NASA's Kepler telescope trails behind Earth in its orbit.
Since 2014, Kepler has been looking outward along the
ecliptic, the plane of our solar system.
In this orientation, the telescope can observe the
superior planets like Neptune, which passed through its field
of view in late 2014 through early 2015.
As we zoom into Kepler's array of detectors, we see the images
taken by the telescope during this period, sped up from over
two months of observations into a matter of seconds.
The dot orbiting Neptune is its largest moon, Triton, while the
smaller moon Nereid trails faintly to the east.
About halfway through the observation, Neptune appears to
stop its westward march and reverse course.
In reality, Neptune always moves east,
but at a slower rate than Kepler.
This causes an apparent retrograde motion, or backwards
step, each time that the telescope laps the planet.
The Kepler Telescope was originally designed to search
for planets in other solar systems
using the transit method.
When a planet passes in front of its star, it causes a small dip
in starlight that can be measured by Kepler's sensitive
detector, revealing the presence and size of the planet.
During the Neptune observations, Kepler instead looked
at changes in reflected sunlight.
It measured fluctuations in Neptune's brightness of less
than a single percent, caused by factors including the planet's
daily rotation, the movement of clouds, and even seismic waves
within the Sun itself.
By measuring tiny variations in Neptune's brightness, Kepler
teased out clues to the planet's hidden dynamics, pushing the
limits of its detector, and laying the groundwork for more
detailed studies of exoplanets in the years ahead.
[Music]
[Satellite beeping]