rate of the other two mechanisms is harder to assess. Although the planetary primaries involved are smaller than 5 Mjup, and thus far more numerous than Super Jupiters, the conditions necessary to acquire habitable exomoons through collision or capture might ensure that such objects are rare.

My hunch is that Earth-size exomoons are much less frequent in our Galaxy than terrestrial planets of 0.3 to 3 Mea. The latter population is already well-represented in the extrasolar census. Even though few to none of them (depending on your definition) have yet been observed in the habitable zone of a Sun-like star, their presence in such orbits is all but assured.

twilight zones

In the early twentieth century, astronomers believed that Mercury and Venus were tidally locked, meaning that each planet always turned the same hemisphere toward the Sun. This arrangement was thought to result from their short-period orbits, which made them vulnerable to the Sun’s gravitational influence. Now we know that both planets actually rotate slowly. Instead of permanent daysides and nightsides, each experiences an extended day/night cycle. A “day” on Mercury is 58.6 Earth days long, whereas a year is 88 days. A day on Venus is 243 Earth days, whereas a year is 225 days.

Most low-mass exoplanets detected to date have orbital periods shorter than Mercury, and almost all have periods shorter than Venus. Theoretical models predict tidal locking for the great majority. This outcome is especially significant for systems around cool stars less massive than 0.60 Msol, corresponding to the range of spectral types from late K through M. For these stellar masses, planets orbiting anywhere in the habitable zone are very likely to be tidally locked (Selsis et al. 2007). Even around higher-mass stars in the range of 0.60-0.89 Msol, the inner regions of the habitable zone are subject to tidal locking.

Figure 3. Insolation on a tidally locked planet

One of my treasured books as a child was Kenneth Heuer’s Men of Other Planets, a popular astronomy text originally published in 1951. Reflecting the consensus of the 1940s and 1950s, Heuer presents Mercury as a tidally locked world: “The country of the Mercurians is really divided into three parts; there is the land of eternal day, one of eternal night, and a borderland of alternate sunshine and shadow.” This “borderland” is also designated “the twilight zone.” As Heuer explains: “There are in reality two lands of sunrise and sunset [. . .] One at the east and the other at the west, they are elliptical regions connected at the poles, and bordering on eternal day and eternal night.”

Applying Heuer’s perspective to an extrasolar locale, let’s imagine an iron/silicate planet like Earth with a synchronous orbit of 150 days. This period places it in the habitable zone of an amber star of type K4. Although our planet is tidally locked, its orbit is slightly eccentric, causing it to librate. With respect to the host star, it rocks back and forth so that the surface area exposed to illumination varies slightly on a regular cycle. Figure 3 shows the relative insolation (exposure to stellar flux) of various regions on the planet’s surface. At the substellar point, insolation is strongest; prevailing winds from the darkside blow toward this point from all directions. Insolation declines as distance from the substellar point increases, with thermal contours shaped by wind patterns. The terminator (dividing line between day and night) shifts back and forth as the planet librates. Nightward of the terminator is the twilight zone, where the sky is still illuminated while the host star hovers just below the horizon.

Figure 4. Twilight over the Bahariya Oasis, Egypt
 Glowing clouds are reflected in the Salt Lake of Bahariya. Photo by Aymen Ibrahem;
Earth Science Picture of the Day

Depending on the planet’s geothermal flux and relative distribution of land and sea, many environments are possible. If a body of water occupies the substellar point, with continents surrounding it, that point will be occupied by a permanent cyclone. Adjacent regions will be tropical, with temperatures generally falling as distance from the center increases. Snowy conditions might prevail along the terminator, while some organisms are bound to colonize the darkside.

As Yang and colleagues have shown, a reservoir of surface water will strongly constrain the climate of a tidally locked planet (Yang et al. 2014). If the darkside is land-locked, water will be carried toward the antistellar point and freeze out of the atmosphere, creating glaciers on that hemisphere. However, as long as seas cover at least 10% of the planet’s surface, its atmosphere will circulate heat to the darkside and prevent a complete freeze-out of water. If the planet has an ocean cover of 90%, ocean currents will circulate heat to all longitudes and maintain warm temperatures throughout the darkside.

If you’re thinking about setting fantastic adventures on a tidally locked planet, you might consider some form of extended quest. One option would be a circuit of the twilight zone, imagined as a necklace of lakes set in windy mountains or a chain of islands in the circling sea. Another would be an ocean voyage from balmy latitudes through monster-infested straits to the cyclonic inferno at the substellar point. For a dose of horror, I might advise a trek beneath the cold, glittering stars of the night lands, around grimly smoldering volcanoes with veins of red lava, through phosphorescent fungal forests haunted by hallucinogenic mists, for a rendezvous with terror at the heart of darkness . . .

monoform worlds

Perhaps the most popular extrasolar destination is what I call a “monoform planet.” Such a planet has a uniform environment that corresponds to a single geographic region on Earth. Thus we have desert planets like Arrakis, Tatooine, and Jakku; jungle worlds like Yavin IV and Dagobah, and ice planets like Gethen and Hoth. Their terrestrial originals are obvious. The producers of Star Wars even went so far as to film desert scenes in Tunisia, rain forest scenes in Guatemala, and snow scenes in Iceland.

