Planets in binary systems

THE BASIC FACTS:

This graph presents an overview of the architecture of all binary systems harboring a confirmed exoplanet on an S-type orbit, that is, a planet orbiting one of the two stars in the system. This list is exhaustive for all binaries of separation <200 AU (fig. 1) updated on 11/07/2018.

Figure 1: All systems with at least one confirmed exoplanet and one confirmed stellar companion with projected separation <200au. Figure from Thebault & Haghighipour (2014), updated on 06/07/2018.
Figure 1: All systems with at least one confirmed exoplanet and one confirmed stellar companion with projected separation <200au. Figure from  Thebault & Haghighipour (2015), updated on 11/07/2018.

N.B. : The graphs might be freely re-used, as long as properly credited to Philippe Thebault and referring to the original Thebault & Haghighipour (2015) article.

The blue circles' location shows the semi-major axis of the exoplanet while the yellow circles show the location of the companion star. The radius of the blue circle is proportional to the estimated radius of the planet (or the cubic root of its mass when only a mass estimate is available). The radius of the yellow circle is proportional to the cubic root of the mass ratio between the companion and the central star (for the sake of visibility, the sizes of the planets have been inflated by a factor 20 with respect to the sizes of the yellow stars).
For both the planets and the stars, the horizontal purple line represents the radial excursion due to the object's eccentricity. Note that, for most stellar systems of separation > 30-50AU, the semi-major axis of the binary is unknown and the only available information is the projected separation between the stellar components. These cases are indicated by a "I" symbol overlaid onto the companion star.
The black vertical line plotted between the planet and the stellar companion represents the outer limit for long-term orbital stability as estimated with the widely used empirical formula of Holman & Wiegert (1999), assuming that the planet and the binary are coplanar. For systems where only the projected separation of the binary is known, the stability limit is computed assuming that the companion star is on a circular orbit whose radius is equal to the projected separation.

TRIPLE AND QUADRUPLE SYSTEMS:

Some of the presented cases are in fact higher-order multiple systems, mostly triple or even quadruple stellar systems. However, almost all of these systems are highly hierarchical, meaning that the third star does not significantly impact the dynamical evolution of the planet. This is why, for the sake of clarity and simplicity, we chose to present them as "binaries", labelling them with an additional "*" at the end of the system's name, with a brief explanation presenting the system's specificity. Most of these hierarchical cases fall into 2 categories:
1) Systems where either the central or the companion star is itself a very tight spectroscopic binary. In this case, the dynamical stability of the planet is computed by merging the two stars into one "effective" central or companion star
2) Systems where the third star is very distant from the central binary, typically more than 10 times the distance of the closer companion star. In this case, the gravitational pull of the third star is ignored when estimating the planet's stability.

OTHER SPECIAL CASES:

For most cases, the planet orbits the more massive component (usually labelled as "A") of the binary. In this case, we simply give the stellar name without adding the "A". For the few systems where the planet orbits the lower mass ("B") binary component, we specify it be adding the "B" at the end of the stellar name.

For a few systems (2 so far), there are planets orbiting each member of the binary. In this case, the system is divided into two "binaries", one where the first star is the "central star" and the other star is the perturbing companion, and another one where the roles are reversed.

We chose a rather conservative policy of excluding all systems with « planetary » objects having a mass (or a minimum mass) higher than 13 MJup. However, we display the > 13 MJup companions for the few systems (2 so far) for which an exoplanet has been detected in addition to them. These >13 MJup objects are drawn in orange instead of blue.

A FEW WORDS OF CAUTION:

As already pointed out, for the vast majority of systems where only the projected separation between the stars is known while the actual orbit of the binary remains unconstrained, we consider the fiducial configuration of a binary having a circular orbit equal to the projected separation. The estimated stability limit thus only gives a first-order estimate and should be taken with caution. However, it can be reasonably considered as a rather conservative assumption with respect to the planet's orbital stability, as it corresponds to the smallest possible physical separation between the stars.

On a related note, for most systems there is another unknown parameter, which is the relative inclination between the planetary and binary orbital planes. We have considered the simplest possible case of a coplanar configuration, which might hold for the tightest binaries, since observations of young binaries have shown that proptoplanetary discs tend to be aligned with stellar equatorial planes for separations up to 30-40AU (see Hale, 1994). But significant inclination values, possibly entailing complex effects such as the Kozai mechanism, cannot be ruled out for most systems. As a matter of fact, some detailed numerical studies have shown that large i values could increase the odds for long term orbital stability for some specific planets observed at the limit (or even beyond the limit) of the coplanar orbital stability (HD196885, HD59686).

