HD 219134: Take Three

Figure 1. Analyses of longitudinal radial velocity data by Johnson & colleagues have confirmed three planets orbiting HD 219134, a small Sun-like star of spectral class K3 located at a distance of only 6.53 parsecs (21 light years). Additional low-mass planets were reported in this system by Motalebi & colleagues (2015) and Vogt & colleagues (2015), but Johnson’s group had insufficient data to confirm or reject those candidates. In the figure, the planets colored aquamarine (b, d) are supported by Johnson et al. 2016, Motalebi et al. 2015, and Vogt et al. 2015; the large blue planet (e (h)) is supported by Johnson et al. 2016 and Vogt et al. 2015; and the orange planet (c) is supported by Motalebi et al. 2015 and Vogt et al. 2015

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I love science! Replication of results, falsification of invalid hypotheses! Teams of scientists working on singular questions, converging on robust answers, shining the light of understanding on the farthest reaches of the universe!

Or at least that’s the ideal.

Since last summer, we’ve seen science in action around HD 219134, an appealing amber star of 0.78 Solar masses (0.78 Msol) located very nearby in the northern constellation of Cassiopeia. That star and its planetary system have been featured in three previous blog posts (here, here, and here), which record unfolding developments both in the number of the reported planets and in the most appropriate characterization of the system architecture.

Two major studies last year presented conflicting analyses based on two different datasets. Motalebi & colleagues (2015; hereafter M15) proposed a four-planet architecture comprising three low-mass planets inside 0.25 AU and a gas giant of 0.19 Mjup (62 Mea) just outside 2 AU. In order of distance from the star, the planets were designated b through e. Almost simultaneously, Vogt & colleagues (2015; hereafter V15) proposed a six-planet architecture with five low-mass planets inside 0.4 AU and a gas giant of 0.34 Mjup (108 Mea) just outside 3 AU. In the manuscript circulated before publication, these six were designated b through g in order of distance from the star, but on publication they were renamed b through h without reference to their distance, omitting e altogether (perhaps reserving that letter for the planet reported by M15?). M15 also proposed a rotation period of 42.3 days for the host star, while V15 proposed a period of approximately 20 days.

Now a third team, one of whose members (Meschiari) also participated in V15, has weighed in on this system (Johnson et al. 2016; hereafter J16). Most of the authors are based at the University of Texas at Austin and involved in the McDonald Observatory Planet Search. A key strength of this work is its radial velocity coverage, which spans 27 years. Another is its data on stellar activity, which span 17 years and were obtained by the Keck/HIRES spectrograph. A third strength is the analytical acumen of the investigative team, which includes Artie Hatzes and Paul Robertson. A notable limitation, as the authors concede, is the low precision of the radial velocity data, which did not permit definitive results regarding any of the proposed low-mass planets apart from the innermost, planet b (see Figure 1).

The most direct contribution of J16 is their finding that HD 219134 has a stellar activity cycle with a period of 11.6 years. This is very similar to the 11-year cycle of our Sun, which manifests in the ebb and flow of sunspots on the Sun and auroras on Earth. Identifying this cycle in HD 219134 assisted the analysis of the radial velocity data, enabling J16 to rule out systematic effects originating in stellar activity. However, they were unable to determine the star’s rotation period, which is essential for a robust data analysis.

As the authors note, a star’s rotation has a critical relationship with its age. For HD 219134, a period of about 20 days would imply an age of about 1.3 billion years. Notably, V15 reported a period of 22.8 days for their planet f – very close to their estimate of the stellar rotation period. A period of 42.3 days, as reported by M15, would imply an age of 4.1 billion years, making the star a bit younger than our Sun. J16 favor a period similar to M15’s estimate, which they say is most consistent with the stellar activity data.

Table 1. Summary of findings on the HD 21934 planetary system

Tags: P = approximate periodicity in days; PL = planet designation; d = reported period in days; Mea = reported mass in Earth units; a = semimajor axis in astronomical units (AU; Earth’s semimajor axis = 1 AU); e = orbital eccentricity.

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In broad terms, J16 support M15’s analysis of the low-mass planets in the inner system and V15’s analysis of the gas giant in the outer system (Table 1).

In the inner system, J16 had no difficulty recovering a signal corresponding to planet b, which was characterized by M15 and V15 as a planet less massive than 5 Mea with a period just over 3 days. J16 also recovered periodicities at 22.8 days and 79.1 days, but they concluded that both signals were caused by stellar rotation (as an alias or harmonic of the period) rather than by planets. They were much more confident of another signal at about 47 days, which corresponds to planet d in the models of M15 and V15. However, the two earlier studies provided conflicting estimates of this object’s minimum mass, with M15 proposing 8.67 Mea and V15 proposing 21.3 Mea. J16 did not comment on this candidate’s mass.

J16 also failed to recover any periodicity corresponding to planet c. Nevertheless, both M15 and V15 characterized this object in very similar terms, while J16 noted that their own radial velocity precision was significantly lower than that of the two earlier studies. Thus it seems safe to regard planet c as validated.

In the outer system, J16 confirmed the small gas giant detected by M15 and V15, noting that the duration of M15’s dataset was too brief to inform a robust estimate of the planet’s orbital period. Benefiting from their extended radial velocity coverage, J16 calculated a period similar to that reported by V15 (2121 days versus 2247 days from V15), but they favored a mass closer to M15 (76 Mea, versus 62 Mea from M15 and 108 Mea from V15). Sadly, they retained the confusing designation of planet h for this object.

I say “sadly” because h implies a system with seven exoplanets, whereas only one extrasolar system with that many companions has been confirmed to date: Kepler-90. The planeticity of this remarkable system is rivaled only by HD 10180, which has six exoplanets (c through h) confirmed by radial velocity measurements and a seventh (b) whose existence has remained tentative ever since the system’s initial publication in 2011. Notably, no study has yet proposed more than six planets for HD 219134, and J16 were dubious (though not dismissive) of two of the six reported by V15. So I contend that planet e is a more appropriate designation for the cool gas giant in this system, on the grounds of logical consistency.

Figure 2. Selected mixed-mass planetary systems

Tags: ME = Earth masses; RE = Earth radii; AU = astronomical units (Earth-Sun separation = 1 AU). Selection criteria: At least two low-mass planets and at least one gas giant on an exterior orbit. In each system with transiting planets, all transiting orbits are co-planar. Note that gas giants Kepler-89d and WASP-47b are also observed in transit, while a single transit is reported for the innermost planet of HD 219134.

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As we learn more and more about its architecture, HD 219134 becomes a more and more interesting example of what I might call “a rich mixed-mass system.” Figure 2 illustrates the construct I have in mind. It updates Figure 3 from a recent posting (Almost Jupiter) by revising the orbit of the outer planet of HD 219134 and adding our Solar System.

These 11 systems look like variations on a single theme. All center on a Sun-like star (o.70-1.25 Msol) and include at least two low-mass planets with at least one gas giant on a wider orbit. In 9 out of 11 systems, at least one low-mass planet orbits inside 0.1 AU, and in 8 out of 11, the gas giant orbits outside 1 AU. In three systems (including our own), at least one additional low-mass planet orbits outside the (outer) gas giant. One system (WASP-47) hosts two gas giants inside the system ice line, such that one dominates the inner cluster of planets and the other orbits in the system habitable zone. Another (Kepler-90) might host two warm giants, since planets g and h both have radii larger than 8 Rea.

Although the Solar System is typically the oddball in any line-up of planetary systems orbiting Sun-like stars, it bears a distinct family resemblance to most systems in Figure 2. Specifically, it is one of nine systems in this sample with a pronounced gap between the inner aggregation of low-mass planets and the cool giant. In our system the gap appears in the void between Mars and Jupiter. Such a gap is absent only from the two most compact systems (Kepler-89, 289), where all known planets orbit inside 0.6 AU.

Even one of our system’s oddest features – the absence of planets inside 0.3 AU – has an extrasolar counterpart, since Kepler-289 reveals a similar dearth of planets inside 0.2 AU. Evidently gaps can appear almost anywhere within 5 AU of a host star.

