exoplanet dynamics, sucking

In group meeting, Dun Wang talked about astrometric calibration of the GALEX Satellite, and Kat Deck (Caltech) talked about the dynamical evolution of exoplanetary systems. She pointed out that we naively expect lots of planets close in period to be locked in resonances, but in fact such resonances are rare, empirically in the Kepler sample. She has explanations for this involving the evolving proto-planetary disk.

After lunch, Deck gave the astro seminar, on planetary system stability and the Kepler planets. She discussed chaos, stability, and heuristic stability criteria. One interesting thing is that there really is no non-heuristic stability criterion: We think of a planetary system as "stable" if there are no catastrophic, order-unity changes to any of the orbital osculating elements. That's not really an equation! And at the talk there was some discussion of the point (counter-intuitive and important) that a system can be stable (by our astronomer definition) for far, far longer than the Lyapunov time. Awesome and important.

At the end of the day, Foreman-Mackey and I made the (astonishing) decision to abort and fail on our NASA proposal: We just ran out of time. I am disappointed; we have an eminently fundable pitch. That said, we just didn't start early enough to make that pitch at the level we wanted. Not sure how to feel about it, but I sure need to catch up on sleep!

finding planets with TTVs

In the tiny bit of research time available today, I spoke with Kat Deck (Caltech) and Foreman-Mackey about finding planets with large transit-timing residuals. These signals aren't precisely periodic, so some searches could miss them entirely. Deck has simple models (based on perturbation theory) for the variations, so we can in principle add only a few parameters and capture a large range in possible variations. This might make us much more sensitive to extremely important and interesting systems hiding in existing data.

The rest of the day was spent writing our NASA proposal, with the exception of lunch with Yann LeCun (facebook) and Léon Bottou (facebook) at the NYC office of facebook Research.

ultra-short-period planets, 2

At group meeting, Roberto Sanchis-Ojeda (Berkeley) and Kat Deck (Caltech) were around to argue exoplanets. We had Sanchis-Ojeda tell us about his ultra-short-period (USP) planet work. He showed us his evidence for a disrupting (or something) USP planet; to me it looks like evidence for a technological civilization making power from a star! But, for some reason, no-one else thought so! We discussed tidal mechanisms and stripping mechanisms, but we concluded that there is no real reason to think of the USPs as being related to hot jupiters or any other kind of planet in any mechanistic way.

ultra-short-period planets

Foreman-Mackey and I spent quality time with Roberto Sanchis-Ojeda (Berkeley) figuring out what we are going to do together, related to ultra-short-period planets. He was pulling for things related to false-positive evaluation, I was arguing for search at lower signal-to-noise ratios. I feel like there are lots of undiscovered systems. The rest of our science time was spent proposal-writing for NASA!

talks all day; Dr Gorbenko

Roberto Sanchis-Ojeda (Berkeley) arrived for a few days of hacking. He is responsible for finding some very short-period transiting exoplanets. We didn't get a chance to set our goals for the day, because the day was packed with talks:

Victor Gorbenko (NYU) gave an absolutely excellent PhD defense. Congratulations Dr Gorbenko. Among other things, he was considering the difference between the theoretical (analytic) action of massless modes on the 1+1 worldsheet of a string (not a 0+1 wordline but s 1+1 worldsheet) and a lattice calculation. In this comparison, he found massive modes. He very nicely connected this work to larger questions in string theory, but also to the origins of string theory in QCD.

At lunch, Guido D'Amico (NYU) spoke about axions, axion–photon interactions, and cosmic transparency. This is related to work I did with Bovy and with More in the optical. He advertised some clever ideas he has about about using the Planck data to constrain this sector using millimeter wavelengths. He has been implementing some of these ideas at #astrohackny.

At the end of the day, Neil Lawrence (Sheffield) spoke about deep machine-learning models built from nested Gaussian Processes. The methods show a lot of promise and connect well to things we are thinking about in CampHogg.

