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Planet in a Quadruple Star System

Planets in multiple star systems intrigue us particularly when we try to imagine the view from the surface. Call it the ‘Tatooine Effect,’ made to order for visual effects specialists and cinematographers. But planets like these also raise interesting issues. Lewis Roberts (JPL) and colleagues have just published a new study of the 30 Ari system, demonstrating that it is a quadruple star system with a gas giant of about four times the mass of Jupiter in a 335 day orbit around its primary star.

We already knew about the planet in the 30 Ari system. What’s new is the discovery of the additional star. At 23 AU from the planet, the newly discovered fourth star would seem to be a factor in the orbital dynamics of the gas giant, but just what effects it has remain to be studied. The paper, which also reports the detection of a stellar companion to the exoplanet host system HD 2638, notes that 30 Ari is the second quadruple system known to host an exoplanet. And interestingly, both HD 2638 BC and 30 Ari BC have projected separations of less than 30 AU, so that the stellar companions may play a key role in the evolution of the exoplanets’ orbits.

multiple_system

Image: A gas giant orbiting a binary star. How planets interact with their primary and other stars in multiple-star systems like these is a question that will demand orbital computations over a long span of observation. Credit: NASA, E. Schwamb.

According to this news release from the University of Hawaii’s Institute for Astronomy, the view from the surface of the 30 Ari planet (or, let’s say, a moon around it) would involve the primary star and two other stars bright enough to be visible in daylight. One of the bright ‘stars’ would actually be a binary system if examined in a telescope. The other known planet in a quadruple system is Ph1b in a system designated KIC 4862625, from which a different view would emerge. Ph1b is on a circumbinary orbit, a giant planet of between 20 and 55 Earth masses orbiting an eclipsing binary made up of a G- and an M-class dwarf, with a second binary star at a distance of 1000 AU.

While quadruple star systems are somewhat unusual (Andrei Tokovinin of the Cerro Tololo Inter-American Observatory in Chile.estimates that about four percent of solar-type stars are in quadruple configurations), the scant number of planets we’ve found in such systems may be the result of how we observe. The Roberts paper, published in The Astronomical Journal makes the point, and notes the inherent observational difficulties:

Known close visual binaries are traditionally excluded from radial-velocity (RV) exoplanet programs because the presence of a visual companion degrades the RV precision. This intrinsic bias complicates statistical inferences about exoplanets in binaries. Moreover, a faint visual companion that is itself a close spectroscopic binary pair can produce periodic low-amplitude RV modulation in the combined light that can be mistaken for an exoplanet. False positive exoplanet detections caused by unrecognized hierarchical multiplicity of their hosts may reach 1–2% (Tokovinin 2014b) of the total exoplanet sample. This is yet another reason to observe exo-hosts with high angular resolution and deep dynamical range.

But despite their difficulties, the fact that exoplanets and the stars they orbit have a common origin means that the more we learn about the multiple star system in question, the more we learn about the exoplanet as well. Just what effects do multiple stars in their various configurations have on the planets around them? One possibility is that gravitational nudges from these nearby stars may affect the protoplanetary disk, producing massive planets on eccentric orbits as the disk is disrupted. To learn more, we need to increase the number of observed binary systems, especially those where the binary separation is small.

Thus far the pattern that has emerged is that the frequency of exoplanets among single stars is roughly the same as that around the components of wide binaries, the latter defined as those having a semimajor axis greater than 100 AU. The evidence suggests that wider binaries have little impact on exoplanet orbital dynamics. But when we get to binaries with separations of less than 100 AU, we find fewer exoplanets but in general more massive ones. No planet has yet been detected in a stellar binary with a separation of less than 10 AU. It’s worth keeping in mind that Centauri A and B, our nearest neighbors, close to within 11 AU at their closest approach.

