The Perils of Proxima

Figure 1. Guillem Anglada-Escude and colleagues have announced a planet candidate with a minimum mass of 1.27 Earth units on a temperate orbit around Proxima Centauri, a tiny red dwarf that happens to be the nearest star to our Sun. This artist’s view shows the planet alongside its red host star, with the binary system of Alpha Centauri visible in the distance. Image credit: Ricardo Ramirez. 

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By now you’ve probably heard the news. Last week, a team led by Guillem Anglada-Escude reported radial velocity data from the HARPS spectrograph supporting the presence of a terrestrial planet orbiting Proxima Centauri, our Sun’s nearest neighbor. Proxima is an especially tiny red star of spectral type M5.5. Its mass is only 12% Solar and its luminosity is less than 1% Solar. Proxima b, as the new object is known, is also quite small for an exoplanet: its minimum mass is estimated at only 1.27 Earth units (1.27 Mea). That finding has inspired journalistic catchphrases such as “second Earth” and “Earth twin” (McKiernan 2016).

Just as newsworthy is the planet’s likely temperature. Even though Proxima b has an orbital period of only 11.2 days and a semimajor axis of only 0.049 astronomical units (AU), its host star is so dim that the planet receives just 65% of the irradiance that bathes Earth. This results in a blackbody equilibrium temperature (Teq) of 235 Kelvin (K) – which is actually cooler than Earth’s Teq of 255 K, and a bit warmer than that of Mars, at 210 K. Proxima b is located squarely in its system’s habitable zone.

Anglada-Escude and colleagues were unable to determine the eccentricity of the new planet’s orbit, offering only an upper limit of 0.35. This is a notable gap in our understanding of the overall system architecture. Nevertheless, the discovery team collected data suggesting a possible second planet orbiting Proxima Centauri with an orbital period longer than 100 days. This result meets expectations, given abundant evidence that small planets like Earth and Proxima b often have companions of similar mass.

Another limitation in our knowledge stems from the fact that Proxima b was detected by radial velocity measurements instead of transit observations. Radial velocity data can provide only a minimum mass, not a true mass, and in the absence of a transit, we have no idea of the radius of Proxima b. Thus, we cannot calculate the planet’s bulk composition, a fundamental determinant of surface conditions.

Nevertheless, despite the sparseness of the available data, and despite the excessive hype that often surrounds announcements of small planets, this is truly big news. Unlike the case of the phantom planet formerly claimed for Alpha Centauri B (Proxima’s next-door neighbor), all commentators seem satisfied with the reality of Proxima b. And if its host star were a G dwarf like our Sun or a K dwarf like HD 219134, I wouldn’t hesitate to identify Proxima b as an extrasolar Holy Grail: a potentially habitable Earth-like planet.

But Proxima Centauri is an M dwarf, and a very puny one at that. It lies near the bottom of the mass range for this spectral type, right above the cut-off for brown dwarfs (which aren’t stars). Given the findings of Luger & Barnes (2015) and Owen & Mohanty (2016) on the evolutionary and energetic characteristics of M dwarfs, my instinct is to discount the possibility that Proxima b could support oceans or complex life.

Then again, I’m not a professional astronomer whose research depends on funding from government agencies and whose career benefits from media attention. Governments and media evidently determined long ago that taxpayers/consumers have no interest in exoplanets unless they resemble Earth, Pandora, or Tatooine. Accordingly, the publication of Proxima’s detection last week in Nature was accompanied by a posse of preprints on its potential formation history and present habitability. All tried very hard to find scenarios that would yield a habitable Earth-like planet, despite the unfavorable conditions predicted for the Proxima Centauri system and others like it.

dim stars are risky bets for life

Before looking at a selection of those preprints, let’s briefly review the unfavorables, which are straightforward and stubborn:

 

First, an evolving M dwarf spends hundreds of millions of years with temperatures far higher and stellar activity more energetic than it will experience when it finally enters maturity on the main sequence (the phase in stellar evolution when hydrogen fusion occurs). This developmental history means that any planet orbiting in a red star’s mature habitable zone will likely experience runaway greenhouse conditions for hundreds of millions of years. Loss of atmosphere and water is probable, forestalling the emergence of life (Luger & Barnes 2015).

