target selection; rock and metal

At Flatiron we have purchased a share in the Terra Hunting Experiment, which will be a big, long-term radial-velocity monitoring program with HARPS3. Today Megan Bedell (Flatiron) and I had a conversation about target selection for that survey. There are many choices that could be made in target selection that could make populations or astrophysics inferences very difficult or even impossible later. These conversations remind me of the great and hard work that went in to target selection in the SDSS family of surveys.

The day ended with a great talk by Leslie Rogers (Chicago) about the things that set planet sizes (as a function of mass). She always phrases her results in terms of what isn't rocky, because of the one-sided-ness of some or most of the composition-related observational uncertainties, but it sure looks to my eyes like the smallest planets are rock and metal, like the Earth. She has one extremely good case, which is orbiting so close to its host star that tidal-disruption arguments come in to play! She also was optimistic that transit-timing information might be informative in the near future. There were jokes about water planets and soda-water planets, because many planets that are rich in water are also expected to be very rich in CO₂.

#dsesummit, day 2

My day started with a long breakfast conversation with Yann LeCun (NYU) about adversarial methods in deep learning. In these methods, a generator and discriminator are trained simultaneously, and against one another. It is a great method for finding or describing complex density functions in high dimensions, and people in the business have high hopes. In particular, it is crushing in image applications. We discussed the problem that is currently on my mind, which is modeling the color–magnitude diagram of stars in Gaia, using one of these adversarial systems, plus a good noise model for the parallaxes. I would love to do that, and it should be much easier than the image problems, because the data are much lower in dimensionality.

I ran a very amusing session at the Summit, in which we had participants bring figures and we crowd-sourced a reaction, critique, and to-do list for each of them. We looked at a figure from politics from Michael Gill (NYU), making a causal claim about regulations and how meeting minutes are kept, a figure from geophysics from Nicholas Swanson-Hysell (Berkeley) showing the data and a model for polar wander, and a figure from neuroscience from Bijan Pesaran (NYU) showing brain region classifications. The feedback from the group was great and useful and constructive (though not always polite; my apologies!). One theme of our discussion ended up being consistency across figure elements. I feel like this crowd-sourcing session was a model for future sessions; it would even be fun to make this a regular event in some forum in NYC.

There was a lot of non-research today, but in the remainder of my research time, I worked on outline material for our growing paper on Hack Weeks.

the best temperature-gravity relationship ever

We have realized that Anna Ho has the most detailed, feature-rich diagram of the temperature–gravity relationship for red giant stars ever created. She has 450,000 red giants from LAMOST labeled with APOGEE and The Cannon. None of these giants is supremely precisely labeled with either temperature or gravity, however, the sheer numbers make for visibility of very subtle features in the diagram that have (perhaps) never been seen before. Anna made visualizations and a gif of the data, and we debated what to do about it all, in the next paper. We are trying to get papers one and two done first.

I performed a code review with Melissa Ness, and we were able to speed up her code by a significant factor. We did it by reducing functionality, of course! But now she can run experiments fast, which is critical in the investigation phase. We made predictions for the figures she should make and what they would look like. She is working on putting far more informative prior beliefs into The Cannon.

Late in the day came the Physics Research Conference (Colloquium). It was by Maria Zuber (MIT), talking about the GRAIL mission to map the geoid of the Moon. The mission was amazingly simple: Two satellites in low orbit around the Moon, sending time-codes to each other and back to Earth. Do the math and figure out the gravitational shape of the Moon. She had many geological (or selenological?) results to discuss. In particular, she has an explanation for the strong morphological differences between the front side and the back side of the Moon. I was impressed that the data analysis was not in the slightest probabilistic: They just did the GPS-like thing (GRAIL is very like a miniature GPS) of finding the best solution to explain the data. It made it clear that a better analysis is possible. Not that I'm volunteering!

NG Next SfL, day 1

I spent the day at Northrop Grumman, where Alberto Conti (NG Aerospace) convened an intimate workshop on the search for life around other stars. It was a remarkably interdisciplinary day, and I learned a huge amount. An incomplete and personal list of highlights follow.

Tom Vice (NG Aerospace Systems President) and Tom Pieroneck (NG) opened up the meeting by talking about hard problems, which is a good way of thinking about building an engineering team that will have work to do for many years; I realized that it is hard problems in data analysis that unites a lot of what we do at Camp Hogg; it attracts good students, postdocs, and collaborators.

Sara Seager (MIT) kicked off the science talks with a completely eye-opening discussion of how we might identify signs of life through spectroscopy. She emphasized that there are many possible false-positive signals. But it would be exciting if we found oxygen, water, and methane in the same atmosphere. The discussion of water-based life vs other liquids. She made a strong case for water! Another great idea (maybe from Lee Feinberg in the audience?) was to look for signs of climate change to identify life!

Sanjoy Som (Blue Marble) gave a great and surprising talk about how we might use the geological record of rocks on Earth to look at life signatures and changes to our atmosphere over time. The coolest was his use of fossilized raindrops, plus some first-year mechanics. His point: The different states of Earth through time are proxies for exoplanets with life. Great point!

Leslie Rogers (Berkeley) talked about inferring planet masses and mass–radius relationships. In a direct-detection experiment we will measure neither directly; is that a problem? In the question period, someone brought up the possibility that it would be the moon of a giant planet in the habitable zone that might be the inhabited object.

Chris Stark (STScI) produced mind-blowing simulations of all possible ExoHab or LUVOIR missions to find habitable planets. His simulations include optimizations of target ordering, exposure time, for starshade and coronographic experiments, all as a function of things like mirror size, mission lifetime, and so on. So much input! He made the comparison to the LHC: We need a mission that produces an interesting answer even if it doesn't detect life signatures. That is a good point. He mentioned that we will be affected by exo-Zodiacal light at unknown levels; we need to figure this out before we settle on final design decisions for anything. Fortunately, this may be addressable from the ground or with WFIRST.

There were so many other interesting things, in talks and discussions: Karin Öberg (CfA) talked about the formation of planets and the chemical and materials properties of the disks in which planets formed; Daniel Apai (Arizona) talked about mapping planet surfaces with time-domain lightcurves (something he has done to great effect); Alicia Berger (Colorado) talked about amino acids and their relationship to biosignatures. In that latter talk, Öberg burst one of my bubbles by noting that despite claims in the literature, amino acids have not been discovered in interstellar spectra. A great day!