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A World Lit by Three Suns

Three Suns of Amara

Habitable planets in multiple star systems are one of science fiction’s great tropes. Find a second star somewhere in the daylight sky and you know you’re not in Kansas anymore. It makes slow going today, but as a kid I was struck with William F. Temple’s The Three Suns of Amara (1962), a story whose questionable science and creaky plot was somewhat mitigated by its striking imagery. For Amara managed to weave its orbit through a triple star system in which each star was a different color. Talk about great visual effects!

Image: My battered copy of Temple’s The Three Suns of Amara, rescued from a closet. Not the best novel I’ve ever read, but it did instill a lifelong fascination with habitable planets in multiple star systems.

Yesterday we looked at Elisa Quintana’s work on habitable planets in binary systems, and while reviewing for that story, I found Temple’s Amara again coming to mind. For it turns out we do have five confirmed exoplanets known to orbit one member of a triple star system. Quintana’s paper discusses the planet HD 188753 Ab, for example. It has a minimum mass 1.14 times that of Jupiter and orbits its primary (slightly over one solar mass) in a tight 3.35 day orbit.

Now get this: Around this star/planet duo orbits a short period binary star system (total mass 1.63 that of the Sun) that comes as close as 6 AU to HD 188753 A. Just how this system evolved its planets is a fascinating speculation, one that will receive much study as we learn more about it, especially as to the existence of other planets there. Amara it’s not, but the exoplanet around HD 188753 A has a lot to tell us. The tight passage of the binary pair offers us a unique laboratory for planet formation.

Three other exoplanet systems, though not triple, are intriguing because their stellar separations are well within the dimensions of our own Solar System. GJ 86 is a binary star with stellar separation somewhere around 20 AU, a situation paralleled by Gamma Cephei and HD 41004. So far we only know about giant planets in these systems, but as we saw yesterday, the separation between each of these stars does allow for planets in their habitable zones, making those SF-style speculations about day and night sky scenarios come inevitably to mind.

I should also mention that Gamma Cephei has been the object of recent study by Philippe Thebault and collaborators. Simulations building from small planetesimals up to Mars size worlds show favorable results, which is not to say that there are terrestrial worlds here but that we’re beginning to work out the parameters of how planets form in complex stellar environments involving more than one star. The paper is Thebault et al., “Planetary formation in the Gamma-Cephei system,” Astronomy & Astrophysics 427 (2004) pp. 1097-1104, available online.

Comments on this entry are closed.

  • roid January 24, 2007, 10:22

    if you wanna keep the dream alive, how about a habitable moon orbiting planet HD 188753 Ab? I’m trying to imagine the day/night cycle that the planet’s shadow would give.

    Just when you thought it was a 3 colour disco inferno already – along comes a strobe effect.

  • andy January 24, 2007, 11:44

    A habitable moon orbiting HD 188753 Ab? There are many reasons that isn’t a realistic possibility, not least because that planet is in a torch orbit with temperatures in excess of 1000 K. And tidal interactions would tend to destroy large moons around short period planets, so any surviving moons wouldn’t be much larger than a few tens of kilometres across.

  • Adam January 25, 2007, 17:13

    Hi Andy

    Wonder what the vapour pressure of silicates is at that temperature range? Could small moons ‘sublimate’ away into space?


  • andy February 23, 2007, 6:49

    HD 188753 Ab may not exist at all: No evidence of a hot Jupiter around HD 188753 A

  • Administrator February 23, 2007, 8:25

    Yes, their conclusion is striking: “From our failure to detect the presumed planet around HD 188753 A and from the available data on HD 188753, we conclude that there is currently no convincing evidence of a close-in giant planet around HD 188753 A.” Nice catch, Andy!

  • andy February 23, 2007, 9:33

    There’s also a response to this from Maciej Konacki, discoverer of HD 188753 Ab.

  • ljk March 29, 2007, 13:08

    News Release: 2007-036 March 29, 2007

    NASA Telescope Finds Planets Thrive Around Stellar Twins

    The double sunset that Luke Skywalker gazed upon in the film “Star Wars” might not be a fantasy.

    Astronomers using NASA’s Spitzer Space Telescope have observed that planetary systems – dusty disks of asteroids, comets and possibly planets – are at least as abundant in twin-star systems as they are in those, like our own, with only one star. Since more than half of all stars are twins, or binaries, the finding suggests the universe is packed with planets that have two suns. Sunsets on some of those worlds would resemble the ones on Luke Skywalker’s planet, Tatooine, where two fiery balls dip below the horizon one by one.

    “There appears to be no bias against having planetary system formation in binary systems,” said David Trilling of the University of Arizona, Tucson, lead author of a new paper about the research appearing in the April 1 issue of the Astrophysical Journal. “There could be countless planets out there with two or more suns.”

    Previously, astronomers knew that planets could form in exceptionally wide binary systems, in which stars are 1,000 times farther apart than the distance between Earth and the sun, or 1,000 astronomical units. Of the approximately 200 planets discovered so far outside our solar system, about 50 orbit one member of a wide stellar duo.

