Exoplanets and planet formation

Planets form in protoplanetary discs around young stars as dust grains collide and stick together to form larger and larger bodies. Protoplanetary discs are observed around most young stars. They play two important roles: they are birth sites of planets and they feed gas to the star via accretion.

At Lund Astrophysics, we study how the discs form and evolve, turbulence in the disc, and the formation of planetesimals.

Planetesimal formation

The formation of km-sized planetesimals from smaller millimeter- to centimeter-sized sized pebbles faces difficulties in the traditional hit-and-stick scenario. Such large pebbles do not stick well and very quickly drift towards the star to evaporate in the inner nebula. The streaming instability provides a possible path to planetesimals despite the poor sticking properties of pebbles.

Planetesimal formation takes place in a complex environment of turbulent gas interacting via drag forces with particles of many sizes. The streaming instability thrives in the systematic relative motion of gas and particles and leads to spontaneous clumping of particles. The gravity from the star is partially compensated by the radial pressure gradient of the gas, causing gas to orbit at a slightly slower speed than Keplerian. Clumping is initiated as initially very low amplitude particle overdensities accelerate the gas towards the Keplerian speed, hence reducing the local head-wind on the particles, which in turn slows the radial drift of the particles. Drifting particles pile up where the head-wind is slower, causing exponential growth of the particle density as the particles continue to increase their drag force influence on the gas.

The process can be compared to how bicycle riders and migrating geese travel together to protect themselves from the headwind of the atmosphere.

The particle filaments become very dense, up to several thousand times the local gas density. The collective gravity of all the particles is high enough to initiate a gravitational collapse to form planetesimals with a characteristic radius of a few hundred kilometers km, similar to the largest asteroids and Kuiper Belt objects.

Growth of planetary systems

Protoplanetary discs have life-times of a few million years before the gas and the dust is either accreted onto the star or blown away by winds driven by photoevaporation or magnetic fields. The cores of gas giants and ice giants must assemble while there is still gas left in the protoplanetary disc. But the accretion of 100-kilometer-sized planetesimals is too slow in outer regions of the protoplanetary disc to compete with the removal of the gas. Accretion of pebbles of millimeter-centimeter sizes can boost the growth of the cores of gas giants.

Pebble accretion is an appealing planetary growth mechanism because of the observations of large amounts of pebbles in protoplanetery discs around other stars with instruments such as ALMA, inferred from the characteristic way the opacity of the emission depends on wavelength when particles have grown well beyond micrometer sizes. Pebbles are accreted much more efficiently than planetesimals because the pebbles experience a drag force from the gas, so when a pebble passes by a growing protoplanet it will lose part of its kinetic energy to gas drag, become gravitationally bound to the protoplanet and be left with no choice but to spiral down towards the protoplanet to become accreted.

The combination of streaming instability and pebble accretion is a promising pathway to grow from pebbles to planetary systems. However, collisions between the growing protoplanets will play an important role in the inner regions of the protoplanetary disc where the terrestrial planets form. The giant impact that formed our Moon is an example of such collisions. Terrestrial planets and their higher mass cousins found to orbit around other stars (super-Earths and sub-Neptunes) thus likely form by a combination of pebble accretion, planetesimal accretion and and giant impacts.

State-of-the-art spectrographs have the ability to observe the chemical signatures of the atmospheres of hot, close-in exoplanets. Transit and day-side emission spectra contain the faint signatures of atoms and molecules, which we use to understand the chemistry, structure and dynamics of the atmosphere. In addition, adaptive-optics assisted slit or integral-field spectroscopy can be used to resolve hot, young planets in wide orbits, providing inputs to planet formation models.

In Lund, we focus on developing spectroscopic methods and models to observe and understand the atmospheres of the hottest gas giant exoplanets exoplanets. With new, powerful spectroscopic facilities on the horizon (JWST, ELT, Ariel, etc.), these insights will help us to study cooler, smaller planets and search their spectra for signatures of habitability and life.

What are extra-Solar planets made of? To answer this, we turn to the dead stars known as white dwarfs. Many have heavy elements detected spectroscopically in their atmospheres. These are delivered by asteroids scattered close to the white dwarf that break up and deposit their material in its atmosphere. Understanding this process is the only means we have of determining the bulk elemental compositions of extra-Solar asteroids and planets.