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The Milky Way and its Exoplanets

Nielsen, Jesper LU (2021) In Lund Observatory Examensarbeten ASTM31 20211
Lund Observatory
Abstract
The detections of over 4000 exoplanets as of today has shown that they exist in a wide range of different configurations, otherwise known as architectures. Recently, studies have been made trying to link the environment of host stars to the architectures of their planetary systems and have found that the architectures of planetary systems may not necessarily be uniform in the Milky Way. Since effects such as photoevaporation may truncate or dissipate protoplanetary discs and thus affect the formation and evolution of planetary systems, we aim to investigate how planetary system architectures are affected by the galactocentric radius at which their host stars were formed. By determining the ages of stars using a precise Bayesian fitting... (More)
The detections of over 4000 exoplanets as of today has shown that they exist in a wide range of different configurations, otherwise known as architectures. Recently, studies have been made trying to link the environment of host stars to the architectures of their planetary systems and have found that the architectures of planetary systems may not necessarily be uniform in the Milky Way. Since effects such as photoevaporation may truncate or dissipate protoplanetary discs and thus affect the formation and evolution of planetary systems, we aim to investigate how planetary system architectures are affected by the galactocentric radius at which their host stars were formed. By determining the ages of stars using a precise Bayesian fitting algorithm and combining these with their metallicities and models of the evolution of the radial metallicity gradient in the Milky Way, the \textit{formation radii} of stars are determined. By binning the stars in formation radius and estimating the occurrence rates in each bin through Markov Chain Monte Carlo simulations of a Poisson process likelihood model which separates different orders of detection, individual multiplicities of planetary systems can be linked with the birth environment of their stars.
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Six-planet systems were found to be the most common multiplicity for all formation radii and was also the highest multiplicity considered although due to very large uncertainties and the expectation that high multiplicities are common, these results will need to be further investigated. No statistically significant trend between the occurrence rate of planetary systems and the formation radius of their host stars was found for any of the multiplicities considered but the large uncertainties make the results still inconclusive.
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KS tests were performed on the orbital period and planet radii distributions of planets around host stars formed inside and outside a given Galactocentric radius. After debiasing for stellar ages, the null hypothesis that the two subsamples of orbital periods and planet radii were drawn from the same, underlying distribution, could not be rejected for any formation radius. These results hints towards the formation process and evolution of planetary systems being independent of the galactocentric radius at which the system was formed although further investigations are needed. (Less)
Popular Abstract (Swedish)
Our Solar System is not the only planetary system in the universe. Since the first detected planet orbiting a star other than the Sun, over 4000 exoplanets have been detected. These exoplanets live in systems with an incredible diversity. The systems can include single, giant planets orbiting very close to the star, or they can have multiple planets packed tightly together. What is causing these different architectures to arise is still unknown to us. What is known is the fact that planets are formed from a disc of gas which orbits around a newly formed star, a protoplanetary disc, and the properties of these discs shape the architectures of the system. The properties of the disc is in turn shaped by the properties of its host star. The... (More)
Our Solar System is not the only planetary system in the universe. Since the first detected planet orbiting a star other than the Sun, over 4000 exoplanets have been detected. These exoplanets live in systems with an incredible diversity. The systems can include single, giant planets orbiting very close to the star, or they can have multiple planets packed tightly together. What is causing these different architectures to arise is still unknown to us. What is known is the fact that planets are formed from a disc of gas which orbits around a newly formed star, a protoplanetary disc, and the properties of these discs shape the architectures of the system. The properties of the disc is in turn shaped by the properties of its host star. The environment in which stars are formed are therefore incredibly important in shaping the structure of the protoplanetary discs and thus the architectures of planetary systems. For example, stars formed in very chaotic environments with lots of stars surrounding them may experience stellar fly-bys which is when two stars fly very close to each other. The gravity from each star can then cause the planetary systems to become so unstable that planets get ejected from the systems. This means that it is expected that planetary system should look different throughout the galaxy since the environment in our galaxy changes whether or not someone is for example close to the center of it, where there are plenty of stars, or if someone is in the outskirts, where there are much fewer stars. Naively then, one might think that it is possible to observe where in the galaxy planetary systems are, and see if they change. The problem with that method is the fact that stars move from their birthplace. If a star is formed, orbiting at some radius $R_1$ from the center of the galaxy, it can, during its lifetime, move to another orbit at radius $R_2$. This means that the environments in which stars are observed in today are probably not the environments in which they were born.

