The Kepler Spacecraft Observes Exoplanets
Kepler was launched into space in 2009 to collect statistical information about exoplanets; however, mechanical issues in 2013 rendered its telescope incapable of maintaining steady pointing. Engineers then revised K2 so it uses solar pressure to continue staring at patches in Cygnus and Lyra sky.
Photometry
Kepler space telescope searches for Earth-sized exoplanets through the transit method – by observing repeated dips in starlight caused by orbiting planets passing in front of their host stars – using high precision starlight detectors capable of measuring shadow cast by these planets as they pass in front of their host star.
Scientists began taking seriously in the 1980s their idea for a space telescope capable of searching continuously for dimming in the sky, using transit photometry. It was decided to observe fields in Cygnus-Lyra because this region offers more stars for easy detection of variations in brightness of single sources.
Kepler uses background pixels collected during every observation to improve photometric precision by creating a model of background flux distribution within its focal plane, then applying this model to target pixels to correct for celestial and instrumental backgrounds such as zodiacal light.
Observations
The Kepler telescope was developed to monitor stars and detect light changes caused by planets transiting (passing in front of) their stars, which astronomers use as indicators of size, shape and orbital characteristics of planets.
The CCD detectors on board the spacecraft are similar to silicon light-sensitive chips found in digital cameras and camcorders, with pairs being assembled using sapphire field-flattening lenses for each pair of CCD detectors.
Kepler was designed to monitor stars outside our galaxy’s ecliptic plane to avoid Earth and its glare; therefore, Kepler observed stars from Cygnus and Lyra constellations which lie well outside its ecliptic plane, free from obscurations, stray light, or gravitational perturbations caused by Earth orbital dynamics. This prevented its own orbit around our Sun from producing gravitational perturbations causing its observations of them to be affected.
In May 2013, one of Kepler’s four reaction wheels failed, leaving it without control over its three axes of freedom (X, Y and Z – roll, pitch and yaw). Engineers devised a new mission design called K2 to search for planets by aiming the telescope at various parts of the sky in a cycle called “campaign.”
Data
The 0.95-m aperture Schmidt telescope onboard the spacecraft houses a focal plane array consisting of 21 science and 4 Fine Guidance Sensor CCD modules, featuring field flattener lenses on each module to map spherical telescope images directly onto CCD chips and create wavelength bandpass filters for overall wavelength bandpass control.
Kepler conducts three 90-day observation periods per year, during which it will monitor 145,000 main-sequence stars continuously and search the data for planets orbiting these stars that must periodically pass in front of their host stars and dim their light as they do so. Scientists then search the data for signs that planets exist around these stars that scientists believe may harbor planets orbiting them – they do this by monitoring when any planet passes in front of its host star and thus dimming its light as its orbit changes over time.
The spacecraft’s 42 CCD detectors will continuously download pixels surrounding selected target stars on a 30- or 1-minute cadence, which are then transmitted back to Earth for analysis in order to detect periodic dimming caused by planets passing in front of their host star. Data Management Center at STScI receives these “raw” Level 1 data for calibration, transit detection and characterization; further observations on candidate planets can be conducted at SAO, USNO, Harvard Lowell Observatory and NASA JPL by scientists.
Discovery
Astronomers have identified thousands of exoplanets circling other stars thanks to the Kepler space telescope, named for 17th-century astronomer Johannes Kepler. This telescope focused on an area roughly the size of Earth’s moon in our galaxy where stars pass directly in front of each other causing starlight dimming due to planet passage in front of their hosts causing their starlight to dim as planets passed in front and caused starlight dimming due to planet presence.
Kepler completed its 3.5-year mission between 2009 and 2012 by surveying an area approximately the size of the moon held outstretched in an outstretched hand, collecting more than 678 gigabytes of data. When one of Kepler’s reaction wheels failed, K2 assumed responsibility for finishing out its lifetime mission.
The K2 mission was created to assist researchers in identifying promising targets to be observed by other telescopes, such as those capable of observing stars with apparent brightness variations due to eclipsing binaries or star spots. Learn more by visiting its Overview page.