Research

I study how planetary systems acquire their architectures, at two levels: population surveys that measure how often different kinds of planets occur, and detailed studies of individual systems. Across both, I combine radial velocities (RVs), astrometry, and imaging to measure what no single technique can.

Planetary architectures

The Solar System has small rocky planets close to the Sun and giant planets farther out. Is that architecture common? For my dissertation, I designed and led the Distant Giants Survey, a three-year Keck RV search for long-period giant planets in systems where TESS had already found a small transiting planet.

Three years is short compared with the orbits of distant giants: a planet on a 10-year orbit appears only as a slow trend in the RVs. To recover these systems, I wrote ethraid, which combines RV trends with the 25-year baseline of Hipparcos-Gaia astrometry and with direct imaging.

  • Distant giants are twice as common in systems with an inner small planet as around the average Sun-like star.
  • These outer giants have lower eccentricities than a comparison population, pointing toward dynamically cool histories that let inner planets survive.
  • In the California Legacy Survey, this enhancement shows no evidence of depending on stellar metallicity.

Papers

  • Survey results: occurrence and eccentricities (2025) · ADS · arXiv
  • No metallicity-dependent enhancement (2024) · ADS · arXiv
  • First giant-planet discoveries (2023) · ADS · arXiv
Masses and orbital periods of small planets and the giant planets found farther out in the same systems
Giant planets often accompany small ones. Each red square is a small planet that TESS found close to its star; each blue square is a giant planet we found much farther out in the same system, joined to its neighbor by a line. Faded points are other known planets, for context. From Van Zandt et al. (2025).
Thumbnail of Judah Van Zandt's UCLA Grad Slam talk Watch: my thesis in three minutes UCLA Grad Slam 2022 talk on how close-in small planets and distant giants are connected (YouTube).

True-mass demographics

RVs measure only a planet's minimum mass, which blurs the boundaries between giant planets, brown dwarfs, and low-mass stars. Adding absolute astrometry from Hipparcos and Gaia breaks this degeneracy. I combined the California Legacy Survey with Hipparcos-Gaia astrometry to refit the orbits of 194 companions and measure their occurrence as a function of true mass and separation.

  • About 40% of RV-identified "brown dwarfs" turned out to be stars.
  • Occurrence varies smoothly from giant planets to brown dwarfs, suggesting that core accretion and gravitational instability produce companions in overlapping mass ranges.
  • The brown dwarf desert extends out to 10 AU.

I apply the same approach to individual systems: characterizing companions to stars flagged by their astrometric accelerations for the GEODES survey, which prepares for Gaia DR4, and revisiting γ Cephei, host of the first exoplanet candidate.

Papers

  • True-mass occurrence of the California Legacy Survey (2026) · ADS · arXiv
  • ethraid: RVs + astrometry + imaging (2024) · ADS · arXiv · code
  • GEODES I: three accelerating stars (2026) · ADS · arXiv
  • The γ Cephei system (2026, accepted) · arXiv
  • Related: eccentricities across the planet–brown dwarf transition (Gilbert et al. 2026) · ADS · arXiv
How common companions are at different masses, from giant planets to brown dwarfs
Heavier companions are rarer, with a gradual boundary between planets and brown dwarfs. The numbers give roughly the percentage of stars with a companion in each mass range; they fall steadily from Jupiter-mass planets toward the "brown dwarf desert." From Van Zandt et al. (2026).
How common giant planets and brown dwarfs are at different distances from their stars
Giant companions are most common beyond about Earth’s distance from the Sun. Few giant planets orbit closer to their host stars than 1 AU; their numbers rise sharply around 1 AU and may fall off again beyond 10 AU. The colored curves are two simple models fit to the data. From Van Zandt et al. (2026).

Stellar mass and planet demographics

Giant planets are more common around more massive stars, but do the distributions of planet mass and orbital separation also change with stellar mass? Core accretion predicts that they should: more massive stars have more massive disks, and their water snow lines lie farther out.

  • Splitting my California Legacy Survey true-mass sample by host-star mass, I found tentative evidence that the planet mass function peaks near Jupiter's mass or below for lower-mass stars, but at several Jupiter masses for higher-mass stars (Van Zandt & Bowler, submitted).
  • I am now studying how companion separations vary with stellar mass and metallicity (in prep.).

Gaia DR4 will enable a controlled test. I have assembled a sample of about 7,000 stars from 0.6 to 1.2 solar masses, in two host-mass groups with matched metallicities, so that any difference in their planet populations can be attributed to stellar mass. With the GEODES team, I am vetting candidates now so that population analyses can begin as soon as DR4 is released.

How common giant planets of different masses are around lighter and heavier stars
Heavier stars may make heavier planets. How common giant planets of different masses are around stars lighter (left) and heavier (right) than the Sun. Around lighter stars, the most common giants are about Jupiter’s mass or smaller; around heavier stars, they are two to three times Jupiter’s mass. The vertical scales differ because giant planets are also more common around heavier stars. The result is tentative; Gaia will test it with a far larger sample. Van Zandt & Bowler (submitted).

All publications →