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The search for extra-solar planets has led to the surprising discovery
of many Jupiter-like planets in very close proximity to their host
star, the so-called ``hot Jupiters'' (HJ). Even more surprising, many
of these HJs have orbits that are eccentric or highly inclined with
respect to the equator of the star, and some (about 25%) even orbiting
counter to the spin direction of the star. This poses a unique
challenge to all planet formation models. We show that secular
interactions between Jupiter-like planet and another perturber in the
system can easily produce retrograde HJ orbits. We show that in the
frame of work of secular hierarchical triple system (the so-called
Kozai mechanism) the inner orbit's angular momentum component parallel
to the total angular momentum (i.e., the z-component of the inner
orbit angular momentum) need not be constant. In fact, it can even
change sign, leading to a retrograde orbit. A brief excursion to very
high eccentricity during the chaotic evolution of the inner orbit
allows planet- star tidal interactions to rapidly circularize that
orbit, decoupling the planets and forming a retrograde hot Jupiter. We
estimate the relative frequencies of retrograde orbits and counter to
the stellar spin orbits using Monte Carlo simulations, and find that
the they are consistent with the observations. The high observed
incidence of planets orbiting counter to the stellar spin direction
may suggest that three body secular interactions are an important part
of their dynamical history.

Hippocampal place cells are considered basic substrates of spatial memory, but the degree to which their ensemble representations of space are stable over long time periods has remained unmeasured. By using an integrated, miniature microscope, and micro-endoscope probes, we performed Ca2+-imaging in behaving mice as they repeatedly explored a familiar environment. This approach allowed us to track the place fields of thousands of CA1 hippocampal neurons over weeks. Spatial coding was highly dynamic, for on each day the neural representation of this environment involved a unique subset of neurons. A minority of the cells (~15–25%) overlapped between any two of these subsets and retained the same place fields. Although this overlap was also dynamic it sufficed to preserve a stable and accurate ensemble representation of space across weeks. These findings raise several important questions: What are the biological mechanisms that drive the turnover in the place cell membership of each day’s coding ensemble? What is the functional relevance of these dynamics to hippocampal memory? Overall, this work reveals a dynamic time-dependent facet of the hippocampal representation of space, and introduces a novel approach for investigating, in a behaving animal, how coding in large neuronal populations changes over long periods of time and as function of experience.