possible Trojan planet?

In group meeting last week, Stefan Rankovic (NYU undergrad) presented results on a very low-amplitude possible transit in the lightcurve of a candidate long-period eclipsing binary system found in the NASA Kepler data. The weird thing is that (even though the period is very long) the transit of the possible planet looks just like the transit of the secondary star in the eclipsing binary. Like just like it, only lower in amplitude (smaller in radius).

If the transit looks identical, only lower in amplitude, it suggests that it is taking an extremely similar chord across the primary star, at the same speed, with no difference in inclination. How could that be? Well if they are moving at the same speed on the same path, maybe we have a 1:1 resonance, like a Trojan? If so, there are so many cool things about this system. It was an exciting group meeting, to be sure.

Terra Hunting Fall Science Meeting, day 1

Today was the first day of the Terra Hunting annual science meeting. One highlight of the day was a presentation by Yan Liang (Princeton), who is modeling stellar spectral variability (the tiny variability) that affects extremely precise radial-velocity measurements. Her method involves a neural network, which is trained to distinguish RV variations and spectral shape variations through a self-supervised approach (with a data augmentation). Then it separates true stellar RV variations from spectral-variability-induced wrong RV variations by requiring (essentially) that the RV variations be uncorrelated with the (latent) description of the stellar spectral shape. This connects to various themes I am interested in, including wobble by Bedell, a spectral variability project by Zhao, and causal structure in machine learning.

Florida, day one

I spent today with Sarah Ballard's group, plus others, at the University of Florida. I gave a talk, to a large, lively, and delightful audience. At the end of this talk I was very impressed by the following thing: Ballard had everyone in the room discuss with their neighbors (turn and talk) for about 3 minutes, after the seminar but before the question period began! This is a technique I use in class sometimes; it increases participation. After those 3 minutes, audience members had myriad questions, as one might imagine.

I spoke with many people in the Department about their projects. One highlight was Jason Dittman, who showed me gorgeous evidence that a particular warm exoplanet on an eccentric orbit has an atmosphere that undergoes some kind of phase change at some critical insolation, as it moves away from its host star on its orbit. Crazy!

Late in the day I discussed n-point functions and other cosmological statistics with Zach Slepian and Jiamin Hou. We discussed the plausibility of getting tractable likelihoods for any n-point functions. We also discussed the oddity that n-point functions involve sums over n-star configurations among N stars (N choose n), but there are mathematical results that show that any permutation-invariant function of any point cloud can be expressed with only a sum over stars (N). That sounds like a research problem!

first-ever Blanton–Hogg group meeting?

Today was not the first-ever Blanton–Hogg group meeting. But it was the first ever for me, since I missed the first two for health and travel reasons. It was great! Tardugno (NYU) showed simulations of gas disks with embedded planets. The planets affect the disk, and the disk causes the planets to interact. Daunt (NYU) showed that his method for inferring (simultaneously) the spectrum, tellurics, and radial velocities in stellar spectra all works. I am stoked! Novara (NYU) showed that he has a bug in his code! But we had a good discussion inspired by that bug about surfaces of section in a real dynamics problem. Gandhi (NYU) showed us a paper with questionable claims about the CMB light passing through galaxy halos?

planets forming in a disk

At the end of last week I had a great conversation with Valentina Tardugno (NYU) and Phil Armitage (Flatiron) about how planets form. I spent the whole weekend thinking about it: If a few planets are forming in a proto-planetary disk, there are all sorts of interactions between the planets and the disk, and the planets and each other, and the disk with itself. You can think of this (at least) two different ways:

You can think of planets which are interacting not just directly with one another, but also with a disk, and with each other as mediated by that disk. This is the planet-centric view. In this view, the planets are what you are tracking, and the disk is a latent object that makes the planets interact and evolve.

Alternatively, you can think of the disk, with planets in it. In this disk-centric view, the planets are latent objects that modify the disk, creating gaps and spiral waves.

Both views are legitimate, and both have interesting science questions. We will explore and see where to work. I am partial to the planet-centric view: I want to know where planetary systems come from!

is a periodic signal in a time series statistically significant?

I had conversations with Nora Eisner (Flatiron) and Abby Shaum (CUNY) today about how we report the significance of a signal we find in a time series. In particular a periodic signal. It's an old, unsolved problem, with a lot of literature. And various hacks that are popular in the exoplanet community (and binary-star community!). My position is very simple: Since all methods for determining significance are flawed, and since when you fit a signal you have to estimate also an uncertainty on that signal's parameters, the simplest and most basic test of significance is the significance with which you measure the amplitude of the proposed signal. That is, if the amplitude is well measured, the signal is real. Of course there are adversarial data sets I can make where this isn't true! But that's just a restatement of the point that this is an unsolved problem. For deep reasons!

