Exoplanets and how to find them

19 March 2012

Dr Andrew Norton spoke about finding exoplanets at the IOP on 14 March 2012.

Institute of Physics News

The possibility of the existence of non-solar system planets was known for a long time. Lucretius, the Epicurean poet and philosopher commented on the possible worlds phenomenon. At that time, it was known that planets orbited stars. 

These insights were followed by eminent philosopher-scientists such as Albertus Magnus in the Middle Ages, Giordano Bruno in the Renaissance and Huygens in the 17th century. 

It was not until 1995 when advances in technology and computing allowed a steadfast progress in this area by detecting and visualizing these planets. 

In October 1995, Mayor and Queloz announced the discovery of an exoplanet orbiting the star 51 Pegasi. The planet was named 51 Pegasi b. 

It orbits very close to the star, is nearly half the mass of Jupiter but with a temperature of 1200ºC (hence the name hot Jupiter). Today, nearly 800 planets have been detected and their number is increasing rapidly.

The lecture then progressed with a discussion of four detection methods for exoplanets:

1. Detection by Direct Imaging, which can be difficult because of the light emitted by the star, (easier to do in the infrared spectrum). 29 planets have been detected by this method.

2. Detection by Doppler Shifts: When an object moves, the light reflecting or emanating from it is shifted in wavelength. This is the Doppler Shift. 

If an object is moving away from a detector, its light is shifted towards shorter, redder wavelengths; if an object is moving away from a detector, its light is shifted towards shorter, bluer wavelengths. 

When a planet orbits a star, it orbits the centre of mass of the system. Whilst the planet does the bulk of the moving, the star wobbles. 

By carefully observing a star and measuring the shift in the colour of light, we can determine how quickly the star is moving. 

From the size of the Doppler Shift we can estimate the mass of the planet and the time it takes to orbit around the stars well as other properties. As the technique gets more refinement, more and smaller planets have been detected by this method.

3. Detection by Microlensing: The pattern of the light will appear to bend as it goes past a massive object like the sun (as predicted by Einstein and later on tested by Eddington).

4. Detection by Transits: Observing the extrasolar planet when passing in front of its parent star. On doing this, the planet will block part of the light from the star. More than 200 planets have been detected by this method. 

The method allows calculations for: mass, radius, density, temperature and shape of the planetary orbit. On other occasions, instead of a planet, the transiting object could be a binary star companion. A good example of this method is the WASP (Wide Angle Search for Planets), detecting 70 planets. It can also detect biospheres in the transiting exoplanets. 

The main purpose of the WASP technique is to detect Earth-like planets, planets that lie in the habitable zone (or Goldilocks zone) and with a similar size. Si far, we know of WASP 18b, WASP 17b, planet D, Corot 7 and the Kepler mission which in December 2011 confirmed Sun-like star (Kepler 20) and two Earth-sized candidates, Kepler-20e and Kepler-20f. In the constellation of Cygnus. 

Kepler's only instrument is a photometer that continually monitors the brightness of 145,000 main sequence stars in a fixed field of view.