by Ruth Titz-Weider (DLR), August 2024
Kepler is a NASA mission named after the German astronomer Johannes Kepler (1571-1630). Like the previous and future space missions in search of exoplanets – CoRoT, TESS, CHEOPS and PLATO – Kepler uses the transit method. In order for a search for a tiny, regular dimming to be successful, a large number of stars are observed simultaneously and their intensity recorded. The aim of the mission was to find Earth-sized planets in the life-friendly zone of sun-like stars and to estimate the frequency of their occurrence, i.e. distribution statistics.
Course of the mission
Kepler was launched from Cape Canaveral on 7 March 2009 on a Delta launcher and was in operation until October 2017. The telescope worked with the expected spatially precise alignment and observed a field of around 190,000 stars in the constellation of Swan for almost five years. Hundreds of transiting planets were found in this data. In May 2013, two of the four drive wheels that were essential for the precise alignment of the telescope failed. This meant that it was no longer possible to observe a field for years and the main mission was officially ended in August 2013.
Artist’s impression of the Kepler satellite with the observation fields of the K2 mission, which were all in the ecliptic plane. Source: NASA / Ames / JPL-Caltech
But the engineers found a way to continue using the satellite, albeit in a different mode: the solar wind was used for stabilisation. This new phase of life was called K2, which was chosen with reference to the second highest mountain on earth. The observations were only possible with restrictions due to the lack of alignment and the ageing instrument. The observation fields were located along the ecliptic and the telescope was aligned to a new region of the sky every 75 days. Twenty fields, most of which did not overlap, were observed between March 2014 and September 2018. It provided new data that was immediately available to scientists around the world. On 30 October 2018, this secondary mission was also ended due to a lack of fuel, and on 15 November Kepler was finally switched off.
Instrument
The telescope was built as a Schmidt telescope with a primary mirror of 1.4 metres and a Schmidt plate diameter of 0.95 metres. The large aperture of the telescope ensured that enough photons could be received from a star. In the focus of the telescope were 42 CCD sensors with a total of 95 million pixels. The CCDs are arranged in pairs to form 21 squares.
A mosaic of 42 CCDs, each with 1024×2024 pixels, was mounted in the focal plane of the Kepler telescope. Credit: Kepler Mission / NASA
Field of view
The Kepler telescope had a very large field of view, about 12° edge length. This corresponds roughly to the area of the sky covered by an outstretched fist. In the first five years, during the nominal mission, the telescope looked continuously at a field in the constellation Swan and observed around 190,000 stars.
The field, which the Kepler telescope observed continuously for almost four years, was located in the constellation cygnus. Credit: Carter Roberts / Eastbay Astronomical Society
In the second mission phase, the K2 mission, in which the telescope could no longer be precisely aligned for a longer period of time, fields were observed along the ecliptic.
Orbit
Kepler did not move around the earth, but with the earth around the sun. The distance was greater than that of the Earth and the orbital period was 375 days. The satellite therefore took a little longer than the Earth to orbit the sun and thus travelled further away over the years. The advantage of such an orbit is that the satellite was never obscured by the Earth and was only exposed to minor interference.
Results
Kepler has discovered several thousand planet candidates, most of which have been confirmed. These include Kepler-22b, a super-Earth with an orbital period of 290 days, which lies in the habitable zone of a sun-like star and whose first transit was recorded shortly after the mission began in 2009, or the planetary system Kepler-187 with Kepler 187f with an orbital period of 130 days and a radius of 1.1 Earth radii. Another find is the Kepler-90 planetary system with eight planets, arranged similarly to the solar system – the smaller ones close to the star, the larger ones on the outside – but everything would fit within the Earth’s orbit, a shrunken planetary system.
The exoplanet Kepler-186f is about the size of Earth and orbits in the life-friendly zone of its star. Credit: NASA / Ames / SETI Institute / JPL-Caltech
The large number of planets discovered and their radius allows statistics on the size distribution and frequency. However, it is not possible to specify a mass for many of these Kepler planets. This is due to the fact that the corresponding stars only appear very faint and therefore no follow-up measurements are possible. Follow-up measurements using the radial velocity method could be used to find out the mass of the planet and thus determine its average density, which would allow an initial classification of whether it is a rocky or gaseous planet.
