Recent Results on Circumbinary Planets

Recent Results on Circumbinary Planets Jerome A. Orosz with thanks to Bill Welsh, Don Short, Gur Windmiller, Dan Fabrycky, Josh Carter, Laurance Doyle, The Kepler EB and TTV Working Groups

Importance of binaries Detection methods Results published to date Summary

Importance of Binaries Binaries and higher-order multiple systems are common. Raghavan et al. (2010) found 54% ± 2% of nearby solar-type stars are single Many of the target stars in large surveys will be binaries

Importance of Binaries There are two classes of binaries with planets: S-type where the planet orbits one of the stars in a wide binary P-type or circumbinary where the planet both stars Several S-type systems are known, mainly from radial velocity surveys where the star with the RV variations has a resolved companion The first clear detections of circumbinary planets has come from the Kepler data

Detection Methods Use eclipse timing variations (ETVs): The binary system orbits the system center of mass (COM), resulting in a displacement along the line of sight Variations in the eclipse arrival times of order a few seconds or less might be expected in favorable situations Variations in the eclipse arrival times of order a few minutes might be expected in cases where there are dynamical interactions

Detection Methods Use eclipse timing variations (ETVs): Most claimed detections based on light travel time (LTT) effects have been controversial. The best case is NN Ser, where two planets with periods of 7.9 and 15.3 years can explain the ETVs

Use transits: Detection Methods

If a planet passes in front of a star, the light is dimmed. The amount of dimming gives the radius ratio of planet to star.

Detection Methods Use transits: Borucki & Summers (1984) argued that one should look at eclipsing binaries (EBs) to find transiting planets: if the orbits are coplanar the probability of a transit is increased around an EB where the orbit is viewed close to edge-on Schneider & Chevreton (1990) and Deeg et al. (1994) showed that transits in a circumbinary system should have unique signatures

The Kepler Mission The Kepler spacecraft is in an Earth-trailing orbit, which allowed it to observe the same region of the sky nearly continuously for about 4 years Extremely precise photometry was obtained for nearly 200,000 stars, including about 3000 EBs

Here is KOI-28, which has periods of 4.049729 days and 4.100871 days Such a circumbinary configuration would be unstable, so this must be a blend of two EBs (e.g. two EBs landed on the same pixel)

This is KIC 7622486, which has periods of 2.279996 days and 40.246503 days A circumbinary configuration would be stable, so how do we tell if this is a blend?

In a triple system, the three bodies move about the system COM If the third body is planetary, the COM is essentially the binary COM Stellar eclipses, if the orbit is seen roughly edge on, occur at the stellar conjunctions At the time of the inferior conjunction of the planet, the stars are not necessarily along the same line-of-sight

The conjunction is when x=0 for the planet. Chances are the star is not at x=0, and the transits may be early or late relative to a linear ephemeris.

The deep events with the 40 day period in KIC 7622486 do not show significant variations from a linear ephemeris, so they must be due to a blended EB

Here is Kepler-16. The orbital period is 41.0777 days. Note the extra transit in Q1.

Here is Kepler-16. The orbital period is 41.0777 days. Note the extra transit in Q1. Also in Q4.

Here is Kepler-16. The orbital period is 41.0777 days. Note the extra transit in Q1. Also in Q4. And in Q6

There are 7 such events through Q16, with a rough period of 221 days

There are transit timing variations (TTVs) of up to 6 days, which indicates that this is not a blend The dips are caused by the third body transits of the primary

We also see transits of the secondary star, which completely rules out a blended EB

We also see ETVs The primary O-C curve has a different slope than the secondary O-C curve, which in an indication the primary and secondary eclipse periods are different. In this case, the two periods differ by 17.76 seconds

Each curve shows a periodicity of about 112 days, which is roughly half of the mean period of the transits.

