Habitable Environments in Late Stellar Evolution Conditions for Abiogenesis in the Planetary Systems of White Dwarfs Author: Dewy Peters Supervisor: Prof. dr. F.F.S. van der Tak A thesis submitted in fulfillment of the requirements for the degree BSc in Astronomy Faculty of Science and Engineering University of Groningen The Netherlands March 2021 Abstract With very high potential transit-depths and an absence of stellar flare activity, the planets of White Dwarfs (WDs) are some of the most promising in the search for detectable life. Whilst planets with Earth-like masses and radii have yet to be detected around WDs, there is considerable evidence from spectroscopic and photometric observations that both terrestrial and gas-giant planets are capable of surviving post-main sequence evolution and migrating into the WD phase. WDs are also capable of hosting stable Habitable Zones outside orbital distances at which Earth-mass plan- ets would be disintegrated by tidal forces. Whilst transition- ing to the WD Phase, a main-sequence star has to progress along the Asymptotic Giant Branch (AGB), whereby orbit- ing planets would be subjected to atmospheric erosion by its harsh stellar winds. As a trade-off, the Circumstellar En- velope (CSE) of an AGB star is found to be rich in organics and some of the simple molecules from which more complex prebiotic molecules such as amino acids and simple sugars can be synthesised. It is found that planets with initial or- bital distances equivalent to those of Saturn and the Kuiper Belt would be capable of accreting a mass between 1 and 20 times that of the Earth’s atmosphere from the CSE and as a result, could obtain much of the material necessary to sustain life after the AGB and also experience minimal at- mospheric erosion. However, the orbital distance evolution of these planets also presents an obstacle to their habitabil- ity in that they would have to migrate from „ 100 AU to „ 0:01 AU to dignify their prebiotic chemistries with the warmer conditions necessary to sustain life at stable WD HZs. In this regard, more work is required to model extreme inward orbital migration or the accretion of a secondary or tertiary atmosphere from the CSE of an AGB star. Article number, page 2 of 77 Contents 4.3 Prebiotic Molecules . 22 4.3.1 Formal Definition . 22 1 Introduction 5 4.3.2 Prebiotic Molecules in Circumstellar Envelopes . 23 2 The Planets of White Dwarfs 6 2.1 Evidence for Planets . 6 5 Prospects for Abiogenesis 24 2.1.1 White Dwarf Pollution . 6 5.1 Planetary Accretion from the Circumstellar Envelope . 24 2.1.2 Circumstellar Disks . 7 5.1.1 Delivery of Envelope Material . 24 2.1.3 Transit Photometry . 8 5.1.2 Orbital Distance Evolution . 25 2.2 White Dwarf Demographics . 9 5.1.3 Masses Accreted from the Circum- 2.2.1 Gaia Data Release 2: White Dwarfs stellar Envelope . 26 within 100 pc.............. 9 5.2 The Synthesis of Higher Level Prebiotic 2.2.2 Detectable Earth-like Planets . 10 Molecules . 27 2.2.3 Montreal White Dwarf Database . 10 5.2.1 Amino Acids . 27 2.3 Planetary Compositions . 11 5.2.2 Carbohydrates . 28 2.3.1 General Trends in White Dwarf Pol- 5.2.3 Nucleobases . 28 lution . 11 5.2.4 Catalysts: Polycyclic Aromatic Hy- 2.3.2 Heavily Polluted White Dwarfs . 12 drocarbons . 28 3 Habitable Zones around White Dwarfs 14 6 Discussion 29 3.1 Key Parameters . 14 6.1 Detectable Earth-like Planets around White Dwarfs . 29 3.1.1 Habitable Zone Orbital Distance . 14 6.2 Stable Habitable Zones around White Dwarfs 29 3.1.2 Roche Limit . 15 6.3 Prebiotic Molecules in Circumstellar Envelopes 30 3.2 Earth-insolation Distances . 15 6.4 The Planetary Circumstellar Envelope Ac- 3.3 Implications of Climate Models . 16 cretion Model . 30 3.3.1 Classical Boundaries . 16 7 Conclusion 33 3.3.2 Methane Extension . 17 A Propagated Error Values for Habitable Zone 4 The Prebiotic Chemistry of Circumstellar Calculations 38 Envelopes 20 A.1 Roche Limits in 10´2 AU . 38 4.1 Stellar Evolution . 20 A.2 Habitable Zones in 10´2 AU . 40 4.1.1 Asymptotic Giant Branch . 20 A.2.1 Earth-insolation Distances . 40 4.1.2 Protoplanetary Nebula . 20 A.2.2 Inner Boundaries . 42 4.1.3 Planetary Nebula . 21 A.2.3 Outer Boundaries . 44 4.2 Circumstellar Envelopes . 21 A.2.4 Methane Outer Boundaries . 46 4.2.1 Carbon-rich Envelopes . 21 A.3 Habitable Zones in RRoche . 48 4.2.2 Oxygen-rich Envelopes . 22 A.3.1 Earth-insolation Distances . 48 Article number, page 3 of 77 A.3.2 Inner Boundaries . 50 A.3.3 Outer Boundaries . 52 A.3.4 Methane Outer Boundaries . 54 A.4 Methane Extension . 56 B Code 58 B.1 Histogram in Introduction . 58 B.2 Estimation of Earth-transits within 100 pc . 58 B.3 Cooling Tracks for 0:6 Md and 0:8 Md White Dwarfs . 