STATISTICAL SIGNIFICANCE:

Although it might be tempting to do so, it is difficult to straightforwardly derive statistics regarding the incidence of planets in binary systems, because the available list of systems is affected by several strong biases. The first one is that, until relatively recently, observational surveys, especially those relying on the radial velocity method, had been strongly biased against binaries, excluding known multiple systems from their potential targets. Another issue is that, for many cases, the binarity of the system was not known at the time of the exoplanet's discovery and was established by later observational campaigns. This means that there should still be a potentially large population of exoplanet hosting "single" stars that are in fact members of a (yet undetected) multiple system. To alleviate this problem, several large-scale adaptive optics surveys are currently underway in order to assess the presence of stellar companions around exoplanet hosts. These surveys have already detected a large number of potential stellar companions (albeit mostly around yet-unconfirmed KOIs ("Kepler Object of Interest")), but the physical link between stellar components (as opposed to chance alignment with background stars) of each individual system remains yet to be established (see, for instance, Wang et al., 2015a,2015b, Kraus et al., 2016, or Ziegler et al., 2017). 

NOTES ON SOME INDIVIDUAL SYSTEMS:

- Gamma Ceph A is a post-main-sequence star (sub-giant). Recent astrometry analysis (Benedict et al., 2018) shows that there is a 70 degrees mutual inclination between the binary and planetary orbits and that the planet probably has a much higher mass than previously expected (9 MJup).

- Ari B is actually part of a wide triple system (component Ari A, see Kane et al., 2015). What we show here is the BC binary. The orbit of the Ari C companion is poorly constrained: with a separation of 21.9AU, the constraint is that e<0.75. I took e=0.75/2 as a reference value. (And the component Ari Aa is actually also a very tight binary)

- Kepler 410 is maybe a triple system with a star "C" between A and B (Gajdos et al., 2017)

- Kepler 444: it is a triple star, so the data corresponds to the merged B+C M-stars (Dupuy et al., 2016). 

- HD8673: The "planet" is at the brown dwarf limit. The projected separation is actually 11AU only. The given orbit is the closest stable one as computed by Roberts et al.(2015)

- HD87646: the 2nd "planet" is potentially a brown dwarf

- HD131399 DISCARDED. It is a hierarchical triple (the companion star is in fact a tight binary, see Wagner et al., 2016). Nielsen et al.(2017) discard the imaged planet as a being a background star 

- HD106515: The binary orbit corresponds to that with lowest possible e and a, but there is a wide range of other possibilities (see Fig. 9 of Desidera et al., 2012). UPDATE: better constraints on the orbit given by Rica et al.(2017)

- Kepler 132 possesses at least 3 planets, and we know that 2 of them (b and c) cannot orbit the same star. But we don't know which planet orbits which star. There is also a 4th planet that is still a KOI. (Lissauer, et al., 2014, Everett et al., 2015)

- HD132563B: The distant A component is a binary of separation 15AU (Desidera et al., 2011)

- Gliese667: Triple System. The planet-hosting C star is linked to the A-B binary at a projected distance of 230AU. The AB binary has an orbit with a =12.6AU and e=0.58. The stability has been calculated for a merged AB component 

- Kelt 4: Kelt-4A is orbited, at a projected separation of 328AU, by a binary (B-C), composed of 2 identical K stars separated by 10.3AU. The stability has been calculated for a merged BC component (Eastman et al., 2015)

- HD65216: The B component is in fact a B-C tight binary of separation 6AU (Mugrauer et aL, 2007)

- Kepler 21, 68 , HD197037, HD217786: binarity discovered by Ginski et al.(2016)

- HD28254: not listed as binary host by  the 2 sites. But listed in Moutou et al. and Lodieu et a.(2014). No info on the companion star type (put M2/M1=1 by default)

- HD30856, HD116029, HD207382, HD86081, HD43691, not listed as binary host by the exoplanet.eu and openexoplanetcatalogue.com sites. All objects described in Ngo et al.(2017). HD30856, 86051, 207382: companions presented for the first time in this paper. HD43691 & 116029: companions confirmed by Ngo.

-Kepler693: not in the 2 online databases exoplanet.eu and openexoplanetcatalogue.com. 150MJup companion found by Masuda

-Kepler13: the B component is in fact a binary (separation 0.410UA)

-HD126614 is a triple star, the third component is beyond 1000AU (Gould and Chamané, 2004)

-HD2638 is a triple star. The planet is actually orbiting HD2628B, the companion being HD2638C. But there is a third, more massive star (called HD2567, not HD2638A) at a projected separation of 839" (45000AU, see Roberts et al., 2015)

FURTHER READINGS:

Online catalogue giving additional data about planets in binaries:
- Catalogue of exoplanets in binary star systems 

Review paper on planet formation in binaries:
- Thebault & Haghighipour, 2015 

Recent statistical studies regarding planet incidence in binaries:
- Wang et al., 2014a, 2014b
- Kraus et al., 2016
- Ziegker et al., 2017

Page developed and maintained by Philippe Thebault