Tools to detect and characterise circumbinary exoplanets

Baycroft, Thomas ORCID: 0000-0002-3300-3449 (2025). Tools to detect and characterise circumbinary exoplanets. University of Birmingham. Ph.D.

Preview

Baycroft2025PhD.pdf
Text - Accepted Version
Available under License All rights reserved.
Download (121MB) | Preview

Abstract

Circumbinary exoplanets, those planets orbiting around a binary star, are a fascinating class of exoplanet which, while prevalent in science fiction (e.g. LucasFilm et al. 1977), were only confirmed to exist in 2011. Circumbinary planets are particularly useful as a controlled experiment for planet formation since certain theorised planet formation mechanisms around single stars are not available to planets orbiting binaries. In this thesis I develop tools better to detect and understand circumbinary planets, and then deploy these across different stages of binary evolution and to various types of data. Chapter 1 provides an introduction to the field of exoplanet research and to circumbinary planets specifically.

Chapter 2 contains an introduction to Bayesian statistics and its use in model comparison for exoplanet detection. The various code contributions that I have made, in particular to kima are also described in Chapter 2. One contribution is the BINARIESmodel, tailored to analysis of radial velocity data of binaries and circumbinary planets. This model includes general relativistic and tidal corrections for close binaries; the apsidal precession of the binary as a free parameter; and the ability to analyse double-lined binaries which have two simultaneous radial velocity timeseries. Other contributions to kima are models to fit eclipse timing variations and Gaia astrometry.

Chapter 3 and Chapter 4 present results on main-sequence binaries. Chapter 3 is focused on double-lined binaries, which includes radial velocities confirming the detection of a circumbinary planet around TIC 172900988 and an analysis of a sample of double-lined binaries showing that new spectral disentangling methods allow us to obtain more precise and more accurate radial velocities than previously. A discussion of future circumbinary planet detections with Gaia data is also included. Chapter 4 divulges results from the BEBOP survey for circumbinary planets. Results include individual systems with planet detections and candidates, the BEBOP-3 system is highlighted. Other results from BEBOP are included, presenting the growing population of circumbinary planets. This population is compared to previous results from samples of transiting circumbinary planets.

Circumbinary planets have been claimed orbiting post main-sequence binaries, I revisit a famous example of a post common-envelope binary HW Virginis, which has claimed planets from eclipse timing variations, in Chapter 5. The eclipse timing variations are studied to test whether there are credible planetary signals in the data, and Gaia-Hipparcos astrometry is used to find marginal evidence for the existence of a planet in the system. Predictions are also made in Chapter 5 for how future Gaia data releases will allow for confirmation or refutation of many of the claimed eclipse timing variation systems like HW Vir.

Chapter 6 describes the use of apsidal precession measurements of binaries in greater detail and how such a measurement can be used to predict the presence of a circumbinary planet and glean information about its orbital configuration. I present the analysis of a rare binary brown dwarf which is found to have a retrograde apsidal precession, and show that ruling out all other possibilities it must be due to a circumbinary planet on an orbit inclined at 90 degrees to the orbital plane of the binary.

Concluding remarks are made in Chapter 7.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):