Looking around for fictional descriptions of multiple star systems as viewed from one of their planets, I come back to a book I mentioned not long after this site began, Stanton A. Coblentz’s Under the Triple Suns (Fantasy Press, 1955). Here’s what Coblentz imagined some sixty years ago, a bit of fun to end this post:

thumb-under_the_triple_suns

The red sun glowed high in the copper heavens. It was as wide as a dozen moons, and of the color and brightness of smoldering embers; and it did not end sharply as a disk should, but terminated in a nebulous crimson fringe. It shed its rays like a dying fire over a great sweep of wooded, partly hilly country, terminated in the distance by saw-toothed mountains, and marked at closer range by the loop of a cascading river and the oval of a lake, and by a cluster of shimmering beehive structures that billowed and fluttered in the breeze.

After a time, above the serrate edges of the far-off ranges, a white illumination began to spread; and the mist-banks about the peaks, ruddy before, took on a sheet-like glare as a globe that seemed of a hand’s width slowly swam into sight. Although much smaller than the red sun, it dominated the scene by its intense hot flame.

The white orb was about fifteen degrees above the horizon when another light began to emerge. Of an almost unbearable brilliance, it looked not much larger than a silver dollar; but its companions seemed almost pale beside its terrible sea-blue incandescence. Evidently the blue sun and the white belonged together, like the earth and the moon; and the three luminaries, along with a Saturn-like ringed fourth that had no fire of its own but glowed red, white or blue according to the influence of the moment, circled with a gradual movement from west to east.

The paper is Roberts et al., “Know the Star, Know the Planet. III. Discovery of Late-Type Companions to Two Exoplanet Host Stars,” The Astronomical Journal Vol. 149, No. 4 (2015), 118 (abstract / preprint).

Comments on this entry are closed.

  • Andrew LePage March 5, 2015, 15:16

    “It’s worth keeping in mind that Centauri A and B, our nearest neighbors, close to within 11 AU at their closest approach.”

    So far, with the exception of the still-unconfirmed Alpha Centauri Bb (which I discussed four months ago in Centuari Dreams), there have been no planets detected orbiting either of these stars. However, all that has been excluded with any degree of confidence are Saturn to Jupiter-size bodies and larger. The jury is still out on the presence of smaller planets. A more detailed discussion can be found here:

    http://www.drewexmachina.com/2014/08/11/the-search-for-planets-around-alpha-centauri/

    http://www.drewexmachina.com/2014/09/25/the-search-for-planets-around-alpha-centauri-ii/

    Similarly, nothing has been found orbiting the third star in this system, Proxima Centauri, some 15,000 AU away from the pair of Sun-like stars. But once again, all that has been definitively excluded are Saturn to Jupiter-size planets or larger.

    http://www.drewexmachina.com/2015/02/23/the-search-for-planets-around-proxima-centauri/

  • Ashley Baldwin March 5, 2015, 17:31

    Very interesting set up here. The planet circling 30 Ari B is at about an AU out . The star is an F6 , so even if the gas giant planet has a moon it will be too close to the star to be in any HBZ zone even if it has a large mass . ( shame, if it was a bit further out it could still still have a stable orbit yet be in the HBZ for any solid terrestrial body, or if the star was a bit dimmer/smaller too) .
    Large planetary mass can be one of many features of multiple stellar systems like this . If you could stand on the surface of such a moon you would see one sun and two bright stars , visible in daylight and one of which could be separated into a pair with a telescope. The planet itself doesn’t seem to extensively perturbed by the presence of its parent stars binary just 22 AU or so away. Ironically not enormously different from the Alpha Centauri pair. Centauri dreams has published on how near to the parent star any planet would need to be to avoid gravitational influence of its orbit in that system with a figure around 2 AU or so being the consensus. We’ve talked about alien life, but what a view you would have sat on the solid moon of a ten Jupiter mass gas giant, 1 AU from an F6 star and 1400 from two other stars ! That’s some vista.

  • Andrew_W March 5, 2015, 19:34

    What, No mention of Asimov’s short story The Nightfall Effect?

