Is the Solar System unique, or are planets ubiquitous in the universe? The answer to this long-standing question implies the understanding of planet formation, but perhaps more relevant, the observational assessment of the existence of other worlds and their frequency in the galaxy.
The detection of planets orbiting other suns has always been a challenging task. Fortunately, technological progress together with significant development in data reduction and analysis processes allowed astronomers to finally succeed. The methods used so far are mostly based on indirect approaches, able to detect the influence of the planets on the stellar motion (dynamical methods) or the planet’s shadow as it crosses the stellar disk (transit method). For a growing number of favorable cases, direct imaging has also been successful. The combination of different methods also allowed probing planet interiors, composition, temperature, atmospheres, and orbital architecture. Overall, one can confidently state that planets are common around solar-type stars, low mass planets being the most frequent among them.
Despite all the progress, the discovery and characterization of temperate Earth-like worlds, similar to the Earth in both mass and composition and thus potential islands of life in the universe, is still a challenging task. Their low amplitude signals are difficult to detect and are often submerged by the noise produced by different instrumentation sources and astrophysical processes. However, the dawn of a new generation of ground and space-based instruments and missions is promising a new era in this domain.
Article
Detection and Characterization Methods of Exoplanets
Nuno C. Santos, Susana C.C. Barros, Olivier D.S. Demangeon, and João P. Faria
Article
Planetary Systems Around White Dwarfs
Dimitri Veras
White dwarf planetary science is a rapidly growing field of research featuring a diverse set of observations and theoretical explorations. Giant planets, minor planets, and debris discs have all been detected orbiting white dwarfs. The innards of broken-up minor planets are measured on an element-by-element basis, providing a unique probe of exoplanetary chemistry. Numerical simulations and analytical investigations trace the violent physical and dynamical history of these systems from astronomical unit (au)-scale distances to the immediate vicinity of the white dwarf, where minor planets are broken down into dust and gas and accrete onto the white dwarf photosphere. Current and upcoming ground-based and space-based instruments are likely to further accelerate the pace of discoveries.
Article
Technosignatures and Astrobiology
Jacob Haqq-Misra
Astrobiologists are engaged in the search for signs of extraterrestrial life in all forms, known as biosignatures, as well as specific signs of extraterrestrial technology, known as technosignatures. The search for technosignatures and biosignatures attempts to identify characteristic evidence of life on other planets that could be detected using astronomical methods. The first scientific searches for technosignatures began in the 1960s, which used radio telescopes to examine nearby star systems for evidence of narrowband transmissions used for communication. The search for extraterrestrial intelligence has continued to search for anomalous radio and optical signals that would indicate intentional or unintentional extraterrestrial communication. Advances in ground- and space-based spectroscopy are also beginning to enable searches for technosignatures in exoplanetary systems such as atmospheric pollution, city lights, large-scale surface structures, and orbiting satellites. Some technosignature searches also attempt to search for nonterrestrial artifacts within the solar system on planetary bodies or in stable orbits. All of these technosignature concepts use known technology on Earth as a starting point for thinking about technology that could be plausible and detectable in extraterrestrial systems.
Technology is a relatively recent phenomenon in the history of life on Earth, so the search for technosignatures also employs methods from futures studies to explore numerous trajectories for extensions of known technology. The range of possibilities considered by technosignature science can include any known or plausible technology that could be remotely detected and would not violate any known physical laws. Megastructures are examples of theoretical large-scale planetary engineering or astroengineering projects that could be detectable in exoplanetary systems through infrared excesses or gravitational effects. Many other technosignatures remain possible, even if they do not draw upon Earth projections, but most astrobiological study of technosignatures focuses on predictions that could be tested with current or near-future missions. The positive discovery of extraterrestrial technology could be of great significance to humanity, but technosignature searches that yield negative results still provide value by placing qualitative upper limits on the prevalence of certain types of extraterrestrial technology.
Article
The Formation and Evolution of the Solar System
Mikhail Marov
The formation and evolution of our solar system (and planetary systems around other stars) are among the most challenging and intriguing fields of modern science. As the product of a long history of cosmic matter evolution, this important branch of astrophysics is referred to as stellar-planetary cosmogony. Interdisciplinary by way of its content, it is based on fundamental theoretical concepts and available observational data on the processes of star formation. Modern observational data on stellar evolution, disc formation, and the discovery of extrasolar planets, as well as mechanical and cosmochemical properties of the solar system, place important constraints on the different scenarios developed, each supporting the basic cosmogony concept (as rooted in the Kant-Laplace hypothesis). Basically, the sequence of events includes fragmentation of an original interstellar molecular cloud, emergence of a primordial nebula, and accretion of a protoplanetary gas-dust disk around a parent star, followed by disk instability and break-up into primary solid bodies (planetesimals) and their collisional interactions, eventually forming a planet.
Recent decades have seen major advances in the field, due to in-depth theoretical and experimental studies. Such advances have clarified a new scenario, which largely supports simultaneous stellar-planetary formation. Here, the collapse of a protosolar nebula’s inner core gives rise to fusion ignition and star birth with an accretion disc left behind: its continuing evolution resulting ultimately in protoplanets and planetary formation. Astronomical observations have allowed us to resolve in great detail the turbulent structure of gas-dust disks and their dynamics in regard to solar system origin. Indeed radio isotope dating of chondrite meteorite samples has charted the age and the chronology of key processes in the formation of the solar system. Significant progress also has been made in the theoretical study and computer modeling of protoplanetary accretion disk thermal regimes; evaporation/condensation of primordial particles depending on their radial distance, mechanisms of clustering, collisions, and dynamics. However, these breakthroughs are yet insufficient to resolve many problems intrinsically related to planetary cosmogony. Significant new questions also have been posed, which require answers. Of great importance are questions on how contemporary natural conditions appeared on solar system planets: specifically, why the three neighbor inner planets—Earth, Venus, and Mars—reveal different evolutionary paths.