Second, during much of their long lifetimes, M dwarfs are subject to frequent flaring events and coronal mass ejections (CMEs), and they emit high levels of extreme ultraviolet radiation. This behavior subsides very slowly with age. Flares and CMEs are likely to erode the atmospheres and volatile contents of any planets orbiting in the inner systems of M dwarfs, where their habitable zones are found. Thus, even if an Earth-size planet survived an intense greenhouse in its infancy, it would still be vulnerable to evacuation of volatiles in its maturity (Luger & Barnes 2015). Again, a sterile desert is more likely than a garden.

Most of the brand-new studies on Proxima b acknowledge these two challenges.

 

Figure 2. Desert Sunset

Landscapes on red dwarf planets are more likely to resemble this photograph of sunset over the Sahara than the watery locales pictures by many optimistic space artists. Source: Wikimedia, with a red filter over the Sun.

proxima centauri sub specie aeternitatis

A team of scientists led by James Davenport, none of them associated with the Proxima discovery team, reported recent observations by the MOST satellite that underscore the harsh conditions sketched above. They found that Proxima Centauri emits flares at a tempo of at least 63 per Earth day, with superflares occurring about 8 times per Earth year (approximately once every 4 orbits of Proxima b). As they remark:

“If these flares regularly impacted Proxima b, the atmosphere would never fully recover. While this is not known to be a ‘show-stopper’ for habitability, it clearly necessitates a more detailed investigation of atmospheric response […] and photoevaporation […] for Proxima b” (Davenport et al. 2016).

Another group led by Gavin Coleman, consisting of a subset of the discovery team, conducted a set of numerical simulations to investigate four potential formation scenarios for the new planet. Each was intended to produce an analog of Proxima b within a specified range of planet masses and orbital periods. One problem with their approach, from my perspective, is that all their simulations assume an implausibly large mass for the host star’s protoplanetary disk, amounting to 4.5% of the stellar mass. By contrast, a large and growing body of observations indicates that a typical protoplanetary disk contains about 1% or less of the mass of its parent star (Williams & Cieza 2011, Andrews et al. 2013).

The first scenario explored by Coleman & colleagues was in situ accretion from a swarm of embryos and planetesimals after the dissipation of the gaseous component of the protoplanetary disk. This is similar to the process invoked to produce the four inner planets of the Solar System. Numerical simulations tended to produce compact multiplanet systems with one or more Earth-mass planets like Proxima or Venus, often with a few lower-mass planets alongside them, smaller than Venus but bigger than Mars. Across simulations, their compositions ranged from water-rich to dry, and their orbits were somewhat eccentric.

The second scenario followed the migration of several embryos with initial masses ranging from 0.05 to 0.2 Earth masses (Mea) – roughly similar to the mass of Mars – within icy and dusty regions of a gaseous protoplanetary disk extending outward to 9 AU – equivalent to the orbit of Saturn in our system. This scenario started earlier in system history than the first one, so the simulations modeled interactions between the growing embryos and the gas disk. Both inward and outward migration were enabled, and forming planets could accrete hydrogen from the disk. This scenario tended to yield one or two Earth-mass planets accompanied by a few smaller planets, as in the first scenario. However, all these planets were rich in volatiles, prompting the designation of “Ocean Planets” (Coleman et al. 2016). Their orbital eccentricities were generally smaller than those in the first scenario.

The third scenario was similar to the second, except that it featured a single migrating embryo that formed at a distance of several AU. The object was accompanied on its inward journey by a swarm of planetesimals. Each simulation in this set produced just one volatile-rich planet, whose mass varied from one to several Earth masses. Many simulation runs failed to produce planets with orbits as tight as Proxima b.

The fourth scenario also featured a single embryo growing in a gas disk, except that it was accompanied by pebbles instead of planetesimals. This scenario had the most difficulty forming an analog of Proxima b, even though the embryo tended to migrate over long distances. The authors observed that the formation of “a true Proxima analog” in this scenario would require a substantially larger disk mass than they assumed (even though they were already using an unrealistically massive disk).

Coleman & colleagues conclude by suggesting observational tests for each of their four scenarios. These tests hinge on obtaining a precise estimate of the orbital eccentricity of Proxima b; establishing the presence or absence of additional planets in the system; and determining Proxima’s bulk composition (dry versus watery, hydrogen versus heavier atmospheric gases). Apart from pointing out the difficulties involved in the single-embryo scenarios, the authors do not attempt to rank their models according to likelihood.

Although I admire Gavin Coleman’s work, I see two limitations in this study. First, as noted earlier, the investigators used an unrealistically large disk mass, and second, they did not consider potential planet masses in excess of 2 Mea, even though Proxima b’s true mass could easily be 4 or 5 Mea instead of 1.3 Mea. How would their results change if they broadened their simulation parameters to include these possibilities?

