Exoplanet Transits

with Radio Telescopes

Benjamin Pope, NYU

NASA Sagan Fellow

Presenting arxiv:1810.11493 with coauthors Joe Callingham (ASTRON), Paul Withers and Marissa Vogt (BU).
Slides available at

Radio Astronomy

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.

Cohen et al. 2018 predict a modulation of stellar coronal radio emission at ~10-250 MHz frequencies from close-in exoplanets...

...but no significant effect at higher frequencies - where our most sensitive telescopes operate.

The Square Kilometre Array (SKA) is a billion-dollar project to build a massive ~ GHz frequency SKA-Mid array in the Karoo Desert in South Africa and a ~ 100 MHz SKA-Low array in the Murchison Desert in Western Australia.

The Next-Generation Very Karge Array (ngVLA) is a similar-scale project under consideration in the USA.

What can the SKA do for exoplanet science?

Radio Stars and Transits

The first and brightest celestial radio source to be discovered after the Milky Way 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)

Ruby Payne-Scott Sun Images

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

Selhorst Figure
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.

Callingham Sensitivities Figure


Twinkling and Refraction

Stars twinkle because of the inhomogeneous atmosphere introducing phase delays - and Jupiter's magnetosphere does the same to quasars.
The magnetosphere of a model hot Jupiter planet is expected to cause both strong scintillation... Hot Jupiter scintillation

... and broadband strong lensing from refraction through its mean density profile.

Hot Jupiter defocusing
In the strong lensing and scintillation regimes we expect modulations of order ~ unity of small-scale surface features such as spots.

M Dwarfs

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 at SKA-Mid frequencies out to tens of parsecs! Gaia indicates there are many hundreds out to this distance, most undetected so far in radio.

M dwarfs are known to be very variable in the radio, with wideband, polarized flares.

I argue that modulation of the amplitude of this variability on an optically-determined exoplanet ephemeris will nevertheless be a strong signal.

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

Low frequency emission is at present poorly constrained - LOFAR will test for SKA-Low..

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?

LOFAR will probe the M dwarf population at low frequencies - how bright are they?

Can we predict their light curves better?

What else might the SKA do for exoplanets?