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Searching for a Double Sunrise

Watching two suns over Tatooine’s sky in the original Star Wars movie was a breathtaking experience, particularly given where most science fiction films were at the time. Here was an attempt to convey a truly alien landscape. But a second thought quickly came unbidden. Was this planet not in an extremely unstable orbit, moving around both stars simultaneously in an obvious habitable zone? The suspicion was that a planet could orbit one or the other members of a binary system, but surely not both unless its orbit were extended so far out into the planetary nether regions as to make life doubtful.

Two suns as seen from Tatooine

Image: The twin suns of Tatooine. Are planetary orbits like this possible? Credit: © Lucasfilm Ltd. & TM. All Rights Reserved.

That was back in the 1970s, of course, but take a look at the situation today. The ‘hot Jupiter’ in the triple system HD 188753 is interesting, but the planet in question orbits but one of the stars. The early discussion of HD 188753 Ab was quick to raise the Tatooine parallel, which was first suggested by Caltech scientist Maciej Konacki. But at 0.04 AU, this gas giant hugs its G-class star, with the other two stars orbiting each other and also — at Saturn-like distance — the same star orbited by the planet.

We do have one known exoplanet that orbits twin stars, but PSR B1620-26 is not your average stellar system, consisting of a pulsar and a white dwarf. Even so, twenty percent of known exoplanets are in multiple systems, and it’s interesting to speculate on whether a planet with a double sunrise like that of Tatooine might exist. Radial velocity techniques avoid short-period binaries of the sort that might make this possible, and the wide orbits of such planets would likely make transit detections quite difficult. But Cheongho Han (Chungbuk National University, Korea) is now arguing that microlensing techniques might work here.

From the paper:

The general geometry of a planet revolving around the stars of a close binary is such that the separation between the stars is much smaller than the star-planet separation… Under this geometry, the lensing behavior of the triple lens system can be greatly simplified because the close stellar binary pair and the planet can be separately treated.

Cheongho Han argues that a planet like this would be involved in perturbations at a common region around the center of mass of the binary stars, creating a detectible microlensing signature. Despite the extreme difficulty of the task, such analysis of microlensing data has already begun. Multiple telescopes at different locations, allowing continuous coverage of microlensing events, should be able to detect a planet that fits this description. Given the difficulty of radial velocity and transit methods for this work, it may be that microlensing will be the method of choice to find a planet that, like Tatooine, would experience multiple sunrises. It would be a view to be savored, though not, one suspects, one that living beings would be around to observe.

The paper is Cheongho Han, “Microlensing Search for Planets with Two Simultaneously Rising Suns,” a draft of which is available here.

Comments on this entry are closed.

  • ljk February 5, 2008, 15:02

    I am waiting for the day we find a system where the suns circle
    a gas giant world.

    Imagine any intelligent beings in that system accepting the
    heliocentric theory for ages, with a few radicals daring to
    say that the suns go round the world!

  • Jason Adams February 5, 2008, 17:57

    This reminds me of Kevin Anderson’s Saga of Seven Suns, where the planet is in a system with — you guessed it — seven suns. The inhabitants are understandably afraid of the dark, since at any given time, at least one of the suns is in the sky.

  • dad2059 February 5, 2008, 18:09

    ljk: Would that even be possible? Not naturally occurring anyway according to present theory.

    But the Universe is a mighty big place.

  • Dave Moore February 5, 2008, 18:19

    I always thought the Tatooine scenario was possible. The two sun’s angular diameter is approximately that of the sun at 1 au, so if the the two suns were mid G dwarfs with approximately one half solar luminosity, then the planet would be recieving the right isolation for 1 au.
    Assuming we are seeing the two suns at near their maximum separation, then they would be orbiting about 3 million miles apart, which would allow for stable orbits around their common center from 15 million miles out.
    The only part of the picture that isn’t correct is that 2 stars 3 million miles apart would distort each other into obvious oblate speriods.


  • Christopher L. Bennett February 5, 2008, 18:22

    “I am waiting for the day we find a system where the suns circle
    a gas giant world.”

    Couldn’t happen. When two bodies orbit each other, they’re actually both orbiting a common center of mass. If one body is much more massive than the other, then the center of mass will be inside it and you can say that it’s being orbited by the other body. So the more massive body is always the one being orbited. And a star is always going to be more massive than a gas giant — because if the gas giant were massive enough, it would undergo fusion and become a star, or at least a brown dwarf. So you’ll never find a star orbiting a gas giant, only the other way around.

