As well as the basic information already discussed, a transiting planet can provide a wealth of detail about the stellar system that it is part of. The combination of photometric detection and
radial velocity follow-up is a powerful one that can reveal a whole host of useful information.
1.3.1
Physical properties
One of the most important properties of a transiting system is the limited range of possible system geometries that is imposed - since transits are visible the inclination of the system must be well constrained. It is this which allows us, in combination with the aforementioned follow- up observations, to place excellent limits on a planet’s mass and radius. This in turn provides us with the information needed to determine planets’ bulk compositions. A remarkable range of exoplanetary structures have been deduced, from gas giants similar to Jupiter or Saturn, to planets with greatly extended atmospheres, massive cores, or a significant proportion of their mass in the form of ices. The existence of ‘ocean planets’, which would have the majority of their bulk composed of water, has also been postulated (Léger et al., 2004), and some may have been discovered (Kaltenegger et al., 2013). Thanks to the contribution of the Kepler satellite, we have even found planets with masses down to a few times that of the Earth, including bodies which appear to be rocky in nature (e.g. Fressin et al., 2012; Barclay et al., 2013).
The planetary interior models used to determine these compositions are commonly de- rived from models of the Solar system planets, for which the composition is well known, with the details of each layer varying from planet to planet depending on their observed proper- ties (Baraffe et al., 2010). The majority of the Jovian exoplanets appear to follow a similar structure: outer envelopes of hydrogen and helium, with some heavier metal enrichment, and a core of heavy elements such as carbon, nitrogen, and oxygen in their volatile, molecular forms. Some studies (e.g. Sudarsky et al., 2000) have also postulated the presence of silicate and iron cloud decks, and it is thought that phase transitions may exist within planetary inte- riors, particularly where hydrogen and helium are present (Chabrier, 2009), which can create challenges for theoretical models of planetary structure.
Eclipse events, both primary (transits) and secondary, can also provide information on the atmospheric composition and structure of the exoplanet involved. During a transit some of the light from the star passes through the atmosphere of the planet, and spectroscopic obser- vations of this light can reveal some of the atmosphere’s constituents. The height at which the atmosphere is opaque to tangential rays is wavelength dependent, which leads to similarly wavelength dependent changes in the ratio of the in- and out-of-transit spectra (Brown et al.,
Figure 1.3: Left: Planetary mass as a function of orbital semi-major axis. Right: Planetary radius as a function of orbital semi-major axis. In both plots the axes use logarithmic scales. Plots are taken from
www.exoplanet.eu
In both plots it is apparent that there are certain regions of parameter space which are more densely populated than others. Planets found inside the red region in the left-hand panel were generally discovered by wide, shallow, ground-based transit searches. The planet marked in blue is WASP-19, one of the systems which will be discussed in Chapter 4.
The advent of the Kepler space mission has pushed the limit of the distribution towards the lower left corner of both plots, whilst micro-lensing experiments probe the parameter space at the opposite corner of the figures.
2002). If the transit is observed to be deeper in a particularly bandpass, then that implies that the species which corresponds to that bandpass is present in the planet’s atmosphere (Charbonneau et al., 2002). Observations at different wavelengths allow the presence, or otherwise, of different chemical species to be deduced. Once a set of measurements for a given planet has been built up they can be compared to model atmospheric spectra to gain further information about the planet. These measurements are very difficult to carry out, and can be subject to controversy (e.g. Swain et al., 2009; Gibson et al., 2011), but there is still great excitement about their potential (Barstow et al., 2013).
1.3.2
Orbital parameters
Geometrical and temporal arguments for the visibility of planetary transits imply a selection effect in favour of large planets at small orbital separations. As previously noted, the fact that transits are visible limits the range of orbital inclinations, with the limiting value being dependent ona/Rs. There is also a generally accepted minimum number of transit detections required before a signal is considered a candidate; the closer the planet is to the star, the more rapidly these repetitions can be seen. Finally, the larger the planet relative to its host star, the greater the change in flux/transit depth, and the lower the SNR for which the signal will be detectable.
It is therefore not surprising that a large number of such planets have been found by transit search programs. Many of them have masses and radii similar to Jupiter, but orbit their host stars with semi-major axes less than or equal to that of Mercury (see the area marked in red in Figure 1.3). Known as ‘hot Jupiters’, they have been discovered down to very small semi-major axes (e.g. Hellier et al., 2009; Hebb et al., 2010; Sasselov, 2003). A large fraction of the known hot Jupiters have been found by wide, shallow, ground-based transit searches, of which they constitute the main harvest. Whilst such surveys were dominant, it was hot Jupiters which were the most common form of known planet. But they are poorly represented in the Kepler sample, indicating that they are actually rather rare.
Current theories of planet formation are unable to produce hot Jupiters in situ, as the protoplanetary disc close-in to the star would be too hot for sufficiently massive cores to form, preventing the planet from accumulating sufficient gas (Melo et al., 2006). The com- monly held view is therefore that hot Jupiters formed further out from the star before moving inwards under the influence of some migration mechanism. There are several competing hy- potheses for what this mechanism might be, but conclusive evidence for one or more of them being dominant has yet to emerge.
In the near future it may be possible to distinguish between them using the spectroscopic transit signature (see Figure 1.1). This phenomenon provides information on the alignment of the planetary orbit to the rotation of the host star, and will be discussed further in Chapters 5 and 6. When the angle between the orbital and rotation axes is measured, it is used to classify the system as ‘aligned’ or ‘misaligned’ according to some pre-existing criterion (e.g. Winn et al., 2010a).
Observations by the Kepler satellite have revealed a fascinating variety in the architecture of planetary systems. Examples with almost every conceivable combination of single and multiple planets, both gaseous and terrestrial, and with a wide range of orbital distances, eccentricities, and periods have been found. The possibilities seem to be endless.