CHAPTER 2 Exoplanets & Planet Formation Are we alone in the universe? Astronomers have discovered thousands of planets orbiting other stars. Their sheer number and amazing diversity—from lava-worlds to super-Jupiters—give us a new perspective on our place in the universe. We now know that Earth is just one rocky planet amongst billions in the galaxy, and that planets range at least from 1/10th to 10,000 times the mass of Earth. Rocky planets that orbit their stars at just the right distance could be temperate enough to allow for the presence of seas of liquid water. Our nearest neighboring star, Proxima Centauri, likely hosts one such planet, but detecting biosignatures on exoplanets is extremely challenging because they are so much fainter than their host stars. The GMT’s sensitivity, resolution, and spectroscopic capabilities will enable us to understand the origins of the wide range of planetary systems we see in the galaxy around us. We will be able to measure the physical properties of those planets to learn if they contain life and to compare our solar system to its neighbors. Chapter Authors Alycia Weinberger (Carnegie Institution for Science, Department of Terrestrial Magnetism) Jayne Birkby (University of Amsterdam) Brendan Bowler (The University of Texas at Austin) Mercedes López-Morales (Harvard-Smithsonian Center for Astrophysics) Jared Males (University of Arizona) Katie Morzinski (University of Arizona) Sharon Wang (Carnegie Institution for Science, Department of Terrestrial Magnetism) CHAPTER 2 Exoplanets & Planet Formation 2 2.0 Exoplanets and Planet Formation Perhaps the most profound and enduring question over all of human history is whether life exists elsewhere in the universe. It is only in the past two decades that we have learned for certain that there are planets orbiting other stars—thousands of them. Earth is likely one rocky planet amongst billions; every star in the galaxy could have a rocky planet (Sumi et al. 2011). Present thinking is that our best chance of finding life is on rocky planets like Earth with liquid water on their surface. Such habitable planets must orbit their stars at the correct distance to maintain a surface temperature near 300 K; remarkably, our nearest stellar neighbor, Proxima Cen, hosts one such planet (Anglada-Escudé et al. 2016). The GMT will make revolutionary exoplanet discoveries. Radial velocity measurements, transit and direct imaging observations, and microlensing surveys have all contributed to finding these thousands of known exoplanets. These techniques will continue to be productive in the next decade, but a larger telescope is crucial to assembling a representative sample of planetary systems and measuring the surface conditions on the most intriguing planets, like those that most resemble the Earth. New space missions such as NASA’s Transiting Exoplanet Survey Satellite (TESS; Ricker et al. 2016), should discover dozens of Earth-sized worlds in the habitable zones of neighboring stars, a rich sample we can probe with the GMT’s unique instruments for high resolution spectroscopy and imaging in visible and infrared wavelengths. The sheer number and diversity of the known exoplanets helps us to appreciate our place in the universe. We have discovered planets with masses as small as Mars and as large as 10,000 Earth masses, and with orbits almost as small as their parent stars to even larger than our solar system. In addition to individual planets, we have also discovered a remarkable variety of exoplanetary systems that are quite different from our own. In some, multiple rocky planets orbit their stars in a space smaller than the terrestrial planet zone of our solar system. The challenge is to understand how this remarkable panoply of systems formed and whether planets frequently form with the inventories of elements necessary for life. Observations with the GMT will allow us to determine the densities and atmospheric compositions of these planets and to discover whether any are truly Earth-like. Earth-sized planets with Earth-like temperatures are common but we have yet to learn if they have Earth or Venus-like atmospheres or no atmospheres at all. Their atmospheres may contain water and even oxygen, key signatures for life on present-day Earth. If past exoplanet studies are any guide, we should expect to be surprised by the variety of atmospheres we discover. Mature planets are key targets, but protoplanetary systems—where planets are born—are also critical targets if we are to understand the great variety of the worlds we see. Disks of gas and dust with sufficient mass to form a planetary system like our own surround more than 80% of young, Sun-like stars. The GMT is essential to studying these young stars because they are found farther away than mature systems. We will learn about planet formation by studying circumstellar disks that are still forming planets, a process that appears to last for a few million