Gravitational Microlensing

Gravitational Microlensing involves a star passing directly between our viewpoint on Earth and a background star. When such an event occurs, the brightness of the background star increases (Gould & Loeb, 1992). Such events typically last for several weeks or months, over which time the brightness of the background star increases, peaks and then decreases smooothly. Gould & Loeb (1992) describe how such an event can be parameterised by the width, maximum magnification, and timing of the event. A microlensing event is shown in Figure 1.12. This microlensing event is taken from Beaulieu et al. (2006) and shows what

happens when the foreground star also hosts an exoplanet. The presence of a planet changes the gravitational field of the lens star, causing the magnification curve to deviate from the predicted model light curve. The timescale over which the planet alters the light curve is around a few days, much shorter than the whole microlensing event.

The event shown in Figure 1.12 shows one clear advantage of gravitational microlensing. The derived planetary mass of OGLE-2005-BLG-390Lb is 5.5 M_⊕ with an orbital separation from its host star of a = 2.6 au. Gravitational microlensing is therefore able to find much smaller exoplanets that are located further from their host star when compared to the radial velocity and transit methods. The major downside to gravitational microlensing is that it requires the chance alignment of a foreground and background star, as such follow up obser- vations are not possible.

1.2.5

Direct Imaging

Directly imaging an exoplanet is incredibly difficult due to the brightness contrast between the host star and the planet. As with the radial velocity and transit methods for detecting exoplanets, direct imaging suffers from the bias that it is best suited to detecting very large, Jupiter sized planets orbiting nearby stars. One distinct difference between the direct imaging and the radial velocity and transit methods however, is that direct imaging works better for finding far-out planets where the planet can be separated in the image from the host star (Chauvin et al., 2004).

Figure 1.13 shows photometry of HR 8799 (Marois et al., 2010). The image clearly reveals the presence of four companion objects orbiting the star. Direct imaging allows the radius of the planet to be derived from the luminosity and temperature; however, the mass can only be derived from models.

1.2.6

Planetary and Exoplanetary Magnetic Fields

In our own Solar system, we know that Earth and the gas giant planets have magnetic fields. These fields protect the atmosphere of the planet from the Solar wind. The magnetosphere of the planet is a cavity in the Solar wind that forms due to the interaction of the Solar wind with the magnetic field (or ionised upper atmosphere) of the planet. For the gas giants and Earth, the interaction is primarily between the dipolar magnetic field of the planet and the Solar wind (Southwood & Kivelson, 2001). The other terrestrial planets in the Solar system such as Mars

Figure 1.13: Detection of four exoplanets orbiting HR 8799 through direct imaging. Image reproduced with permission from Marois et al. (2010).

and Venus do not have a magnetic field; rather, the magnetosphere is caused by the Solar wind interacting with the upper atmosphere of the planet. The size of the magnetospheric cavity is principally determined by pressure balance between the planet’s magnetic field and the pressure from the Solar wind.

Figure 1.14 shows an illustration of the Solar wind interacting with Earth’s magneto- sphere. At the boundary between the Solar wind and Earth’s magnetosphere a bow shock forms. Typical Solar wind conditions result in this distance being approximately 10R_⊕. For Jupiter, the Solar wind pressure is lower because the planet is located further from the Sun. The strength of the magnetic field is also considerably stronger than Earth’s magnetic field due to the different rotation rate and internal structure of the planet. This results in the mag- netosphere of Jupiter being much larger than Earth’s. Typically, the bow shock forms at a distance of approximately 50R_J from the surface of the planet.

All the magnetically active planets exhibit radio emission. The radio emission from Earth appears in two rings, one surrounding the north pole, and one surrounding the south pole. These rings are linked with the electron beams that cause the aurora. The emission has been dubbed ‘Auroral Kilometric Radiation’ (Kurth et al., 1975). Figure 1.15 shows the Southern

Figure 1.14: Schematic showing the interaction between the Solar wind and Earth’s mag- netosphere. Image from SOHO, NASA.

Auroral Oval overlaid onto an image of the Earth.

Recently, this emission has been viewed from space by the Cluster mission (Mutel et al., 2008). It has been proposed that searching for radio emission could provide a method for detecting magnetic fields on exoplanets (Zarka, 2007). However, there have as yet been many unsuccessful attempts to detect radio emission from exoplanets (see for example, Bastian et al. (2000); Jardine & Cameron (2008); Smith et al. (2009)). There are a number of reasons that these attempts may have been unsuccessful, instrumental effects including a lack of sensitivity or geometric effects such as the beam of emission not being orientated so that we can observe it from Earth will all factor in our ability to detect radio emission from exoplanets.