Brad Hansen


Office: 3-913, Physics and Astronomy
Phone: (310) 825-5924
Fax: (310) 206-2096


Physics and Astronomy, and
Mani L. Bhaumik Institute for Theoretical Physics ,

Positions held in the past:

Hubble Fellow, Princeton University
Research Associate, Canadian Institute for Theoretical Astrophysics


Ph.D., 1996, California Institute of Technology  
Undergraduate: University of Natal, Durban
High School: St Henry's College, Durban


Curriculum vitae (PDF)

Current Graduate Students ( History )

Grant Weldon (Third year)

Current Undergraduate Students

Jacob Levine, Gemma Greene

Prior Graduate Students

Tze-Yeung Yu (2023)
Thesis: Exoplanet Orbital Dynamics: From Resonance Chain Instabilities to Modeling the Host Stars Magnetic Effect on Protoplanetary Disk and Its Effects on the Close-in Super Earths and Sub-Neptunes

K. Hayakawa (2022)
Thesis: Debris Rings from Extrasolar Irregular Satellites

J.K. Zink (2021)
Thesis: Utilizing Kepler and K2 to Advance Exoplanet Demographics

A.M. Vinson (2020)
Thesis: Exoplanetary Spin States in Cool Dwarf Systems: Effects of Mean Motion Resonances and Atmospheric Tides

S.F.N. Frewen (2015)
Thesis: Eccentric Planets around Evolved Stars

I.J.M. Crossfield (2012)
Thesis: Infrared Observations of Exoplanet Atmospheres

F.E. Koch (2009)
Thesis: Numerical Orbital dynamics in classical and relativistic systems

S.J.Berukoff (2009)
Thesis: Problems in Computational Astrophysics

E.Y.Chen (2008)
Thesis: Evolution of White Dwarfs with Some Hydrogen


Administrative Assignments

Chair, Comprehensive Exam Committee 2023

Guidelines for the UCLA Astronomy Comprehensive Exam can be found here and here .


Research Interests: Theoretical Astrophysics

Extrasolar Planets are my principal subject of interest today. My students and I pursue mostly theoretical but also some observational programs on a variety of topics.

Recent theoretical work has focussed on the dynamics and origin of extrasolar planetary systems as well as to how our own Solar system fits into this broader context. Observations now suggest that the dominant population of planets in our galaxy may be composed of highly multiple systems of low mass stars, especially if such systems are found around low-mass M dwarfs, which are the dominant stellar demographic in the Galaxy. Along with Norm Murray, at CITA, I have been studying the in situ assembly of rocky planets on sub-AU scales around stars of different mass. Our models are able to provide an encouraging match to the observed distribution, and a much better fit that the more conventionally assumed models based on the concept of migration. More detail on this work can be found here .

I am also interested in the dynamics of systems containing multiple giant planets. Understanding the secular structure of these systems may shed light on the formation processes, especially in those systems that exhibit substantial eccentricities and/or inclinations. I am also interested in the attendant populations, such as large moons or trojan planets, as they may also provide insight into the formation of such planets. See here for a non-technical discussion of some of these issues. I also have students working on models for the migration of giant planets ( Matt Yu ) and the production of dust in debris disks ( Kevin Hayakawa ). These two projects are done in collaboration with Yasuhiro Hasegawa at JPL.

I also have an ongoing interest in how our own Solar system fits into the framework of the planets discovered around other stars. I have worked on the formation of the terrestrial planets and am currently working on the formation of moons in the Solar system and around other stars.

I also maintain a long-standing interest in the existence and survival of planets around evolved stars. This dates back to my days as a graduate student, when I worked on models for the formation of the pulsar planets, and continues today with studies of planetary systems around white dwarfs. My former graduate student, Shane Frewen, studied the evolution of planetary systems around both white dwarfs and subgiant stars. More detail on this work can be found here .

On the observational side, along with my former student Ian Crossfield , and long-time collaborator Travis Barman, we have pursued a program over the last several years to observe transmission spectra of extrasolar giant planets. These observations offer the promise of constraining the chemical compositions of planetary atmospheres, and the techniques we develop will hopefully prove useful one day in performing similar measurements on lower mass, earth-class planets. More detail on this work can be found here . In the last few years, my role in this particular enterprise has diminished as Ian has successfully assumed leadership of this and related projects. A more recent student observational project is that of Jon Zink, who is working on the production of a uniform catalogue of planets from the K2 mission, in collaboration with Jessie Christiansen, at IPAC.

