EXOPLANETS 2012 ASTRO SUMMER SCHOOL

Historical Background In the sixteenth century the Italian philosopher Giordano Bruno put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. He was burned at the stake by the Roman Inquisition in 1600, though his views on astronomy were not the main reason for his condemnation.

First Confirmed Discovery On 21 April 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR This discovery is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.

First planet around a main sequence star On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first detection of an exoplanet orbiting 51 Pegasi. It was discovered via the transit method.

As a planet orbits a star, both objects orbit around the center of mass. The movement of the star can be detected by looking at its spectral lines – we can now detect velocities below 1ms -1. This method has so far found the highest number of exoplanets, although it can only be used to find a lower limit on the planet’s mass, not the actual mass Doppler Method

Transit Method If a planet crosses in front of the star, it will create a small drop in the apparent brightness. This enables us to measure the size of the planet but only works for systems in the correct orientation.

Transit Timing Variation

Gravitational Microlensing Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Planets orbiting the lensing star can cause detectable anomalies in the magnification as it varies over time.

Pulsar Timing Pulsar timing can detect planets less than a 10 th of the mass of Earth. Unfortunately these planets would not be habitable.

Direct image of exoplanets around the star HR8799

Infrared image of 2M1207 and 2M1207b

Formalhaut b

How many stars have planets around them? It is difficult to give a definitive answer to this. We have only just started our discovery of exoplanets but so far, the numbers look something like this: 1-1.5% of sun-like stars have hot jupiters. 20% of sun-like stars have at least one giant planet. 40% of sun-like stars have planets of lower mass. In total it is estimated that the Milky Ways contains billion planets.

< 300 light years

What type of stars have planets? Most currently known planets orbit around sun-like stars, but this is mainly because current observing programs tend to concentrate on these types. The smallest type M stars probably don’t have planets, the largest O type are so hot that they produce a photo- evaporation effect that stops planets from forming.

What is the mass of most planets? As of 2012, all but 50 known exoplanets are over 10 times the mass of the Earth. This is because larger planets are easier to detect. Initial results from the Kepler spacecraft show that low mass planets are probably far more common than high mass.

Predicted sizes of different planet types

Isolating a Planet’s Spectrum

Comparing the spectrum to models

Detecting water in HD b

Measuring the temperature of the surface

Which exoplanets are habitable?

Upsilon Andromedae d

The Kepler Mission

Kepler’s focal plane (camera)

Kepler Results As of 28 th June 2012, Kepler has discovered and confirmed 74 planets. There are currently 2321 planet candidates that need to be followed up with more observations.

Data Visualisation