The big arrow is how the planet pulls on the COM of the binary The small arrows are the tidal force the planet puts on the stars The m=2 tidal pattern rotates at the planet s orbital period, and every half an orbit the stars have to climb up and down the tidal potential

Their phases are successively delayed and advanced every period of the planet, so the observed effect is twice the planet s frequency

Here is an other example (Kepler-38). Note the transit duration variations

In general, the TTVs and the durations of circumbinary transits vary cyclically with the binary phase, which can be understood analytically

The transit duration depends on the radius of the star (or more precisely the length of the chord), and on the relative transverse velocities of the star and planet V planet = τ i = 2(R planet + R ) 1 b 2 i V planet + V 2πGM EB P planet 1/ 3 const V = M 2 M EB 2πGM EB P EB 1/ 3 esin ω+ sin(θ + ω) 1 e 2 In the above, e and ωare the eccentricity parameters for the binary, and b i is the impact parameter for the i th transit.

Near the phase of the primary eclipse, the planet and the star are moving in opposite directions, hence the transits are shorter Near the phase of the secondary eclipse, the planet and the star are moving in the same direction, hence the transits are longer

What We Have Learned So Far Given enough events, the transits of a circumbinary body will show large TTVs, which can rule out a blend The circumbinary body can transit both stars, and if the primary and secondary eclipses are not equal, the transits across the primary and secondary will likewise be unequal---this also rules out a blend Under certain conditions, the circumbinary body can perturb the binary, leading to observable effects

Details, Details Since the binary is not a point mass, the planet s orbit is not exactly Keplerian Since the planet may perturb one or both stars, their orbits may not be strictly Keplerian Given the above, the standard codes to model eclipsing binaries need to be modified to account for the gravitational interactions The first such code was devised by Josh Carter My ELC code has been modified to include the necessary dynamical effects

Details, Details The transit depths give you the ratio of the planet s radius to the stellar radius The eclipses give you the ratio of the stellar radii The radial velocity curve of at least one star is needed to establish the scale of the system If the scale of the system is known, the actual radii of the stars and the planet can be found If ETVs are observed, then mass of the planet can be found

Initial Discoveries Between October, 2012 and October, 2013 we announced the discovery of five EBs with circumbinary planets

In Kepler-16 the planet orbits outside the two stars. This is the first known transiting circumbinary planet.

Kepler 16 We have 7 primary transits observed with Kepler, plus one additional one from the ground. The planet s period is 228.3 days

Kepler 16 The primary eclipses and secondary eclipses (which are total) are well fit. The EB period is 41.08 days.

Kepler 16 The system is barely double-lined. The stellar masses are 0.68 and 0.20 solar masses. The planet s mass and radii are 90 and 8.5 (Earth units).

Kepler-16 The discovery of Kepler-16 showed that close-in circumbinary planets could exist around relatively short period binaries The planet orbits very close to the binary---if the orbital period were about 14% shorter, the system would be unstable The planetary orbit is about the size of the orbit of Venus However, since the primary is much less luminous than the Sun, Kepler-16b receives only 30% of the insolation the Earth does

We announced the discoveries of Kepler-34 (left) and Kepler-35 January of 2012

Kepler 34 Transits of both stars are evident. The planet s period is 289 days.

Kepler 34 The primary eclipses and secondary eclipses are well fit. The EB period is 27.80 days, and the eccentricity is relatively large at e=0.52.

Kepler 34 The system is double-lined. The stellar masses are 1.05 and 1.02 solar masses. The planet s mass and radii are 53 and 8.8 (Earth units).

Kepler 35 Transits of both stars are evident. The planet s period is 132 days.

Kepler 35 The primary eclipses and secondary eclipses are well fit. The EB period is 20.73 days.

Kepler 35 The system is double-lined. The stellar masses are 0.90 and 0.81 solar masses. The planet s mass and radii are 36 and 8.1 (Earth units).

Kepler-38 Image credit: Grace Mervin (SDSU)

Kepler 38 Transits of only the primary are evident. The planet s period is 106 days.

Kepler 38 The primary eclipses and the weak and total secondary eclipses are well fit. The EB period is 18.80 days.

Kepler 38 The system is single-lined. The stellar masses are 1.05 and 0.26 solar masses. The planet s mass and radii are 69 and 4.4 (Earth units).

NASA/JPL-Caltech/T. Pyle We announced Kepler-47 in the Fall of 2012. It is the first circumbinary system with more than one planet.

