Anders Johansen
Professor
Cooling the Envelopes of Gas Giants : Accretion, structure formation and observability
Author
Summary, in English
We know now that discs of gas and dust, so-called protoplanetary discs, form planets as side-products of the naturally occurring star formation process throughout the Milky Way. Young, growing planets in such discs are commonly referred to as protoplanets. In about 10\% of the so-far observed stars, this planet formation process leads to the occurrence of giant planets, such as Saturn, Jupiter, or even more massive ones. During their early growth, protogiants are expected to be enshrouded in an extremeley extended atmosphere which is still connected to the parent disc, called the envelope. This envelope controls the inflow of mass as well as the cooling properties of the protogiant and later collapses to provide most of the mass of the planet itself. The planetary systems hosting those planets during their envelope growth are tremendously affected by them, be it through their influence on the gas and dust reservoir for further planet formation, or during the late dynamical evolution of the entire planetary system. Due to those factors it is important to know how rapidly and under which protoplanetary disc conditions gas giant envelopes grow.
During my thesis, I used a state-of-the-art simulation code in order to study the growth process, structures and observability of the extended gaseous envelopes of those young planets. At first, I had to solve significant numerical and technical difficulties in order to begin simulating the growth of gas giants. Overcoming those led to a requirement for the numerical resolution of envelopes in order to yield correct growth rates. Subsequently, I was able to simulate a full mass sequence, following protoplanetary growth from Neptune-mass planets up to Jupiter-mass giants. Those simulations show short overall growth timescales for gas giants ranging from hundreds to tens of thousands of years, whereas circumplanetary discs disappear on timescales of a few million years. Hereby it is the growth of dust acting to accelerate this process as large dust cools envelopes efficiently. This emphasizes that the most time-consuming process in nature must be the assembly of a gas giant's solid core.
Furthermore, my work shows that the cooling capability provided to envelopes by dust particles of millimeter-sizes will lead to the formation of circumplanetary discs. These are discs which orbit their host planet and are thought to be the birth sites of regular moon systems around giant planets. During the last episode of my research I compared my simulation data with actual observations of the protoplanet-hosting PDS 70 system. The latter results confirm the presence of a suspected circumplanetary disc around PDS 70c. I further find that the width of the protoplanetary spectrum can be explained by a population of dust grains that have grown significantly to sizes of millimeters.
During my thesis, I used a state-of-the-art simulation code in order to study the growth process, structures and observability of the extended gaseous envelopes of those young planets. At first, I had to solve significant numerical and technical difficulties in order to begin simulating the growth of gas giants. Overcoming those led to a requirement for the numerical resolution of envelopes in order to yield correct growth rates. Subsequently, I was able to simulate a full mass sequence, following protoplanetary growth from Neptune-mass planets up to Jupiter-mass giants. Those simulations show short overall growth timescales for gas giants ranging from hundreds to tens of thousands of years, whereas circumplanetary discs disappear on timescales of a few million years. Hereby it is the growth of dust acting to accelerate this process as large dust cools envelopes efficiently. This emphasizes that the most time-consuming process in nature must be the assembly of a gas giant's solid core.
Furthermore, my work shows that the cooling capability provided to envelopes by dust particles of millimeter-sizes will lead to the formation of circumplanetary discs. These are discs which orbit their host planet and are thought to be the birth sites of regular moon systems around giant planets. During the last episode of my research I compared my simulation data with actual observations of the protoplanet-hosting PDS 70 system. The latter results confirm the presence of a suspected circumplanetary disc around PDS 70c. I further find that the width of the protoplanetary spectrum can be explained by a population of dust grains that have grown significantly to sizes of millimeters.
Department/s
- Lund Observatory - Has been reorganised
Publishing year
2020-04
Language
English
Full text
- Available as PDF - 10 MB
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Document type
Dissertation
Publisher
Lund
Topic
- Astronomy, Astrophysics and Cosmology
Keywords
- Planet formation
- Hydrodynamics
- Giant planets
Status
Published
Supervisor
- Anders Johansen
- Michiel Lambrechts
- Bertram Bitsch
ISBN/ISSN/Other
- ISBN: 978-91-7895-481-0
- ISBN: 978-91-7895-480-3
Defence date
15 May 2020
Defence time
09:00
Defence place
Lundmarksalen, Astronomihuset, Sölvegatan 27, Lund (Live streaming: https://youtu.be/TpXtmrYuw_Q)
Opponent
- Wilhelm Kley (Professor)