La adolescencia: el reconocimiento del ser

The night sky has captivated human curiosity and imagination for millennia. The ancient Greeks noticed a handful of celestial objects that seemed to move with respect to the other stars and called them "planetae" (wanderers). Today we know of the eight planets in the Solar System along with numerous moons, asteroids, dwarf planets and comets. Not only that, but in the last decades we have discovered a plethora of planets orbiting around other stars, referred to as exoplanets. Up to date there are close to 3000 confirmed exoplanets, with another∼2500 exoplanet candidates from the Kepler mission1awaiting confirmation. Figure 1.1 shows planet mass and orbital radius met- rics for over 5000 of these exoplanets, collected from the Exoplanet Data Explorer2(Han et al., 2014).

Figure 1.1: Planet mass as a function of orbital radius for over 5000 exoplanets. Red data points indi- cate planets discovered with transiting method, blue - radial velocity, green - microlensing, brown - direct imaging. Planetary masses for transiting planets without corresponding radial velocity measurements are estimated using mass-radius relations as described in Han et al. 2014 and references therein. The plot shows two large populations - Jupiter-sized (and larger) planets in various orbits, as well as Neptune-sized planets orbiting very close to their stars. We have not discovered any Solar System analogues yet. Data collected from the Exoplanet Data Explorer.

1_{https://www.nasa.gov/mission_pages/kepler/overview/index.html} 2_{http://exoplanets.org/}

1.1. Exoplanets and the Solar System - what do we still not know? The different colours in the figure represent different observing techniques (which I will not discuss here). It is immediately obvious that the known exoplanetary systems are very different from our Solar System. There is a large population of giant planets with masses between 1 and 10 Jupiter masses at distances between 1 and 10 AU from their host stars (blue dot area in plot). In other words, we have discovered many Jupiter or bigger sized planets, existing on anywhere between Earth and Saturn-sized orbits around their host stars. What is more remarkable, there is a host of so-called "hot Jupiters" - planets of Jupiter size with orbital periods of only a few days (separations of<0.1 AU). So there is an entire population of gas giants with orbits smaller than that of Mercury! But that is not all - the majority of Kepler candidates (bottom left quadrant in Figure 1.1) appear to be Neptune-size objects with orbits smaller than the Earth’s.

Why have we discovered such an overwhelming number of big planets so close to their stars? What is their history - did they form where we see them now or did they travel in from further out in their systems? If they did form in the same place we see them now - how was it possible for that to happen so close to their stars? At small distances away from the central star the gas and dust that build a planet are cleared out relatively fast, so would the planets have the time to grow to the size we observe them to have today? And if they did migrate from further out - what is the physical mechanism for that? And why did the migration stop and the planets didn’t orbit into their stars? And of course, if giant planets close to their stars are so common - why do we not see them in the Solar System? More detailed reviews on some of these topics can be found in Morbidelli & Raymond (2016), Johansen et al. (2014), Raymond et al. (2014), Helled et al. (2014).

There are still a number of mysteries within our own Solar System as well. The Solar System has a tri-modal structure - the inner Solar System where the small, rocky planets live, followed by the asteroid belt and finally the realm of the gas giants. How did this particular structure come to be? The inner terrestrial planets consist of Mercury, Venus, Earth and Mars. Terrestrial planets are said to require a Mars-sized embryo to form from (see Morbidelli et al. 2012 and references therein). The present day inner planets are not significantly more massive than Mars and assuming they were indeed formed from Mars-sized embryos, it follows that the rocky planets have not grown by much since they were formed. This may indicate that there was a depletion of solids beyond 1 AU in the early stages of formation of the Solar System (Hansen, 2009; Raymond et al., 2009). Yet the gas giants, which live well beyond 1 AU, needed accretion of solid cores of 10-20 Earth masses in order to form (Pollack et al., 1996), which contradicts the solids depletion idea. And so, how do we explain the current distribution of masses of planets in the Solar System?

Chapter 1. Introduction

More questions about the formation of the Solar system arise from analysis of meteorite sam- ples. Meteorites found on Earth can generally be separated into two major groups - iron and stone ones. Iron meteorites (meteorites which are mostly composed of an iron-nickel alloy) are esti- mated to have formed in the first My (106yr) after the formation of the first solids in the Solar System (Kruijer et al., 2012). On the other hand, chondritic meteorites (a type of stone meteorite predominantly composed of round grains of minerals) are thought to have formed after about 3-4 My (Villeneuve et al., 2009). As both kinds of meteorites come from the asteroid belt and aster- oids are remnants from the planet formation phase, that raises the question of whether there were (at least) two generations of planetesimals (planet seeds) forming in the same area of the Solar System. And if that was not the case, but instead the current asteroid belt is composed of bodies that originally formed in separate places - how did they get transported and trapped in their current location?

The questions laid out in the previous paragraphs are only a small selection of unsolved mys- teries in the field of star and planet formation. The majority of these problems lead back to more fundamental questions such as what is the structure and composition of young stellar systems, how and under what conditions can we build large bodies (planetary embryos) from dust particles, and once formed, how do the embryos interact with their surrounding environment? These problems clearly demonstrate the need to study the early stages of stellar systems’ lives. This thesis will focus on studying the environments and behaviour of stars in their infant ages.

Let us next examine what the early life of a star is like according to the current scientific ideas. The theories presented in the following sections refer only to low-mass stars, roughly defined as having masses of.2 M. The low-mass range is of particular interest as low mass stars are the most commonly found type of stars in the Galaxy. They are also the favoured type of host star for searching for Earth-sized exoplanets with current instrumentation, with the most recent success being the TRAPPIST-1 system, where seven terrestrial planets were found orbiting around a 0.08 Mstar (Gillon et al., 2017).