O-minus-C inanity

In the exoplanet (and, before that, eclipsing-binary) communities, transit-timing variations are described in terms of a quantity called O−C (pronounced “oh minus sea”), which is the difference between the observed transit time and the “computed” transit time. Right now, Abby Shaum (CUNY) and I are using this terminology in our manuscript about phase variations in coherent pulsators with companions, at the behest of Keaton Bell (CUNY). Okay fine! But O−C has this terrible property, which is that the C part depends on the period or frequency you assume. You can completely change the appearance or morphology of an O−C plot just by slightly tweaking the period. And there is no true period of course! There is just whatever estimates you can make. Which are, in turn, affected by what you use to model the O−C. So it is absolutely awful in every way. Not a stable observable, people! Not even identifiable.

classification to save labor

I spent part of the day discussing with Valentina Tardugno (NYU) and Nora Eisner (Flatiron) the goals of a machine-learning classification that Tardugno is creating to help the PlanetFinders project. The deal is: Citizen scientists find candidate planets and (currently) a human (Eisner) has to vet them, to remove contamination by various sources of false positives. This turns out to be a hard problem! When problems are hard, it becomes critical to very precisely specify what you are trying to achieve. So we spent time discussing what, exactly, it is that Eisner needs from a classifier. Is it to find good planets? Is it to remove obvious contaminants? Are some contaminants more problematic than others? Is it to save her hours of wall-clock time? Etc.

Phi-M radio

I worked today with Abby Shaum (CUNY) on her paper about her phase-demodulator to find exoplanet and substellar companions to stars by the timing of asteroseismic modes. I suggested that we highlight the incredible simplicity of her project by writing the method as an algorithm of just a few lines.

extreme infrared excesses

Gaby Contardo (SISSA) showed up in Heidelberg today to make progress on our project on infrared excesses in normal, non-young FGK stars. Because we are using NASA WISE data (along with ESA Gaia and NASA 2MASS), we are only sensitive to bright, hot infrared excesses, much hotter and brighter than typical debris disks around old stars. We have some candidates, which range in temperature from 300 to 1500 K and are reprocessing maybe one percent or a fraction of a percent of the stellar light. (Warning: I haven't calculated this; this is just a guesstimate based on looking at plots.) What are those things? Today we figured out that they can't be warm substellar companions, so they have to be dust (I guess??).

a likelihood for our Phi-M radio

There are AM radios and FM radios and (if you are a nerd) PCM radios. But Abby Shaum (CUNY) and I have built a Phi-M radio, which demodulates phase variations in a carrier signal. We (with Keaton Bell, CUNY) are using it to find binary companions and planets around stars that show coherent pulsation modes in their photometry. Today I wrote down a noise model for the output of our demodulator. It isn't completely trivial. But it's good, because we can make a likelihood function for fitting our companions. Our model will end up being a limit of the more general model called Maelstrom by Dan Hey (Hawai'i).

wobble, star spots, quasar dipole

[Time to try to re-start this forum.]

I spent this morning on three different small activities. One was giving feedback to Matt Daunt (NYU) who is trying to re-build the wobble concept for stellar radial-velocity measurement in jax. He has annoying optimization issues, which are very hard to diagnose! Optimization is always nasty, in my experience.

Another activity was working on the abstract for Lily Zhao's (Flatiron) upcoming paper on stellar variability in the spectral domain, generated by rotating, spotty stars. She is concerned that the paper is too conceptual. I love conceptual papers! I think science moves forward through concepts and implentations, and no individual paper has to do it all.

My third activity this morning was working through the mathematics on a project of Abby Williams (NYU, Caltech) to measure the kinematic dipole in the all-sky Quaia quasar catalog. There are so many different ways to measure it. I think I have a justifiable likelihood function approach, and one in which we could marginalize out—or profile out—the uncertainties in the selection function we have estimated. It's a controversial subject, so I would like to do things correctly.

how to maximize the yield of planets?