years, and by attempting to connect the origins of these protoplanets to the properties of mature planets orbiting older stars. The frequency of planets and multi-planet systems derived from current surveys, particularly NASA’s Kepler mission, provides insight into planet formation mechanisms. The migration of solid material during planet formation will PAGE 32 The Giant Magellan Telescope Science Book 2018 Exoplanets & Planet Formation 2 CHAPTER determine the final masses, locations, and compositions of planets, but the physics of planetesimal growth and migration is poorly understood. The origins of the planets, asteroids and comets in our own solar system are also uncertain. Neptune must have migrated into the Kuiper Belt to generate the population seen there today (Hahn & Malhotra 2005), but perhaps Jupiter and Saturn also migrated substantially from their birth locations (Gomes et al. 2005). Using the GMT to study the chemical makeup of small bodies in the solar system—the Kuiper Belt and beyond—will answer many questions about the birth of our world and its siblings. The planet formation process must be capable of producing the wide variety of known planets; this variety will only increase with new discoveries. We estimate that 50% of Sun-like stars host a planet with a radius less than four times that of the Earth and an orbital period less than 100 days (Fressin et al. 2013; Petigura et al. 2013; Foreman-Mackey et al. 2014). Although the most common type of planet discovered so far is a mini-Neptune with a short orbital period, we do not know if such planets are common in the habitable zone and beyond. Giant planets are apparently rare, but they may provide important clues about the formation of planetary systems. Planets larger than Earth may form from condensed ices far from the star. The traditionally hypothesized mechanism of planet growth by accretion cannot form the massive planets found at large distances from solar-type and low-mass stars. Instead, self-gravitating cores may collapse to become giant planets (Boss 1997). In the following discussion, we will explain how the GMT will contribute to our search for other worlds and other life, and how they came to be. Section 2.1 will focus on the characterization of extrasolar planets, including measurements of mass, density, and atmospheric composition. Section 2.2 will focus on the formation of planets and how observations of the gas and dust disks that surround young stars will answer the fundamental questions of planet formation and planet diversity. 2.1 Toward Earth: Planet Characterization The GMT will study the demographics of mass and composition for small planets, especially Earth-sized and smaller, the occurrence rates of small planets with orbital periods longer than 100 days and, in particular, the frequency of Earth-analogs and super-Earths inside the habitable zones of stars (see Figure 2-1). We know little about the nature of planets receiving stellar energy similar to what the Earth receives, i.e. at a temperature where liquid water could persist on rocky surfaces—also commonly referred to as sitting in the “habitable zone.” We have measured the densities for only three such planets. We know nothing about whether such planets typically host atmospheres like Earth or Venus, or any atmospheres at all. The observations required to characterize such temperate planets will take a variety of forms, all described in this section. To measure the densities of transiting planets, which distinguish between water or vapor balls, iron cores, and truly rocky planets, we will need the GMT to measure masses via the radial velocity method. To go beyond density to measure light reflected off the planets’ surfaces, we will need the GMT’s combination of sensitivity and spatial resolution. Ultimately, we want to understand the composition of planetary atmospheres, and even search for gases that would suggest whether a planet is habitable. The GMT could detect molecular oxygen, whose discovery could suggest that a planet is not only habitable, but inhabited. The Giant Magellan Telescope Science Book 2018 33 PAGE CHAPTER 2 Exoplanets & Planet Formation Measuring Masses of Earth-like Planets The G-CLEF high resolution spectrograph on the GMT will make crucial and decisive measurements of the nature of Earth-sized planets. New space missions, such as NASA’s TESS (Ricker et al. 2016), and ESA’s PLATO1, to be launched around 2024, are expected to discover dozens of Earth-sized planets in the habitable zones of nearby stars. These space missions can measure the radii of the planets they discover. However, to learn which of these planets are rocky, for example, we must measure densities, which means combining the sizes with measurements of the planets’ masses. Such density measurements provide potent insight to the structure and bulk composition of a planet, as well as information about the detectability of its atmosphere.