The Latest Superstar Couple: Closing in on Kepler-36

Kepler 36c, a newly discovered gas giant, would appear impressively large in the sky of its nearby neighbor, Kepler 36b, a smaller, rocky inner planet. Courtesy of Harvard-Smithsonian Center for Astrophysics, Illustration by David Aguilar .

Imagine looking up at the night sky and seeing a gas giant three times the size of our ordinary moon. This is how Kepler-36c, a newly discovered gaseous planet, appears from the surface of Kepler-36b, its rocky neighbor in the Kepler-36 planetary system. The Kepler-36 system is the first planetary group comprised of planets of such strikingly contrasting densities and compositions.

“Are we alone in the universe?” This question first captured the imaginations of scientists, theologians, and philosophers hundreds of years ago, and fuels the current exploration of planets outside our solar system. As of now, a total of 838 confirmed extra-solar planets have been discovered. But Kepler-36 mystified astrophysicists who wondered how two vastly different worlds ended up in such close orbit. As Dr. Sarbani Basu of the Department of Astronomy at Yale explained, “The pivotal moment in our project was when we figured out that these two very different planets are orbiting around the same star. That’s when we started to look at the system as a whole, rather than as just two more extra-solar planets.” Extra-solar planets are nothing new to astronomers like Basu, but strikingly different planets in close proximity presented scientists with a yet-undiscovered type of planetary system.

“One of These Things is Not Like the Other”

Modern astronomy hypothesizes that large gas-giant planets cannot form close to their host stars because stellar wind would blow away most of the surrounding gas in the disc, causing the planet to lose mass quickly. Therefore, this discovery of a massive Jupiter-like extra-solar planet, or exoplanet, near a main-sequence star has opened new horizons to studying planetary formation. When researchers studying this system first landed on the data relayed back from NASA’s Kepler spacecraft, they were no longer surprised to see the gas giant Kepler-36c close to its parent star. They were surprised, however, to see planets as different as Earth-like Kepler- 36b and gas-giant-like Kepler-36c coexisting within such close orbital planes. The discovery of diverse planets separated by a mere 0.013 AU revolutionized astronomical thinking on how planetary systems form and evolve.

In our solar system, there is a clear differentiation between rocky and gaseous planets; the former are confined to the inner part of our system, and the latter found in the outer parts. This trend is in accord with condensation theory, which hypothesizes that interstellar dust is an essential ingredient in the formation of planets. Areas closer to the sun are at higher temperatures and are home to the hotter, rocky planets. Farther away from the sun, colder temperatures allow gases to move slowly and are thus more affected by gravity. As a result, interstellar dust grains come together to form the foundation of gas giants.

The curves above represent theoretical models for planets of a given composition, with dotted curves modeling terrestrial bodies and dashed curves modeling Earth-like solid cores surrounded by hydrogen or helium envelopes. Courtesy of Science.

The detection of giant planets close to other stars proves that this pattern is not universal; planetary orbits can indeed change significantly after their formation. Basu’s international team of approximately 40 scientists from five different countries set out to uncover the intricacies of this anomaly: the Kepler-36 system and its two starkly contrasting planetary members. Kepler-36b, nicknamed a “Super-Earth,” is rocky like our home planet but is a staggering 4.5 times more massive with a radius 1.5 times greater than that of Earth. Kepler-36c, is a gaseous planet 8.1 times more massive than Earth with a radius 3.7 times greater. Kepler-36b and Kepler-36c are 20 times more closely spaced and have a larger density difference than any adjacent pair of planets in our solar system.

Joining Forces: From Planetary Closeness to Scientific Collaboration

The planetary subgroup of the team, led by Josh Carter, a Hubble fellow at the Harvard Smithsonian Center for Astrophysics, discovered the larger planet and its host star during a first quick look at data from the Kepler spacecraft, a space observatory launched by NASA to discover Earth-like planets orbiting other stars. They noticed the the larger planet Kepler-36c as it transited in front of the host star, blocking some of its light to give a characteristic dip or transit signal. However, while Kepler planetary data can tell you how big a planet is relative to its star, it cannot determine how big this planet is with certainty until the size of the star is determined. This is where the stellar physics subgroup that Basu is involved with came in. Using asteroseismology, the study of stars by observing their natural solar-like oscillations that occur as a result of sound wave excitation by turbulence in the star, they determined the properties of the system’s parent star. By measuring various oscillations, the team was able to calculate the size, mass, and age of the host star to exquisite precision. Once the group gave the stellar data to the planetary team, Carter and his colleagues were able to further analyze Kepler-36c to discover its size and composition.

The Power of the Human Eye

At first, the Kepler-36 team did not realize Kepler-36c had company, let alone close company. The smaller Kepler-36b planet is so small that it does not leave much of a signature in the amount of light it blocked from Kepler-36a. Consequently, it was thus rejected by the automatic code of the Kepler data analysis center, which makes the usual assumption of periodic orbits and cannot detect imprecise periods of relatively smaller planets such as Kepler-36b’s. However, Kepler-36b is so remarkably close to its massive neighbor that it alters the gravitational field felt by the smaller planet and changes the strict period nature of trasits. This is known as transit timing variation. Because this period was not well determined, Carter’s team had to collect data by hand instead of using the usual programs. They used an algorithm known as “quasi-periodic pulse detection” to methodically check planetary systems already in the Kepler data, and in this way stumbled upon Kepler-36b. Basu emphasized that this discovery would never have happened without persistent manual follow-up to the team’s practiced intuitions. “The major implication in my mind is to not believe in automated pipelines. Nothing can substitute for the human eye. If Josh hadn’t looked at this system by eye, we wouldn’t have known that there was this second rocky planet sitting there.”

Colored pixels represent relative flux increasing from blue to green, with each row symbolizing an individual transit light curve. The Kepler 36 planets’ curved bands convey fluctuating periods, a product of their gravitational pulls on each other. Courtesy of Science.

The Future of Kepler-36

The discovery of the close two-planet system has shed light on extreme violations of traditional orbit-composition patterns. The international team studying this system galvanized the astronomy community with greater interest in understanding how planets with such different compositions can fall into such astonishingly close orbits. Kepler-36b and Kepler-36c have managed to achieve orbital stability at fascinatingly close range. The team has announced its determination to continue analyzing more Kepler data to locate similar planetary systems in the hopes of unearthing similar close encounters.

About the Author
Li Boynton is a junior in Calhoun College double majoring in Molecular, Cellular and Developmental Biology and East Asian Studies. She is the Production Manager for the Yale Scientific, and works in Dr. Anjelica Gonzalez’s lab using bioengineered transmigration models to study immunological responses to inflammatory signals.

The author would like to thank Professor Sarbani Basu for sharing her knowledge on astronomy and the Kepler-36 system.

Further Reading
Carter, Joshua A., et al. “Kepler-36: A Pair of Planets with Neighboring Orbits and Dissimilar Densities.” Science 337 (2012): 556-59. AAAS. Web. 25 Sept.