All stars are formed from the interstellar medium or ISM, gas floating around between each star system in the Galaxy. The metal contents of the ISM, its metallicity, at the specific time and place in which a star is formed is then kept within that star. The ISM is not homogeneous, instead its metallicity is changing throughout the galaxy and evolving over time as stars explode in supernovae, expelling all their formed elements into the gas. This means that the metallicity in the ISM is unique for a specific time and place in the galaxy meaning that the metallicity of a star can be seen as a sort of fingerprint from its birthplace. If both the age and the metallicity of a star is known, it will be possible to estimate where it was formed, its formation radius.

In this project, the aim is to estimate where in the galaxy stars which are hosting planets are formed. Then, it will be possible to say something about how the planetary systems look like depending on where they were formed. However, it is important to note that it is not possible to detect all planets which are orbiting around any given star. Most planets today have been detected using the so-called transit method where light from a star is recorded and if a planet is passing in front of the star from our point of view, the light dims. By observing how much the light dims, it is possible to determine how large a planet is. However, since the sizes of planets are incredibly small compared to that of stars, only large planets orbiting very close to their stars are able to be detected. This means that the planets we have detected right now are probably not representative of all of the planets which exists out there in the whole wide universe. To combat this in this project, a statistical model is set up. It is possible to calculate the probabilities for us to detect a planet around any given star and by comparing the number of planets we would expect to detect compared to the number of planets we have detected, it is possible to estimate the occurrence rates of planets which is the fraction of stars that are expected to host planets. By comparing the calculated occurrence rates with the formation radii of their host stars, it is finally possible to understand how planetary system change depending on where in the galaxy their host stars were formed which might bring useful insight in what different environmental aspects which might affect the formation and evolution of planetary systems. (Less)
Please use this url to cite or link to this publication:
author
Nielsen, Jesper LU
supervisor
organization
course
ASTM31 20211
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Planets and Satellites, Exoplanets, Occurrence Rates
publication/series
Lund Observatory Examensarbeten
report number
2021-EXA177
language
English
id
9049566
date added to LUP
2021-06-07 10:31:41
date last changed
2021-06-07 10:31:50
@misc{9049566,
  abstract     = {{The detections of over 4000 exoplanets as of today has shown that they exist in a wide range of different configurations, otherwise known as architectures. Recently, studies have been made trying to link the environment of host stars to the architectures of their planetary systems and have found that the architectures of planetary systems may not necessarily be uniform in the Milky Way. Since effects such as photoevaporation may truncate or dissipate protoplanetary discs and thus affect the formation and evolution of planetary systems, we aim to investigate how planetary system architectures are affected by the galactocentric radius at which their host stars were formed. By determining the ages of stars using a precise Bayesian fitting algorithm and combining these with their metallicities and models of the evolution of the radial metallicity gradient in the Milky Way, the \textit{formation radii} of stars are determined. By binning the stars in formation radius and estimating the occurrence rates in each bin through Markov Chain Monte Carlo simulations of a Poisson process likelihood model which separates different orders of detection, individual multiplicities of planetary systems can be linked with the birth environment of their stars. 
\\
\\
Six-planet systems were found to be the most common multiplicity for all formation radii and was also the highest multiplicity considered although due to very large uncertainties and the expectation that high multiplicities are common, these results will need to be further investigated. No statistically significant trend between the occurrence rate of planetary systems and the formation radius of their host stars was found for any of the multiplicities considered but the large uncertainties make the results still inconclusive. 
\\
\\
KS tests were performed on the orbital period and planet radii distributions of planets around host stars formed inside and outside a given Galactocentric radius. After debiasing for stellar ages, the null hypothesis that the two subsamples of orbital periods and planet radii were drawn from the same, underlying distribution, could not be rejected for any formation radius. These results hints towards the formation process and evolution of planetary systems being independent of the galactocentric radius at which the system was formed although further investigations are needed.}},
  author       = {{Nielsen, Jesper}},
  language     = {{eng}},
  note         = {{Student Paper}},
  series       = {{Lund Observatory Examensarbeten}},
  title        = {{The Milky Way and its Exoplanets}},
  year         = {{2021}},
}