Nevertheless, the Solar System’s two cold gas giants make it unique among the systems considered here. Ours is also the only one with confirmed or even proposed objects of habitable mass (i.e., 0.3-3.0 Mea) orbiting in the liquid water zone. Three other systems in this line-up harbor planets in that favored region (WASP-47, Kepler-68, HD 10810). Unfortunately, none are habitable: two are gas giants and the third, HD 10810 h, is more massive than Neptune.

What about the seven systems where the gap occupies the habitable zone? In one of them (Kepler-90), an adjacent planet might prevent habitable planets from forming or surviving. But in six other systems (HD 219134 and Kepler-48, 87, 89, 167, 289) the habitable zone looks empty. For each of these, I’d love to see dynamical simulations of the long-term stability of hypothetical Earth-mass planets on habitable orbits.

migration through a radially structured disk

We have lots of theories about the evolution of our Solar System. These days, the most popular hinge on the unique relationship between Jupiter and Saturn. As noted above, however, none of the other systems in Figure 2 contain such a power couple. Could some other aspect of system evolution, some factor independent of primordial resonance between two gas giants, be responsible for the structural commonalities visible in these systems?

Gavin Coleman and Richard Nelson recently produced two studies in quick succession on their developing model of system evolution (2016a, 2016b; hereafter CN16A and CN16B). Each study reports a set of N-body simulations of planet formation around a star of 1 Msol in a realistic protoplanetary disk that includes gas, pebbles, boulders, planetesimals, and a cavity inside a radius of 0.05 AU. The simulations modeled key physical processes known to affect planet formation and disk evolution, such as magnetohydrodynamic turbulence at the inner edge, aerodynamic drag on planetesimals, gas accretion on planetary cores, Type I migration, Type II migration, and photoevaporation. Both sets of simulations produced planetary systems similar to examples in Figure 2.

CN16A reported an ensemble of 72 simulations. Each one began with a disk of radius 40 AU containing 52 protoplanets (also called planetary embryos) of 10% Earth mass each (0.1 Mea), traveling on orbits between 1 and 20 AU. The full ensemble used initial gas disks that were either 1, 1.5, or 2 times the mass of the minimum mass Solar nebula, with disk metallicities of either 0.5, 1, or 2 times Solar.

This set of simulations readily formed Hot Jupiters, sometimes with low-mass companions on adjacent orbits; the latter systems recall WASP-47. The same setup also produced compact systems of low-mass planets similar to Kepler-11. However, it failed to produce any gas giants with periods longer than 10 days.

The second study, CN16B, reported an unspecified number of simulations with disks initially containing 44 planetary embryos of 0.2 Mea each on orbits between 1 and 20 AU. The full ensemble used initial disk masses that were either 1 or 2 times the minimum mass Solar nebula, with metallicities of either 0.5, 1, or 2 times Solar. Disk lifetimes ranged from 3.5 to 8.5 million years.

The most significant change in the new setup was the inclusion of radial structures in the protoplanetary disk arising from discontinuities in viscous stress and the local surface density of solid particles. Such structures can also be described as planet traps or dead zones: well-defined regions with finite lifetimes where migrating solids become stranded and mutual interactions result in the accretion of planetesimals and protoplanets. Each simulation in CN16B included four radial structures whose location and duration varied from run to run. As one structure subsided, another formed at a different radius. All were located outside the system ice line at semimajor axes wider than 5 AU (see Figure 5 in CN16B).

Figure 3. The protoplanetary disk around TW Hydrae

This young star has a huge protoplanetary disk whose fortuitous alignment enables a face-on view from the Solar System. The disk is about 5% as massive as our Sun (0.05 Msol), and its radius is about 90 AU. Radial structures can be observed throughout (Nomura et al. 2016, Debes et al. 2016).

