THE exoplanets

Extrasolar planets (or exoplanets) have been extensively searched during the last two decades. As a result of this effort, since 1995 -when the first exoplanet was discovered by Mayor & Queloz, and Marcy and Butler there are now more than 700 confirmed exoplanets (to date, 763 exoplanets are listed in the Extrasolar Planets Encyclopaedia).

Exoplanet artistic impression orbiting two stars

Although a very interesting picture of this rich "zoology" is emerging, several important questions remain unanswered: what is the diversity of planetary systems, including long periods of planets and exotic systems? What is their dynamical evolution? What are the properties of exoplanets (orbits, masses, and properties of the atmosphere)? What are the frequency and properties of belt planetesimales and what are the implications for the frequency of formation of terrestrial planets, the habitability of planetary systems, and water distribution in the region of the terrestrial planets? How common are planets similar to Earth? These and other questions are of vital importance from an astrobiological point of view because they can help us to place the evolution of the Solar System and the establishment of basic habitability conditions on at least one of its planets, the Earth, into perspective. This project aims to answer some of these issues by addressing the problem from an observational approach.

Most of the planets have been detected with the radial velocity method (using high resolution spectrographs), but other techniques are also good to detect extrasolar planets. The second most productive indirect technique of detection is the transit method. When a planet crosses (or transits) in front of its parent star, the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors.

In most cases the exoplanets found are giant and hot, usually found in very early stages of formation, and very close to its star. These properties are closely related to the observational bias introduced by the radial velocity method. The stars with known exoplanets are rich in metals compared to those that do not harbor planets. The number of stars with giant planets around seems to increase with metallicity of the star. For most of the exoplanets found we only know orbital parameters with large errors. Taking advantage of the potential use of a robotic telescope, it is intended to study the parameters of the known exoplanets to follow the curves of light and orbital cycles and phases of maximum and minimum so as to reduce drastically error bars. This will enable us to determine with greater precision the masses, sizes, and distances to the central star of the planets. Thus, we will select in a best approach those that might be within the habitable zone of the star. Also, such an observation would help to discriminate among the possible causes for the effect of the relationship between metallicity and the presence of planets in a star.

HD 209458b transit light curve

The main objective of the project is to detect new exoplanets, and especially to characterize known exoplanets by photometric measurements and systematic monitoring of their transits. The telescope is suitable to carry out this project.

The goal is to characterize exoplanets by the Transit Method. The working method is to use the telescope for monitoring beforehand selected stars by fast optical photometry. Usually we will work to obtain observational confirmation of the candidates from exoplanet hunter projects such as TrES or SuperWASP. However, we also work over our own select sky fields to find exoplanets. A previous work of selection may be done to know where to look for, the first stage with solar type stars and after that with M type stars or Planetary Nebulae.

Transit light curve of TrES 3 obtained with the 50-cm CAB's robotic telescope (twin to CESAR's Telescope) on Calar Alto Observatory in the R-band. Differential magnitude is plotted versus phase.(Credit: CAB)

Nearly continuous periods observing selected star are needed to try to find some changes in the light from the star. Comparison of photometry data from one night to another will be done to look for the slight variations that could indicate a possible transit. After that, the candidates will be reobserved to confirm results.

But not only is the search for exoplanets important but also the characterization is a goal. In this sense, we will develop a reliable database of parameters for exoplanets. The working method is similar to the previous goal. In this case we will observe stars with known transit exoplanets to obtain a light curve from which information about its parameters may be extracted. The repeated observation of these transits will substantially reduce the error bars and will provide a data set with accuracy so that they can be collated, thus allowing to draw conclusions about the nature of exoplanets and its formation conditions. Previously, we will select the exoplanets with more favorable conditions to be observed during their transits according to the characteristics of the telescope: large size, low stellar temperature, low magnitude, and long transit. In this way we will restrict the sample to a particular group of exoplanets, which makes more sense to compare their parameters for their homogeneity.