JPL, day 3

I spent the morning with Leonidas Moustakas (JPL), a bit of Foreman-Mackey (in Pasadena for the Sagan Fellowship Symposium), and a bit of Andrew Romero-Wolf (JPL) and Curtis McCully (LCOGT) by phone, discussing projects related to strong-lensing time-delay measurements. We discussed two challenging projects. The first is to determine (from as many as we can construct) the best model for quasar time-domain variability. There are claims in the literature that the damped random walk is the best model, but that (very sensible) model hasn't really been competed against all that much. We know how to do this, lets do this! The reason they want good probabilistic generative models is that they want to determine time delays as precisely as possible, using a probabilistic approach.

The second project is to perform high-quality photometry on the (overlapping from the ground) images of a strongly lensed quasar. In this case—when the point-spread functions overlap—you have to do your photometry by simultaneous fitting, but with the variable (and badly known) PSF of ground-based astronomy, I have never seen such photometry that really looks right: There are always fitting-induced covariances of the overlapping-source light curves. I think this is caused by model mismatch (under-fitting), but I don't really know. Romero-Wolf and McCully pointed out that image differencing methods work well in crowded fields, so I formulated an image-modeling approach to the photometry that is as close to image differencing as possible. I promised to write it up into a document. I am kind-of excited about it; it is still just image modeling, but it makes use of the power of image differencing technologies to get flexibility to fit the real PSF as it is.

Late in the day, Adam Miller (JPL) showed me the JPL Mission Control center and a few high bays, filled with awesome stuff (including some fake Mars!).

LCOGT

I spent the day at LCOGT today, hosted by Diana Dragomir. While she was giving me a tour of the facilities, I met the founder and president, Wayne Rosing, who was in the shops working on the 1-m telescopes. The director, Todd Boroson, explained to me the idea behind LCOGT: It operates not as a set of independent observatories, but as a single telescope that happens to be a network of apertures (and capabilities). That's interesting; and it permits the telescope to make continuous measurements in a way that few other telescopes can. Most of the science is in the time domain. After this I had many great conversations. Some highlights follow.

One of the big issues for LCOGT is scheduling. I had a great conversation about this with Eric Saunders, who said various things that were music to my ears: One is that they have tried to specify the scheduling problem as an integer programming problem. This separates the objectives from the method or algorithm used to optimize it. The other musical thing he said is that they optimize using industrial-grade operations research packages. They are so much better than anything you could write by hand.

With Dragomir I discussed exoplanet atmospheres, and her evidence for Rayleigh scattering in a planet they are writing up now. She is struggling with inconsistencies in the data, which come from different telescopes working at different wavelengths: Are they real or, if not, how to model or remove them?

Late in the day, Andy Howell, Iair Arcavi, and Curtis McCully showed me the Supernova Exchange, which is a project they are working on to coordinate supernova follow-up. They are following up so many supernova of so many types, with so many collaborating projects, that they can't keep track without some sophisticated software. This has to be flexible enough to encode the planned and completed observations and their status, and also comments and discussion, and also tags that represent the reasons behind the follow-up. This latter point is something I have always been interested in: For many statistical questions, you need to know why something was observed.

Northrup Grumman

I spent the day visiting Alberto Conti at Northrup Grumman today. NG is building JWST. I got to see it being assembled in the (huge) clean room. I also saw a full-scale model of the sun shade that they use for testing deployment. It is just straight-up absolutely enormous!

The point of the visit was to learn about NG's new proposals and concepts for NASA-funded astrophysics missions. I learned that distant, formation-flying star shades are actually practicable, especially at L2, and that ground-based testing is extremely promising. The reason the formation-flying is possible is that L2 is very flat, gravitationally. That said, it does take days or weeks to switch the pointing of the "telescope", which consists of two spacecraft 10,000 km apart! And the craziness of how the star-shade works makes me want to reconsider my physical optics!

