Saturday, April 9, 2016

Daydream Destinations, Part 1

Figure 1. Retro Venus, the ultimate daydream vacation. This artistic take on Venus as a lush jungle planet reflects the pulp fiction of C.L. Moore in the early 1930s (“Black Thirst”) and Leigh Brackett in the late 1940s (“The Moon That Vanished”). Image by Steve Thomas. Prints are available for purchase here

Science fiction was the gateway drug for many of us who now share a passion for exoplanetary astronomy. I suspect we’re hoping – consciously or unconsciously – that science will eventually find worlds like the ones we grew to love through speculative novels, comics, pulps, movies, and television.

This two-part posting indulges those collective hopes with some lucid daydreaming about popular extrasolar destinations. Part 1 looks at systems with multiple stars, including circumbinary and conjoined systems. Part 2 looks at exomoons, tidally locked planets, and monoform worlds.

circumbinary systems

You can be sure you’re on an alien planet if you can see two suns in the sky. If you’re lucky you might even see three! (Figure 2) Ever since Star Wars, a multiplicity of host stars has defined mass cultural expectations for extrasolar planets. Thanks to George Lucas, objects in orbit around a pair of stars are now frequently known as Tatooine planets, not just among publicists but even in astronomical circles. The more technical modifier for such a planet is “circumbinary,” meaning that it orbits both members of the binary rather than just one.

Figure 2. Sunset over Trisol

The planet Trisol orbits a triple star system (types G, K, and M) located in the Forbidden Zone of the Galaxy of Terror. (Futurama, Season 1: episode “My Three Suns,” originally aired May 4, 1999)

In the full stellar population, the likelihood of stellar multiplicity increases with mass. As the least massive stars, M dwarfs tend to be single, while virtually all of the most massive stars – types O and B – occur in multiple systems. Binaries are the most common type of multiple star, with larger assemblies becoming progressively less frequent. Among star systems of all spectral types inside 10 parsecs, the RECONS survey reported 185 single stars (71%), 55 binary systems (21%), 15 triples (6%), 3 quadruples (1%), and 1 quintuple (less than 1%).

Because our Sun is well above the median stellar mass, 44% of the Sun-like stars (spectral types F6-K3) located inside 25 parsecs occur in multiple star systems (Raghavan et al. 2010). Within that volume, 33% of Sun-like stars occur in binaries, 8% in triple systems, and 3% in quadruple systems or higher. Raghavan & colleagues noted that higher-order multiples tend to be younger than binaries, suggesting that systems with three or more stars in their youth typically lose stars over time.

Even before the Kepler mission, many exoplanetary host stars had known binary companions. Among non-Kepler binaries with planets, the separation between the two stars ranges from about 20 AU (similar to the distance of Uranus from our Sun) to 10,000 AU (Raghavan et al. 2010). Until very recently, only one star in each planetic binary was known to host planets. Now we know of at least two conjoined systems in which each star hosts its own planetary system (XO-2 and WASP-94).

These days, however, discussions of binary host stars often center on circumbinary planets. Although theory predicted that very close binaries could jointly host a single planetary system, Kepler was the first telescope actually to detect such a configuration. These systems are rare: only nine have been identified to date (Table 1). Eight out of nine host single planets, which include puffy gas dwarfs, baby gas giants, and one Jupiter-sized giant (KOI-2939b). Except for the latter planet, the radii of these singletons range from 4 to 8.6 Rea, staking out a poorly populated area of the overall Kepler distribution. The remaining system (Kepler-47) hosts a family of three small planets, all of low density. Two-thirds of the parent binaries in these nine systems comprise a Sun-like star plus an M dwarf; one-third comprise a pair of Sun-like stars of similar mass. Habitable Earth-size planets might be possible in systems like these, but they are clearly more difficult to observe than larger planets, and they may also be less frequent. Notably, 40% of the known binary planets occupy their system habitable zones (Kostov 2015). One of them is KOI-2939b, which is also an excellent candidate for hosting large exomoons.

Table 1. Circumbinary systems as of April 2016

Tags: Msol = star mass in Solar units; period = period in days; a = semimajor axis in Earth units; e = orbital eccentricity (0 = circular); Rea = planet radius in Earth units; Mea = planet mass in Earth units. Distance = distance in parsecs

Theory and observation alike show that stable multiple star systems are built from binary units arranged in a strict hierarchy (Figure 3). Given the progressively increasing scarcity of a) triple or higher-order multiples of any spectral type, b) Earth-like planets, and c) circumbinary planetary systems, the following prediction seems safe: Habitable planets or moons orbiting all members of a multiple star system are much more likely to have binary hosts than hosts that are triple (single + double) or quadruple (double + double).

Figure 3. Orbital configurations of selected quadruple and quintuple star systems

These three systems are located within 25 parsecs. They illustrate the hierarchy of binary orbits that constitute higher-order multiple star systems. Based on Figures 23 and 24 of Raghavan et al. 2010.
wide binaries

Although I’m sensitive to their rarity and appeal, I’m not as fascinated by circumbinary planets as the mass media expect me to be. Planets in relatively wide binaries are not only vastly more common, but vastly more interesting – not least because of their science fictional possibilities.

At least two potential scenarios are available. The first involves climatic fluctuations induced by an eccentric binary orbit.