    The new Spitzer study focuses on binary stars that are a bit more snug, with separation distances between zero and 500 astronomical units. Until now, not much was known about whether the close proximity of stars like these might affect the growth of planets. Standard planet-hunting techniques generally don’t work well with these stars, but, in 2005, a NASA-funded astronomer found evidence for a planet candidate in one such multiple-star system


  • ljk July 18, 2007, 8:46

    Stability of Triple Star Systems with Highly Inclined Orbits

    Authors: S. A. Khodykin, A. I. Zakharov, W. L. Andersen

    (Submitted on 22 Feb 2003 (v1), last revised 17 Jul 2007 (this version, v2))

    Abstract: It is well established that certain detached eclipsing binary stars exhibit apsidal motions whose value is in disagreement with with calculated deviations from Keplerian motion based on tidal effects and the general theory of relativity. Although many theoretical senarios have been demonstrated to bring calculations into line with observations, all have seemed unlikely for various reasons. In particular, it has been established that the hypothesis of a third star in an orbit almost perpendicular to the orbital plane of the close binary system can explain the anomalous motion in at least some cases. The stability of triple star systems with highly inclined orbits has been in doubt, however.

    We have found conditions which allow the long term stability of such systems so that the third body hypothesis now seems a likely resolution of the apsidal motion problem. We apply our stability criteria to the cases of AS Cam and DI Her and recommend observations at the new Keck interferometer which should be able to directly observe the third bodies in these systems.

    Comments: edited to match published version

    Subjects: Astrophysics (astro-ph)

    Journal reference: Astrophys.J. 615 (2004) 506-511

    Report number: ENMU-03001

    Cite as: arXiv:astro-ph/0302464v2

    Submission history

    From: William Levi Andersen [view email]

    [v1] Sat, 22 Feb 2003 00:03:07 GMT (413kb)

    [v2] Tue, 17 Jul 2007 16:42:21 GMT (421kb)


  • ljk July 25, 2007, 0:07

    Spitzer Finds Evidence For Planets With Four Parents

    by Staff Writers

    Pasadena CA (JPL) Jul 25, 2007

    How many stars does it take to “raise” a planet? In our own solar system, it took only one – our sun. However, new research from NASA’s Spitzer Space Telescope shows that planets might sometimes form in systems with as many as four stars. Astronomers used Spitzer’s infrared vision to study a dusty disk that swirls around a pair of stars in the quadruple-star system HD 98800. Such disks are thought to give rise to planets.

    Instead of a smooth, continuous disk, the telescope detected gaps that could be caused by a unique gravitational relationship between the system’s four stars. Alternatively, the gaps could indicate planets have already begun to form, carving out lanes in the dust.

    “Planets are like cosmic vacuums. They clear up all the dirt that is in their path around the central stars,” said Elise Furlan, of the NASA Astrobiology Institute at the University of California at Los Angeles. Furlan is the lead author of a paper that has been accepted for publication in The Astrophysical Journal.

    HD 988000 is approximately 10-million years old, and is located 150 light-years away in the constellation TW Hydrae.

    Full article here:


  • ljk October 8, 2007, 17:23

    Planetary Stability Zones in Hierarchical Triple Star Systems

    Authors: P.E. Verrier (Cambridge), N.W. Evans (Cambridge)

    (Submitted on 5 Oct 2007)

    Abstract: A symplectic integrator algorithm suitable for hierarchical triple systems is formulated and tested. The positions of the stars are followed in hierarchical Jacobi coordinates, whilst the planets are referenced purely to their primary. The algorithm is fast, accurate and easily generalised to incorporate collisions. There are five distinct cases — circumtriple orbits, circumbinary orbits and circumstellar orbits around each of the stars in the hierarchical triple — which require a different formulation of the symplectic integration algorithm. As an application, a survey of the stability zones for planets in hierarchical triples is presented, with the case of a single planet orbiting the inner binary considered in detail. Fits to the inner and outer edges of the stability zone are computed. Considering the hierarchical triple as two decoupled binary systems, the earlier work of Holman & Wiegert on binaries is shown to be applicable to triples, except in the cases of high eccentricities and close or massive stars. Application to triple stars with good data in the multiple star catalogue suggests that more than 50 per cent are unable to support circumbinary planets, as the stable zone is almost non-existent.

    Comments: 16 pages, MNRAS, in press

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0710.1167v1 [astro-ph]

    Submission history

    From: N. W. Evans [view email]

    [v1] Fri, 5 Oct 2007 13:31:38 GMT (256kb)


  • Roman September 2, 2008, 4:57

    Hallo people! I am very interested in Alpha Centauri and especially a fictional planets around it. I saw the cover of sci-fi – Three suns of Amara. Is it possible to obtain this book in paper or electronical form? Please mail to: vida.roman@gmail.com (Excuse me for my English, I am Slovak)

  • Administrator September 2, 2008, 8:25

    Roman, The Three Suns of Amara is now out of print, but I occasionally see it in used bookstores. If I run across a copy, I’ll be glad to pass it along. You might also want to check eBay in case it turns up there, and see if any of the Amazon booksellers have it available. I just checked and there do seem to be copies at a low price: