Recent improvements in radial velocity precision

Since the first discovery of an extrasolar planet around a Solar-type star ten years ago (Mayor and Queloz, 1995), research in this field has been very productive and has led to the detection of more than 140 exoplanets. The vast majority of these discoveries has been made with the radial-velocity (RV) technique, i.e. the precise measurement of the RV wobble that a planet induces in its parent star due to its orbital movement. A major effort to improve the accuracy of the RV measurements has been undertaken by several groups, since this is absolutely necessary to detect the RV signatures of giant planets, in the range 1–100 ms−1. Two main techniques were developed: one using a ThAr calibration simultaneously with each observation (Baranne et al., 1996) to track instrumental drifts, and one using an iodine absorption cell, superimposing a reference spectrum on the stellar spectrum (Butler et al., 1996). Both techniques have been able to deliver RV precision at the level of ∼3 ms−1, opening the way to the discovery of many planetary systems.

Over the past decade, the exoplanet group at Geneva Observatory has been operating two high-resolution spectrographs able to achieve high RV precision, namely the ELODIE instrument mounted on the 1.93 m telescope at Observatoire de Haute-Provence (France), and the CORALIE instrument installed on the Swiss 1.2 m telescope at La Silla Observatory (Chile). Both ELODIE and CORALIE are high-resolution (R = 50 000), fiber-fed echelle spectrographs.

Introduction

Planets form within circumstellar disks composed of a mixture of gas and dust grains. These disks result from the gravitational collapse of rotating molecular cloud cores. They are initially rather massive and consist of about 0.3 M*, where M* is the mass of the central star (e.g. Yorke et al., 1995). In contrast, the minimum mass required to build the planets of our Solar System is only about 0.01 Solar masses (M⊙). Evidently, there are processes that redistribute the mass, transform the dust to larger particles, and disperse much of the gas and dust.

The processes which are responsible for the dispersal of the gas influence the formation of planets. For example, the timescale for gas dispersal as a function of the disk radius affects the composition of the resulting planetary system. As long as the dust particles are small enough to be tightly coupled to the gas, they follow the gas flow. If the gas is dispersed before the dust particles have had a chance to grow, all the dust will be lost and planetesimals and planets cannot form. Even if there is time for particles to coagulate and build sufficiently large rocky cores that can accrete gas (Pollack et al., 1996; Hubickyj et al., 2004), the formation of gas-giant planets like Jupiter and Saturn will be suppressed if the gas is dispersed before the accretion can occur.

Introduction: geoscience meets astronomy

Much of our knowledge about the formation of planets in the Solar System and in particular concepts and ideas about the origin of the Earth are derived from studies of extraterrestrial matter. Meteorites (Sears, 2004; Lauretta et al., 2006; Krot et al., 2006) were available for laboratory investigations long before space probes were sent out for in situ investigations of planetary surfaces, or Moon rocks were brought back to Earth. Meteorite studies provided such important parameters as the age of the Earth and the time of formation of the first solids in the Solar System (Chen and Wasserburg, 1981; Allegre et al., 1995; Amelin et al., 2002), as well as the average abundances of the elements in the Solar System (Anders and Grevesse, 1989; Palme and Jones, 2004). Traditionally, the study of rocky material requires techniques that fundamentally differ from astronomical techniques. While electromagnetic radiation from stars is analyzed by spectroscopy, the solid samples of aggregated cosmic dust and rocky matter from planetary surfaces require the use of laboratory instruments that allow the determination of their chemical and isotopic composition. Planetary surface materials are present in polymineralic assemblages. Formation conditions and thermal stability of individual minerals provide important boundary conditions for the genesis and history of the analyzed materials. Such studies require a thorough mineralogical background. The abundances and properties of the rock-forming elements, the focus of geo- and cosmochemical research, are, however, not necessarily of major concern to astrophysicists.

Introduction

Given that brown dwarfs are usually much more massive than planets (see Section 15.6), it is somewhat surprising that the first incontrovertible discovery of a brown dwarf (Nakajima et al., 1995) and the discovery of the first extrasolar planet (Mayor and Queloz, 1995) were announced simultaneously in 1995. Over the past decade, the rapid progress made in both fields has been extraordinary. There are now more than 150 extrasolar planets known, including more than a dozen multiple-planet systems. The first brown dwarf, Gliese 229B, was found in orbit around an M-dwarf, but in the same year other candidates, later confirmed to be free-floating brown dwarfs, were announced (e.g. Teide 1 by Rebolo et al., 1995), along with PPl 15 which was later discovered to be a binary brown dwarf (Basri and Martin, 1999). Observations now suggest that brown dwarfs are as common as stars, although stars dominate in terms of mass (e.g. Reid et al., 1999).

Since the rest of this book is devoted to the topic of planets, in this chapter I will review the properties and potential formation mechanisms of brown dwarfs, comparing and contrasting them with planets, but referring the reader to the other chapters of the book for detailed information on planets.

Masses

The most fundamental parameter of a brown dwarf is its mass.

Introduction

According to the core-accretion model for planet formation, the building blocks of planets are formed by the coagulation of dust grains, growing from the initial sub-micron sizes inherited from the interstellar medium to the 100 kilometer sizes of full-grown planetesimals. This is a growth process over 12 orders of magnitude in size and 36 orders of magnitude in mass. The physics of dust is of crucial importance for the study of planet formation. It also plays a major role in the structure and evolution of protoplanetary disks, since the dust carries most of the opacity of the dust–gas mixture in these disks and provides the surface for chemical reactions. Moreover, infrared and (sub)millimeter observations of dust continuum emission from these disks can be used as a powerful probe for the disk structure and mineralogical composition. A deep understanding of the physics of dust and the coagulation of grains is therefore of paramount importance for the study of the formation of planets and the circumstellar disks in which they are formed.

The study of grain coagulation and the formation of planetesimals has a long history. At the start of the twentieth century an equation for coagulation of colloidal particles was formulated by Smoluchowski (1916), though not related to astrophysical applications. The continuous form of that equation was later used to study the size distribution of fog particles in the Earth's atmosphere (Shumann, 1940).

Introduction

Rather few facts can be considered as acceptable to all who are working in the field of planet and planetesimal formation. Starting there, we will explore the possible pathways as suggested by experiments. It is certainly undisputed that the regular mode of planet formation is connected to protoplanetary disks. These disks consist mostly of gas, which makes up about 99% of their mass. The remaining 1% resides in the form of dust and – depending on the temperature – in the form of ice. As terrestrial planets are mostly built from heavier elements it is natural to assume that they are somehow assembled from the dust component in the disk.

Whatever model is placed between the dust and the planets, collisions between the solid bodies are unavoidable. In fact a large part of the process of planet formation can be based on collisions which can and (at least partly) will lead to the formation of larger bodies.

In the following sections we will review experiments that have studied these collisions and eventually put these results in a rough sketch of planetesimal formation. It is sometimes argued that collisions of large bodies might be too energetic to lead to the formation of a still larger body (Youdin and Shu, 2002). As described in this chapter it is true that collisions can lead to erosion rather than growth. However, we will show that this is not necessarily so for all collisions.

Introduction

The observation of a transit in our own Solar System is a long-lasting experience. Historical events related to transits can be traced back to Ptolemy who mentioned in his “Almagest” that the lack of detections of transits was not in contradiction with Mercury and Venus being closer to the Earth than the Sun (in the geocentric system) simply because they could be either too small to be detected or their orbital plane could be slightly tilted to the Solar one (Gerbaldi, personal communication). In 1607 Johannes Kepler thought he had directly observed a predicted Mercury transit but in fact only followed sun spots. He did, however, predict the next transits of Venus and Mercury to take place in 1631 following the extremely accurate observations of the planets by Tycho Brahe. The first transit to be observed was the Mercury transit in 1631 with the best observations leading Pierre Gassendi to evaluate its diameter to be less than 20 arcsec, much smaller than ever thought before. All the following transit observations led to new ephemerides and estimates of the size of the Solar System, but not as accurate as expected because of the difficulty of locating in time the entrance and exit of the planetary disk over the Solar one. First pictures of the Venus transit were made as early as in 1874 (Fig. 9.1). The transit of Mercury was also observed with the Solar and Heliospheric Observatory (SOHO) spacecraft from the L5 Lagrange point of the Earth.

Introduction

The observed characteristics of molecular clouds from which stars form can be reproduced by simulations of magnetohydrodynamic (MHD) turbulence, indicating the vital role played by magnetic fields in the processes of star formation. The fields support dense cloud cores against collapse, but they cannot do so indefinitely, because only charged particles couple to the field lines while neutral atoms and molecules can freely slip through. Through this process, called ambipolar diffusion, the cores slowly contract. The recombination rate in denser gas increases, causing the ionisation degree of the core to decrease. According to available observational data, once the core has contracted to ∼0.03 pc it decouples from the magnetic field and enters the dynamic collapse phase. During the collapse the angular momentum is locked into the core and remains unchanged (Hogerheijde, 2004).

Protostellar collapse and formation of disks

The typical specific angular momentum of a core on the verge of dynamic collapse, jc, amounts to ∼1021 cm2 s−1, and is many orders of magnitude larger than the typical specific angular momentum of a star (Hogerheijde, 2004). The inevitable conclusion is that the protostellar object resulting from the collapse must be surrounded by a large, rotationally supported disk (hereafter, protoplanetary disk) in which the original angular momentum of the core is stored. The outer radius of the disk, rd, may be roughly estimated based on Kepler's law.

This conference truly reflects a microcosm of an explosive revolution in the quest to understand the origin of planet and star formation. The diverse nature of this wide-open field necessitates a multi-facet attack on all relevant issues. In this pursuit, it is particularly important to find the missing links between the many seemingly independent observations as circumstantial clues around a global picture. The development of a comprehensive coherent interpretation requires an integrated approach to identify the dominant physical processes which determine the physical characteristics of planets and the dynamic architecture of planetary systems.

On the basic concept of planetary origin, there is very little difference between modern theories and the original Laplacian hypothesis. The coplanar geometry of all the major planets' orbits hardly needs any extrapolation for theorists to postulate the scenario that the planets formed long ago in a rotational flattened disk which is commonly referred to as the Solar Nebula. Today, we have direct images and multi-wavelength spectra of protostellar disks within which planet formation is thought to be an ongoing process. Perhaps the biggest advancement in the past decade is the discovery of over 100 planets around nearby stars other than the Sun. For the first time in this scientific endeavor, the Solar System reduces its unique importance to a single entry in the rapidly growing database of planetary-system census.

Galactic scale phenomena relevant to life on a terrestrial planet are reviewed. The habitability of the Earth for complex life is surprisingly dependent on a diverse collection of processes ranging from Galactic chemical evolution to Galactic nuclear activity to comet impacts. The combined effect of these is to restrict the time and space that complex life can exist on a terrestrial planet. That region in the Milky Way is termed the Galactic Habitable Zone.

Introduction

The introduction of the Circumstellar Habitable Zone (CHZ) concept in the late 1950s (Huang 1959) and later refinements (Hart 1979; Kasting et al. 1993; Franck et al. 2000) have permitted the study of life in the universe to be systematized to some degree. However, discussion of habitability on the scale of the Milky Way Galaxy has received less attention. Trimble (1997a) considered habitability in the context of Galactic chemical evolution. Clarke (1981) discussed the possible effects on habitability of a Seyfert-like outburst in the Galactic center. In addition, many papers have been written about the possible threats to life by nearby supernovae (e.g. Ellis & Schramm 1995). While these studies have been helpful studies, they do not attempt to systematize the concept of habitability on the Galactic scale.

Before beginning any discussion about habitability, it is important to be up front about assumptions regarding life. As in CHZ studies, we assume Earth-like life in exploring Galactic-scale habitability constraints.

I examine some recent findings in cosmology and their potential implications for the emergence of life in the Universe. In particular, I discuss the requirements for carbon-based life, anthropic considerations with respect to the nature of dark energy, the possibility of time-varying constants of nature, and the question of the rarity of intelligent life.

Introduction

The progress in cosmology in the past few decades leads also to new insights into the global question of the emergence of intelligent life in the Universe. Here I am not referring to discoveries that are related to very localized regions, such as the detection of extrasolar planetary systems, but rather to properties of the Universe at large.

In order to set the stage properly for the topics to follow, I would like to start with four observations with which essentially all astronomers agree. These four observations define the cosmological context of our Universe.

1. (i) Ever since the observations of Vesto Slipher in 1912–1922 (Slipher 1917) and Hubble (1929), we know that the spectra of distant galaxies are redshifted.

2. (ii) Observations with the Cosmic Background Explorer (COBE) have shown that, to a precision of better than 10−4, the cosmic microwave background (CMB) is thermal, at a temperature of 2.73 K (Mather et al. 1994).

3. (iii) Light elements, such as deuterium and helium, have been synthesized in a high-temperature phase in the past (e.g. Gamow 1946; Alpher, Bethe, & Gamow 1948; Hoyle & Tayler 1964; Peebles 1966; Wagoner, Fowler, & Hoyle 1967).

4. […]

A planetary system may undergo significant radial rearrangement during the early part of its lifetime. Planet migration can come about through interaction with the surrounding planetesimal disk and the gas disk—while the latter is still present—as well as through planet-planet interactions. We review the major proposed migration mechanisms in the context of the planet formation process, in our Solar System as well as in others.

Introduction

The word planet is derived from the Greek word “planetes,” meaning wandering star. Geocentric views of the Universe held sway until the Middle Ages, when Copernicus and Kepler developed a better phenomenological explanation of planetary wanderings, which with small modifications has withstood the test of time. Kepler's first law of planetary motion states that planets travel along elliptical paths with one focus at the Sun. Thus, although planets wander about the sky, in this model their orbits remain fixed and they do not migrate. In his physical model of the Solar System, Newton theorized that planets gradually altered one another's orbits, and he felt compelled to hypothesize occasional divine intervention to keep planetary trajectories well-behaved over long periods of time. In the early 1800s, Poisson pointed out that planetary-type perturbations cannot produce secular changes in orbital elements to second order in the mass ratio of the planets to the Sun, but Poincare's work towards the end of the 19th century suggests that the Solar System may be chaotic.

It is becoming increasingly clear, based on a combination of observational, theoretical, and laboratory studies, that the interstellar medium (ISM) is not chemically “inert.” Instead, it contains a variety of distinct environments in which chemical synthesis and alteration are constantly occurring under the aegis of a number of different processes. The result of these different processes is an interstellar medium rich in chemical diversity. The discussion found here will concentrate on those materials and molecular species built from the elements C, H, O, and N, with particular emphasis on those compounds that may be of prebiotic interest. Furthermore, there is excellent evidence that the products of interstellar chemistry are not restricted solely to the ISM, but that some fraction of these materials survive the transition from interstellar dense clouds to planetary surfaces when new stars and planets form in these clouds. This raises the interesting possibility that molecules created in the interstellar medium may play a role in the origin and evolution of life on planetary surfaces.

Introduction

A variety of organic and volatile compounds are now known or suspected to exist in a number of different space environments including stellar outflows, the diffuse interstellar medium, dense molecular clouds, and protostellar nebulae.

Stellar nucleosynthesis of heavy elements, followed by their subsequent release into the interstellar medium, enables the formation of stable carbon compounds in both gas and solid phases. Spectroscopic astronomical observations provide evidence that the same chemical pathways are widespread both in the Milky Way and in external galaxies. The physical and chemical conditions—including density, temperature, ultraviolet radiation and energetic particle flux—determine reaction pathways and the complexity of organic molecules in different space environments. Most of the organic carbon in space is in the form of poorly-defined macromolecular networks. Furthermore, it is also unknown how interstellar material evolves during the collapse of molecular clouds to form stars and planets. Meteorites provide important constraints for the formation of our Solar System and the origin of life. Organic carbon, though only a trace element in these extraterrestrial rock fragments, can be investigated in great detail with sensitive laboratory methods. Such studies have revealed that many molecules which are essential in terrestrial biochemistry are present in meteorites. To understand if those compounds necessarily had any implications for the origin of life on Earth is the objective of several current and future space missions. However, to address questions such as how simple organic molecules assembled into complex structures like membranes and cells, requires interdisciplinary collaborations involving various scientific disciplines.

Introduction

Life in the Universe is the consequence of the increasing complexity of chemical pathways which led to stable carbon compounds assembling into cells and higher organisms.

Transits of the planets Mercury and especially Venus have been exciting events in the development of astronomy over the past few hundred years. Just two years ago the first transiting extra-solar planet, HD 209458b, was discovered, and subsequent studies during transit have contributed fundamental new knowledge. From the photometric light curve during transit one obtains a basic confirmation that the radial velocity detected object is indeed a planet by allowing precise determination of its mass and radius relative to these stellar quantities. From study of spectroscopic changes during transit it has been possible to probe for individual components of the transiting planets atmosphere. Planet transits are likely to become a primary tool for detection of new planets, especially other Earth-like planets with the Kepler Discovery Mission. Looking ahead, the additional aperture of the James Webb Space Telescope promises to allow the first possibility of studying the atmosphere of extra-solar Earth-analogue planets, perhaps even providing the first evidence of direct relevance to the search for signs of life on other planets.

Transits in history

Transits happen when an obscuring body passes in between us, the observers, and a background luminous source. Historically, both of the planets interior to Earth in the solar system have been observed while transiting the Sun. Mercury transits the Sun from our perspective frequently, Venus transits the Sun from the vantage point of the moving Earth only twice in every 130 years given current orbits.

Interstellar material surrounding an extrasolar planetary system interacts with the stellar wind to form the stellar astrosphere, and regulates the properties of the interplanetary medium and cosmic ray fluxes throughout the system. Advanced life and civilization developed on Earth during the time interval when the Sun was immersed in the vacuum of the Local Bubble and the heliosphere was large, and probably devoid of most anomalous and galactic cosmic rays. The Sun entered an outflow of diffuse cloud material from the Sco-Cen Association within the past several thousand years. By analogy with the Sun and solar system, the Galactic environment of an extrasolar planetary system must be a key component in understanding the distribution of systems with stable interplanetary environments, and inner planets which are shielded by stellar winds from interstellar matter (ISM), such as might be expected for stable planetary climates.

Introduction

Our solar system is the best template for understanding the properties of extrasolar planetary systems. The interaction between the Sun and the constituents of its galactic environment regulates the properties of the interplanetary medium, including the influx of interstellar matter (ISM) and galactic cosmic rays (GCR) onto planetary atmospheres. In the case of the Earth, the evolution of advanced life occurred during the several million year time period when the Sun was immersed in the vacuum of the Local Bubble (Frisch & York 1986, Frisch 1993). Here we use our understanding of our heliosphere to investigate the astrospheres around extrasolar planetary systems.