≡ Menu

A Planet Each for Stars in Binary System

The WASP Consortium (Wide Angle Search for Planets) has come up with an interesting find: Two new Jupiter-class worlds, one around each star in a binary star system. Both are ‘hot Jupiters,’ a class of planets that is susceptible to discovery by transit methods, as in this case, and radial velocity as well. Consisting of two robotic observatories, one on La Palma (Canary Islands) and the other in South Africa, WASP has a proven track record, having found over 100 planets since 2006. WASP-94A and WASP-94B, like all WASP candidates, were confirmed by radial velocity techniques through a collaboration with the Geneva Observatory.

The two stars are about 600 light years away in the constellation Microscopium. In this case, it was the WASP-South survey team that noticed dips in the light of WASP-94A, the mark of a likely hot Jupiter, with WASP-94B being found by the Geneva team during the confirmation process for the first planet. “We observed the other star by accident, and then found a planet around that one also!”, says Marion Neveu-VanMalle (Geneva Observatory), lead author of the paper on this work.


Image: A WASP planet transiting its host star. Credit: Mark Garlick.

We’ve sometimes speculated in these pages about a close binary system like Centauri A and B, wondering whether there could be planets around each star, a matter that remains undecided. WASP-94A and WASP-94B are in a much different situation — the estimated separation between the two is 2700 AU. The paper lists three other binary systems with pairs of planets. Like WASP-94, HD20782/HD20781 is a wide binary, with HD20782 hosting a Jupiter-mass planet and HD20781 two Neptune-class worlds.

We also have Kepler-132, an interesting system hosting three ‘super-Earths,’ with an angular separation too small to allow us to tell which of the two stars the planets are transiting, although researchers believe that the two planets with the shortest periods cannot be orbiting the same star. Finally, there is XO-2, another wide binary in which one star hosts a transiting hot Jupiter, while the other hosts two gas giants, one Jupiter-class, the other the mass of Saturn.

We may learn something interesting about the formation of hot Jupiters from the WASP-94 system. Planets like these should form far enough from their primary to allow ices to freeze out of the protoplanetary disk, while being later forced, presumably through interactions with another star or planet, into the inner system. The paper comments: “The discovery of a hot Jupiter around each star suggests that the same formation process took place and that similar favorable conditions boosted the migration of the planets.” Interactions between the two stars are problematic given the large separation but this may help us stretch our theories:

Even though at this stage nothing can be proven, there are recent dynamical theories relevant to this system. Moeckel & Veras (2012) described interactions in which a planet orbiting one component of a stellar binary can ‘jump’ to the other star. If the two giant planets were formed around the same star, planet-planet scattering could have occurred. This would have pushed one of the planets near the host star and ejected the second one, which could then have been captured by the stellar companion. As we do not know the eccentricity of the stellar system, we can also consider the ‘flipping machanism’ described by Li et al. (2014), in which a coplanar system leads to very high eccentricities for the planet.

We may also find the WASP-94 system valuable on other grounds. Like most of the WASP planets, WASP-94A and WASP-94B orbit stars that are relatively bright — most of the Kepler stars, by contrast, are faint. This Keele University news release quotes the university’s Coel Hellier speculating on the possibility of atmospheric studies through transmission spectroscopy, where the atmosphere of the transiting world can be analyzed as it moves onto and off the stellar disk during a transit. I can scarcely imagine what John Herschel, who first observed this stellar system back in 1834, would have made of possibilities like these.

The paper is Neveu-VanMalle et al., “WASP-94 A and B planets: hot-Jupiter cousins in a twin-star system,” in press at Astronomy & Astrophysics (preprint).


Comments on this entry are closed.

  • NS October 10, 2014, 12:38

    Probably a silly question, but as a boy I recall seeing speculations that in a binary system a planet might orbit both stars, either in a distant orbit with the stars near the center or a figure 8 with the stars toward each end. Is any of that still taken seriously or is the assumption a planet will always be in orbit around one star or the other?

  • Eric October 10, 2014, 14:52

    Yes. See the Kepler discovery of the ‘Tatooine’ system, Kepler 16b:


  • Thomas Tarrants October 10, 2014, 15:26

    Circumbinary planets have been reported at 2MASS J01033563-5515561, DP Leo, FW Tau, HU Aqr, HW Vir, Kepler-16, Kepler-34, Kepler-35, Kepler-38, Kepler-413, Kepler-47, Kepler-64, KIC 9632895, NN Ser, NY Vir, OY Car, PSR B1620-26, Ross 458, ROXs 42B, RR Cae, SR 12, and UZ For.

