Venus observed like an exoplanet
Thanks to this rare opportunity, the Venus' upper atmosphere could be observed by the scientists working on Picard. It was also a great occasion to test the detection models of the exoplanets' atmospheric compositions. In fact, the scientists who try to discover new exoplanets do use the so-called transit method: a planet passing in front of its host star results in a small luminous intensity decrease. The star's light, by passing through the exoplanet's atmosphere, keeps spectrographic traces which are the reflection of its composition. By using this method on the Venus transit in front of the Sun, the scientists could study the received signals and compare them to the similar ones obtained with exoplanets: an opportunity to improve the models. It was also a great opportunity to detect possible changes in the Venus' atmosphere which has been studied by the European Venus Express probe in orbit around Venus since 2006.
Venus is probably one of the most well-known planets of our solar system. It is the second nearest planet from the Sun. Over time, a lot of similarities with the Earth were found by the astronomers: size, mass, atmospheric structure and formation period.
The first modern measurements date back to Galilean, who observed it for the first time using its astronomical telescope. Since then, the data provided by the telescopes and satellites disclosed a lot of information about Venus.
For example, its orbital period is 224.7 days. Just like the Earth and Mars, it is a telluric planet. It hosts a low magnetic field and does not have any natural moon. The peculiarity of Venus is probably its almost spherical shape and retrograde rotation, i.e. in the opposite direction of that of the Earth. Finally, among all the planets of the solar system, it is the one with the most circular orbit.
Since the orbits and periods of Venus and the Earth are different, they can only be aligned with the Sun when Venus crosses the plan of the ecliptic. These scenarios occur successively at the following intervals: 8 years, 105.5 years, 8 years and 121.5 years before resuming the cycle. It means that the cycle lasts 243 years.
Seen from the Earth, Venus only hides a very little part of the solar disk during these transits. So this phenomenon occurs in pairs at 8-year intervals. For the current cycle, the first transit pair took place in June 2004 and June 2012 and the next ones will take place in 2117 and 2125.
Measuring the Earth-Sun distance thanks to the Venus transits
Johannes Kepler was the first to predict a transit of Venus in 1631 but since its prediction was not accurate enough, Gassendi could not observe the phenomenon: the transit occurred by night. Eight years later in 1639, always according to the Johannes Kepler predictions, the first real observations could be achieved by Jeremiah Horrocks and William Crabtree.
Studying the transit of Venus had great interest in the past because, by using the parallax method established by Thales, it was possible to pinpoint the Sun-Earth distance and obtain a tremendous value of the solar system size. This calculation, useful not only in astronomy and optics but also in measurement instrument calibration, translates into the impact of the observer's point of view on the object observation. In astronomy, the parallax is the angle under which a reference length can be seen from a moon.
By knowing the distance between two people looking at the transit on Earth, the angle formed by these two observers and Venus can be determined. This angle then makes the Sun-Earth distance determination possible thanks to Thales and the 3rd law of Kepler. In fact, the observers see a small black spot on the solar disk which can thus be assimilated to a screen. By linking the different successive positions of Venus materialized by these black spots, two distinctive traces appear some distance apart. The Sun-Earth distance can thus be determined by this length associated with the observation angle.?
The young Halley anticipated the phenomenon thanks to the Kepler predictions and sent an appeal to all the astronomers of his time who had the opportunity to live the 1761 transit. Unfortunately, in this time in the midst of a war, the results were only partly successful and quite imprecise. But the sun ray refraction observed by the Russian astronomer Lomonossov allowed the atmosphere existence on Venus to be confirmed. This detection principle called "Lomonosov effect" is still being used nowadays. (The 1874 and 1882 transits did give clearly improved precision even if it was still insufficient).
Of course, the astronomers of the time could not imagine that the humanity of the twenty-first century would, thanks to improved technology, possess the most effective tools ever developed by the human kind to observe the 2004 and 2012 transits. They did not except either that the Sun-Earth distance would be known already. In fact, the parallax has been established with high precision. During the 2004 transit, the parallax calculation only had a pedagogical goal and was performed by thousands of amateur astronomers. The US TRACE satellite telescope took an accurate picture of the gout phenomenon, which was often responsible of calculation errors in the eighteenth century, at the beginning of the Sun occultation by Venus.
But the 2012 transit gave to the scientists a new opportunity to improve the star science. Thanks to the French Picard satellite dedicated to the Sun observation and launched in 2010 with its high-precision SODISM telescope, the efficiency of the exoplanet detection system using the occultation principle can be put into concrete application by the CNRS' scientists. They could thus improve their measurement system which was already operational on the terrestrial telescopes and satellites like CoRoT. This time, during the transit, Venus played the role of an exoplanet, i.e. a planet located in another solar system.
The detection technique is finally very simple. The measurement instruments of the telescopes calculate the light variations looking a star. If a planet transits in front of a star, the instruments detect a light intensity decrease. Using the Lomonossov effect principle and looking at the intensity decrease variations, the scientists can pinpoint the physical and atmospheric specifications of the transiting planet.
The Picard satellite and the resulting information are among the last most precise measurements ever established of this rare phenomenon. The humanity had to wait more than 100 years to once again observe the phenomenon.
These transits occur in pairs at 8-year intervals and then disappear for more than a century: the next Venus transit between the Sun and the Earth will take place in 2117.