extreme precision radial velocities

I continued working on my document about release of data and code. Twitter (tm) continues awesome.

Research highlight of the day was a long discussion with Megan Bedell (Chicago) about the consistency of exoplanet-host radial-velocity measurements order-by-order in a many-order high-resolution echelle spectrograph. The measurements are made by cross-correlation with a binary (ugh) template, and some orders are consistently high and some are consistently low, and we very much hope there are other more subtle regularities to exploit. Why are there these discrepancies? Probably because the model is inflexible and wrong. Unfortunately we don't have access to it directly (yet) so we have to live with the cross-correlation functions. We discussed simple methods to discover regularities in the order-by-order offsets and results, and sent Bedell off with a long to-do list.

I ended the day with a long conversation with Kat Deck (Caltech). Among other things, we discussed what we would do with our lives if exoplanet research evolves into nothing other than atmosphere transmission spectroscopy and modeling. Of course neither of us considers this outcome likely!


  1. Thought it's a losing battle, I fight it nonetheless:

    We shouldn't call it "transmission spectroscopy". Transmission spectroscopy is when you have an absorption cell, and every photo the comes out the other side has been transmitted through the cell.

    In *transit spectroscopy*, 99% of the photons (in the case of a Jupiter — more otherwise) didn't touch the planet at all. Of those that did touch the planet, nearly all of them were entirely blocked by the planet. The fraction of photons that were transmitted through the thin patina of atmosphere is nearly zero.

    1. How do you distinguish between a spectrum of a star that is in transit from a spectrum that *does* contain information about the atmosphere? You could have a transit spectrum with no information about the atmosphere; that doesn't convey what I intend to convey, which is the change to the spectrum caused by having some of the light passing through the exoplanet atmosphere.

    2. A slightly clunkier term, which I think is still better than "transmission spectroscopy", is "transit radius spectroscopy". The advantage of this term is that it's very close to being what is *actually* measured.  We have no idea which photos were transmitted through the planet's atmosphere and therefore no way to take a spectrum of those photons. What we do is measure the transit radius of the planet as a function of wavelength.