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Multi-transiting Planet Systems Part I

Today’s post is part one of a two part series by guest blogger by Darin Ragozzine. He’ll be talking about multi-planet transiting systems. A  related Talk update, we’ve labeled all the published multi-planet systems with the tag Kepler multiplanet candidate located below the plotted light curve in the lower left corner.

Darin Ragozzine is a Caltech Ph.D. and currently a postdoc at the Smithsonian Astrophysical Observatory. Expert on planetary orbital dynamics and transit light curves. Member of the Kepler TTV/Multiples Working Group and Kepler Science Team Collaborator, co-author on Kepler-9, 11, and multiple other Kepler Team papers. And occasional PH user and commenter.

Hello Planet Hunters! I’m glad to be making this guest blog post to tell you to keep looking for more planets, especially around stars that already have planet candidates.

In a nicely written article about the basics of transiting exoplanets, MIT exoplaneteer Josh Winn quoted astronomer Henry Norris Russell (of Hertzsprung-Russell fame), who said in 1948: “From immemorial antiquity, men have dreamed of a royal road to success—leading directly and easily to some goal that could be reached otherwise only by long approaches and with weary toil. Times beyond number, this dream has proved to be a delusion…. Nevertheless, there are ways of approach to unknown territory which lead surprisingly far, and repay their followers richly. There is probably no better example of this than eclipses of heavenly bodies.”

Transiting planets are a brilliant example of Russell’s royal road of rich repayment. There are a huge list of measurements and of physical characterization that is possible with transiting exoplanets that are not possible with any other technique. For this reason, in the search for potentially habitable worlds, non-transiting planets barely get mentioned. Even the direct imaging of ~30 years from now will not be able to measure the radius of non-transiting planets: only the brightness can be measured. And as we all know from the demise of Pluto, bright objects can either be large and non-reflective (Pluto, circa 1930) or small and shiny (Pluto, circa 2000), leading to serious confusion on the question of habitability.

Measuring the radius in combination with an estimate from the mass gives you a density, which is a peek into the internal composition, the formation conditions, the atmospheric history, and the potential habitability of these distant worlds. Transiting planets are information-rich, because they can be physically characterized, with radius and density being only one example. Therefore, transiting planets are unique and valuable. This is why the Kepler Space Telescope was chosen as a NASA Discovery Mission, because it can learn valuable things about planets as small as Earth with present day technology. The value of transiting planets is what motivates me and thousands of other Planet Hunters to look for those wonderful U-shaped dips.

In the first decade since the discovery of the first transiting planet around HD209458 by Charbonneau et al. 2000, the vast majority of transiting exoplanets were single hot Jupiters. As interesting as this population is, I have a bit of a bias against them because I am a dynamicist. Orbital dynamics (or celestial mechanics) is just not that interesting when there are only two bodies (one star and one planet). When we discover multiple planets in the same system, a strong synergy teaches us much more about the formation and evolution of planetary systems. From the orbital architecture we can see if planets are packed close together or spaced far apart. From the non-circularity of orbits, we can see the signature of past epochs of system-wide instability. Many more examples can be given of insights gleaned from the architecture of multi-planet systems.

Furthermore, when there are multiple planets, there are additional unique measurements that can be made. For example, it is possible to detect the weak gravitational interaction between the planets, providing a measurement of the objects’ masses. When the masses can’t be measured, a maximum mass can be determined by noting that if everything were too massive then the whole system would go unstable, contradicting the long-term and orderly procession we are observing. This maximum mass is almost always low enough to exclude stars, proving that the transiting objects do indeed have planetary masses.

As of about a year ago, these two fruitful worlds – the world of transiting planets and the world of multiple planetary systems – were entirely disconnected, except for some theoretical possibilities. Kepler has fantastically bridged this gap with the discovery of 170 (!!!) systems with multiple candidate transiting planets. This includes 115 two-candidate systems, 45 three-candidate systems, 8 four-candidate systems, and one system each with five and six candidates. Wow! There are now way more candidates in multi-transiting systems than non-Kepler transiting planets, and more candidate planetary systems than discovered by all the other techniques over their entire histories combined. This is the power of the Kepler Space Telescope and the data that we’ve been analyzing. Note that CoRoT also just announced their first system with multiple transiting planets.

These systems are *the most information-rich planetary systems outside our solar system* because they combine the value of physical characterization of the planets with the understanding gained by using the tools of orbital dynamics. I am so excited about these systems and the future they afford to our understanding of planetary systems that I wrote an entire paper on the subject, talking about every interesting aspect of these multi-transiting systems that I could think of. In the next blog post, I’ll give some examples of the value of these systems that we’ve already explored on the Kepler Team and give you Planet Hunters some tips in looking for additional planets. For now, let me strongly encourage you to NOT STOP when you see one set of dips and to keep looking for additional transits regardless of what else has been found in the light curve.

Go Planet Hunters!

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