Two other new studies focused on the habitability of Proxima b. One included members of the discovery team joined by several other distinguished researchers, mostly affiliated with European institutions (Ribas et al. 2016). The other was conducted by astronomers who were not involved with the discovery; all are affiliated with U.S. institutions (Barnes et al. 2016). Notably, the latter group includes Rodrigo Luger and Rory Barnes, who wrote that widely cited study on the likelihood of extreme water loss for M dwarf planets (2015).

proxima centauri as a habitable planet: barnes & colleagues

Barnes & colleagues (2016) begin with an expansive review of our knowledge of Proxima Centauri. The first question they consider is whether Proxima is the third member of a triple star system centered on Alpha Centauri A and B, which are both Sun-like stars chemically enriched in metals. In the present epoch of our Galaxy, Proxima is separated from our Sun by 1.3 parsecs, but its distance from Alpha Centauri AB is only 15,000 AU. The chances are one in a million that Proxima could be found so close to the binary unless all three stars are physically associated. Indeed, observations over the past 100 years show that Proxima shares a common motion through space with its brighter neighbors. Yet not even the latest measurements, using the most sophisticated instruments and analytic methods, have been able to establish any curvature in Proxima’s trajectory.

Thus, this ruddy little twinkler might simply be part of a moving group that includes the Alpha Centauri binary as well as other star systems. As such, Proxima might or might not have formed in the same molecular cloud as Alpha Centauri AB. If it did, then we can assume that its age and metallicity are similar to those of the dazzling binary: age about 3 to 6 billion years (preferred value 4.8 billion years) and [Fe/H] about +0.25. These numbers imply a star that is slightly older and far richer in heavy elements than our Sun. Barnes & colleagues argue that stellar enrichment in metals is evidence that Alpha Centauri formed substantially closer to the Galactic Core than did our Sun, since the local concentration of metals increases with proximity to the Core.

Given the possibility that Proxima is bound to Alpha Centauri AB as a third member of the star system, the authors point to the recent work of Kaib & colleagues (2013) on the consequences of wide stellar orbits. Barnes & colleagues consider it likely that Proxima’s planetary system has been disrupted by a close encounter with the bright binary at some point during its history, especially if the hypothetical Alpha Centauri trinary migrated to its present Galactic orbit from the inner Milky Way. If it did, Proxima b might once have followed a wider orbit around Proxima Centauri, but was driven closer to the star by perturbations induced by Alpha Centauri AB. Such perturbations might have excited the planet’s orbital eccentricity and inclination.

Therefore, Barnes & colleagues find that Proxima b could exist in two different orbital regimes: either 1) it is tidally locked, with one hemisphere perpetually assaulted by flares and CMEs and the other in endless night, or 2) it is engaged in a 3:2 spin-orbit resonance like the planet Mercury, spinning 3 times for every 2 orbits around the host star. The former regime would be consistent with a circular orbit, the latter with a moderately eccentric orbit.

Having reviewed the planet’s motion through space, the authors explore several additional factors, including atmospheric escape, tidal evolution, and radiogenic heating, before they get down to business. As they tell us, their aim is to investigate “plausible evolutionary scenarios, focusing on cases that allow the planet to be habitable.” Their bias in favor of habitable outcomes is explicit.

Their approach is analytic, based on an original software package called VPLANET. They consider two different formation histories for Proxima b. In one, it is a water-rich planet with a hydrogen envelope amounting to 1% or less of its total mass. In the other, it is a water-rich planet without any gaseous hydrogen. Their primary analysis assumes a default planetary mass of 1.27 Mea, but they also consider more massive cases.

Barnes & colleagues conclude that, if Proxima b achieved its present orbit at the time of its formation, it would have to support at least 10 Earth oceans in order to retain 1 Earth ocean today. If the planet’s water content were any smaller, given the intense stellar flux, “it is likely desiccated today.” They also find that a hydrogen envelope any smaller than 1% would readily dissipate, but that larger concentrations would linger, with negative consequences for water and life. Larger planet masses would be an additional factor preventing the escape of a primordial hydrogen envelope.

