Extra-Solar Planets
Keith Horne, Andrew Cameron, Jane Greaves, Martin Dominik
The search for extra-solar planets, the determination and prediction of their abundance, as well as the understanding of how these form and how life can develop constitutes the core research of the Astrobiology Initiative within the Astronomy and Space Physics Theme of the Scottish Universities Physics Alliance (SUPA). It will provide clues on how frequent life might be in the Universe and where to look for it.
In St Andrews, we are currently carrying out experiments for detecting extra-solar planets based on two different techniques – transits and microlensing –, while also studying debris disks around nearby stars, which constitute candidate systems for the presence of planets. Our research featured in an exhibit ``Is there anybody out there? Looking for new worlds'', which was concepted for the 2008 Royal Society Summer Science Exhibition.
TRANSITS: Doppler surveys show that 1% of nearby main-sequence stars host "hot Jupiters" in 1-4 day orbits with radii less than 0.05 AU. 10% of those should have orbits close enough to edge-on for the planet to transit in front of the star. Transiting planets are uniquely useful because the drop in light level as the planet crosses the star gives us precise measurements of the planet's radius. As a leading member of the UK SuperWASP project, we are conducting a wide-angle survey of bright stars for transits of large, short-period planets with two identical robotic observatories giving access to both the northern and the southern sky. Working in collaboration with the Geneva planet-search team in the southern hemisphere and the SOPHIE radial-velocity spectrometer team in the north, the SuperWASP team has discovered 60 transiting planets between 2006 September and 2011 August. We have been allocated time on the Hubble Space Telescope and the infrared Spitzer Space Telescope to make precise measurements of the sizes and atmospheric temperatures of many of these planets. Using the HARPS spectrometer on the ESO 3.6-m telescope, we have discovered that the orbits of many of these planets are strongly tilted or even retrograde relative to the stellar spin axis, providing important dynamical clues to their origins. Through SUPA-2, the University of St Andrews has contributed to the construction of the Geneva-led HARPS-North radial-velocity spectrometer, to be commissioned at the 3.5-m Telescopio Nazionale Galileo (TNG) on La Palma early in 2012. Its primary goal will be to measure the masses of low-mass planet candidates discovered by the NASA Kepler mission.
MICROLENSING: At any given time, only one in a million stars in the Galactic bulge is significantly brightened due to the bending of its light caused by the gravitational field of an intervening foreground star, a phenomenon known as (galactic) gravitational microlensing. However, daily monitoring of hundreds of millions of stars by survey teams (such as OGLE or MOA) results in about 1000 on-line alerts of ongoing microlensing events per year. A planet orbiting the foreground 'lens' star can reveal its existence by producing a brief deviation on the observed light curve, lasting from days for a Jupiter down to hours for an Earth. Since 1997, members of our group have been involved in the PLANET (Probing Lensing Anomalies NETwork) collaboration, which was the first systematic effort to hunt for extra-solar planets by means of high-cadence round-the-clock follow-up observations on ongoing microlensing events. While PLANET used a network of staffed 1m-class optical telescopes, we pioneered the use of 2.0m robotic telescopes with RoboNet-1.0 and now RoboNet-II, as well as the deployment of algorithmic schemes for optimal target selection and anomaly detection resulting in the ARTEMiS (Automated Robotic Terrestrial Exoplanet Microlensing Search) system. A fully-deterministic microlensing campaign with RoboNet-II and MiNDSTEp (Microlensing Network for the Detection of Small Terrestrial Exoplanets) will lead to a census of cool planets down to Earth mass and even below orbiting either Galactic disk and bulge stars, and thereby probe models of planet formation and orbital evolution in a region that is not reasonably accessible by any other means.
Our efforts were crucial to the discovery of OGLE-2005-BLG-390Lb, the first cool rocky/icy exoplanet, which had a huge echo in the international news media. With 3-10 Earth masses orbiting at a few AU, OGLE-2005-BLG-390Lb occupied a unique position among the known exoplanets closest to the terrestrial planets of the Solar system at the time of its discovery. With a planetary signal that corresponds to a current snapshot, gravitational microlensing can reveal the presence of planets whose orbital period is too long for other techniques being applicable, but which are still too close to their parent star for a direct detection. The detection of a first Solar system look-alike by means of the microlensing event OGLE-2006-BLG-109 showed that planetary systems should be considered the rule rather than the exception, and that the Solar system is unlikely to be unique in the Universe.
DEBRIS DISKS: Submillimetre-wavelength observations allow us to detect belts of 'debris' around nearby stars. The debris is generated by collisions between extrasolar comets, giving us a way to infer the presence of bodies as small as a few km across.
The structure within the belts can be used to pinpoint the locations of perturbing planets. In addition, the inferred number of comets tells us what the environment might be like for any Earth-like planets around these stars, for example whether impact events are common or rare. The figure shows the debris belt around the nearby star epsilon Eridani.