The Shaw Prize in Astronomy for 2015 is awarded to William J Borucki for his conception and leadership of the Kepler Mission, which has greatly advanced knowledge of both extrasolar planetary systems and stellar interiors. He is the Principal Investigator of the Kepler Mission, NASA’s Ames Research Center.

In 1984, William J Borucki and Audrey Summers published a paper assessing the potential for detecting extrasolar planetary systems by transit photometry. The key concept is to simultaneously monitor the brightness of a large number of stars. Planets are revealed by the dips in brightness they produce when they pass in front of (transit) their host stars. Successive transits by a planet are spaced by its orbit period, which helps to distinguish transits from other sources of stellar variability. Transit depths determine the ratio of the planet’s surface area to that of its host star. William J Borucki and Audrey Summers emphasized that detection of Earth-size planets would require observations from above the atmosphere.

Subsequently, William J Borucki began a long quest to convince the astronomical community and the US National Space and Aeronautics Administration (NASA) that a modest space mission could discover planets potentially capable of harbouring life. Four proposals submitted between 1992 and 1998 were rejected before the fifth was selected in December 2001 as Discovery Mission #10. Mission development began in 2002 and launch occurred in March 2009.

An important milestone in gaining acceptance for the Kepler Mission was the demonstration that photometry from space would be precise enough to detect Earth-size planets transiting Sun-size stars. Since Earth’s radius is about one hundredth of Sun’s radius, this requires measuring brightness variations smaller than one part in ten thousand. Kepler routinely achieves higher precision and as a result has discovered planets even smaller than Earth. Five years after launch, the Kepler Mission has discovered thousands of planets. Many stars harbour multiple planets. Two stars were found to have at least six. Similar to the solar system, systems of multiple planets are flat. Given geometric biases affecting transit detections and the limited time since launch, planets discovered by Kepler tend to orbit close to their parent stars.

Kepler also accomplished an important secondary goal, the precise measurement of photometric variations due to stellar oscillations. Determination of oscillation frequencies informs us about stellar ages, masses, radii, and internal rotation rates.

Further technical explanations
In classical astronomy, a transit refers to a phenomenon in which a planet (which must be an inferior planet, i.e., one that is closer to the Sun) moves between the Sun and the observer (namely the Earth). Because the solar disk can be resolved, a transit is revealed as a small black dot moving across the solar disk. The inferior planets Mercury and Venus were known since antiquity, and the transit method was widely used in the 19th century to search for other inferior planets — of course none were found.

The same idea can be applied to other stars. If there is a planet orbiting a host star, and the planet moves between the host star and the observer (namely us), part of the light is blocked. But because these stars are far away, their surfaces cannot be resolved, and we do not see a black dot moving across the stellar disk. All we can observe is that the star’s brightness is slightly decreased, by the ratio between the area of the planet and the area of the star. The work of Borucki and his team was to detect this tiny but periodic decrease in the brightness.

Astronomy Selection Committee
The Shaw Prize

1 June 2015  Hong Kong