actions or observables? forbidden planet radii

The highlight today of our Gaia DR2 prep meeting was a plenary argument (recall that this meeting is supposed to be parallel working, not plenary discussion, at least not mainly) about how to find halo substructure in the data. Belokurov (Cambridge) and Evans (Cambridge) showed some nice results of searching for substructure in something close to the raw data. We argued about the value of transforming to a space of invariants. The invariants are awesome, because clustering is long-lived and stark there. But clustering is terrible because (a) it introduces unnecessarily wrong assumptions into the problem and (b) normal uncertainties in the data space become arbitraily ugly noodles in the action space. We discussed whether there are intermediate approaches, that get the good things about working in observables, without too many of the bad things of working in the actions. We didn't make specific plans, but many good ideas hit the board.

Stars group meeting contained too many results to describe them all! It was great, and busy. But the stand-out result for me (and this is just me!) was a beautiful result by Vincent Van Eylen (Leiden) on exoplanet radii. As my loyal reader knows, the most common kinds of planets are not Earths or Neptunes, but something in-between, variously called super-Earths and mini-Neptunes. Now it turns out that even this class bifurcates, with a bimodal distribution—there really is a difference between super-Earths and mini-Neptunes, and little in between. Now Van Eylen shows that this gap really looks like it goes exactly to zero: There is a range of planet radii that really don't exist in the world. Note to reader: This effect probably depends on host star and many other things, but it is incredibly clear in this particular sample. Cool thing: The forbidden radii are a function of radius, and the forbidden zone was (loosely) predicted before it was observed. Just incredible. Van Eylen's super-power: Revision of asteroseismic stellar radii to get much more precision on stars and therefore on the transiting planets they host. What a result.