    Some, like the Kepler planets, are secure detections, but others, like HU Aqr and HW Vir, are much less certain due to some difficulty in finding stable fits for the eclipse timing measurements of those binary stars.

  • Andrew LePage October 10, 2014, 15:42

    @NS October 10, 2014 at 12:38

    Planets that orbit around both stars of a primary (what are called P-type orbits – while orbits around just one component of a binary are S-type) are still considered seriously by the scientific community. In fact, there are even examples of such circumbinary planetary systems that have been found: Kepler 16 (a Saturn-mass planet with a 0.70 AU orbit around a K and M-dwarf stars which are locked in a 0.22 AU orbit around each other) and Kepler 47 (three planets orbiting a tight binary). There is even a fair body of scientific literature discussing the possibility of habitable zones around such stars.

    As for the figure 8-like orbit, while theoretically possible it is an improbable configuration. It would require two stars of similar mass in near circular orbits with a planet orbiting at just the right distance from them. While not impossible, it is unlikely that all the right conditions can be met simultaneously (it would make for an interesting theoretical calculation to determine the probability).

  • Thomas October 10, 2014, 15:47

    Hey NS, this would be called a “circumbinary” planet, and they do exist:


  • Antonio October 11, 2014, 4:56

    I’m curious… why the planets aren’t named WASP-94 Ab and WASP-94 Bb?

  • NS October 11, 2014, 5:52

    Thanks for the replies!

    I had read (and obviously, forgotten) about the Kepler-16b discovery, but was unaware of the other “circumbinary” planet detections, and hadn’t heard of the P- and S-type orbit terminology either. Comment in haste and repent at leisure…

  • Michael Spencer October 11, 2014, 6:15


    Based on the comments I am seeing this morning, your site (our site?) would benefit from some sort of layering like Disqus?

    In any case I noticed the 2700 AU: as our own Oort cloud is thought to extend to 50,000 AU, this system has a might active mix. One could easily imagine objects perturbed in Oort A hurling headlong into Sun B, for example.

    I wonder, though, in the absence of long-period port-type objects exactly how much influence a star at 2700 AU would have on planets at, say, 1AU? And I wonder how each star would appear to an observer, again at 1 AU around each sun.

    As I have said, it is a great time to be alive–well, other than, say, about a millennium from now!

  • EricSECT October 11, 2014, 7:24

    With all the exoplanet discoverys, have we detected a correlation between the probability of a star having a planet(s) with the rate of spin that a star has? There was speculation many years ago that this link existed; slow rotation equaling planets.

  • Andrew LePage October 11, 2014, 10:40

    @Antonio October 11, 2014 at 4:56

    Well, the “WASP 94” designation is for the 94th star system of interest observed by the WASP (Wide Angle Search for Planets) consortium. The “A” and “B” designations are the standard astronomical designations for the first and second stars recognized in this system (with any subsequent stars in that system receiving the upper case letters that follow). And the “b” is the standard astronomical designation for the first planet discovered around that star (with any subsequent planets discovered receiving the following lower case letters). So “WASP 94 Bb” is the first planet discovered orbiting the second star of the 94th star system of interest in the WASP program. Other observing programs (e.g. Kepler) use identical nomenclature.

  • Andrew LePage October 11, 2014, 10:45

    @EricSECT October 11, 2014 at 7:24

    To answer your question, no, there is typically no correlation between a star’s rotation rate and the presence of planets. There *IS* a correlation between a star’s rotation rate and its age with older stars tending to rotate more slowly. There is also evidence to suggest smaller stars will have the evolution of their spin states affected by particularly large planets in very tight orbits. But generally, a slow stellar rotation rate only implies that the star is older not that it necessarily has any planets.

  • Alex Tolley October 11, 2014, 14:11

    @EricSECT – I think this was the Tommy Gold hypothesis. It assumed that the sun threw off the material to form the planets and that this momentum transfer slowed down the sun much like a spinning ice skater slows down by throwing out her arms. This model is no longer accepted, AFAIK.

  • andy October 12, 2014, 16:35

    Isn’t a figure-8 orbit unstable, i.e. the slightest perturbation would cause the disruption of the system?

    I suspect other “exotic” configurations may also be poor prospects: Trojan orbits are unstable for typical binary star mass ratios (stability requires a mass ratio above ~25). According to this reference, horseshoe orbits are an even more hopeless case, requiring mass ratios exceeding ~1200. Not sure about what the requirements are for quasi-satellites.

  • Antonio October 15, 2014, 8:01

    I know the standard naming conventions. I simply was wondering why the article uses a different convention, like in: “Like most of the WASP planets, WASP-94A and WASP-94B orbit stars that are relatively bright — most of the Kepler stars, by contrast, are faint”.