59 B.4 [Fe/Mg] in Polluted White Dwarfs . 60 B.5 Habitable Zones . 62 B.5.1 Initialisation . 62 B.5.2 Coefficients . 63 B.5.3 Habitable Zone Calculations . 63 B.5.4 Error Propagation . 64 B.5.5 Generating LaTeX Tables of Errors . 66 B.5.6 Plot of Earth-equivalent Distances . 66 B.5.7 Plot of Classical Habitable Zones . 67 B.5.8 Plot to compare with Methane Boundaries . 68 B.5.9 Plot of the Methane Extension . 69 B.6 Accretion Model . 69 Article number, page 4 of 77 1. Introduction atoms fused in and convected from the the interiors of the AGB stars (Habing & Olofsson 2013). Juxtaposing this biologically relevant molecular diversity with the prospect Since the discovery of a planet orbiting the Main Sequence of WDs facilitating habitable planetary environments (MS) star 51-Pegasi (Mayor & Queloz 1995), exoplanetol- warrants an investigation into whether life could evolve on ogy has very rapidly grown as a sub-field of astronomy and the planet of a WD and how it could do so. in part has been driven by the search for extraterrestrial life. As of the 17th of February 2021, 4341 exoplanets In light of the points discussed above, the central ob- have been confirmed (Akeson et al. 2013; NASA 2021), jective of this thesis is to determine the extent to which and it has become increasingly evident that Earth is by carbon-based life is likely to arise on the planet of a WD no means unique in being a terrestrial planet (see Fig. 1). from material contained within the circumstellar envelope It has also become increasingly clear that Earth is by no of its AGB progenitor through the PN phase. In order to means unique in occupying the Habitable Zone (HZ) of its do so, Section 2 examines the demographics of WDs and host star (eg. Anglada-Escudé et al. (2016)): that is, the current constraints on the compositions of their planets; distance at which a planet is able to sustain liquid water on 1 Section 3 then investigates the whether WDs are capable its surface given sufficient atmospheric pressure (Kasting of exhibiting continuous habitable zones beyond the orbital et al. 1993). Interestingly, evidence for exoplanets orbiting distances at which an Earth-like planet would be destroyed stellar remnants predates that of MS stars: Wolszczan & by tidal forces; Section 4 probes the prebiotic molecular Frail (1992) confirmed two planets orbiting the pulsar PSR content of Circumstellar Envelopes (CSEs) and Section 5 1257+12 and observational evidence for a planet(esimal) treats both how the molecules found in CSEs could be used having been accreted onto the photosphere of a White in synthesising those uniquely found in living organisms Dwarf (WD) was recorded as far back as 1917. The planets and how this material could be delivered to a nearby planet of stellar remnants have seldom been targets in the search surviving through to the WD phase. Bringing findings for habitable worlds, owing to former-HZ planets being from all these disparate investigations together, in Section engulfed in late stellar evolution or subjected to harsh 7, a conclusion is sought on the likelihood of life arising on radiation fields (Villaver & Livio 2007). In spite of this, a WD planet and what constraints it would be subjected recent theoretical work has suggested that if the planets to. Finally, the implications of these findings and possible of WD undergo tidal migration, they may indeed be improvements are discussed in Section 6. capable of sustaining life (Kaltenegger et al. 2020). The first transit detection of a giant intact2 planet (WD 1856b) orbiting a WD has also demonstrated that planetary mi- gration is possible beyond the MS (Vanderburg et al. 2020). The appeal of habitable WD planets is rooted in their very high potential transit depths. These would be conducive to the in-depth scrutiny of their atmospheric constituents including possible biosignatures (Agol 2011; Loeb & Maoz 2013; Kaltenegger et al. 2020). Since WDs are expected to be the evolutionary end-point for 97% of stars in the Milky Way (Fontaine et al. 2001), studying their planets also allows us to infer the future of the vast majority of planetary systems, including the Solar System. However, the lowest mass progenitors (with spectral classes K and M) both have lifetimes greater than the current age of the Universe. Higher mass progenitors which have bequeathed their remnant cores as WDs (0:68 Md ă M ă 8 Md (Dob- bie et al. 2006)) have likely all done so by shedding their circumstellar envelopes on the Asymptotic Giant Branch (AGB): that is the phase of stellar evolution whereby a MS star has already become a red giant, reached its maximum luminosity and is losing a considerable amount of mass ´8 ´5 ´1 („ 10 ´ 10 Md yr ).