So what's the upshot? For now, we haven't identified a single single parameter that cleanly distinguishes giant planets from brown dwarfs. But it looks like their mass regimes probably overlap. More work is needed! 9/

Paper I: arxiv.org/abs/2511.12816
Paper II: arxiv.org/abs/2511.11818

Stay tuned for a third paper in this series, where Judah Van Zandt analyzes occurrence rates of these object near the ice line, where planet formation is thought to be enhanced. 8/

Together, these trends suggest a gradual transition from giant planets to brown dwarfs. A straightforward interpretation is that both core accretion and gravitational instability create (rare) objects between 1-20 Mjup. 7/

Applying a hierarchical Bayesian model, I found a gradual change in the eccentricity distribution with mean eccentricity <e> ~ 0.2 for Jupiter-mass objects to <e> ~ 0.5 for brown dwarfs above the deuterium-burning limit. 6/

Applying a change-point model, Steven identified a transition in host star metallicities at 25 +/- 10 Mjup. This measurement is in contrast to previous analyses which identified transitions at ~6 Mjup and ~42 Mjup. Notably, Steven's constraint is much broader as well, suggesting a gradual change. 5/

In a pair of papers, Steven Giacalone and I analyzed the California Legacy Survey of Doppler-detected planets, searching for trends in mass, semi-major axis, host star metallicity, and orbital eccentricity. 4/

Let's consider planet-like "bottom-up" formation (i.e. core accretion) vs star-like "top-down" formation (i.e. gravitational instability) as a possible avenue for better classifying brown dwarfs vs giant planets.

Do these mechanisms leave observable signatures? 3/

We typically draw the dividing line between brown dwarfs and super-giant planets at 13 Jupiter-masses, the minimum mass for Deuterium fusion. But can we do better? 2/

If you want all the details, you can read my paper in PNAS (www.pnas.org/doi/10.1073/...) and Sheila’s paper is on arXiv (arxiv.org/abs/2507.07169).

In contrast, Sheila’s analysis of M-dwarfs does not detect this feature. The M-dwarf sample size is small (236 planets), so non-detection is not necessarily evidence of non-existence. Nevertheless, giant planets are rare around small stars, so there is reason to think the trend could be real.

We do see one difference though. My analysis of FGK stars detected tentative (2-sigma) evidence for elevated eccentricies in the so-called exoplanet radius valley, which we hypothesize arises from giant impacts mediated by giant planets.

The straightforward conclusion is that the astrophysics of planet formation are largely similar for cool stars (M-dwarfs) compared to more Sun-like stars (FGK dwarfs).

Now, UF graduate student Sheila Sagear has demonstrated that the same trends hold for planets orbiting smaller M-dwarf stars.

A conspicuous eccentricity rise at approximately 3.5 Earth-radii also coincides with known transitions in occurrence rates and host star metallicities, providing clues to formation physics.

The eccentricity-radius relation holds for both single-transiting and multi-transiting systems, suggesting these singles and multis belong to the same parent population.

A few months ago, I published a paper demonstrating that planets larger than Neptune have elevated orbital eccentricities compared to smaller planets. Our analysis measured eccentricities for 1646 transiting planets orbiting FGK stars, by far the largest sample of exoplanet eccentricities to-date.