    About a planet in a multiple star system whose inhabitants only experience night rarely when their planet is thrown into darkness through an eclipse, where upon they all go mad burning their civilization to the ground in order to get some illumination.

    A story written to show the opposite effect depicted in the Ralf Waldo Emerson quote:
    “If the stars should appear one night in a thousand years, how would men believe and adore; and preserve for many generations the remembrance of the city of God which had been shown.”

  • Andrew_W March 5, 2015, 19:38

    Correction: the story is called “Nightfall”, Asimov described “the nightfall effect” as people going mad at the sight of – if not stars – the advocating of people doing stuff in space.

  • ljk March 6, 2015, 10:08

    The people of the planet Lagash in Asimov’s story didn’t just go crazy from the darkness which they were not used to (six suns in the sky will keep things pretty bright all the time, except for once every 2,049 years when five of the suns are on one side of the planet and the last star is eclipsed by an unknown moon at the same time), but the discovery during the darkness that they were in a star cluster of thousands of suns.

    Before then a few of their astronomers speculated there might be a few other suns besides their own six, but that was radical talk equivalent to when Copernicus (and Aristarchus of Samos long before him) proposed that Earth went around Sol rather than everyone orbiting our world.

    Ironically yesterday was the anniversary of when the Vatican’s Sacred Congregation of the Index added Nicolaus Copernicus’s “De revolutionibus orbium coelestium” (“On the Revolutions of the Heavenly Spheres”), first published in 1543, to its list of banned books in 1616.

    If you want an amusing but very informative and well done story on how we first began to grasp the true nature of the Universe around us, check out this site:

    http://tofspot.blogspot.com/2013/08/the-great-ptolemaic-smackdown.html

    If you want to read Nightfall (the story, not the subsequent expanded novel from 1990 co-authored with Robert Silverberg, which is fine in its own right), see here:

    http://web.ics.purdue.edu/~rebeccal/lit/238f11/pdfs/Nightfall_Asimov.pdf

    Good article on the story and its underlying subject:

    http://www.theguardian.com/books/2012/dec/19/darkness-nightfall-isaac-asimov

  • Mark Zambelli March 6, 2015, 10:40

    Fictional Suns seen from the surface…
    Stanislav Lem’s ‘Solaris’ featured a red/blue binary. My wife reminded me that ‘The Dark Crystal’ featured a multiple star system coming into alignment. An absolutely awesome visualization of being illuminated by the tints of different suns is in the movie ‘Pitch Black’, a favourite of mine. The films depiction is well worth seeing although I can’t help but feel as if the writers based their film on the short Niven story ‘Flare Time’ in the ‘Limits’ collection… not only for the multiple system but also for the creature-antagonists. But, as I can’t recollect just how Harlan Ellison set up Medea, it’s lifeforms and system, I unable to recall how much came from Larry Niven in the ‘Flare Time’ story. Excellent depictions of living under more than one sun anyway.

  • Joe Wooten March 6, 2015, 10:52

    I always like Poul Anderson’s novel Fire Time about a planet in a binary system and the effects on it of the close approach of the red dwarf companion star.

  • Paul Gilster March 6, 2015, 12:48

    Joe, thanks so much. I’m delighted to be reminded of a Poul Anderson work that I haven’t read. Something to look forward to indeed!

  • andy March 6, 2015, 13:08

    At 23 AU from the planet, the newly discovered fourth star would seem to be a factor in the orbital dynamics of the gas giant, but just what effects it has remain to be studied.

    One thing to watch out for: the separation given is the projected separation, the true separation depends also on the (unknown) relative distance along the line-of-sight.

  • Etienne March 6, 2015, 13:33

    Fire time! Yes, definitely worth reading, a great Poul Anderson novel! Enjoy it

  • Paul Gilster March 6, 2015, 13:40

    andy writes:

    One thing to watch out for: the separation given is the projected separation, the true separation depends also on the (unknown) relative distance along the line-of-sight.