Kepler 47 The first planet transited the primary 24 times. That planet s period is 49.5 days.

Kepler 47 The second planet transited the primary 6 times. That planet s period is 187 days.

Kepler 47 The third planet transited the primary 4 times. That planet s period is 303 days.

Kepler 47 The primary eclipses and the weak and total secondary eclipses are well fit. The EB period is 7.45 days.

Kepler 47 The system is single-lined. The stellar masses are 0.96 and 0.34 solar masses.

Kepler 47 The planetary masses are about 2, 19, and 3 Earth masses, and the planetary radii are about 3, 7, and 4.7 Earth radii. The densities are all less than ~0.4 g/cc.

Additional Systems Kepler-64 (aka Planet Hunters-1, Schwamb et al. 2013) Kepler-413 (Kostov et al. 2014) Kepler-453 (Welsh et al. 2015) Kepler-1647 (Kostov et al. 2016)

Kepler-453 Illustration copyright Mark Garlick

Kepler 453 Three transits of the primary are evident. The planet s period is 241 days.

Kepler 453 The primary eclipses and weak and total secondary eclipses are well fit. The EB period is 27.32 days.

Kepler 453 The system is single-lined. The stellar masses are 0.94 and 0.94 solar masses. The planet s mass and radii are <16 and 6.2 (Earth units).

Kepler-1647

Brightness Primary Eclipse 20% loss Secondary Eclipse 15% loss Planet Transit 0.2% loss Time

Image: Billy Quarles Image: Lynette Cook Kepler-1647b has a mass and radius similar to that of Jupiter (all other transiting circumbinary planets are smaller than Saturn). The size of its orbit is more than twice as large as the orbits of the other known circumbinary planets. The period is about 1107 days.

The score so far: The Big Picture 11 planets in 9 eclipsing binaries published 2 more eclipsing binaries with a single planet each in the works The developing picture, based on all systems: The primary star masses range from 0.69 to 1.53 solar masses (Kepler-16, Kepler-64) The eccentricities of the binaries range from 0.023 to 0.521 (Kepler-47, Kepler-34) The periods of the binaries range from 7.49 to 41.08 days (Kepler-47, Kepler-16)

The Big Picture The developing picture, based on all systems: The primary star masses range from 0.69 to 1.53 solar masses (Kepler-16, Kepler-64) The eccentricities of the binaries range from 0.023 to 0.521 (Kepler-47, Kepler-34) The periods of the binaries range from 7.49 to 41.08 days (Kepler-47, Kepler-16) All systems are within a few degrees of being coplanar The stellar spin axis of Kepler-16 and Kepler-47 are roughly aligned with the orbital angular momentum

The Big Picture

The Big Picture The developing picture, based on all systems: With three exceptions (two outer planets in Kepler- 47 and Kepler-1647), the planets orbit very close to the critical radius for stability With one exception (Kepler-1647), all planets have radii much smaller than Jupiter (keep in mind larger planets are easier to detect owing to deeper transits)

The Big Picture

The Big Picture The developing picture, based on all systems: The planet with the largest mass and radius (Kepler- 1647) has the orbit that is by far the largest---the other planets with lower mass orbit very near the stability limit Pierens & Nelson (2008) predicted such a tendancy, based on simulations of orbital evolution of planets in a circumbinary disk (Jupiter-mass planets tend to be unstable)

The Big Picture The developing picture, based on all systems: About half of the EBs in the Kepler sample have periods of a few days or less The shortest period circumbinary system has a period of 7.5 days (Kepler-47), and typical periods are 20 days or more Is this an observational bias? Short-period systems should have more transits, and should be easier to detect On the other hand, shorter-period systems tend to have more stellar activity and other complications

The Big Picture The developing picture, based on all systems: About half of the EBs in the Kepler sample have periods of a few days or less The shortest period circumbinary system has a period of 7.5 days (Kepler-47), and typical periods are 20 days or more Is this an observational bias? The apparent lack of planets around short-period binaries ay be related to the mechanism that removed the angular momentum from the stellar orbit and allowed the stars to orbit so closely

Questions? 問題