An artist’s conception shows Kepler-36c as it might look from the surface of neighboring Kepler-36b. Credits: Harvard-Smithsonian Center for Astrophysics/David Aguilar.
1,530 light years away in constellation Cygnus, Kepler-36 is a sun-like star orbited by 2 known alien worlds. The inner planet, Kepler-36b is a so-called “super-Earth,” ie larger than our home planet but smaller than Neptune; the larger Kepler-36c, resembling the solar system’s outermost planet, is described as a “mini-Neptune.” What is unusual about this planetary system is that these 2 exoworlds have very close orbits, separated only by 0.013 astronomical units (AU)—5X the Earth-moon distance. Princeton scientists are trying to determine physical conditions and understand the evolution process of this curious, distant system.
Based on the available data from NASA’s Kepler exoplanet-hunting spacecraft, James Owen from Institute for Advanced Study in Princeton and Timothy Mortom of the Princeton University performed hydrodynamic calculations to obtain a detailed evaporation model constraining the possible “birth” composition of this system. The model shows direct connection between the system’s present-day observed properties and formation theories like present-day mass and radius as a function of core-mass, core composition, initial envelope-mass fraction, and initial cooling time. “We calculate the evolution of each planet independently, including evaporation and bolometric irradiation by the central star. As we evaluate this evolution on grids of initial physical conditions, we are able to use the inferred posterior distribution of the planets’ present-day properties calculated from the transit timing variations to constrain these initial conditions,” they said.
They found Kepler-36b has an evaporatively stripped core, while Kepler-36c has retained some of its initial envelope due to its higher core mass and both exoworlds could have had a similar formation pathway. With core-mass of 4.4 Earth masses, the inner planet has an initial envelope-mass fraction of less than 10%. The outer planet’s envelope-mass fraction is 15 – 30% and its core mass is ~approximately 7.3X mass of Earth.
Overall, the model gives important information about exoplanet structure shortly after formation.
“A well described birth envelope-mass, core-mass relation along with any intrinsic scatter would provide a strong constraint for any planet formation model,” the paper reads.
The researchers also discovered Kepler-36c had a long initial cooling-time, >30 million years. This points to a dramatic cooling process that took place early in the planet’s life which could be also caused by inefficient heat transport, possibly occurring after giant impacts. What puzzles the researchers is that although the 2 planets are very close to each other, there is a significant disparity in densities. They suspect that some process must have occurred in order to produce such a system after 6 billion years of evolution.
The findings suggest that planets with larger core masses accrete a larger initial envelope, which is not unexpected theoretically. The scientists hope that with a reasonable sample of well constrained planet masses and radii, it would able to infer the birth envelope-mass/core-mass relation and to provide crucial insights into the evolution process of extrasolar planets.
“The observations necessary to begin this exploration has already been provided by Kepler, and the upcoming TESS (Transiting Exoplanet Survey Satellite) mission promises to deliver an even larger, possibly age-dependent sample,” they concluded. http://arxiv.org/abs/1511.07385
http://phys.org/news/2016-02-physical-conditions-exoplanets-kepler-.htmljCp
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