Greg Gilbert

Hey folks - if you are interested in astronomy news, you should be following @caltechipac.bsky.social!!! IPAC hosts the NASA Exoplanet Archive, the NASA Extragalactic Database, the InfraRed Science Archive, as well as data from ongoing missions like Euclid, SPHEREx, and upcoming missions like Roman!

Orbital Eccentricities Suggest a Gradual Transition from Giant Planets to Brown Dwarfs To date, hundreds of sub-stellar objects with masses between $1-80\ M_{\rm Jup}$ have been detected orbiting main-sequence stars. The current convention uses the deuterium-burning limit, $M_c \approx ...

Paper day! What the difference between a giant planet and a brown dwarf? Maybe not so much. Read on to find out more. 1/

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

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/

Eccentricity distributions P(e) vs e for five mass bins between Mc = 0.8 - 80 Mjup. The lowest mass bin has mean eccentricity <e> ~ 0.2, gradually increasing to <e> = 0.5 for the highest mass bin. The shape of the distribution also changes smoothly.

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/

A figure showing [Fe/H] (dex) versus Mc (Mjup). Scatter points from the CLS sample. A change-point model identifies a transition at 25 +/- 10 Mjup. The mean [Fe/H] of the low-mass "planet-like" distribution is ~0.2 whereas the mean [Fe/H] of the high-mass "star-like" distribution is ~0.0.

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/

Orbital Eccentricities Suggest a Gradual Transition from Giant Planets to Brown Dwarfs To date, hundreds of sub-stellar objects with masses between $1-80\ M_{\rm Jup}$ have been detected orbiting main-sequence stars. The current convention uses the deuterium-burning limit, $M_c \approx ...

Paper day! What the difference between a giant planet and a brown dwarf? Maybe not so much. Read on to find out more. 1/

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

JESUS, MARY, AND JOSEPH! This gave me old Google back! It killed the AI results dead!

Scaling K2 VIII: Short-Period Sub-Neptune Occurrence Rates Peak Around Early-Type M Dwarfs We uniformly combined data from the NASA Kepler and K2 missions to compute planet occurrence rates across the entire FGK and M dwarf stellar range. The K2 mission, driven by targets selected by guest ...

Kepler mission: smaller stars have more short-period, small #exoplanets.

Theory: the smallest stars won’t have enough disk material to make small planets so there must be a turnover.

Kepler+K2: We have found a turnover!

Check out our newest Scaling K2 paper: arxiv.org/abs/2508.05734

🧵 1/9
🔭🧪☄️

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).

There is no evidence of elevated eccentricities for planets in the radius valley of M-dwarf stars.

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.

The relationship between <e> and adjusted radius shows tentative evidence of an eccentricity peak in the radius valley.

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.

Trends in occurrence rate, [Fe/H], and <e> as a function of Rp hold for M-dwarf 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).

The <e> - Rp relationship for M-dwarf vs FGK-dwarf planets.

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

Small planets are common, large planets are rare. Large planets need high metallicity, small planets do not. Small planets have low <e>, large planets have elevated <e>.

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 relationship between <e> and Rp for single- vs multi-transiting Kepler systems

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

The relationship between <e> and Rp

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.

The eccentricity (ellipticity) of a planet’s orbit is a relic of its formation history. We measured eccentricities of 1646 planets with sizes ranging from 0.5 to 16 Earth-radii (R⊕). On average, large planets (4–16 R⊕) are four times more eccentric than small planets (0.5–4 R⊕), pointing to distinct formation chan- nels for these two size groups. Small planets typically form on nearly circular orbits and experience minimal perturbations, while large planets are more likely to experience eccentricity excitation. Small planets are bifurcated into at least two groups, super-Earths (1.0–1.5 R⊕) and sub-Neptunes (2.0–3.0 R⊕), with few planets in between. The planets that fall between these two populations may also have elevated eccentricities, pointing to dynamically exotic formation histories.

Want to learn about the relationship between planet size and orbital eccentricity? Read this thread! 🧪 🔭 🪐

Astronomers may have just discovered the third interstellar object passing through the Solar System!

ESA’s Planetary Defenders are observing the object, provisionally known as #A11pl3Z, right now using telescopes around the world.

Massive cuts to NASA science proposed in early White House budget plan The preliminary version of President Donald Trump’s budget proposal to Congress, known as a “passback,” would cut the agency’s science budget funding nearly in half.

First the rumour was a 20% budget cut. Then, 50%. Now the president's NASA budget is out and it's a 68% cut to astrophysics ($1.5B to $487M).

Even if this gets reversed in four years, we will *never* recover the missions, partners, people who will be gone.

www.washingtonpost.com/science/2025...

Thanks for the write-up, @dtstarkid.bsky.social

I’m working on a piece about funding basic science and if you’ve never explored spinoff.nasa.gov, I would really encourage you to do so. Even though I’ve been doing this for 20 years, it really is humbling and informative to see the ways in which space science works its way into our daily lives.

World's darkest and clearest skies at risk from industrial megaproject On December 24th, AES Andes, a subsidiary of the US power company AES Corporation, submitted a project for a massive industrial complex for environmental impact assessment. This complex threatens the pristine skies above ESO’s Paranal Observatory in Chile’s Atacama Desert, the darkest and clearest of any astronomical observatory in the world [1]. The industrial megaproject is planned to be located just 5 to 11 kilometres from telescopes at Paranal, which would cause irreparable damage to astronomical observations, in particular due to light pollution emitted throughout the project’s operational life. Relocating the complex would save one of Earth's last truly pristine dark skies.

The best dark sky in the world - the Atacama Desert in Chile, which hosts many @eso.org telescopes - is under threat from an industrial project. 🔭

Astronomers can sign a petition in favour of moving the planned industrial project: docs.google.com/forms/d/e/1F...

But more accurately...

Physics major → existential crisis → the improv years (TM) → teaching high school → start grad school → get sick → in-and-out of the hospital for 3 years → finish grad school → astrophysics postdoc

Just a reminder that winding paths can look straight when zoomed out

If you went to college...
1. what was your career goal when you started?
2. your initial major?
3. if you changed majors, what did you change to?
4. what do you do now, professionally?

1. Physics research
2. Physics
3. Physics
4. (Astro)physics research

A model image of what our home galaxy, the Milky Way, might look like face-on: as viewed from above the disc of the galaxy, with its spiral arms and bulge in full view. In the centre of the galaxy, the bulge shines as a hazy oval, emitting a faint golden gleam. Starting at the central bulge, several glistening spiral arms coil outwards, creating a perfectly circle-shaped spiral. They give the impression of someone having sprinkled pastel purple glitter on the pitch-black background, in the shape of sparkling, curled-up snakes.

A model image of what our home galaxy, the Milky Way, might look like edge-on, against a pitch-black backdrop. The Milky Way’s disc appears in the centre of the image, as a thin, dark-brown line spanning from left to right, with the hint of a wave in it. The line appears to be etched into a thin glowing layer of silver sand, that makes it look as if it was drawn with a coloured pencil on coarse paper. The bulge of the galaxy sits like a glowing, see-through pearl in the shape of a sphere in the centre of this brown line

The ESA #Gaia mission has delivered the best Milky Way maps to date and taken its last starlight before spacecraft retirement 🔭

www.esa.int/Science_Expl...