Transcript for "Kepler Stares at Neptune"
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.