Figure 5. Artist’s impression of a Carboniferous landscape

Artist: Ludek Pesek

Earth itself has evidently sustained a succession of relatively uniform environments at different phases in its history. At early times our planet experienced extreme glacial periods (e.g., the Huronian, Marinoan, and Ordovician) during which it probably resembled an ice planet. According to one popular hypothesis, the Neoproterozoic brought about Snowball Earth conditions, when even the global ocean was covered by ice or slush. Earth has also been a jungle planet, notably during the Carboniferous, when global temperatures were higher, the atmosphere was denser, and tropical forests thrived over a large area, despite lingering glaciation (Figure 5). Although Earth hasn’t yet seen desert conditions from pole to pole, its ultimate fate is complete desiccation as the Sun evolves and heats up. According to some prognosticators, our planet’s hydrosphere will evaporate within a billion years.

Fortunately, water still covers more than three-quarters of Earth’s surface, inspiring perennial visions of ocean planets. Science fictional incarnations include Perelandra and Solaris, each the titular presence in a classic novel (by C.S. Lewis and Stanislav Lem, respectively). But the ocean planets imagined by exoplanetologists are quite different from those in science fiction narratives, which typically assume an environment like the Pacific hemisphere (Figure 6). Instead of an average ocean depth of 4.28 km, however, the ocean planets proposed by Leger & colleagues (2004) have watery envelopes 100 km deep over a layer of high-pressure ice. This layer would prevent liquid water from mixing with heavy elements in the mantle, a situation unfriendly to the emergence of life (Alibert 2014). If we want an exoplanet with extensive oceans, volcanic islands, and marine organisms, we’re really looking for Earth 2.

Figure 6. A well-known Ocean Planet

North is on the right. Image credit: NASA/JPL-CalTech

Our planet has presented many different faces over the course of its long history. We should expect no less from Earth-like planets orbiting other stars.

other exotic destinations

The itinerary of extrasolar daydream worlds can be extended to include terrestrial planets with rings, carbon planets made of diamonds, planets suspended in the event horizons of black holes, rogue planets hurtling starless through the void, and probably others that Google will gladly bring to your screen.

We might also want to consider Earth-like planets in open clusters, where bright stars like our Sun are packed more tightly than in local space and stellar populations are rich in binaries and triple star systems. Clusters make excellent settings for federated planets, interstellar empires, and most other tropes of space opera. Sadly, they have two drawbacks: 1) the vast majority break up within a billion years, such that only the most massive and heavily populated can survive as long as the current age of the Earth, and 2) any cluster that endured for 4.6 billion years would have witnessed a succession of stellar cataclysms as its brightest stars evolved off the main sequence and potentially fried everything within a few light years.

Then there are the artificial worlds, like Freeman Dyson’s Sphere, Larry Niven’s Ringworld, and Iain Banks’ Orbitals and Shellworlds. All marvels indeed, but beyond my bandwidth today!

Agnor C, Hamilton D. (2007) Neptune’s capture of its moon Triton in a binary–planet gravitational encounter. Nature 441, 192-194.
Alibert Y. (2014) On the radius of habitable planets. Astronomy & Astrophysics 561, A41.
Boue G, Laskar J. (2010) A collisionless scenario for Uranus tilting. Astrophysical Journal Letters 712, L44-L47.
Canup RM, Ward WR. (2006) A common mass scaling for satellite systems of gaseous planets. Nature 441, 834-839.
Heller R, Williams D, Kipping D, Limback MA, Turner E, Greenberg R, Sasaki T, et al. (2014) Formation, Habitability, and Detection of Extrasolar Moons. Astrobiology 14, 798-835..
Heuer, Kenneth. (1951, 1963) Chapter 4, “Mercurians and Venusians,” in Men of Other Planets. Collier Books.
Jewitt D, Haghighipour N. (2007) Irregular satellites of the planets: Products of capture in the early Solar System. Annual Review of Astronomy and Astrophysics 45, 261-295.
Leger A, Selsis F, Sotin C, et al. (2004) A new family of planets? “Ocean Planets.” Icarus 169, 499-504. Abstract:
Raymond SN, Mandell AM, Sigurdsson S. (2006) Exotic Earths: forming habitable planets with giant planet migration. Science 313, 1413-1416.
Selsis F, Kasting JF, Levrard B, Paillet J, Ribas I, and Delfosse X. (2007) Habitable planets around the star Gliese 581? Astronomy & Astrophysics 476, 1373-1387.
Tinney CG, Wittenmyer RA, Butler RP, Jones HR, O’Toole SJ, Bailey JA, Carter BD, Horner J. (2011) The Anglo-Australian Planet Search. XXI. A gas-giant planet in a one year orbit and the habitability of gas-giant satellites. Astrophysical Journal 732, 31.
Yang J, Liu Y, Hu Y, Abbott DS. (2014) Water trapping on tidally locked terrestrial planets requires special conditions. Astrophysical Journal Letters 796, L2. doi:10.1088/2041-8205/796/2/L22

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