Imagine an inhabited planet whose parent star is an early K dwarf with a brighter, hotter binary companion of spectral type F7. The binary orbit is wide and extremely eccentric, and its period is longer than a thousand Earth years. When the two stars are at periastron (i.e., their closest approach, which would last a few decades) the planet heats up. For several years during periastron passage, the day/night cycle alternates between two seasons: One features hot, bright days with both stars visible in the sky, followed by dark nights with neither star. The other season sees endless light, with the K star shining by day and the F star turning the nights white. Then the two stars would separate, the white star would shrink and grow dimmer, and temperatures would cool again. Assuming that the planet has polar caps and oceans, at periastron the ice would shrink, sea levels would rise, and weather would become unusually volatile. Moving toward apastron (the two stars’ widest separation) the reverse would happen. The effects of this cycle on geology, climate, organisms, psychologies, and cultures would be momentous.

Figure 4. The Helliconia Trilogy

The overall impact would depend on the planet’s baseline climate near apastron, where it would spend most of its orbit. If the baseline is temperate, periastron might occasion fiery disasters, while if the baseline is glacial, periastron might herald a brief, benign epoch of abundance. Brian Aldiss has explored such an eccentricity-induced climate in his Helliconia Trilogy (1982-1985; Figure 4), and George Martin seems to be playing with similar ideas in his ongoing Song of Ice and Fire series (1996-?). However, as Martin has warned us, the irregular climate of Westeros has more to do with magic and dragons than with astrophysics (see also Kostov et al. 2013).

conjoined systems

The second binary scenario that floats my boat involves conjoined planetary systems. The separations between the two stars in such a configuration will be measured in astronomical units (AU) instead of parsecs. Their proximity will enable interstellar travel within a human lifetime through the use of propulsion technologies currently in development. Imagine a binary system comprising one G2 star like our Sun and a cooler G8 star separated by 150 AU. Each system has a different architecture, but both feature at least one inhabited planet. The inhabitants might be indigenous to each system, or indigenous to one and pioneers in the other, or colonists on both from a far-future version of Earth. Conjoined systems plus space travel will likely give rise to a federation of planets, rather like those envisioned in the space operas of the 1930s and 1940s and later updated by both iterations of Battlestar Galactica.

Figure 5. Detail of the Cyrannus quadruple system, home of the Twelve Colonies

Here I’m thinking of C.L. Moore’s tales of Northwest Smith, an outlaw rocketeer whose wanderings take him to Venus, Mars, and an unnamed moon of Jupiter, all of which have indigenous life. Moore’s vision of a densely inhabited Solar System, as presented in her fiction of the early 1930s, was resurrected a decade later in dozens of stories by her friend Leigh Brackett. The latter created a similar outlaw hero, Eric John Stark, an Earthling who was raised from infancy – much like Tarzan – by a tribe of hairy Mercurians. His adventures regularly bring him to Mars and Venus, which as in Moore’s stories are envisioned respectively as a decadent desert planet and a savage jungle planet.

The creators of Battlestar Galactica modernized this many-worlds setting by leaving our Solar System altogether and situating the human race in an extrasolar locale called the Twelve Colonies. These are twelve inhabited worlds in the conjoined planetary systems orbiting a quadruple star system. Google searches returned several maps of this remarkable construct; Figure 5 shows a detail of one. A similar approach was adopted by the people behind the short-lived but much-loved Firefly series when they created their wild extrasolar frontier. Known as the Verse, this locale appears to be a quintuple star system that seriously violates the laws of binary hierarchy (a characteristic shared with Serenity’s crew). Figure 6 shows the official map, which highlights inhabited exomoons and exoplanets.

Figure 6. The Verse, scene of the action in Firefly and Serenity

Next to these impossible wonders, WASP-94 and XO-2 offer thin gruel indeed. But at least they’re real. And the odds are that somewhere in our Galaxy, there’s at least one pair of conjoined systems with a habitable planet or moon between them.

Kostov V, Allan D, Hartman N, Guzewich S, Rogers J. (2013) Winter is Coming. Submitted for publication in Oldtown Journal of Evil Omens. Abstract: 2013arXiv1304.0445K
Kostov V, Orosz JA, Welsh WF, Doyle LR, Fabrycky DC, Haghighipour N, et al. (2015) KOI-2939b : the largest and longest-period Kepler transiting circumbinary planet. Astrophysical Journal, in press. Abstract: 2015arXiv151200189K
Raghavan D, McAlister HA, Henry TJ, Latham DW, Marcy GW, Mason BD, Gies DR, White RJ, ten Brummelaar TA. (2010) A survey of stellar families: Multiplicity of Solar-type stars. Astrophysical Journal Supplement Series 190, 1-42. Abstract: 2010ApJS..190....1R
Welsh WF, Orosz JA, Short DR, Cochran WD, Endl M, Brugamyer E, et al. (2015) Kepler 453 b – The 10th Kepler transiting circumbinary planet. Astrophysical Journal 809, 26. 2015ApJ...809...26W
Welsh WF, Orosz J, Quarles B, Haghighipour N. (2015) Kepler-47: A Three-Planet Circumbinary System. American Astronomical Society, ESS Meeting #3, id.402.01. Abstract: 2015ESS.....340201W

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