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For disks of sufficient mass and metallicity, the improved setup produced both Hot Jupiters and long-period gas giants. Overall results replicated the widely reported “period valley” between 0.1 and 0.5 AU (the range of semimajor axes where gas giant planets are rare). Final architectures included systems with a Hot Jupiter plus an outer giant companion; compact inner systems of low mass planets bounded by a gap with a cool giant orbiting outside it (siblings of HD 219134 and Kepler-167); and systems with two cool gas giants analogous to Jupiter and Saturn. This setup also produced compact low-mass systems, some with inner gaps resembling those around Kepler-289 and our Sun.

As Coleman & Nelson readily concede, they’re working with a “toy model” that isn’t intended to exactly recapitulate planet formation or predict the relative frequency of system architectures. Nevertheless, their model incorporates the latest theoretical perspectives and observational insights (e.g., high-resolution imaging of the protoplanetary disks around HL Tauri and TW Hydrae), and its most recent iteration compares very well with other available approaches. As far as I’m aware, it’s the only evolutionary model proposed to date that can produce system architectures whose diversity rivals reality. I’m eager to see the next study this team produces.

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REFERENCES
Coleman G, Nelson R. (2016a) On the formation of compact planetary systems via concurrent core accretion and migration. Monthly Notices of the Royal Astronomical Society 457, 2480-2500. Abstract: 2016MNRAS.457.2480C
Coleman G, Nelson R. (2016b) Giant planet formation in radially structured protoplanetary discs. Monthly Notices of the Royal Astronomical Society. In press. Abstract: 2016arXiv160405191C
Debes JH, Jang-Condell H, Schneider G. (2016) The inner structure of the TW Hya disk as revealed in scattered light. Astrophysical Journal Letters 819, L1.
Johnson MC, Endl M, Cochran WD, Meschiari S, Robertson P, MacQueen PJ, Brugamyer EJ, Caldwell C, Hatzes AP, Ramírez I, Wittenmyer RA. (2016) A 12-year activity cycle for the nearby planet host star HD 219134. Astrophysical Journal 821, 74. Abstract: 2016ApJ...821...74J
Kasting JF, Kopparapu R, Ramirez RM, Harman CE. (2014) Remote life-detection criteria, habitable zone boundaries, and the frequency of Earth-like planets around M and late K stars. Proceedings of the National Academy of Sciences 111, 12641-12646. Abstract: http://adsabs.harvard.edu/abs/2014PNAS..11112641K
Kipping DM, Torres G, Henze C, Teachey A, Isaacson H, Petigura E, Marcy GW, Buchhave LA, Chen J, Bryson ST, Sandford E. (2016) A transiting Jupiter analog. Astrophysical Journal 820, 112. Abstract: 2016ApJ...820..112K
Lovis C, Ségransan D, Mayor M, Udry S, Benz W, Bertaux J-L, & al. (2011) The HARPS search for southern extra-solar planets. XXVIII. Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems. Astronomy & Astrophysics 528, A112. Abstract: 2011A&A...528A.112L
Motalebi F, Udry S, Gillon M, Lovis C, Ségransan D, Buchhave LA, Demory BO, Malavolta L, Dressing CD, Sasselov D, et al. (2015) The HARPS-N Rocky Planet Search I. HD 219134 b: A transiting rocky planet in a multi-planet system at 6.5 pc from the Sun. Astronomy & Astrophysics 584, A72. Abstract: 2015A&A...584A..72M
Nomura H, Tsukagoshi T, Kawabe R, Ishimoto D, Okuzumi S, Muto T, et al. (2016) ALMA observations of a gap and a ring in the protoplanetary disk around TW Hya. Astrophysical Journal Letters 819, L7.
Vogt SS, Burt J, Meschiari S, Butler RP, Henry GW, Wang S, Holden B, Gapp C, Hanson R, Arriagada P, Keiser S, Teske J, Laughlin G. (2015) A six-planet system orbiting HD 219134. Astrophysical Journal 814, 12. Abstract: 2015ApJ...814...12V

 

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).

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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.

exomoons

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

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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

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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

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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!
 

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REFERENCES

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: http://adsabs.harvard.edu/abs/2003ESASP.539..253L

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