Galactic properties of exoplanet host stars
(2025) FYSK04 20251Department of Physics
Astrophysics
- Abstract
- Context. The radius valley is the bimodal distribution of planet radii Rp = 1 − 3.9R⊕, which separates super-Earths from sub-Neptunes. One possible explanation is that stellar clustering in the birth environment of the host star could cause sub-Neptunes to shed their gaseous envelope and turn into super-Earths (Kruijssen et al., 2020). Using the calculated phase space density of host stars as a measure for stellar clustering in their birth environment, Kruijssen et al. (2020) finds that super-Earths are absent in systems of a low phase space density. In contrast, Mustill et al. (2022) suggests that phase space overdensities are a result of less kinematic heating, due to a low stellar age, rather than a remnant from the birth environment.... (More)
- Context. The radius valley is the bimodal distribution of planet radii Rp = 1 − 3.9R⊕, which separates super-Earths from sub-Neptunes. One possible explanation is that stellar clustering in the birth environment of the host star could cause sub-Neptunes to shed their gaseous envelope and turn into super-Earths (Kruijssen et al., 2020). Using the calculated phase space density of host stars as a measure for stellar clustering in their birth environment, Kruijssen et al. (2020) finds that super-Earths are absent in systems of a low phase space density. In contrast, Mustill et al. (2022) suggests that phase space overdensities are a result of less kinematic heating, due to a low stellar age, rather than a remnant from the birth environment. The aim of this thesis is to verify the result of Kruijssen et al. (2020).
Methods. The method of calculating phase space densities of known exoplanet host stars follows from Winter et al. (2020b), and uses astrometric data from the Gaia DR3 release. Cuts are made on the host mass, host age, orbital period, and planet radius. Furthermore, the classification of phase space density is done by fitting a two-component Gaussian mixture model. This is done for stars with at least 400 neighbours within 40 pc, and for which the probability of belonging to a one-component Gaussian model is < 0.05.
Results. I find that super-Earths are present in systems of low phase space density. Moreover, I find that the median stellar age of low phase space density systems is generally larger than in high phase space density systems, and that the fraction of systems in high versus low phase space density is a function of stellar age. This indicates that phase space overdensities are not remnants of the birth environment but a result of less kinematic heating and a lower stellar age.
Conclusion. I do not find any evidence for super-Earths being absent from systems in low phase space density, as was found by Kruijssen et al. (2020). Nor do I find any relation between the measured host star phase space density and the density of its birth environment. In addition, I find that the phase space density is likely a function of stellar age. I then conclude that the impact of stellar clustering in the birth environment can not be determined by measuring the phase space density of systems in the field, as suggested by Kruijssen et al. (2020). (Less) - Popular Abstract (Swedish)
- Mänskligheten har länge varit fascinerad av jordliknande planeter som kretsar kring avlägsna stjärnor. Vi har skickat teleskop ut i rymden för att studera dessa planeter, och för att hitta mönster bland deras egenskaper. Kepler-teleskopet var ett av de första sådana teleskop, och avslöjade den stora variationen av planeter i vår galax. De vanligaste typerna av planeter som hittades var så kallade ”superjordar” och ”subneptuner”. Superjordar är mer eller mindre vad de låter som: som jorden men större. Subneptuner är däremot endast lika Neptunus i storlek. I verkligheten tror man att de är mer lika superjordar i sammansättning, med stora kärnor gjorda av sten som möjligen innehåller vatten och andra flyktiga ämnen, men med mycket tjockare... (More)
- Mänskligheten har länge varit fascinerad av jordliknande planeter som kretsar kring avlägsna stjärnor. Vi har skickat teleskop ut i rymden för att studera dessa planeter, och för att hitta mönster bland deras egenskaper. Kepler-teleskopet var ett av de första sådana teleskop, och avslöjade den stora variationen av planeter i vår galax. De vanligaste typerna av planeter som hittades var så kallade ”superjordar” och ”subneptuner”. Superjordar är mer eller mindre vad de låter som: som jorden men större. Subneptuner är däremot endast lika Neptunus i storlek. I verkligheten tror man att de är mer lika superjordar i sammansättning, med stora kärnor gjorda av sten som möjligen innehåller vatten och andra flyktiga ämnen, men med mycket tjockare atmosfärer. Märkligt nog verkar superjordar saknas kring stjärnor i en