    Excellent point! Thanks.

  • Andrew LePage March 6, 2015, 14:24

    @andy March 6, 2015 at 13:08

    Very true – the apparent separation of 22.3 AU is the minimum current distance between 30 Ari B and C since it does not take into account the unknown distance component along our line of sight. However, Robertson et al. mention in their paper that the statistical or most likely period of this binary is about 80 years and that the actual period is probably within a factor of three of this value. This implies that the semi-major axis of the orbit of 30 Ari BC is probably within a factor of two of the 22.3 AU apparent separation. Fortunately, the orbital period is probably short enough to allow the orbital properties of this pair to be better constrained with just a decade or two of additional observation.

  • Joe Wooten March 6, 2015, 14:26

    You’re welcome Paul.
    I went and looked at it quickly again, and I was wrong, it was a triple star system…….It’s been 10 years since I read it last.

  • stephen March 6, 2015, 16:16

    Medea: Harlan’s World, was a planet orbiting one of the components of the Castor system. About a dozen different major SF authors got together and collaborated on different aspects of the system, its planets, life, the sentients, the anthropology, etc.

    I wish they’d mentioned the petroleum resources so they could discuss Castor Oil…

    Anyway, I was wondering, under what circumstances could a star gravitationally capture another star? And could a binary system capture another star or another binary star, so it becomes a triple or quadruple system? Has anybody worked that out?

  • Andrew LePage March 7, 2015, 11:46

    The very brief, qualitative description of how the sky would appear from 30 Ari Bb with four suns in the sky that was given in the University of Hawaii press release disappointed me a bit because of its lack of detail. The physicist in me just could not resist doing some calculations on my own given what is known about these stars. I wish to share the results of what I found with the group:

    http://www.drewexmachina.com/2015/03/07/a-sky-with-quadruple-suns/

  • andy March 7, 2015, 13:20

    The other thing to watch out for is that we only have a minimum mass for the planetary candidate 30 Arietis Bb. That minimum mass is around 10 Jupiter masses, so it is not out of the question that it might be a brown dwarf or a low-mass star. According to Reffert & Quirrenbach (2011), the Hipparcos astrometric data puts a 3σ upper limit of 162.2 Jupiter masses, so while the possibility that it is a stellar object is geometrically improbable, it has not yet been ruled out.

  • Andrew LePage March 7, 2015, 18:12

    @andy March 7, 2015 at 13:20

    Given a random orientation of the orbit of 30 Ari Bb, there are about even odds that its actual mass exceeds 12 Jupiter masses to be a brown dwarf and less than a 1% chance it is a small star. With an apparent separation of only 0.025 arc seconds, we won’t get any pictures of 30 Ari Bb anytime soon but Gaia astrometric measurements should be able to measure its mass and settle the question.

  • Mark Zambelli March 8, 2015, 13:11

    Andrew LePage,
    Thanks for the involved view with those magnitudes included… you paint a much more accurate and vivid picture.

  • andy March 8, 2015, 17:27

    @Andrew LaPage: the low chance from purely geometric concerns were why I mentioned it is geometrically improbable. The occurrence rate of objects as a function of mass in the regime concerned appears to be rather non-uniform (the brown dwarf desert), so I didn’t post the numbers. Nevertheless despite the low probabilities from geometric considerations, there have already been a couple of examples of massive RV exoplanet candidates turning out to be low-mass stars, e.g. the case of HD 33636 where the m*sin(i)=9.3 Jupiter masses planet candidate turned out to be a 0.14 solar mass star.

    From what I can tell, 30 Ari B is faint enough for Gaia to observe, so yes hopefully there will be some results for this system. Not sure if there’s any hope of figuring out the 30 Ari A binary properties: from what I can tell the inclination and mass of 30 Ari Ab are also unknown but with an orbital period of only 1.1 days, the separation between the two stars is going to be very small.