Cold Friends of Hot Jupiters

Identifying additional planets in transiting-planet systems, whether transiting or not, yields a wealth of information about the dynamics of the system (Fabrycky 2009). By firmly establishing the existence of additional planets in other transiting systems, it will be possible to determine if, as astronomers suspect, multipleplanet systems are common.

We propose to constrain migration mechanisms for known transiting extrasolar planets with short orbital periods by searching for stellar and/or planetary companions in long-period orbits. This search requires radial velocity monitoring with a precision of 2-3 m/s over multiple years, and we utilize the legacy of Keck HIRES data already acquired on these systems to identify systems with existing radial velocity trends as well as misaligned or eccentric orbits. These systems are prime candidates for migration processes involving interactions between either a massive stellar companion (i.e., Kozai migration) or multiple outer planets (secular migration and planet-planet scattering). Constraints on the presence or absence of additional objects in these systems therefore provide a crucial test for these migration mechanisms. These same measurements also allow us to improve current estimates for the masses and orbital eccentricities of the known short-period transiting planets in these systems.

Many of the almost 500 confirmed extrasolar planets differ significantly from their solar system counterparts. Hot Jupiters orbit at distances of less than 0.05 AU and have temperatures as high as 3000 K, comparable to cool stellar atmospheres. These planets pose a considerable challenge for planet formation models, as we know that they could not have formed at their present-day locations but instead must have migrated inward from beyond the ice line (Lin et al. 1996). By carrying out this radial velocity search for massive outer companions in systems with known short-period transiting planets, it will be possible to test competing migration models.

Hot Jupiter migration models can be broadly divided into several classes, including disk-driven migration, star-planet interactions, and planet-planet interactions. In the simplest disk migration models, including both type I and II migration, we expect the resulting short-period planets to have largely circular orbits that are well-aligned relative to the stars spin axis (e.g., Goldreich & Tremaine 1980, Tanaka et al. 2002, Lin & Papaloizou 1986). Star-planet interactions can drive inward migration in the case where the planet orbits one star in a widely separated stellar binary and where the planet’s orbit is highly inclined relative to the orbit of the second, outer star (e.g., Malmberg et al. 2007, Fabrycky & Tremaine 2007). This process is known as Kozai migration, and it causes the planet to oscillate between a highly inclined and a highly eccentric orbit; as a result we expect these systems to exhibit high orbital eccentricities and orbits that are significantly misaligned relative to the star’s spin axis. The presence of one or more additional planets in the system can produce a similar effect if the orbits of the planets are significantly inclined with respect to one another, while planetplanet scattering events can also send one planet spiraling rapidly inwards, also with a high orbital eccentricity (e.g., Chatterjee et al. 2008, Nagasawa et al. 2008). Lastly, long-term transfer of angular momentum via secular interactions in systems with three or more planets can lead to a scenario in which an initially circular planet far from its host star acquires a high orbital eccentricity and then circularizes at a new, short orbital period through tidal damping (e.g., Wu & Lithwick 2010).

The Rossiter-McLaughlin effect, where the planet produces an apparent radial velocity shift as it first occults the approaching and then the receding limbs of the rotating star, will give the spin-orbit alignment for transiting-planet systems. Significant misalignment with respect to the star's spin axis favor migration models involving either a second star or multiple planets for a significant fraction of hot Jupiters (Morton & Johnson 2011). If this is the case, we will detect the presence of additional planets or stars at wide separations in these systems using long-term radial velocity imaging. Indeed, the absence of such objects in these systems would render migration processes driven by secular instabilities unlikely.

It will also be possible to make solid inferences about the internal structures of the innermost planets. Specifically, Batygin et al. (2009b) have demonstrated that a dynamical analysis of such a system combined with models of the inner planet’s interior structure can constrain the core mass and tidal quality factor, Q, of the transiting planet. Insight into the internal structure of transiting objects can, in turn, help to constrain possible formation scenarios for the objects (Guillot 2008).

The prototypical example of such a system is the transiting planet of HATP-13. In addition to its transiting hot Jupiter, ongoing Doppler monitoring of the HAT-P-13 system (Bakos et al. 2009, Winn et al. 2010) has revealed the presence of a second and third massive perturbing planet (HAT-P-13c and HAT-P-13d) on long-period eccentric orbits. This system presents a unique opportunity to study the internal density structure as well as the efficiency of tidal dissipation in the inner transiting planet. Batygin et al. (2009b) have combined a tidal-secular orbital evolution model for HAT-P-13 with interior planetary evolution models of the inner planet, to constrain the planetary core mass to less than 120 Mearth and constrain the tidal quality factor Q to between 104 and 3105. This constraint on an exoplanetary Q factor is better than that for Jupiter. The fact that astronomers understand the interior structure of some exoplanets better than planets in our own solar system is testimony not only to the power of the proposed technique which forms the basis for this project, but also the advanced state of exoplanetary science in general. The discovery of the HAT-P-13 system has launched numerous follow-up observing campaigns that seek to refine the measurement of the planet’s radius and further constrain the core mass and Q value.

The GJ 436 system presents another set of challenges for dynamicists and theorists alike, and will likely benefit from ongoing radial velocity follow-up observations. Gliese 436b, a transiting hot Neptune with an eccentricity of 0.15, poses an unique puzzle. With no additional planets in the system, its Q factor would be larger than 107, the highest ever inferred for any planet. In the presence of additional planets, however, this object would possess a Q of order 105, similar to our ice giants (Batygin et al. 2009a). The lack of a perturber in this system may shed light on the relation between tidal Q and orbital frequency – something that has been nearly impossible to infer previously.

Professor Knutson had her proposal for observing relevant stars accepted at Keck. Her one night per semester will be added to a queue of nights award to the California Planet Search (CPS) program, led by Professor Geoff Marcy at UC Berkeley and Professor John Johnson at Caltech. A list of twenty targets was chosen, for either (1) a non-zero eccentricity, (2) a known spin-orbit misalignment, or (3) a long-term RV trend.