viss sorts miljö. Målet med detta arbete är att ta reda på om detta är fallet, och varför. Föreställ dig en stjärna i en tät region, med många stjärnor i dess närhet. Denna stjärna skulle ha en hög rumslig täthet i sin omgivning. Föreställ dig nu en stjärna med bara några få grannar, med en låg rumslig täthet. Du kanske tror att dessa två stjärnor skulle kunna ha samma typ av exoplanet. I så fall skulle du ha fel (åtminstone enligt en studie). Det visar sig att tätheten i värdstjärnans omgivning kan avgöra vilken typ av exoplanet den kan ha. Hastighetstäthet kan också spela en roll i detta. För att förstå hastighetstäthet, föreställ dig en grupp stjärnor där varje stjärna är långt ifrån de andra stjärnorna, med en låg rumslig täthet. Om alla dessa stjärnor rör sig med liknande hastigheter, skulle de anses ha en hög hastighetstäthet. En ny studie upptäckte att superjordar endast verkar kretsa kring stjärnor med en hög täthet i rymd och hastighet. En förklaring till detta är att subneptuner kan förlora sin atmosfär om deras värdstjärna befinner sig i en miljö av hög densitet. Och vad är en subneptun utan större delen av sin atmosfär? En superjord. En annan fråga är varifrån den höga tätheten i rymd och hastighet faktiskt kommer ifrån. En studie föreslår att stjärnor kan röra sig tillsammans som en grupp, med en hög hastighetstäthet, om de föddes ur samma stjärnkluster. En annan studie föreslår att stjärnor blir mindre koncentrerade i rymd och hastighet över tid, genom interaktioner med galaxens spiralarmar. Så även om det fanns kvarlevor från födelseklustret som färdas med hög hastighetstäthet, skulle dessa snabbt ha spridit sig i hastighet. Man kan också ifrågasätta om superjordar verkligen saknas hos stjärnor med låg täthet i rymd och hastighet. I detta arbete kommer jag att göra om denna studie med fler exoplaneter och mer välbestämda egenskaper hos värdstjärnorna. Förhoppningsvis kan detta visa oss om det finns ett samband mellan stjärnans täthet i rymd och hastighet och vilka typer av planeter vi kan hoppas hitta i dess omloppsbana. Detta kan vara särskilt viktigt när man letar efter liv på andra planeter. Sammanfattningsvis, att ta reda på varför superjordar saknas i system med låg täthet kan hjälpa oss att förstå mer om andra planetsystem. Ett annat mysterium att lösa är varför typen av planet i ett system överhuvudtaget är relaterad till tätheten i värdstjärnans omgivning. I detta arbete kommer jag att undersöka om superjordar fortsätter att vara frånvarande i sådana system, för ett mycket större urval av exoplaneter. (Less)
- Popular Abstract
- Humanity has long been fascinated by the prospect of Earth-like planets orbiting distant stars. We have sent telescopes to space to study these planets, and to find patterns among them. The Kepler telescope was one of the first such telescopes, which uncovered the vast variety of planets in our galaxy. The most common types of planets found were so-called ’super-Earths’ and ’sub-Neptunes’. Super-Earths are more or less what they sound like: like Earth but bigger. However, sub-Neptunes are only similar to Neptune in size. In reality, they are thought to be more like super-Earths in composition, with large rocky cores possibly containing water and other volatile elements, but with far thicker atmospheres. Oddly enough, super-Earths appear to... (More)
- Humanity has long been fascinated by the prospect of Earth-like planets orbiting distant stars. We have sent telescopes to space to study these planets, and to find patterns among them. The Kepler telescope was one of the first such telescopes, which uncovered the vast variety of planets in our galaxy. The most common types of planets found were so-called ’super-Earths’ and ’sub-Neptunes’. Super-Earths are more or less what they sound like: like Earth but bigger. However, sub-Neptunes are only similar to Neptune in size. In reality, they are thought to be more like super-Earths in composition, with large rocky cores possibly containing water and other volatile elements, but with far thicker atmospheres. Oddly enough, super-Earths appear to be missing around stars in a certain kind of environment. The goal of this thesis is to figure out if this is the case, and why. Imagine a star in a dense region, with many stars in its vicinity. This star would have a high density in space. Now, imagine a star with only a few neighbours, with a low density in space. You might think that these two stars could have the same type of exoplanet in their orbit. In that case, you would be wrong (at least according to one study). It turns out that the density of the host star’s environment could determine which type of exoplanet it can have. The density in velocity could also play a role in this. To understand density in velocity, picture a group of stars where each star is far apart, with a low density in