White Dwarfs are the end product of stellar evolution for most stars (except for those massive enough to explode as supernovae). As such, the study of old white dwarfs can tell us a lot about the history of star formation in our Galaxy. By the time a star has reached the white dwarf stage, it has exhausted all the nuclear burning resources with which it was born. With no remaining reservoir of energy supply white dwarfs slowly cool and fade over time. I have worked a lot on theoretical models for the evolution and appearance of white dwarfs of different kinds. With my observational collaborators, I have used these models to estimate the ages of various stellar components of our Galaxy, including both the stars in the immediate vicinity of the sun and some of the nearest globular clusters. One of our recent results is a setting a lower limit on the age of our Galaxy of approximately 11 billion years.

Neutron Stars and Black Holes are the first objects I worked on, during my PHD. My most recent interest in this subject concerns the black hole at the center of our Galaxy and the immediate environment. With my students Steve Berukoff and Elliot Koch, I have been studying the dynamics of thousand-solar mass black holes orbiting the main black hole. We believe these `intermediate mass' black holes may have played an important role in the transport of young stars to the Galactic center and they will have an important influence on the resulting dynamics.

Recent Lead Author Publications: (last 5 years)

``Bound circumplanetary orbits under the influence of radiation pressure: Application to dust in directly imaged exoplanet systems"

B. Hansen & Kevin Hayakawa The Open Journal of Astrophysics, 7, (2024)

``Consequences of dynamically unstable moons in extrasolar systems''

B. Hansen, Monthly Notices of the Royal Astronomical Society, 520, 761 (2023)

``Unbound close stellar encounters in the Solar neighbourhood''

B. M. S. Hansen, The Astronomical Journal, 163, 44 (2022)

``Minimal conditions for survival of technological civilizations in the face of stellar evolution''

B. M. S. Hansen & B. Zuckerman, The Astronomical Journal, 161, 145 (2021)

``The Stationary Points of the Hierarchical Three Body Problem''

B. Hansen & S. Naoz, Monthly Notices of the Royal Astronomical Society, 499, 1682 (2020)

``Formation of Exoplanetary Satellites by pulldown capture''

B. Hansen, Science Advances, Vol 5, eaaw8665 (2019)

Recent Papers led by Graduate Students

``Debris Rings from Extrasolar Irregular Satellites''

K. T. Hayakawa & B. M. S. Hansen , Monthly Notices of the Royal Astronomical Society, 522, 2115 (2023)

``Halting Migration in Magnetospherically Sculpted Protoplanetary Disks''

T.-Y. M. Yu , B. Hansen & Y. Hasegawa , submitted to MNRAS (2023)

``The Great Inequality and the Dynamical Disintegration of the Outer Solar System'',

J. K. Zink , K. Batygin, F. C. Adams , Astronomical Journal, 160, 232 (2020)

``Scaling K2. III. Comparable Planet Occurrence in the FGK Samples of Campaign 5 and Kepler'',

J. K. Zink , K. K. Hardegree-Ullman; J. L. Christiansen; E. A. Petigura; C. D. Dressing; J. E. Schlieder; D. R. Ciardi; I. J. M. Crossfield, , Astronomical Journal, 160, 94 (2020)

``Scaling K2. II. Assembly of a Fully Automated C5 Planet Candidate Catalog Using EDI-Vetter''

J. K. Zink , K. Hardegree-Ullman, J. Christiansen, C. Dressing, I. Crossfield, E. Petigura, J. Schlieder, & D. Ciardi, , Astronomical Journal, 159, 154 (2020)

``The Chaotic Nature of TRAPPIST-1 Planetary Spin States''

A. M. Vinson , D. Tamayo & B. M. S. Hansen , Monthly Notices of the Royal Astronomical Society, Vol 488, 5739 (2019)

``Accounting for Multiplicity in Calculating Eta Earth''

J. K. Zink & B. M. S. Hansen , Monthly Notices of the Royal Astronomical Society, Vol 487, Page 246 (2019)

``Accounting for Incompleteness due to Transit Multiplicity in Kepler Planet Occurrence Rates''

J. K. Zink , J. L. Christiansen & B. M. S. Hansen , Monthly Notices of the Royal Astronomical Society, Vol 483, Page 4479 (2019)

Recent Papers led by Undergraduate Students :

``Predicting the Dominant Formation Mechanism of Multiplanetary Systems''

C. Shariat , Y. Hasegawa, Hansen, B., T.-Y., Yu & R. Hu
Astrophysical Journal, 964, L13 (2024)