The formation of exoplanets

Introduction

The idea that our Solar System may not be unique, and that there might be planets orbiting other stars (or exoplanets), has been around for a long time. Important principles that underpin exoplanet research today were foretold by key discoveries in the eighteenth and nineteenth centuries. In 1783, an unseen companion was presented as an explanation of the peculiar periodic dimming observed for the bright star Algol, and in 1844 the observation of a periodic change in position of the bright stars Sirius and Procyon uncovered their two unseen companions. The concept of detecting and exploring an unseen object by studying its influence on a nearby astronomical source has also been applied to exoplanets and their host stars.

However, exoplanets were expected to be extremely hard to observe in practice. Using the orbits and size of planets in our Solar System as a guide, the influence of an exoplanet on its host star was predicted to be very small. Indeed, it took until the late twentieth century for technological advancements to enable the first exoplanet to be identified, and the result was surprising: 51 Pegasi b (named after its Sun-like host star 51 Pegasi in the Pegasus constellation) was unlike anything in our Solar System. 51 Pegasi b is a hot Jupiter: a planet with a similar mass to Jupiter, but with an incredibly high surface temperature as its orbit takes it very close to its host star. The fact that a planet like 51 Pegasi b exists triggered what became a radical overhaul of theories of planet formation and evolution. But the discovery of a hot Jupiter was also encouraging, as it showed that exoplanet detection was perhaps not quite as impossible a challenge as many had assumed. All astronomers needed to do was start looking for something that is different from the planets of the Solar System.

Since the discovery of 51 Pegasi b, thousands more exoplanets have been discovered by a variety of techniques. The main outcome of exoplanet searches and characterisation studies carried out using these techniques is a known exoplanet population with a wide variety of physical and orbital characteristics. This course explores the key astrophysical concepts involved in planet formation and how they can be used to explain the great diversity of configurations observed.

Section 1 focuses on the birthplaces of planets: the protoplanetary discs. Section 2 describes the core-accretion scenario for planet formation. This was initially developed to explain the existence of Jupiter, but has, over the years, become a more general model of planet formation for its ability to account for a large diversity of planetary outcomes, from Earth-sized planetary cores that form first, to ice and gas giants that evolve later. Lastly, Section 3 focuses on the final stages of planet formation, and explores the core-accretion scenario further, as well as an alternative disc-instability scenario, where the formation of massive objects occurs first from the collapse of gas into clumps by self-gravity.

This course material will refer to masses of stars in terms of the mass of the Sun (represented by M_☉ = 1.99 × 10³⁰ kg), and masses of planets in terms of the mass of the Earth (represented by M_⊕ = 5.97 × 10²⁴ kg) and the mass of Jupiter (represented by M_Jup = 1.90 × 10²⁷ kg). Orbital distances from stars will be expressed in terms of the distance of the Earth from the Sun; this is 1 astronomical unit (1 au = 1.496 × 10¹¹ m).

This OpenLearn course is an adapted extract from the Open University course S384 Astrophysics of stars and exoplanets [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .