Now that well over 100 planets have been discovered, we no longer need to ask the question "Are there planets out there?" Instead we ask, "How common are planets?" and "What are their properties?" In short, we have moved from the initial discovery phase to the characterization phase.

The objective of XO is to find additional hot-Jupiters (like HD 209458b) that transit stars bright enough for detailed follow-up studies to be done with existing telescopes such as HST, Keck, and the VLT. Given that hot-Jupiters are in transit roughly 3% of the time, and given statistics of stars and hot-Jupiters, at any given time we could observe one bright (mv < 11) star being transited, if only we knew where to look.

Transit observations by themselves provide no information about the mass of the planet and radial velocity studies can yield only minimum masses. However, the combination of the doppler obit plus the photometric detection of transits will yield a full solution for the physical characteristics of the candidate.

To acquire new candidates for high-precision studies of exoplanets, either a diminishing return scenario will play out on ever larger telescopes using the radial velocity technique, or else the transiting planets will be discovered photometrically.

A transiting planet is interesting in at least three important ways:

  1. The orbital inclination i is known, so thereby are the physical characteristics of the planet such as its mass (not m sin i) and density and "surface" gravity.
  2. Absorption of starlight is a much larger signal than reflected starlight, so for example absorption spectroscopy has already permitted detection of an exoplanet's atmosphere (Charbonneau et al 2001).
  3. The rapid and precisely predictable on/off nature of the transit permits excellent calibration, which for two examples, permit us to search for satellites and Saturn-like rings orbiting the transiting planet and to attempt to measure the planet's albedo by observing the reflected light of the planet being blocked by the star (Brown et al 2001).

If a Jovian planet, located ~1 AU from a solar type star, has a satellite, then it may happen that the first "habitable world" outside our solar system may be detected prior to the launch of Kepler, by timing of or precise photometry of that Jovian planet's transits. Such a satellite's orbit is not dynamically unstable in 4.6 Gyr if its host planet's semi-major axis is roughly ≥ 0.3 AU, i.e. with a period roughly ≥ 2 months for a solar-type host star (Barnes & O'Brien 2002).

By now it is apparent that we know little about the range of possible planetary configurations. The surprising discovery of hot Jupiters demonstrates that we still have many surprises ahead and much to learn about how planets are formed or how planetary systems evolve dynamically.

At present time, only transits show promise for extending the very successful velocity technique. The direct imaging approach is likely to be limited by technology to a few special cases for the foreseeable future and the micro-lensing method suffers an "Achilles heel" in that the events never repeat, so follow up studies are impossible. (The nearly non-repeatable nature of planetary orbits with periods much longer than an astronomer's career makes study of planets like Saturn difficult). Like the radial velocity technique, the planetary transit technique favors the discovery of large planets in small orbits, but finding transits of large planets in larger orbits requires more observations, not necessarily better ones. To find smaller planets, better observations are required (i.e. the Kepler mission).

Interpreting transits is straightforward; once the observational errors are understood, there are only two major physical considerations, the effects of stellar activity and the frequency of planetary systems. Although large numbers of observations are required, they are relatively easy to make and reduce, and the statistics depend entirely on simple geometrical and orbital considerations. Stellar activity is not a show-stopping issue for Kepler and especially not for large-amplitude transits of gas giants (Borucki and Summers 1984). With enough observations, we can deduce the frequency of planets of various sizes in various orbits.