There were discussions this week at University of Warwick about the Terra Hunting Experiment strategy and likely detection capability. Various take-homes include that we need to mitigate lots of stellar noise, and that we care deeply about the covariance (as a function of separation in time) of adjacent measurements. I advocated that we split our ten-year survey into two or three surveys, of varying length. In the first, we learn about the stars, and in the last, we go to town on the very most promising targets. There was general agreement that this is a good idea. But now we need a very specific plan for what this means. As my loyal reader knows, in my view, the decisions must be based on repeatable operations, so that we have some hope of learning statistical things about populations in the end.

predicting RVs from SDO imaging

I'm at the Terra Hunting annual Science Working Group meeting, held this year at University of Warwick. There were many great talks today, some technical and some science. My mind was blown by Ben Lakeland (Exeter), who showed Solar Dynamics Orbiter data of the Sun, and then showed that, from these images, he can predict the magnetic-activity-generated RV signals in simultaneous EPRV measurements of the Solar RV. That's pretty exciting. He also showed that much of the time, the RV variations are dominated not by magnetic activity per se. If we are going to beat one meter per second, we are going to have to correct for convective shifts. Somehow!?

a well-posed problem in gastrophysics

Magda Siwek (Harvard) gave an execellent NYU Astrophysics Seminar today, about evolution of binary systems when the binary is accreting from a circumbinary disk. She sets a few (just a few) disk parameters, and then sets the mass ratio and eccentricity of the binary, and seeks steady-state (low disk-mass or low accretion-rate) solutions. By ignoring electromagnetic fields and various bits of microphysics, she can create a setup that is completely scale-free, so it applies (approximately) from all scales from exoplanets to super-massive black holes. That's brilliant. She finds that the eccentricities are in general driven to non-zero steady-state values, which depend (strongly) on mass ratio and (maybe weakly) on disk parameters. That's a nice problem, and observationally relevant to projects we are doing right now.

causal structure in ML

Today I had the honor on serving on the PhD advising committee of Yan Liang (Princeton), who is designing her PhD project. She is adding causal structure to an autoencoder such that it can separate stellar variability-induced radial-velocity signals from exoplanet-induced signals in extreme precision radial-velocity data. Her method design is novel, and tests suggest that it might work. The committee recommended adding even more causal structure and physics knowledge (more is probably always better, provided that it isn’t incorrect)! As my loyal reader knows, I think this is the frontier for machine learning in the natural sciences: adding causal structure.

First Science Results from JWST, day two

Today was day two of the First Science Results from JWST meeting at STScI. Once again, it was a blast of results from all different fields. Some things I'll think about more going forward include: Something like 3 percent of white dwarf stars show an infrared excess that is consistent with them having a Saturn-like ring system? How did I not know this previously? It makes me want to find a WD with a transiting exoplanet to map the rings and maybe even ring gaps! There is a huge class of red luminous outbursts that appear to be the result of mergers of binary stars (maybe often when one of the binary pair starts to go off the main sequence and engulf its partner). Some of these, for energetic and other reasons, look like they are created not by binary-star systems but instead by star–planet systems. I wonder if the populations can be connected to the population of stars with weird lithium and refractory abundances?

can you see the orientation of a star?

Stars don't have uniform surfaces, and they rotate. Can you see the orientation of the star in a single spectrum? Of course the answer is no: You don't have a coordinate system! But if you have some previous spectra of the star, can you establish a rotation period and define an angular coordinate system, and then follow that by taking a new spectrum and saying where the star is in its rotational phase? It looks like the answer to this question might be yes, based on experiments that Lily Zhao (Flatiron) is doing. Of course we don't really care about the stellar orientation. What we care about is capturing or correcting the artificial radial-velocity signals introduces to the data from the rotating, non-uniform surface.

phase and frequency variations

If a star has a (relatively) coherent oscillation mode, and you can monitor it over a long period of time, then orbital motion of the star can be seen as either phase or frequency variations of that mode. I've been working on this in different collaborations, with Dan Hey, with Simon J Murphy, with Abby Shaum (CUNY), and recently with Nora Eisner (Flatiron). Right now, Shaum, Eisner, and I are looking at signal-processing approaches that look like demodulators. What I'm interested in—at least in terms of me learning about signal processing—is how can we make a demodulator that demodulates both phase and frequency simultaneously. There must be hybrid and combined approaches. I'm also interested in what we can measure from incoherent oscillators.

Korg

Today Andy Casey (Monash) joined a regular meeting I have with Megan Bedell (Flatiron) and Lily Zhao (Flatiron) about things related to precision spectroscopy. We discussed projects we can do with surface spectra of the Sun, one from the quiet part, and one from a spot. Casey is involved in the Korg project led by Adam Wheeler (OSU); we discussed fitting both spectra with Korg, and learning about the physical differences between the quiet and active regions in the Sun. We also discussed Zhao's projects to empirically correct for stellar activity in time-domain spectroscopy looking for planets.