PLANETS IN BINARY SYSTEMS (independent database & information page)
(This Page is for Planets on S-type orbits that orbit one of the stars. For Circumbinary Planets, click here)
.... CURRENTLY 197 SYSTEMS IN THE DATABASE
=> A MACHINE READABLE TABLE of all the binaries-with-planets' main characteristics (sorted by increasing binary separation) is available for download. This table is complete for all planets on S-type orbits in binaries of separations up to 500au (an equivalent table for circumbinary planets (P-type orbits) can be found here)
=> For Latest Updates, click here
THE BASIC FACTS :
Figure 1: Architecture of confirmed 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 figure is exhaustive for all binaries of separation <200 AU and for all known stellar companions of mass >75MJup (a more complete figure, with separations up to 500au, can be found here).
Detailed description of Figure 1: All systems with at least one confirmed exoplanet and one confirmed stellar companion (or with non-background probability of <1%) with projected separation <200au. 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 "|" 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. // Planets detected by the radial velocity method are written in black, planets detected by transit are in blue, and planets detected by other methods are in red.
Figure updated from an original version published in Thebault & Haghighipour (2015) (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). Custom-made graphs can be requested by emailing the author (firstname.lastname@example.org)
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 (see here) 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.
A or B COMPONENTS: 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.
NON-INCLUSION OF BROWN DWARFS: I 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 (3 so far) for which an exoplanet has been detected in addition to them. These >13 MJup objects are drawn in orange instead of blue. Likewise, I chose to only consider stellar companions that have a mass >75MJup, thus excluding, here again, brown dwarfs. Note also that, for most planets detected with the RV method, what is estimated is in fact m.sin(i) (i being the (usually unknown) angle between planetary orbit and the plane perpendicular to the line of sight), so that the real planetary mass might fall into the >13 MJup regime.
A FEW WORDS OF CAUTION REGARDING ORBITAL STABILITY: As already pointed out, for the vast majority of systems for which 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 inclination 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).
(LACK OF) 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/speckle 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 Objects of Interest") or TOIs ("TESS Objects of Interest")), but the physical link between stellar components (as opposed to chance alignment with background stars) of some individual systems often remains yet to be established (see, for instance, Wang et al., 2015a,2015b, Kraus et al., 2016, Hirsch+(2017), Ziegler et al., 2017,2020, Howell+2021, Lester+2021). Very recently, a new and powerful way of finding bound stellar companions to exoplanet hosts (in most cases TOIs) has been to mine the GAIA catalogue (see for instance Mugrauer&Michel2020, Fontanive+2021 or Michel&Mugrauer2021). The El-Badry+(2021) catalogue of binaries derived from GAIA DR3 is now routinely used to assess the presence of stellar companions to exoplanet hosts (but this catalogue is not complete for binary separations < 100au).
Let me present, however, a histogram of the semi-major axis (ab) distribution of all the (r<500au) S-type binaries in our datase. As a comparison, I also present, as a solid line, the normalized canonical distribution of binaries in the solar neighbourhood, as estimated by Raghavan+(2010) . As can be cleary seen, there is a striking difference between the two distributions, which is probably the result of a real depletion of planet-hosting tight binaries (see for instance Kraus et al., 2016, Hirsch et al., 2017 or Ziegler+2021) combined to the observational biases discussed above. Note that, for the numerous systems for which only the projected separation r of the binary is known, I estimate a "statistical" semi-major axis using the statistical relation log(ab) = log(r) + 0.13 derived by Raghavan+(2010) (hence the X axis extending up to 675au instead of 500)
An illutrative way to show that the depletion of planet-hosting tight binaries is not only due to observational biases is to plot the distribution of projected angular separations as a function of distance to the system (see graph below, where the 3 lines correspond to physical separations of 10au, 50au and 500au). As can be seen, there is a population of r<10-20au planet-bearing binaries at d≤100pc with an angular separation >0.1" that should have been detectable by current instruments (because such >0.1" planet-bearing binaries have been osberved in the d≥100pc domain) but which were not detected. The (at least qualitative) conclusion is that there is a real physical depletion of <20au binaries with planets (the void of objects in the upper-right part of the graph is artificial and is the result of our cut at r=500au in the database)
Online catalogue giving additional data about planets in binaries:
- Catalogue of exoplanets in binary star systems (last updated in May 2022)
Recent statistical studies regarding planet incidence in binaries:
- Wang et al., 2015a, 2015b
- Kraus et al., 2016
- Hirsch et al., 2017
- Ziegler et al., 2017, 2020
- Matson et al., 2018
- Moe & Kratter, 2021
- Michel&Mugrauer, 2021
- Fontanive et al., 2021
- Lester et al., 2023
- Michel&Mugrauer, 2024
.... Page developed and maintained by Philippe Thebault
Figure 2: Same as Figure 1, but this time for all exoplanet-hosting binaries with separations up to 500au: