Why is it so hard to tell forming gas giants from brown dwarfs in JWST images?

[ElizabethL]

I was looking at the latest images from the James Webb Space Telescope, specifically the ones of the protoplanetary disks around young stars, and I got to wondering how we can actually tell the difference between a forming gas giant and a brown dwarf from so far away. The mass ranges seem to overlap so much in those early stages, and I’m just not clear on what the definitive observational signature is.

[BrandonRM]

Yeah, there isn't a slam dunk signature. In practice people try to pin a mass by combining how the disk looks in infrared with how the gas moves. JWST can spot bright spots where a circumplanetary disk may be feeding the planet, and spectroscopy can hint at temperature and gravity of the companion, but you can't turn that into a clean mass cut. The real kicker is dynamical mass from the velocity field: if you can see a localized perturbation in the disk's rotation, you can model it to get a mass estimate. But even that depends on the disk's inclination, temperature, and how the gas couples to the planet. So you end up with ranges rather than a clear line between a planet and a brown dwarf.

[GregoryCJ]

One disk I looked at had a bright point-like feature that looked like a CPD, but the spectrum was noisy and I couldn't separate star light from the feature. The accretion signature (like Brackett lines) suggested there was gas falling onto something, but it could be a low-mass planet rather than a more massive substellar object. It left me with a guess that the companion might be tens of Jupiter masses at most, but that's just a guess.

[Kevin.L]

Sometimes the issue is age and luminosity. A young object can glow as bright as a forming giant planet, and in a disk environment the brightness depends on how we are seeing the accretion heat. You can get a lot of overlap in late formation stages. The only thing that feels more decisive is a robust dynamical mass, which you really need very good velocity maps plus a model of the disk physics. Without that, you’re stuck with a range.

[Dennis.L]

Do you think the real problem is that we are chasing a single diagnostic and hoping for a clean cutoff, or is the bigger issue that many of the candidates we see aren't even bound to the star in the first place?