They discuss seven possible atmospheric states for Proxima b in the present epoch. In the “habitable but dry” case, the planet avoids a hydrogen greenhouse and retains a small quantity of water, but it is much dryer than Earth – more like Dune or Barsoom than a Cryogenian snowball or a Carboniferous jungle. In the “Venus-like” case, it retains a thick CO2 atmosphere but is completely desiccated and uninhabitable. In the “Neptune-like” case, it is similarly desiccated and hellishly hot, but the culprit is an extensive hydrogen envelope that failed to dissipate, either because the planet is more massive than 1.27 Mea or because the original envelope exceeded 1% of the bulk composition. In the “abiotic oxygen” case, photolysis of the planet’s original water content led to a complete loss of hydrogen, while the liberated oxygen accumulated in the atmosphere and saturated global geochemistry. In this case, they argue, the intensely oxidized environment would prevent the emergence of life, even if quantities of water escaped photolysis (with that possibility defining the “water and oxygen, but uninhabitable” scenario).

They also discuss an outcome they consider especially unlikely: the “no atmosphere” case. While they concede that the host star’s intense flaring activity is capable of stripping an Earth-like atmosphere from Proxima b, they argue that such a catastrophe would be followed by outgassing from the planet’s interior, which could re-establish an atmosphere. Only if the planet’s core has solidified, quenching planetary magnetism, or if the star is a few billion years older than their preferred estimate of 4.8 billion years, providing enough time for the mantle to completely devolatilize, would its atmosphere be permanently destroyed.

The authors devote the most time and space to the “Earth-like” case, consistent with their stated aims. Within this case they find three pathways to a happy ending. In one, Proxima b originally formed as an Earth-like planet on an orbit well outside the system’s mature habitable zone, neatly avoiding ablation of volatiles. After the star settled down on the main sequence, a close encounter between Proxima and Alpha Centauri AB scattered the planet into its present orbit, where it managed to maintain its atmosphere and water despite continuing flares and CMEs. In the second pathway, Proxima b achieved its present orbit in primordial times and suffered desiccation as in the “Venus-like” case, but then a close passage involving Proxima and the bright binary launched icy asteroids and comets on a collision course with the desert planet. This bombardment re-hydrated the environment and enabled oceans and life. In the third pathway, Proxima b started its existence as a gas dwarf with a hydrogen envelope comprising about 0.1% of its bulk composition, plus a water inventory amounting to 4.5 Earth oceans. The host star then blew off the envelope and evaporated most of the water, and thus unveiled, the planet brought forth life. This is the “habitable evaporated core” scenario, which Barnes & colleagues consider the likeliest of the three pathways to Eden.

In their closing remarks, nonetheless, they concede that life-friendly outcomes represent a small subset of the possible scenarios for our new neighbor. Having reviewed the many mechanisms by which the planet can end up a blasted, lifeless desert, they “identify the retention of water as the biggest obstacle for Proxima b to support life.”

proxima centauri as a habitable planet: ribas & colleagues

Ignasi Ribas led another group including Reiners, Morin, and Anglada-Escude from the discovery team, as well as Sean Raymond, Jeremy Leconte, Franck Selsis, Emeline Bolmont, and others. Their study is substantially briefer and less expansive than that of Barnes & colleagues, but they cover much of the same ground, while reaching somewhat rosier conclusions.

They begin with formation scenarios, considering various possibilities also discussed by Coleman & colleagues and Barnes & colleagues: formation in situ, formation in situ with late water delivery by bombardment, and formation by accretion with long-distance migration. They also consider a subset of the evolutionary outcomes explored by those studies, including Ocean Planets, completely desiccated planets, and of course Earth-like planets. Among likely orbital states, they agree with Barnes & colleagues regarding the possibility of two regimes: a “synchronous” or tidally locked case and a 3:2 spin-orbit resonance.

In their exploration of stellar irradiation and the potential for erosion of atmosphere and water, Ribas & colleagues offer a broad range of outcomes. Although they recognize that the host star’s troublesome behavior could desiccate a temperate planet even if it originally supported 21 Earth oceans, they also find cases where the planet could lose less than a single Earth ocean. It’s no surprise that this special case assumes special prominence in their overall findings. As they put it, the “general conclusion from our study is that Proxima b could have liquid water on its surface today and thus can be considered a viable candidate habitable planet.” I’d say the fix is in.

 

wrapping it up

After typing these thousands of characters, I’ve only scratched the top layers of the new literature on Proxima b. Members of Ribas’ group have also produced a study of the planet’s potential climate (Turbet et al. 2016), members of Barnes’ group just circulated a lengthy treatment of its potential environments and their observational signatures (Meadows et al. 2016), while a solo author offers a study of heat distribution (Goldblatt 2016). So many experts contributing so much brain power to that pale red dot!

