The JWST Rocky Worlds DDT Program reveals GJ 3929b to likely be a bare rock

TL;DR

The JWST Rocky Worlds DDT program observed GJ 3929b and found it to likely be a bare rock with no significant atmosphere, based on secondary eclipse measurements and temperature analysis, indicating atmospheric loss.

Contribution

This study provides the first JWST secondary eclipse measurements of GJ 3929b and demonstrates it is likely a bare rock, refining its properties and ruling out thick CO2 atmospheres.

Findings

GJ 3929b has a dayside temperature consistent with a bare rock.

The planet lacks a significant secondary atmosphere, with CO2-rich atmospheres thicker than 100mbar ruled out.

Two additional non-transiting planets were identified in the system.

Abstract

We report first results from the JWST Rocky Worlds Director's Discretionary Time program. Two secondary eclipses of the terrestrial exoplanet GJ 3929b were recently observed using MIRI photometric imaging at 15 um. We present a reduction of these data using the updated SPARTA pipeline. We also refine the planet mass, radius, and predicted time of secondary eclipse using a new sector of TESS data and new, high-precision radial velocities from the MAROON-X spectrograph. For the two JWST observations, we recover secondary eclipse depths of 177+47-45ppm and 143+34-35ppm at times consistent with a nearly circular orbit, as expected from the radial velocity data. A joint fit of the two visits yields a dayside brightness temperature Tp,dayside = 782+/-79K for GJ 3929b, which is consistent with the maximum brightness temperature Tmax = 737+/-14K for a bare, black rock (i.e., assuming zero Bond albedo and no heat redistribution). These results rule out CO2-rich atmospheres thicker than 100mbar at >3sigma, suggesting that GJ 3929b has lost any significant secondary atmosphere. The radial velocity data also indicate two additional non-transiting planets in the system: a previously-identified planet in a 15.0d orbit, and a newly-identified planet candidate in a 6.1d orbit.