space. If all of these stars move with similar velocities, they would be considered to have a high density in velocity. A recent study found that super-Earths only seem to orbit stars with a high density in space and velocity. One explanation for this is that sub-Neptunes could lose their atmosphere if their host star is in a crowded environment. And what is a sub-Neptunes without most of its atmosphere? A super-Earth. Another question is where the high densities in space and velocity actually come from. One study suggests that stars can move together as a group, with a high density in velocity, if they were born from the same cluster of stars. Another study suggests that stars become less ”clustered” in phase space over time, by scattering off the galaxy’s spiral arms. Thus, even if there were remnants of the birth cluster moving together, these would have quickly been scattered in velocity. One could also question whether super-Earths are truly missing from stars in low density. For this thesis, I will redo this study with more exoplanets and more well-known host star properties. Hopefully, this could tell us whether there is a relation between the phase space clustering of a star and what kinds of planets we could hope to find in its orbit. This could be especially important when looking for life on other planets. In conclusion, finding out why super-Earths are missing from low-density systems will help us understand more about other planetary systems. Another mystery to solve is exactly why the type of planet in a system is related to the density of the host star in the first place. In this thesis, I will investigate whether super-Earths continue to be absent in such systems, for a much larger sample. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/student-papers/record/9194464
- author
- Coger, Tilde LU
- supervisor
- organization
- course
- FYSK04 20251
- year
- 2025
- type
- M2 - Bachelor Degree
- subject
- keywords
- Exoplanet systems, Exoplanet formation, Stellar dynamics, Radius valley, Planet formation, Planetary systems, Galactic dynamics, Stellar kinematics, Exoplanets
- report number
- 2025-EXA247
- other publication id
- 2025-EXA247
- language
- English
- id
- 9194464
- date added to LUP
- 2025-06-18 11:55:35
- date last changed
- 2025-06-18 11:55:35
@misc{9194464, abstract = {{Context. The radius valley is the bimodal distribution of planet radii Rp = 1 − 3.9R⊕, which separates super-Earths from sub-Neptunes. One possible explanation is that stellar clustering in the birth environment of the host star could cause sub-Neptunes to shed their gaseous envelope and turn into super-Earths (Kruijssen et al., 2020). Using the calculated phase space density of host stars as a measure for stellar clustering in their birth environment, Kruijssen et al. (2020) finds that super-Earths are absent in systems of a low phase space density. In contrast, Mustill et al. (2022) suggests that phase space overdensities are a result of less kinematic heating, due to a low stellar age, rather than a remnant from the birth environment. The aim of this thesis is to verify the result of Kruijssen et al. (2020). Methods. The method of calculating phase space densities of known exoplanet host stars follows from Winter et al. (2020b), and uses astrometric data from the Gaia DR3 release. Cuts are made on the host mass, host age, orbital period, and planet radius. Furthermore, the classification of phase space density is done by fitting a two-component Gaussian mixture model. This is done for stars with at least 400 neighbours within 40 pc, and for which the probability of belonging to a one-component Gaussian model is < 0.05. Results. I find that super-Earths are present in systems of low phase space density. Moreover, I find that the median stellar age of low phase space density systems is generally larger than in high phase space density systems, and that the fraction of systems in high versus low phase space density is a function of stellar age. This indicates that phase space overdensities are not remnants of the birth environment but a result of less kinematic heating and a lower stellar age. Conclusion. I do not find any evidence for super-Earths being absent from systems in low phase space density, as was found by Kruijssen et al. (2020). Nor do I find any relation between the measured host star phase space density and the density of its birth environment. In addition, I find that the phase space density is likely a function of stellar age. I then conclude that the impact of stellar clustering in the birth environment can not be determined by measuring the phase space density of systems in the field, as suggested by Kruijssen et al. (2020).}}, author = {{Coger, Tilde}}, language = {{eng}}, note = {{Student Paper}}, title = {{Galactic properties of exoplanet host stars}}, year = {{2025}}, }