By Professor William Chaplin, Professor of Astrophysics
When you look up at the sky on a clear night do you ever ask yourself: how many of the twinkling stars have planets, like the planets orbiting our own Sun? And did you know that stars like the Sun ring, like musical instruments?
Thanks to the launch of the NASA Kepler Mission the past three years has seen dramatic progress in the study of other stellar systems in our galaxy, and scientists at Birmingham are playing a leading role in this exciting work. Kepler has been continuously monitoring the brightness of around 150,000 stars in our galaxy and has to date discovered over 2,000 candidate planets by the miniscule dimming of light from the stars as planets transit, or pass across, their visible faces. In order to properly understand the formation, the evolution over time, and the frequency of habitable systems like our own, we must not only find and measure the properties of small rocky planets, but also fully characterise the properties of their host stars. Kepler’s exquisite data have revolutionized the study of stars, in particular thanks to the application of a powerful new technique called asteroseismology, the study of stars by observation of their natural resonances which manifest as surface oscillations.
Asteroseismology is a rapidly growing field of astronomy, and over 500 scientists are using Kepler data to study stars as part of the international Kepler Asteroseismic Science Consortium (KASC). Professor Chaplin has been leading the 200-strong part of the KASC which is responsible for the study of stars similar to our own Sun; and also a smaller team which has been working directly with the Kepler Mission’s exoplanet Science Team to use asteroseismology to constrain the properties of stars that are newly discovered exoplanet hosts. Professor Elsworth and Dr Miglio lead international research using data from Kepler, and another space telescope called CoRoT, on studies of red giants, stars like the Sun nearing the end of their life cycles.
Stars resonate like musical instruments. Sound is made naturally in the outermost layers of Sun-like stars. This sound is trapped, with some sound waves penetrating all the way to the centres of the stars. The sound waves are able to reinforce to make the stars resonate at their natural frequencies, like waves inside a wind instrument. The compressions of the trapped sound makes the stars oscillate and we are able to detect this gentle breathing by observing small, periodic changes in brightness as stars get slightly hotter as they are compressed, and then cooler as they relax. By measuring the properties of the oscillations, we may use this “music of the spheres” to not only estimate the fundamental properties of the stars (e.g., size, mass and age, to level of precision and accuracy that cannot usually be reached in astrophysical observations) but we may also peel away their surface layers to probe the structure and dynamics of their normally hidden interiors.
When a planet is discovered by the transit method the tiny dip in the amount of light received from the star provides a measure only of the size of the planet relative to the star. Thanks to asteroseismology, we can measure the size of the star extremely precisely, allowing the size of the planet to be fixed with a high level of confidence.
The international team, led by Professor Chaplin, performed the stellar characterisation work of the planet-hosting star Kepler-37, which in turn led to confirmation of the extremely small size of one of its planets, as reported in 'A sub-Mercury-sized exoplanet' published in Nature (Volume 494, Issue 7438, pp. 452–454). The host star is smaller and cooler than the Sun, and hosts three planets. Because it is so small, one of the planets was barely detected by the Kepler space telescope.
Thanks to asteroseismology, it was possible to measure the radius of the star to a level of uncertainty of just a few per cent; and only with those tight constraints was it then possible to say with some confidence that the tiny planet orbiting Kepler-37 has a radius that is smaller than Mercury, and not much larger than our Moon. This research shows for the first time that other stellar systems host planets smaller than anything in our solar system, providing further information for scientists working on the formation and evolution of planetary systems, and also helping to place our own solar system in a wider context.
The Best Publication of the Month for February was awarded to Professors Bill Chaplin and Yvonne Elsworth and Dr Andrea Miglio, for the paper 'A sub-Mercury-sized exoplanet' published' in Nature (Volume 494, Issue 7438, pp. 452–454).
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