Exoplanets and SETI

The discovery of exoplanets has both focused and expanded the search for extraterrestrial intelligence. The consideration of Earth as an exoplanet, the knowledge of the orbital parameters of individual exoplanets, and our new understanding of the prevalence of exoplanets throughout the galaxy have all altered the search strategies of communication SETI efforts, by inspiring new "Schelling points" (i.e. optimal search strategies for beacons). Future efforts to characterize individual planets photometrically and spectroscopically, with imaging and via transit, will also allow for searches for a variety of technosignatures on their surfaces, in their atmospheres, and in orbit around them. In the near-term, searches for new planetary systems might even turn up free-floating megastructures.Comment: 9 page invited review. v2 adds some references and v3 has other minor additions and modification

Planetary Dynamics and Habitable Planet Formation In Binary Star Systems

Whether binaries can harbor potentially habitable planets depends on several factors including the physical properties and the orbital characteristics of the binary system. While the former determines the location of the habitable zone (HZ), the latter affects the dynamics of the material from which terrestrial planets are formed (i.e., planetesimals and planetary embryos), and drives the final architecture of the planets assembly. In order for a habitable planet to form in a binary star system, these two factors have to work in harmony. That is, the orbital dynamics of the two stars and their interactions with the planet-forming material have to allow terrestrial planet formation in the habitable zone, and ensure that the orbit of a potentially habitable planet will be stable for long times. We have organized this chapter with the same order in mind. We begin by presenting a general discussion on the motion of planets in binary stars and their stability. We then discuss the stability of terrestrial planets, and the formation of potentially habitable planets in a binary-planetary system.Comment: 56 pages, 29 figures, chapter to appear in the book: Planets in Binary Star Systems (Ed. N. Haghighipour, Springer publishing company

Galactic Effects on Habitability

The galactic environment has been suspected to influence planetary habitability in many ways. Very metal-poor regions of the Galaxy, or those largely devoid of atoms more massive than H and He, are thought to be unable to form habitable planets. Moreover, if such planets do form, the young system is subjected to close stellar passages while it resides in its stellar birth cluster. Various potential hazards remain after clusters disperse. For instance, central galactic regions may present risks to habitability via nearby supernovae, gamma ray bursts (GRBs), and frequent comet showers. In addition, planets residing within very wide binary star systems are affected by the Galaxy, as local gravitational perturbations from the Galaxy can increase the binary's eccentricity until it destabilizes the planets it hosts. Here we review the most recent work on the main galactic influences over planetary habitability. Although there must be some metallicity limit below which rocky planets cannot form, recent exoplanet surveys show that they form around stars with a very large range of metallicities. Once formed, the probability of star clusters destabilizing planetary systems only becomes high for rare, extremely long-lived clusters. Regarding threats to habitability from supernovae, GRBs, and comet showers, many recent studies suggest that their hazards are more limited than originally thought. Finally, denser regions of the Galaxy enhance the threat that very wide binary companions pose to planetary habitability, but the probability that a very wide binary star disrupts habitability will always be substantially below 100% for any environment. While some Milky Way regions must be more hospitable to habitable planets than others, it is difficult to state that habitable planets are confined to any well-defined region of the Galaxy or that any other particular region of the Galaxy is uninhabitable.Comment: Invited review chapter, accepted for publication in the "Handbook of Exoplanets"; 19 pages; 2 figure

Direct exoplanet investigation using interstellar space probes

Experience in exploring our own solar system has shown that direct investigation of planetary bodies using space probes invariably yields scientific knowledge not otherwise obtainable. In the case of exoplanets, such direct investigation may be required to confirm inferences made by astronomical observations, especially with regard to planetary interiors, surface processes, geological evolution, and possible biology. This will necessitate transporting sophisticated scientific instruments across interstellar space, and some proposed methods for achieving this with flight times measured in decades are reviewed. It is concluded that, with the possible exception of very lightweight (and thus scientifically limited) probes accelerated to velocities of ∼0.1c with powerful Earth-based lasers, achieving such a capability may have to wait until the development of a space-based civilization capable of leveraging the material and energy resources of the solar system