Travel across 14 billion light years to better understand our research and what remains to be discovered, step by step, throughout the Universe
The internal structure of the Sun and the source of its energy are well understood today. But many details concerning solar activity, with its important repercussions for the Earth, need clarifying. And we still do not know what heats the upper atmosphere of the Sun–the corona–to a temperature of about a million degrees–nor how the solar wind, which propagates through the solar system, is emitted. The Sun is an important research topic at the Paris Observatory.
The striking feature of the planets and their satellites is their extreme diversity. Considerable progress has been made recently in our understanding of the formation of the solar system and the diversity of its members, both via numerical simulations involving celestial mechanics and by ground- and space-based observations. This work leads us to basic questions concerning the presence of water and the origin of life on Earth. The quest is far from over, and there is no lack of technical challenges in our search for the answers.
These are the most primitive bodies in the Solar System. Comets in particular are made from the original gas and dust cloud, and have changed little since: That is why they are so interesting. Space exploration, and in particular the Rosetta mission, have led to considerable progress in our understanding of this material, but has also brought its surprises and problems. How are these bodies made? Having brought us part of our water, which is in our oceans, could they have also sown the Earth with molecules from which life evolved?
The Sun is the source of the solar wind, a very rarefied and hot gas. About 15 billion km from us (100 times the distance from the Earth to the Sun), this wind meets with the very rarefied and much cooler interstellar gas which surrounds the solar system. The region where these two gases meet has been traversed in recent years two Voyager probes, launched in 1977. Many questions remain to be answered concerning the limits of the solar system. It is extremely difficult to explore or even just observe the bodies which are there, and many discoveries have yet to be made.
Stars are born through the gravitational collapse of gas and dust clouds. Considerable progress has been made recently in our understanding of this phenomenon, thanks to observations in the infrared and radio spectral domains, which enable us to see what is inside these clouds, thus revealing unsuspected phenomena such as the production of jets associated with the formation of stars. The internal structure of stars is increasingly well understood, thanks to progress in astero-sismology and to increasingly precise imagery.
Just as in the case of our Sun, most stars in the Milky Way are accompanied by planets. Since the first discovery in 1995, then with space telescopes such as the French Corot telescope and the American Kepler telescope, which were specifically designed for planet hunting, we are discovering more and more exoplanets. We knew of 1,000 exoplanets in 2013, and about 3,500 today, and upcoming space missions promise to reap a rich harvest, as the Gaia satellite has shown us. Over and above these discoveries, ongoing research is enabling us to better understand these distant planetary systems, their extraordinarily diverse exoplanets and their exocomets. Some of these planets could well harbour life–this remains to be discovered.
The Milky Way is a flat disc which turns around an axis, a mass of gas, dust and stars with a spiral structure. We thought that we understood it pretty well, but recent observations have cast doubt on our established ideas. The Milky Way is not an unchanging structure; it changes shape, stars migrate from one place to another and matter is often exchanged with neighbouring galaxies. We are entering a new era of exploration, in which large-scale mappings of the whole sky and the Gaia satellite are playing a major role. Progress in radio-astronomy is leading to a better understanding of the structure and composition of our galaxy.
Galaxies exist in all kinds of shapes and sizes, and the relative amounts of gas and stars change from galaxy to galaxy. This is a sign of important differences in their origins and evolution. Moreover, galaxies are most often in groups or clusters, and gravitational interactions as well as exchanges of matter occur frequently. This is the source of many as yet unanswered questions. When were the first galaxies formed? Were there stars which could have been a source of heavy elements? Questions which today have only the beginnings of an answer.
Foreseen within the theory of general relativity, black holes are objects which are so dense that even light cannot leave them. They are thus invisible, but their mass can be detected indirectly when matter falls into them. Some black holes are members of a binary star system; others, much more massive, are in the cores of galaxies. The one which is in the Milky Way is “only” 4 million times more massive than the Sun, but in certain other galaxies the mass can be a billion times higher.
Today, there is no longer any doubt that the Universe is expanding. Observations made with the European satellite Planck have clearly determined the geometry and mass, while emphasizing a major problem: This mass is very much larger than that of the galaxies, which suggests the presence of a kind of dark matter whose nature remains mysterious in spite of much research. Moreover, the expansion is accelerating, which shows the presence of a dark repulsive energy, whose nature is also the subject of work at the Observatory. Scientists there are trying to identify this material via simulations and so clarify our understanding of the Universe.