Unveiling Planetary Processes with the Solar System and Exoplanets

Abstract

Both solar system and exoplanet science study the same class of objects to advance our knowledge of fundamental planetary processes. Our solar system provides a detailed view of a single planetary system in one epoch. In contrast, exoplanets offer sparser data from thousands of diverse systems in various evolutionary states. Holistic research on planetary objects must draw from both complementary sources. After introducing this mindset in Chapter 1, Chapters 2-4, each highlight a case study with this philosophy.

Chapters 2 & 3 draw on an emergent pathway to connect the solar system and exoplanets: interstellar small bodies. The discovery of 1I/`Oumuamua in 2017, the first-detected planetesimal of extrasolar origin, provided an unprecedented chance to study exoplanetary material at high-resolution. However, the curious properties of `Oumuamua did not suggest any solar system analogs; to date, there remains no scientific consensus on the nature of `Oumuamua. Nonetheless, these attributes raise new questions on the physical, chemical, and dynamical processes that sculpt small bodies.

For example, `Oumuamua's shape was more elongated than nearly all known solar system objects of similar size. Because this shape manifests as lightcurve variability, wide-field discovery surveys may be biased for or against aspherical asteroids. Thus, Chapter 2 uses this property of `Oumuamua for insight on solar system asteroids (Levine & Jedicke, 2023); population estimates for near-Earth asteroids can inform models of planetesimal collisions. Via injection-recovery tests, I find that elongated asteroids are less discoverable than their spherical counterparts. Shapes are not included in standard debiasing procedures, so I show that published asteroid populations may be incorrect. My results suggest that shapes may be causing longstanding discrepancies between surveys on the near-Earth asteroid population.

Next, Chapter 3 examines processes that may sculpt small bodies during their host stars' asymptotic giant branch (AGB) phase (Levine et al., 2023). Although an unlikely explanation for `Oumuamua -- its inbound kinematics instead indicated an origin from a young planetary system -- the AGB mass-loss may generate interstellar small bodies by unbinding long-period orbits. Therefore, the questions arise of whether these objects (1) could comprise a sizeable fraction of galactic small bodies and (2) would be distinguishable from small bodies that were ejected during the star's main sequence. I use analytic and numerical arguments on the orbital and thermal evolution of small bodies to prove that both hypotheses are plausible. In addition, I model the search sensitivity of the Legacy Survey of Space and Time (LSST) to elucidate how its (non)detection of such objects will constrain the average mass budget of extrasolar Oort Cloud analog regions; this region is among the most difficult to detect in our solar system.

Finally, Chapter 4 uses aeronomy, the study of upper atmospheres, to examine hydrodynamic outflows of exoplanets (Levine et al., 2024). I argue that aeronomy is required to explain exoplanet radii, as stellar XUV may drive photoevaporative mass-loss. Furthermore, hydrodynamic escape likely sculpted early Earth and Venus but no longer occurs in the solar system. Thus, exoplanet research is necessary to empirically constrain this process. Fruitful work in aeronomy has observed a natural experiment of the Sun-Earth system over time; changing solar XUV is the independent variable to which Earth’s thermosphere reacts. Applying this approach to exoplanets, I conduct a case study of WASP-69b with contemporaneous Palomar infrared transit observations and Swift Observatory X-Ray data. I show that the planetary metastable helium signal from escaping high-altitude gas dropped in tandem with the star’s X-Ray flux. By calculating the expected planetary response to the X-Ray change via a Parker wind formalism, I attribute the mass-loss variability to WASP-69’s stellar activity cycle, a canonical result in aeronomy but the first documented detection of this phenomenon on an exoplanet.

To conclude, Chapter 5 discusses future research that will bridge solar system and exoplanet science. In the coming years, surveys like the LSST and NEO Surveyor will expand the small body census by five-fold. Determining the colors, orbits, and albedos for sub-km Main Belt asteroids will statistically test models of planetesimal collisions. Understanding the interplay of mineralogy and planetesimal growth is imperative for connecting the composition of dust in protoplanetary disks to the densities of mature exoplanets. In addition, the prolific use of the James Webb Space Telescope (JWST) for characterizing exoplanet atmospheres will further constrain the physics of hydrodynamic escape. NIRISS/SOSS has measured the metastable helium tracer for >20 targets, and a homogeneous analysis of that catalog will probe of planetary mass-loss across a broad parameter space.