Finding Exoplanets – 24 April 2014

At our April meeting Andrew Norton of the Open University spoke to us about Finding Exoplanets.

The idea of exoplanets has been around for a long time, but it is only in more recent years that they have been identified through a variety of methods. Giordano Bruno (1548-1600) surmised there were ‘countless earths around countless stars’ and Huygens (1629-1695) realised the limitations of his telescopes for exploring beyond our solar system. Otto Struve predicted that oscillation changes and eclipses would lead to dimming of stars and could indicate the presence of orbiting planets

In 1995 the first exoplanet was discovered by Mayor and Queloz, Swiss astronomers. This planet orbits the sun-like star 51 Pegasi, 0.05 AU from the star with an orbital period of 4.25 days. It has a mass half that of Jupiter, and is known as a Hot Jupiter Star. Hot Jupiters are the easiest exoplanets to discover as they generate oscillation in the star being massive planets relatively close to their stars.

There are now more than 1000 confirmed exoplanets: 3 Merucian, 7 Subterran, 11 Terran. 114 Super Terran, 148 Neptunian and 77 Jovian.

There are various methods for finding exoplanets: direct imaging, Doppler Shift, microlensing and detection by transits.

Direct imaging is easier to do in infra-red. The light from the star drowns out the reflected light from the planet. 47 exoplanets have been discovered by this method, usually a small star, e.g. a brown dwarf, and a large planet.

The first exoplanet was discovered using Doppler Shift. The star and the planet orbit the centre of mass of the combined objects and the Doppler Shift of spectral lines reveal orbital motion. This provides a good idea of the mass of the star, and the orbital period, distance and temperature of the planet. Different types of planets give different Doppler Shifts, but the intrinsic variability of stars means that differences in Doppler Shift may not be due to planets.

Microlensing is possible because massive objects distort light, and a planet around a star will distort gravitational lensing. There is a huge amount of data, but 28 planets have been discovered using this method

Detection by transit is the most effective method of discovering exoplanets finding more than 400. A large planet passing in front of a star provides information about the radius of the star, but nothing about its mass. Atmospheric studies are possible because some starlight shines through the atmosphere.

SuperWASP is a collaboration of various universities including the OU, Leicester, Keele and Warwick. Mulitple cameras on La Palma and in Sutherland employ rapid slewing of and 8 degree field of view image all night, every night. These images measure the brightness of stars and provide light curves. Dips in these light curves and detected automatically, but results have to be checked by eye. 1 in 1000 such dips in light curve are confirmed as exoplanets. Some objects can imitate exoplanets, for example: a white dwarf passing in front of a star or grazing eclipsing binaries.

By 2013 SuperWASP had imaged 31 000 000 unique objects and identified 100 exoplanets, all of which are Hot Jupiters.

Ultimately the hunt for exoplanets is the search for signs of life. Our speaker suggested that within 20 years techniques will develop such that it could be confirmed whether there is life on some exoplanet. For example: Gliese 581 has six exoplanets, one of which is located in the habitable zone.

The search continues: in 2024, PLATO (Planetary Transits and Oscillations of Stars) will image half the sky, utilising astro-seismology.

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