NG also has great concepts for new missions that would be ambitious but affordable. One is a telescope that starts small and grows with robotic servicing missions. Another is a pathfinder telescope on the ISS. Another is a telescope that is highly asymmetric in mirror shape, to get good angular resolution but still fit in a launch-vehicle fairing. It was a dramatic set of presentations, in part because the hardware engineering is so impressive, but in part because they are thinking of full system engineering (including software–hardware trade-offs) to control budget and make things fundable.

JPL, day 2

My second day at JPL included conversations with Gautam Vasisht (JPL) about adaptive optics systems, Geoff Bryden (JPL) about the abundance of Earth analogs, and Alina Kiessling (JPL) about intrinsic alignments and weak lensing. On the latter we discussed the problem that most of the theory is based on dark-matter-only simulations, but this is precisely a problem where the baryons matter a lot! With Vasisht I learned that JPL has a "clock" up in one of the buildings that shows you the current counts of exoplanet candidates, confirmed exoplanets, and planets in the habitable zone! Plus a huge model of the Mars Lander. Awesome!

At the end of the day, Leonidas Moustakas (JPL), Curt Cutler (JPL), and I argued about the flow from experimental design (think: satellite astrophysics project) to quantitative results on the parameters or scientific questions of greatest interest. They are thinking about making standards and principles for doing this flow, thereby strengthening the quantitative arguments in their proposals for new (and complex) projects. We discussed the challenges of doing something of general value, but they have decided (very sensibly) to start with a few very specific projects to use as "poster children" for the idea. One challenge is doing this with the right "language" such that people from different scientific backgrounds can agree on what's being said at each stage (think of words like "bias" and "noise" and "model" and "systematic" and so on).

JPL, day 1

I am visiting JPL this week, hosted by Leonidas Moustakas. I gave a seminar today, about our work with Kepler, with a focus on the noise modeling aspects of the project. In the rest of the day I had too many great and interesting conversations to list, but highlights were the following:

With Roland de Putter I discussed parameter estimation and inference in large-scale structure projects. We talked a bit about correlation functions and Gaussian kernels. He had some nice intuitions and scaling arguments about the Gaussianity of measurements of the power spectrum that helped me understand (and corrected some of my errors in) my thinking about my cosmological inference projects. He also helped me formulate the simplest possible demonstration that what is traditionally done (in, say BAO projects) to mock up a "likelihood function" is formally (and maybe substantially) wrong and can be replaced. We also talked about inferring the initial conditions of the Universe from what we see. I pitched a "deep learning" version of this problem that I love so much I will have to write it down on the ideas blog!

Francis-Yan Cyr-Racine (JPL) told me about an absolutely awesome project to understand the statistical effect of substructure on lenses without the requirement that the substructures be detected individually. That's right up my alley, and the alley of Brewer as well. He has broken the problem into parts, combining the weak parts into a Gaussian noise and doing the strong parts the hard way. Genius.

I argued for randomized (or other regular but clever) observing strategies for WFIRST and Euclid with Jason Rhodes (JPL) and the dark-matter and dark-energy group. We lamented the lack of good observing-strategy simulations for these projects, which would make such arguments quantitative, open, transparent, and efficient.

responding to referee(s)

Evidently the ApJ has switched to double refereeing for papers that are interdisciplinary between the domains of astrophysics and the methodological domains. This has happened to me twice now. Today I spent time working on responding to the methods-oriented referee for our paper on The Cannon. The referee made some great points about related methodologies in statistics and machine learning to which we should be calling out in the paper; that got me doing some serious reading.

The Right Thing To Do (tm) for asterosismology

Sarbani Basu (Yale) showed up and gave a great talk on the current state of helioseismology and asteroseismology. Before her talk—which showed off the awesome of Kepler and its ability to separate the different post-main-sequence phases of stars using normal modes—she was subjected to a two-hour grilling in CampHogg group meeting. We asked her about the narrowness of modes, the time dependence of amplitudes, the principal observables and how they are determined, and so on.

