In this section, we seek to inform on the art of hunting exoplanets, and speculate on the nature of the geology of exoplanets.
The exploration of exoplanets begins with long range detection methods, which to this day remain our only way to study them. Classically the identification of celestial bodies involves the detection of light reflected off the surface, as with the planets in our solar system, asteroids and comets. When you consider the tiny size of most planets in comparison with their associated star, the amount of light reflected and available for detection at an unknown point and time in space is just too small and difficult to make sense of with current instrumentation and methods. Thus, alternative approaches had to be developed.
This approach takes advantage of the simple truth that when you hold an object in front of a light source, fewer photons of light reach the observer. This can be extrapolated to an astronomical scale for the detection of exoplanets by using sensitive instrumentation to constantly monitor the luminescence of a star over time. Regular dips in light are the tell tale sign of an exoplanet. Due to the nature of this approach, you can deduce and extrapolate a fair amount of information about a planet and its system, such as the actual size (radius) and mass of an exoplanet, orbital axis, stellar mass and radius, orbital inclination and eccentricity. Excitingly sometimes one can even learn about the atmospheric content of a planet! For limitations of the use of this approach, the first big one is of course, there must be a direct line of observation between the researcher and the orbital plane of the system in question. Secondly the approach generates a fair number of false positives (almost half!), and so as with any research, the approach is most correctly paired to a second confirmation using a separate approach. It should also be mentioned that tiny stars like white and brown dwarves and especially massive stars pose their own limitations to using transit photometry.
To really do this kind of analysis you need some of the most state-of-the-art equipment available today. The CoRoT mission launched by the French Space Agency back in 2006 founded the approach and identified dozens of planets before the satellite stopped functioning. Later in 2009 NASA began its Kepler mission to practice the same approach, and this has been a massively successful venture with hundreds of planets identified, including dozens of earth mass size in the habitable zone of a solar system. Recently the Transiting Exoplanet Survey Satellite was launched in 2018 and we expect some mind blowing results from this effort!
Image Credit: NASA/JPL
When a planet orbits a star, both have mass gravitational attraction to each other and both influence each other. In fact, even the small mass of a planet can have an effect on the orbital velocity of a star, and this effect can be monitored via the suns radial velocity. This is done by monitoring specifically the spectral displacement of its radiance (Doppler spectroscopy.) Radial velocity has proven to be a top technique over the years and has led to a significant number of planets being identified. It is however not without its downsides: for instance, the nature of this approach requires a higher signal to noise discrimination, and so this technique may be appropriate only for our local galactic space up to as far away as 100 to 150 light years depending, or until the advent of more sensitive instrumentation. Also this technique requires relatively massive planets such as the size of Jupiter for accurate detection (and conversely, less massive stars for best effect.). Finally, care has to be taken about the study of multi star and multi planet systems as this can complicate this analysis approach. Depending on the circumstances of the system being studied, proper identification could take years of data collection. Ultimately radial velocity can give you an idea of the existence of a planet, sometimes the radial velocity and its general mass, especially when used in parallel with other techniques.
There are a variety of approaches and available hardware for this technique, including the ESO 3.6 m telescope at La Silla Observatory in Chille or the High Accuracy Radial Velocity Planet Searcher spectrophotometer at the Keck telescopes (FIX), or simply via externally dispersed interferometry.
Image Credit: NASA/JPL