  • andy February 5, 2008, 18:57

    Such a planet may have already been detected: this paper gives evidence (from variations in the timing of eclipses) that a jovian planet may be in orbit around the eclipsing binary CM Draconis, though at present the candidate is poorly constrained due to incomplete orbit coverage.

    There’s also the HD 202206 system, which contains a >2.44 Jupiter mass planet exterior to a >17.4 Jupiter mass companion, a system which might be regarded as a sunlike star+brown dwarf binary with a circumbinary planet, or a superplanet and planet in orbit around the sunlike star.

    (Plus, the HD 188753 planet may not exist, see this paper for details. I would add that playing around with the RV data for this system on systemic also suggests the postulated planet in this system doesn’t exist – though alternative solutions to the data are possible)

  • James M. Essig February 5, 2008, 21:17

    Hi Folks;

    The double sun rise configuration might indeed make many would be planetary configurations unstable. However, a single star with planets the mass of Jupiter or Saturn or larger with Earth like moons would be very cool. How about the concept of gas giant planets with Earth-like moons which inturn have smaller sattelites in orbit around them.



  • ljk February 5, 2008, 23:49

    Jason Adams – the Kevin Anderson’s Saga of Seven Suns
    you mention reminds me of Isaac Asimov’s famous story,
    Nightfall, about a society living on a planet with six suns
    where there is never night – except once every 2,049 years,
    when they suddenly discover that their universe is much
    bigger than they ever imagined….

    As for the idea of a star going around a giant planet at the
    center, well, I still think it is a neat idea, and you never know
    what a really advanced civilization might want to do with the
    star systems some day.

  • Adam February 6, 2008, 2:14

    Hi All

    Jim, Moons of Moons are inherently unstable and either crash or escape eventually. But there can always be co-orbitals, like several of Saturn’s moons.

    Larry, you’ve raised that idea before and I’ve dumped on the idea before – as Chris pointed out a planet is always lighter than its star, so it’s unlikely we’ll see stars orbitting planets. Stars – not counting brown dwarfs – have masses above a certain limit, while planets are unlikely to get that high and remain planets. Gas giants are made of fusible elements – hydrogen, helium etc. – and thus will become stars when too big.

    The only option is for a gas giant to be orbitted by an artificial star – which would be a very BIG find indeed.

  • Adam Rosenblatt February 6, 2008, 4:13

    Yes. Break the speed of light record and we’re there.

    Speed of light…..has to be conquered

  • James M. Essig February 6, 2008, 11:32

    Hi Adam;

    It is interesting to note, although a bit obvious, that there have been discovered a total of scores of moons in our solar system, and several big ones larger than the Earth’s moon. Perhaps a multiplicity of moons is commonplace around other planetary star systems. With lots of moons potentially the size of Earth’s moon or larger, the prospects for finding extrasolar homes for humanity is amazing. Obviously, some of these moon’s might have beautiful ecosystems. I remember the images of the moon(s) in the original Star Wars Trilogy motion pictures series especially the one that contained trees that seemed to drawf in size almost all of the trees on Earth except perhaps the Giant Red Wood trees along in the West coasts states of the U.S.. In short, the potential beauty of the ecosystems of such would be moon’s becons me to say in my mind over and over again to the current administration and the next several administrations, the phrase “Interstellar Travel! Interstellar Travel! Interstellar Travel!”. To bad they can’t hear me. It should be a real interesting next few decades as many national juristdictions reach out to our moon again including the U.S., ESA, China, Russia, etc.


    Your Friend Jim

  • george scaglione February 6, 2008, 16:56

    jim,yes sir! all of those moons might prove to be a great boone to us indeed once we got off our desk chairs and head out there to have a better look at our solar system.there are those even at nasa who feel we first have to do that before the stars.and i think they are probably very close to the truth of what will happen.lol though it seems that there are only a couple of dozen of us who are in a hurry!! no kidding everybody,your friend george ps all – there are just so many ideas for star ship propulsion systems around ! wish everybody would state their opinions in that area also.i’d be happy to see that. ;) g

  • ljk February 6, 2008, 23:22

    Experimental design and model selection: The example of exoplanet detection

    Authors: Vijay Balasubramanian, Klaus Larjo, Ravi Sheth

    (Submitted on 4 Feb 2008)

    Abstract: We apply the Minimum Description Length model selection approach to the detection of extra-solar planets, and use this example to show how specification of the experimental design affects the prior distribution on the model parameter space and hence the posterior likelihood which, in turn, determines which model is regarded as most `correct’.

