Planets in Binary Systems

There are two common ways in which planetary bodies (exoplanets) can exist in binary star systems (see Figure 5.1). Firstly, the planet orbits well outside a pair of stars in a close binary orbit. This is referred to as a P-type (or planetary type) orbit. In this case there exists a critical value of the semimajor axis of the planet's orbit around the pair. Too close and the planet is subject to competing pulls from both stars; too distant and the gravitational link vanishes.

Secondly, the planet orbits one or other of a wide pair of stars where the distance of the planet from its sun is much less that the stellar separation. This is an S-type (or satellite-type) orbit and here the semimajor axis of the planetary orbit must be less that a certain critical value if the perturbations from the second star are not to be too disruptive. In other words if the planet wanders too far from its sun during its orbital revolution it will come under the influence of the companion star. All known exoplanets have S-type orbits.

At the time of writing, out of the 100 or so planets discovered so far, 17 are known to orbit stars in 13 binary and triple systems. In almost every case planets have been discovered by the reflected variation in radial velocity of the primary star but a recent observation of the star GJ 876 by the Hubble Space Telescope


-y-N Planet >

_,--q Planet


\ \ \

\ ▼—OB

1 1




Star B



Figure 5.1. Location of stable planetary orbits: a the P-type (planetary-type) and b the S-type (satellite-type).

has revealed the astrometric "wobble" of the primary star to amount to only 0.5 mas caused by the more massive of the two known planets in the system. All the planetary orbits known to date are S-type and are listed in Table 5.1 below. The M sin i column lists the minimum mass (in Jupiter masses) that the planet has, and the sin i term represents the unknown inclination of the planetary orbit. Only in one case known to date does an exoplanet eclipse the parent sun giving sin i = 1 (i = 90°), so the true planetary mass equals the minimum mass.

The first discovery was a planetary companion to one of the stars in the wide pair 16 Cyg. The planet was detected orbiting the fainter of the two stars which themselves are separated by some 39'' on the sky, equivalent to a linear separation of 700 astronomical units at the distance of 70 light years. The orbital period is very long and nothing is known about the orbit of the two stars about the centre of gravity. 16 Cyg B is a dwarf star, somewhat earlier in spectral type than the Sun. The planet orbits star B at a distance of about 1.72 AU with a period of 800 days but the orbit is very eccentric (0.63). The recent discovery of a very faint star close to A, which is probably physical, means this is the first triple star known to have a planetary companion

55, p Cnc is accompanied by a distant M dwarf star which was first identified by W.J. Luyten. The stars make up the system LDS6219. Currently the separation is about 83'' and has shown little change since 1960. The primary star has an annual proper motion of about 0.5'' so it is clearly a physical pair but the orbital period is going to be of the order of thousands of years. Two further planets were confirmed in summer 2002, one of which has the smallest value of M sin i yet found (0.22).

T Boo has a faint (magnitude 11.1) M2 companion which was discovered by Otto Struve at Pulkova. At that time (1831) the separation of 15'' was such that the pair could be relatively easily seen. The distance has closed significantly and the current value is around 3''. An orbit was computed in 1998 by A. Hale and a period of 2000 years derived. This is very uncertain but the determination of the binary star orbital elements is significant because from these observations the inclination of the orbit can be determined. If we assume that the planetary orbit around T Boo is coplanar with that of the two stars then a direct measure of the star's orbital inclination will allow the mass of the planet to be determined directly. If the binary orbit inclination is correct and the tilt of the planetary orbit to the line of sight is also 50° then the sin i factor is 0.77, giving a value of 3.0 Mj for the planet in this system.

The brightest component of the pair STF1341, HD80606, is now known to have a planetary companion with a period of 111.8 days. The eccentricity of the orbit (0.927) is the highest yet found and it is possible that this is due, like that of the planet of 16 Cyg B, to perturbations by the second star in the system.

The wide pair STF2474 consists of two 8th magnitude stars separated by 16''. McAlister found the primary to be a close pair with a period of 3.55 years and recently Zucker et al. found a planetary mass companion to star B which is a G8 dwarf star of 0.87 solar mass.

The bright star y Cep is a spectroscopic binary of very long period - in fact the longest yet found. Roger Griffin2 gives the period as 66 years with an uncertainty of 1 year. The planet has a period of 903 days and its average distance from star A is 2.1 AU.

The first planetary discovery made by Italian astronomers with the 3.5-metre Telescopio Nazionale Galileo on La Palma is a low-mass planet orbiting the fainter component of the pair STF 2995 - currently separated by 5.2''. The large proper motion of the bright component and the small change in separation since 1820 confirm that the stellar pair is a binary one.

Table 5.1 summarises the data that we have at present for the binary systems which have planets. The first column gives the popular name of the binary component with the planet, followed by the double star catalogue name, the approximate separation of the two stars (in astronomical units), a letter representing the planet (b = nearest the star, c is next most distant, and so on), and finally the minimum mass of the planet (in terms of the mass of Jupiter). If it were possible to observe the planet by direct imaging, we could determine the inclination of the planet's orbit and hence its mass. If the orbital plane of the planet is in the line of sight then sin i = 1 and the mass of the planet can be determined exactly. This is the case in only one out of the 100 or so planetary systems found to date.

A recent paper by Lowrance et al.3 lists 11 binary and triple systems which have a planetary companion or planetary system in orbit around one of the stars. Recent discoveries include two more planets in the

Table 5.1.

Planets in known double-star systems (January 2003).





M sin i




16 Cyg B





Triple with 16 Cyg A

and a

55, p Cnc









t Boo

STT 270 A




HD 80606





GJ 86





STF 2474B




Triple with STF 2474

A and a

94 Cet

HJ 663 A




HD 142





HD 195019

HO 131 A




u And




Optical companion?





HD 89744




Companion is a brown


Y Cep

HD 222404




Planet orbits primary


HD 219542

STF 2995B



Stars form physical pair




B is a late M dwarf

HD 3651

STT 550



A is nearby (11 l.y.)

and K0V

55 Cnc system, a new stellar component to u And which already has three planets, a faint stellar companion to HD 114762 and a sub-Saturnian mass planet to HD 3651 whose faint stellar companion is a field star.

The website maintained by Jean Schneider4 at Paris Observatory is kept up to date with new planet discoveries.

Planet discovery is proceeding apace and many further examples are bound to be found in the near future when the upcoming space interferometer missions such as SIM and DARWIN, which are designed to seek out Earth-sized planets, start operation. We will soon know whether such planets exist in double or even multiple star systems.


1 Bate, M. et al. release3.shtml

3 Lowrance, P.J., Kirkpatrick, J.D. and Beichman, C.A., 2002, Astrophys. J., 572, L79.

4 Schneider, Jean

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