Stars are variable sources over timescales which depend on the phenomena triggering this variability. These phenomena are typically produced by the interaction between the stellar magnetic field and its plasma. Typical examples, observed and studied in great details in the Sun, are: flares (sudden release of a large amount of energy by the magnetic field which heats the gas in
The corona is the outermost part of the atmosphere in stars with intermediate and low mass. In this region, which is extended for several stellar radii, the plasma can reach temperatures of several million degrees, emitting mainly X-ray radiation. The coronal temperature varies over a range of several orders of magnitude as a function of several stellar parameters, such as
The search for young exoplanets is crucial to our understanding of the planetary formation process and the early evolution of planets. The disc of gas and dust from which planets form, in fact, typically disperse in a few million of years. Before that, the young planets interact with all the material orbiting around the star within the disk, triggering important
Some of the 4903 exoplanets discovered and confirmed so far (https://exoplanets.nasa.gov/) orbit very close to their stars. We learnt with the very first planet discovered around a star different than the Sun (51 Pegasi b, discovered in 1995) that planets with very close orbits are possible in nature. 51 Pegasi b, in fact, orbits at about 0.05 Astronomical Units (AU,
The study of the architecture of planetary systems, and in particular that of the spin-orbit angle (which is the inclination of the planetary orbit with respect to the stellar rotation axis) can unveil important details on the dynamical evolution of planetary systems. In particular, young planets typically migrate radially, changing their distance to the host star and interacting with the
About 20% of the exoplanets discovered so far (October 2021) were found with measurements of radial velocity (data from https://exoplanets.nasa.gov/). This method is based on spectroscopic observations of stars with planets, from which it is possible to measure the Doppler effect produced by periodic oscillations of the star, due to the gravitational attraction between star and planet along the orbit of
According to the data released by NASA, to date (2021 October 1st) we have confirmed 4525 exoplanets in 3357 planetary systems. The vast majority of these planets (75%) were discovered with the methods of transits, while about 20% with that of radial velocity. With the former technique, planets are discovered when they transit in front of their stars along our line
Hot Jupiters are gaseous giant planets that orbit very close to their stars (within a typical distance of less than 0.1 Astronomical Units, AU, where 1 AU is the average distance between Earth and Sun, about 150 million of km). These planets are of great interest for astronomers both because of the effects due to the intense incident stellar radiation,
M stars are the most common stars in the Universe. They have a mass ranging between 0.6 and 0.08 Solar masses in the main sequence, and an effective temperature ranging between 3900 K and 2400 K. In these stars, the signals due to the presence of planets, such as radial velocity (periodic oscillations of the star from its rest position
The study of planets orbiting around young stars (younger than 100 million years) can help astronomers understanding the physical processes setting the early evolution of planetary systems. However, young stars are typically characterized by rapid rotation and intense magnetic activity, phenomena which produce photometric and spectroscopic signals that can mimic and confuse those due to the presence of planets. It