    Our analysis shows how conditioning on the experimental design can render a non-compact parameter space effectively compact, so that the MDL model selection problem becomes well-defined.

    Comments: 12 pages, 2 figures; To appear in the Festschrift for Jorma Rissanen; UPR-1187

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.0498v1 [astro-ph]

    Submission history

    From: Klaus Larjo [view email]

    [v1] Mon, 4 Feb 2008 21:01:05 GMT (27kb)


  • ljk February 8, 2008, 10:39

    Planetesimal and gas dynamics in binaries

    Authors: S.-J. Paardekooper, P. Thebault, G. Mellema

    (Submitted on 7 Feb 2008)

    Abstract: Observations of extrasolar planets reveal that planets can be found in close binary systems, where the semi-major axis of the binary orbit is less than 20 AU. The existence of these planets challenges planet formation theory, because the strong gravitational perturbations due to the companion increase encounter velocities between planetesimals and make it difficult for them to grow through accreting collisions. We study planetesimal encounter velocities in binary systems, where the planetesimals are embedded in a circumprimary gas disc that is allowed to evolve under influence of the gravitational perturbations of the companion star. We find that the encounter velocities between planetesimals of different size strongly depend on the gas disc eccentricity. In all cases studied, inclusion of the full gas dynamics increases the encounter velocity compared to the case of a static, circular gas disc. Full numerical parameter exploration is still impossible, but we derive analytical formulae to estimate encounter velocities between bodies of different sizes given the gas disc eccentricity. The gas dynamical evolution of a protoplanetary disc in a binary system tends to make planetesimal accretion even more difficult than in a static, axisymmetric gas disc.

    Comments: 18 pages, 13 figures, accepted for publication in MNRAS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.0927v1 [astro-ph]

    Submission history

    From: Sijme-Jan Paardekooper [view email]

    [v1] Thu, 7 Feb 2008 09:19:41 GMT (333kb)


  • ljk April 1, 2008, 9:27

    Two Yellow Supergiant Eclipsing Binary Systems Discovered:
    First Of Their Kind Ever Found

    ScienceDaily (Apr. 1, 2008) — Astronomers have spied a faraway star system that is so unusual, it was one of a kind — until its discovery helped them pinpoint a second one that was much closer to home.

    In a paper published in a recent issue of the Astrophysical Journal Letters, Ohio State University astronomers and their colleagues suggest that these star systems are the progenitors of a rare type of supernova.

    They discovered the first star system 13 million light years away, tucked inside Holmberg IX, a small galaxy that is orbiting the larger galaxy M81. They studied it between January and October 2007 with the Large Binocular Telescope (LBT) on Mt. Graham in Arizona.

    Full article here:


  • ljk May 12, 2008, 13:19

    Planet Formation in Binary Stars: The case of Gamma Cephei

    Authors: Wilhelm Kley (1), Richard Nelson (2) ((1) University of Tuebingen, (2) University of London)

    (Submitted on 9 May 2008)

    Abstract: Over 30 planetary systems have been discovered to reside in binary stars. For small separations gravitational perturbation of the secondary star has a strong influence on the planet formation process. It truncates the protoplanetary disk, may shortens its lifetime, and stirs up the embedded planetesimals. Due to its small semi-major axis (18.5 AU) and large eccentricity (e=0.35) the binary $\gamma$ Cephei represents a particularly challenging example.

    In the present study we model the orbital evolution and growth of embedded protoplanetary cores of about 30 earth masses in the putative protoplanetary disk surrounding the primary star in the $\gamma$ Cep system. We assume coplanarity of the disk, binary and planet and perform two-dimensional hydrodynamic simulations of embedded cores in a protoplanetary disk. The presence of the eccentric secondary star perturbs the disk periodically and generates strong spiral arms at periapse which propagate toward the disk centre. The disk also becomes slightly eccentric (with e_d = 0.1-0.15), and displays a slow retrograde precession in the inertial frame. For all initial separations (2.5 to 3.5 AU) we find inward migration of the cores. For initial semi-major axes (a_p \gsim 2.7), we find a strong increase in the planetary eccentricity despite the presence of inward migration. Only cores which are initially far from the disk outer edge have a bounded orbital eccentricity which converges, roughly to the value of the planet observed in the $\gamma$ Cep system.

