The Keplerian revolution: stars and exoplanets with the NASA Kepler Mission

Tuesday 19th January, 2016

Our speaker was Professor Bill Chaplin from Birmingham University who is a Professor of Astrophysics in the School of Physics and Astronomy. He specialises in studying stars using the technique of astroseismology, which is a method of examining the internal structure of stars by recording their various pulsations. He came to talk to us about "The Keplerian revolution: stars and exoplanets with the NASA Kepler Mission".

Prof Chaplin explained that just like boiling water in a saucepan the ionized gases in a layer beneath our Sun's surface convect as energy is transferred from the Sun's core to its outer layers. This convection causes turbulence which, in turn, produces various pulsations or resonances which travel throughout its interior. These pulsations or resonances occur at a range of frequencies and are recorded as undulations in the star's surface. Just as a geologist can use the seismic waves generated by earthquakes to interpret the Earth's inner structure an asteroseismologist can study a star's pulsations to peer deep into its interior.

He commented that stars can be thought of as "out of tune" instruments that become more dissonant with age. The frequency of the pulses is related to the age of the star with larger stars pulsing at a slower rate compared with small stars. Also different frequencies penetrate to different depths and analysing the data reveals not only the size of the star but also other properties such as its total mass and density variations. It is this ability to accurately measure the properties of a specific star that is aiding NASA's Kepler mission in the search for Earth-like planets.

Prof Chaplin uses data from the Kepler spacecraft which was initially designed to target a portion of the sky around the constellations of Cygnus and Lyra. It launched in 2009 and for over four years it monitored over 150,000 stars searching for dips in their brightness that would signify a planet crossing in front of the star's disc. Unfortunately, in 2013 it was no longer able to stay tightly locked on its target as it lost the use of a second of its four reaction wheels. These are essentially a type of flywheel that a spacecraft uses to maintain its orientation in space instead of using thrusters and a finite supply of fuel.

However, by carefully reorienting the spacecraft so that the pressure of the solar wind acts to stabilise it, NASA's engineers have been able to restart observations. It is now orientated with its main body parallel to its orbital path around the Sun. It can study a specific area of the sky for about 75 days before it needs be rotated to prevent sunlight entering its telescope. The new mission has been named K2 and may continue operating until 2018.

One way that the general public can help with Kepler's mission is to visit the Planet Hunter's website (www.planethunters.org) to examine light curves, looking for dips which you mark with a box. Last autumn participants spotted a strange event when the light from a star dimmed by 20% of its normal brightness for a long period and in very irregular pattern. This generated much speculation in the media as to whether we had spotted some vast alien structure or just a large field of comets breaking up. The answer to this intriguing observation is still not known.