Here’s what I think. Given the limited available data, it’s premature to speculate about the possibility of Earth-like conditions on Proxima b. Given my understanding of human behavior under the regime of desiring-production enforced by terminal commodity capitalism, however, such speculations are inevitable. We want excitement, and we want it now!

Before Proxima b was announced, we already knew that M dwarfs readily supported Earth-size planets in their classical habitable zones, and we already knew that the likelihood of life on such planets was far lower than on their counterparts orbiting Sun-like stars. The announcement of Proxima b hasn’t changed any of that. But it has inspired a lot of cogitation and calculation, and fortunately, that’s not likely to stop.

 

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REFERENCES

Andrews SM, Rosenfeld KA, Kraus AL, Wilner DJ. (2013) The mass dependence between protoplanetary disks and their stellar hosts. Astrophysical Journal 771, 129.

Anglada-Escude G, Amado PJ, Barnes J, Berdinas ZM, Butler RP, Coleman GAL, de la Cueva I, Dreizler S, Michael Endl M, Giesers B, and 21 others. (2016) A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature 536, 437-440.
Barnes R, Deitrick R, Luger R, Driscoll PE, Quinn TR, Fleming DP, Guyer B, McDonald DV, Meadows VS, Arney G, Crisp D, Domagal-Goldman SD, Lincowski A, Lustig-Yaeger J, Schwieterman E. (2016) The habitability of Proxima Centauri b I: Evolutionary scenarios. In press. Abstract: 2016arXiv160806919B
Coleman GAL, Nelson RP, Paardekooper SJ, Dreizler S, Giesers B, Anglada-Escude G. (2016) Exploring plausible formation scenarios for the planet candidate orbiting Proxima Centauri. Monthly Notices of the Royal Astronomical Society, in press. Abstract: 2016arXiv160806908C
Davenport JRA, Kipping DM, Sasselov D, Matthews JM, Cameron C. (2016) MOST observations of our nearest neighbor: Flares on Proxima Centauri. In press.
Goldblatt C. (2016) Tutorial models of the climate and habitability of Proxima Centauri b: A thin atmosphere is sufficient to distribute heat given low stellar flux. In press.
Hansen B, Murray N. (2013) Testing in situ assembly with the Kepler planet candidate sample. Astrophysical Journal 775, 53. Abstract: 2013ApJ...775...53H Kaib N, Raymond S, Duncan M. (2013) Planetary system disruption by Galactic perturbations to wide binary stars. Nature 493, 381-384. Abstract: 2013Natur.493..381K
Luger R, Barnes R. (2015) Extreme water loss and abiotic O₂ buildup on planets throughout the habitable zones of M dwarfs. Astrobiology 15, 119-143. Abstract: 2015AsBio..15..119L
McKiernan K. (2016) Earth’s twin beckons: Scientists say planet proves it’s a “Star Trek” universe. Boston Herald, Thursday, August 25, 2016.
Meadows VS, Arney GN, Schwieterman E, Lustig-Yaeger J, Lincowski AP, Robinson T, Domagal-Goldman SD, Barnes RK, Fleming DP, Deitrick R, Luger R, Driscoll PE, Quinn TR, Crisp D. (2016) The habitability of Proxima Centauri b: II: Environmental states and observational discriminants. In press.
Owen JE, Mohanty S. (2016) Habitability of terrestrial-mass planets in the HZ of M Dwarfs. I. H/He-dominated atmospheres. Monthly Notices of the Royal Astronomical Society 459, 4088-4108.
Ribas I, Bolmont E, Selsis F, Reiners A, Leconte J, Raymond SN, Engle SG, Guinan EF, Morin J, Turbet M, Forget F, Anglada-Escude G. (2016) The habitability of Proxima Centauri b I. Irradiation, rotation and volatile inventory from formation to the present. Astronomy & Astrophysics, in press.
Turbet M, Leconte J, Selsis F, Bolmont E, Forget F, Ribas I, Raymond SN, Anglada-Escude G. (2016) The habitability of Proxima Centauri b II. Possible climates and observability. In press.
Williams JP, Cieza LC. (2011) Protoplanetary disks and their evolution. Annual Review of Astronomy and Astrophysics 49, 67-117. Abstract: 2011ARA&A..49...67W

 

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

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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.

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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

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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