Exoplanet Transits

with Radio Telescopes

Benjamin Pope, NYU

NASA Sagan Fellow

Presenting work in collaboration with Joe Callingham (ASTRON) and Tim White (Aarhus).

Slides available at
benjaminpope.github.io/talks/astron/astron

Transiting Planets

Exoplanet-style transit light curve of Venus from James Gilbert on Vimeo.

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 Cancri e - subject of 367 papers in the last decade!

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 the upcoming James Webb Space Telescope.

Kepler Photometry

The Kepler Space Telescope, launched in 2009, looks for planets by the transit method, and also does asteroseismology.

After the failure of a reaction wheel in 2012, it is now operating as the 'K2 Mission', with very unstable pointing (hence the shaking in the videos you'll see).

To get the photometry, you can just sum the pixel values in a window containing the whole PSF...

but the pixels have different gains ("inter- and intra-pixel sensitivity variation")...

and the pixel window doesn't necessarily track the whole PSF perfectly ("aperture losses").

In our group's pipeline we use Gaussian Process models to detrend the flux time series with respect to the position of the star.

Raw - GP in position - GP in time

By subtracting the GP time and spatial components, we can find a transiting planet!

Alternatively, the EVEREST model (Luger et al, 2016) does 'pixel-level decorrelation', fitting a linear combination of pixel time series to the data, getting excellent photometry.

We will be motivated by this here.

For sufficiently bright stars, though, light fills the CCD wells with electrons that spill up and down the column, ruining the photometry as they leave the aperture.

Kepler saturates on stars brighter than ~ 11th mag (log scale: 5 mag = factor of 100) - but we want to look at stars 10k times brighter.

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 instead minimize the Total Variation

\[\begin{align} TV \equiv \dfrac{\sum_i |f_i - f_{i-1}|} {\sum_i f_i } \end{align} \]

This is the L1 norm on the derivative of the time series.

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

Pleiades

Πλειάδες, the Seven Sisters

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

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

White, Pope et al., 2017

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!

Radio Stars and Transits

Many papers have discussed low-frequency searches for exoplanetary radio emission, with no detections so far.

Theorists now say that expanded ionospheres of hot Jupiters might self-absorb this emission down to undetectable levels.

What else can radio astronomy do for exoplanetary science?

The first and brightest celestial radio source to be discovered was the Sun - brighter than 10,000 Jy across the radio spectrum.

The first radio images of the Sun revealed that emission is dominated by active regions (sunspots)

Using 17 GHz maps of the Sun Selhorst et al predicted deep transits across active regions as seen by ALMA.

Alas, nearby main sequence stars have ~μJy brightnesses in ALMA bands - compare to ~mJy sensitivities for 1 hr integrations with ALMA.

How well might we do with the SKA?

Using SKA design specifications, we calculate the sensitivity of the SKA to transits around solar-like stars (using VLA fluxes of ε Eridani and the MWA SED of the Sun) and M dwarfs (scaled from LHS 3003), we predict the sensitivity of the SKA to transits.

As we can see, it is very challenging to detect solar-like stars at more than a few parsecs, let alone their transits.

Transits across active M dwarfs, on the other hand, can be seen out to tens of parsecs! Gaia indicates there are many hundreds out to this distance, most undetected so far in radio.

With very significant radio variability and complicated magnetic field topologies, it is harder to see what transits across these will look like.

The magnetosphere of a hot Jupiter planet is expected to cause both strong scintillation...

... and broadband strong lensing.

The Future

TESS will deliver thousands of new planetary candidates - and hopefully including some close enough to detect transits with the SKA.

Can we improve the sensitivity to radio transits by stacking many transits? By improved observing strategies or instrumentation?

Can we predict their light curves better?

What else might the SKA do for exoplanets?