High-precision studies of extrasolar planets and low-mass stars

In 1995 we finally detected planets around other stars; we now know that our own Solar system is not unique. Most of these exoplanets have been found by measuring the changes they cause in the velocity of the parent star as they orbit it. This, though, does not give much information about an individual planet: only how close it is to its star and a lower limit on its mass. Fortunately, some planets periodically pass in front of their parent stars: these valuable objects are called transiting exoplanets. During a transit they block a small amount of light from their star, and this amount tells us their diameter. As we are also able to measure their mass, rather than just a lower limit, for these exoplanets we can calculate their density and the strength of the gravity at their surface. This in turn indicates what the planet is made of, and how it might have formed. Therefore we can now study the formation and evolution of planets, without having to rely on just those in our Solar system. Understanding their formation is a vital step towards finding out how many there are in the Universe, and if some might support life. We currently know of 370 planets, of which about 60 are transiting. The next step is to detect Earth-like planets. My own research concentrates on measuring the masses and diameters of transiting exoplanets as accurately as possible. Firstly I measure how much fainter a star becomes when its planet passes in front of it. These transit events last for generally a few hours so are easily observed in one night, but are very shallow so require extremely precise brightness measurements. I had the idea that a poorly-focussed telescope might actually give better results than a well-focussed one, because a lot of light could be detected from the star without saturating parts of the light detector. After a year of experimenting I have made this approach work well: my set of measurements of WASP-2 now hold the overall record for the most precise observations of a star (other than our Sun) from a ground-based telescope. I am now using this technique to study many of the known exoplanets in order to measure their masses and diameters. But I am not the only person doing this kind of work: quite a few people study exoplanets. Unfortunately they all analyse their data in slightly different ways, so their results cannot be compared directly. I am therefore studying the data obtained by other researchers as well as myself, using exactly the same methods for every planet to ensure my results are completely consistent. My results can then be used for detailed statistical analyses, which are a vital way of investigating how planets form and evolve. But there remains a problem with analyses of exoplanets. We measure their properties relative to the properties of their parent star, but we do not always understand the stars as well as we would like. The third and final part of my work concentrates on studies of stars like those which host planets, but are instead orbited by a second star rather than a planet. These are eclipsing binary systems. Because we can detect light from both stars it is possible to measure their masses and diameters from purely observed quantities. Eclipsing binaries form the foundation of our understanding of almost all stars, and so help us to understand those stars which host planets. In turn this allows us to study the planets themselves, and thus continue to improve our knowledge of how star and planets, and perhaps even alien life, form and evolve.