    We have shown that under the condition protoplanetary cores can form at around 2.5 AU, it is possible to evolve and grow such a core to form a planet with final outcome similar to that observed.

    Comments: 12 pages, 17 figures, accepted by Astronomy & Astrophysics

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.1354v1 [astro-ph]

    Submission history

    From: Willy Kley [view email]

    [v1] Fri, 9 May 2008 14:21:50 GMT (554kb)


  • ljk June 5, 2008, 6:42

    Integral reveals exotic and dusty binary systems

    ESA’s orbiting gamma-ray observatory, Integral has revealed
    a new population of exotic and dusty binary stars which might
    represent a brief evolutionary period in a binary star’s life.
    The findings bring to light a gap in our knowledge of the
    formation and evolution of such binary star systems.

    More at:


  • ljk June 24, 2008, 11:02

    Surprising dissimilarities in a newly formed pair of ‘identical twin’ stars

    Authors: Keivan G. Stassun, Robert D. Mathieu, Phillip A. Cargile, Alicia N. Aarnio, Eric Stempels, Aaron Geller

    (Submitted on 18 Jun 2008)

    Abstract: The mass and chemical composition of a star are the primary determinants of its basic physical properties–radius, temperature, luminosity–and how those properties evolve with time. Thus, two stars born at the same time, from the same natal material, and with the same mass are ‘identical twins,’ and as such might be expected to possess identical physical attributes. We have discovered in the Orion Nebula a pair of stellar twins in a newborn binary star system. Each star in the binary has a mass of 0.41 +/- 0.01 solar masses, identical to within 2 percent.

    Here we report that these twin stars have surface temperatures that differ by ~300K (~10%), and luminosities that differ by ~50%, both at high confidence level. Preliminary results indicate that the stars’ radii also differ, by 5-10%. These surprising dissimilarities suggest that one of the twins may have been delayed by several hundred thousand years in its formation relative to its sibling. Such a delay could only have been detected in a very young, definitively equal-mass binary system3 such as that reported here. Our findings reveal cosmic limits on the age synchronisation of young binary stars, often used as tests for the age calibrations of star-formation models.

    Comments: Published in Nature, 19 June 2008

    Subjects: Astrophysics (astro-ph)

    DOI: 10.1038/nature07069

    Cite as: arXiv:0806.3089v1 [astro-ph]

    Submission history

    From: Keivan G. Stassun [view email]

    [v1] Wed, 18 Jun 2008 20:36:02 GMT (1209kb)


  • ljk October 8, 2012, 11:09

    Alien Landscapes

    i Excellent

    What would we see if we stood on an alien world?

    As the word ‘alien‘ originally meant ‘foreigner, stranger, one who is not a naturalized citizen of the country in which they are living,‘ we naturally expect everything alien to be different, and perhaps it will be, but the laws of physics are universal so we should expect some similarities. The classic sci-fi interpretation is often as though a blue/purple/black filter (or blood red filter) had been placed over the lens of a camera just for the sake of it. And although there is some scientific basis to this assumption, as different stars will emit different intensities of radiation, it is an overly simplistic way of coming up with something ‘different‘ for the sake of being different. We have black plants here on Earth, though, making them not-quite-as-alien as we thought.

    The danger is that science fiction may be trying too hard to come up with something radically different just so it seems alien, with alien being synonymous for jaw-dropping and unexpected. Reality may be stranger than fiction in that the laws of physics and chemistry are what determine biology, and as the laws of physics and chemistry are universal, astrobiology too may be much more similar than we’d expect.

    Theoretically, life could arise in a multitude of places we wouldn’t anticipate, using chemistry we haven’t seen exploited on Earth (like silicon-based lifeforms), but as a university professor recently pointed out to me, the odds on favorite has to be carbon from a purely chemical perspective. Carbon is the most versatile atom for forming complex molecules.

    So what would alien landscapes look like? Well, they’re going to be bound by the same laws of physics and chemistry we have on Earth, although probably in different proportions, as it’s unlikely there will be the same gravitational strength, or atmospheric mix, etc.

    I’d venture to say with the astounding diversity of environments we see on Earth, there’s enough versatility to give us a good idea about alien landscapes.