After her talk, Angus, Foreman-Mackey, and I argued about what a generative probabilistic model of a stellar light curve might do for asteroseismology. Right now the data analysis chain is: Take a periodogram of the data, identify modes, identify the maximum-amplitude frequency and various frequency differences, compare those to models thereof. An alternative approach would be: Take a model of a star, compute the expected frequency spectrum and variations around that, produce a probability distribution over lightcurves that could be produced by this spectrum, and then boom, likelihood function! We are nowhere close to having this, but some of the key applied-math pieces we might need are starting to come together.

At the end of the day I spoke at the Westchester Amateur Astronomers, about exoplanet search and discovery, and the prevalence of Earth analogs. I got absolutely great questions from the (very knowledgeable) crowd.

Dr Foreman-Mackey

It was a great day at CampHogg today! Foreman-Mackey defended his PhD thesis with a great talk. Although his thesis also contained chapters about MCMC sampling, exoplanet populations, and finding isolated transit events in lightcurves, he concentrated his oral presentation on the discovery of periodic transits in the K2 data. This permitted him to touch on many aspects of his work, including especially the marginalization out of systematics (instrument-induced signals). He made a very strong case that we need to fit the systematics simultaneously with the signals of interest, because in any other approach (any pre-filtering or data conditioning method), you end up either very strongly restricting the systematics model (so it doesn't do what you need) or else you end up over-fitting and distorting your transit signals. The thesis talk was followed by extensive questioning and the signing of forms.

As my loyal reader can infer from what's written in the many posts on this blog, Foreman-Mackey has had a huge impact on every aspect of my scientific life. I have learned far more from him than he has learned from me. He has achieved a lot, in tool building and in the production of valuable scientific results. I still get a few more weeks of Foreman-Mackey but then he is off to Seattle to start his Sagan Fellowship.

#astrohackny, APOGEE, chaos, brown dwarfs

At #astrohackny, Ben Weaver showed us more development he has done on fitting the individual-exposure radial velocities in the APOGEE data. It looks to me very likely that he will be able to improve the APOGEE data analysis with these, and maybe also discover some cool things.

After the hacking, I spent time with Price-Whelan and Johnston talking about chaos. I admitted that I have no idea what constitutes a chaotic orbit! There are differences, it appears, between diffusion in frequency space and Lyapunov times; these aren't in contradiction, exactly, but the differences are surprising given my (trivial, incomplete) intuition. Something is odd to me about the related but seemingly very different points that a chaotic orbit is 5-space-filling (not just 3-torus-filling) and that it has a positive Lyapunov exponent and that it diffuses in frequency space. These all seem like three very different manifestations of chaos.

Kopytova and I spoke about fitting brown-dwarf spectra. We identified a set of easy projects, one of which is to fit the spectral data with a flexible calibration (or, equivalently, continuum) model. For the longer term, we talked about making data-driven improvements to the models, which seem to be incomplete or in need of work in the H band.

are GRBs beamed?

It was all GRBs today, with Maryam Modjaz (NYU) at coffee talking about this paper that finds a gamma-ray burst by its optical emission (that is, not by having a gamma-ray or x-ray trigger). Then at lunch, more GRBs with Modjaz talking about the GRB progenitors, about which we only have indirect information from looking at environments (and especially abundances). I asked my usual questions about beaming: We know from models of the emission that GRBs should be highly beamed, and that the optical should be much less beamed than the gamma-rays. But that means that there must be many GRB-like optical transients that don't have gamma-ray counterparts. None have been found! What gives? In general, the empirical evidence for beaming in astrophysical sources has always been weak (think of all the arguments about quasar unification), with pulsars an exception. Of course the detailed answers to this question involve the relative brightnesses, time scales, and spectral energy distributions of the way-off-axis emisssion, so there is quite a bit of theory to do to interpret the observations, but right now it is looking hard to reconcile all the theory and observations around GRB beaming.