Finding Exoplanets

Transiting Naked-Eye Stars

Benjamin Pope, NYU

Sagan Fellows Symposium

Slides available at
My Sagan Fellowship project is to search for planets transiting naked-eye stars (V mag < 6.5) in order to find ideal targets for characterization with JWST.
Large searches for exoplanets like the Kepler mission have shown planets to be common in our Galaxy - now we want to learn about their atmospheres and compositions.

The best options are those around bright stars, like 55 Cnc e - subject of 367 papers in the last decade!

Kepler saturates on stars brighter than ~ 11th mag, but with Tim White we have recovered light curves of stars as bright as first mag by using the unsaturated scattered light halo.

Halo Photometry

What if we just look at unsaturated pixels?
Flux \(f_i \) at cadence i is a sum over j pixels \(p_{ij}\) with weights \(w_j\):

\[ f_i \equiv \sum\limits_i w_j p_{ij} \]
In the OWL method (Hogg & Foreman-Mackey), you choose weights to minimize variance of the final light curve - but this has limited success.
To find the appropriate weights, we minimize the Total Variation

\[\begin{align} TV \equiv \sum_i |f_i - f_{i-1}| \end{align} \] subject to constraints \[\begin{align}\forall_j w_j &> 0\\ \sum_{i=1}^{N} f_i &= N.\end{align} \]

This is the \(L_1\) norm or 'taxicab metric' on the derivative of the time series.

This has analytic derivatives you can compute with autodiff - easy to optimize.


Πλειάδες, the Seven Sisters

Alcyone, Atlas (dad), Electra, Maia, Merope, Taygeta, Pleione (mum)

Combined Figure

Atlas lightcurve: raw (top) and halo (bottom)

White, Pope et al., 2017

Combined FigureCombined Figure

Lightcurves of All Seven Bright Pleiades

White, Pope et al., 2017

I am currently searching all bright stars in K2 for transiting planets - none so far, but plenty of asteroseismology!


α Tauri

الدبران ,the follower

... follows the Pleiades!

Hatzes & Cochran, 1993 claimed an early detection of a \(628.96 \pm 0.90\) d RV planet around Aldebaran - finally confirmed in Hatzes et al., 2015!
All Data
A Gaussian Process reanalysis of this data by Will Farr detects p-mode acoustic oscillations at 2.2 μHz - can we confirm this with K2?

Yes! We get the same frequency with K2!

Aldebaran K2 Light Curve
Without this asteroseismology, we have

\[M = 1.27^{+0.24}_{-0.2} \, \mathrm{M_{\odot}}\] and age \(4.86^{+3.56}_{-2.04} \, \rm Gyr \)
With this new constraint, we have

\[M = 1.16^{+0.07}_{-0.07} \, \mathrm{M_{\odot}}\] and age \(6.38^{+1.42}_{-1.12} \, \rm Gyr \)

Using MESA isochrones & stellar tracks, we find that on the main sequence Aldebaran b had a semi-major axis of \(1.507 \pm 0.03 \) AU and Aldebaran had a luminosity \(2.0 \pm 0.7 \, L_\odot \)...

so Aldebaran b had an insolation comparable to Earth when its star was on the main sequence.

The Future

Thanks to the K2 GO office, we have more than a hundred halo light curves and are working our way through them.
The method generalizes well to simulated TESS data - which saturates at ~ 6th mag.
Numbers from Fressin et al., 2013.
We still don't have an explanation for the unreasonable effectiveness of the L1 norm in this algorithm - help us understand!
All our code is open source at - play with it!
We have a solution looking for problems - we want to extend this to TESS, JWST, and even ground-based missions.

Let us know if we can help.