Exoplanets and how to find them Andrew Norton Image Copyright: Mark A. Garlick. Science Credit: Carole Haswell & Andrew Norton (The Open University).

20 years ago – astronomers knew of one planetary system – our own 4 rocky inner planets / belt of rocky asteroids / 4 gas giant planets each with dozens of rocky satellites / belt of rocky+icy small bodies including Pluto / Oort cloud of comets extending half-way to next nearest star Astronomers expected other planetary systems may look similar – good science / bad science ? Wikipedia – Creative Commons Author: Lsmpascal

Mass? Size? Length of year? Temperature? Composition? Atmosphere? That all changed in 1995 with the discovery of the first exoplanet – a “hot Jupiter” system Since then around 1000 exoplanets have been discovered (and many other candidates) by a variety of techniques Mass? Size? Length of year? Temperature? Composition? Atmosphere?

The ultimate goal is to find earth-like planets in earth-like orbits around sun-like stars - as potential habitats for life. These are all artists impressions – planets only reflect light, but all we ever see (in the vast majority of cases) is a pin-point of light – a star. How can we detect exoplanets, let alone measure their properties? By the influence a planet has on the star it’s orbiting.

SuperWASP This is SuperWASP – Wide Angle Search for Planets – two systems (La Palma & South Africa) – project involving astronomers from OU. Monitor almost the whole sky every night. Each image contains hundreds of thousands of stars. Measure the brightness of every star on every image, many times per night for many years. Then string the brightness measurements of each star to form so-called “lightcurve” Then hunt through the lightcurves of millions of stars looking for tell-tale dip in brightness caused by an exoplanet passing in front of the star. So far SuperrWASP has found about 100 exoplanets in 30 million lightcurves / 300 billion data points. © David Anderson

“wink” First – from the observed brightness and colours of stars – we can work out their masses, sizes, luminosities & temperatures pretty well. How often the dips repeat tells us the orbital period (year length) of the planet From which we can work out the distance of the planet from the star & its likely temperature. The depth of the dip tells us the size of the planet (relative to the star) – Jupiter-size planet + Sun-size star = 1% dip The duration of the dip tells us the angle of inclination of the orbit So – just from transit lightcurve : year length, size of orbit, angle of orbit, planet size, planet temperature – remember: just a point of light 2.4%

“wobble” Unseen planet + 240 m/s Follow-up transiting planet candidates with spectroscopy Look for Doppler shift in light of star as it is tugged back and forth by orbiting unseen planet Measure Doppler shift at different times – if a genuine planet: will see this repeat on same period as transits. From amplitude of Doppler shift - work out mass of planet From spectrum during transit – may detect chemical signature of exoplanet’s atmosphere. So – from combined transit lightcurve & spectroscopy – work out: year length, size of orbit, angle of orbit, planet size, planet mass, planet density, bulk composition, planet temperature, planet’s atmospheric composition – remember: just a point of light! 240 m/s

Watch this space … http://science-people.open.ac.uk/a.j.norton A thousand billion stars in our galaxy – best chance of detecting life is probably looking for chemical signature in spectrum of a transiting exoplanet. Maybe just green slime you could scrape off a rock with your finger – but it would be life. I believe it will be found in the next 20 years – if it exists. http://science-people.open.ac.uk/a.j.norton Credit & Copyright: Hubble: NASA, ESA, & D. Q. Wang (U. Mass, Amherst); Spitzer: NASA, JPL, & S. Stolovy (SSC/Caltech)