Lightning on Exoplanets

Title: Is lightning a possible source of the radio emission on HAT-P-11b?

Authors: G. Hodosán, P. B. Rimmer, Ch. Helling

First Author’s Institution: SUPA, School of Physics and Astronomy, University of St Andrews, UK

Status: Published in MNRAS [open access]

Take a guess at how many lightning strikes are happening at this moment on Earth. The answer is amazingly around 100 every second! That’s billions of flashes per year. You can actually monitor the lightning occurring over the world in real time here. Lightning discharging generates electromagnetic waves over a broad frequency range, including optical flash and radio pulses. Albeit being a transient and local phenomenon, with such a high occurrence rate, lightning alters the chemistry of Earth’s atmosphere, converting Nto NO and NO2. Besides our Earth, lightning has also been observed on Jupiter and Saturn, identified by the optical flash or the radio signal. In today’s bite, the authors take an even further step. They analyze whether lightning can explain the mysterious radio signals from the exoplanet HAT-P-11b.

HAT-p-11b is a Neptune sized exoplanet orbiting very close (~0.05 AU) to its star. In 2009, A. Lecavelier des Etangs and coworkers detected a radio signal of 3.87 mJy at 150 MHz from HAT-p-11b. They suggested that the signal is due to the interactions between the planetary magnetic field and the stellar coronal. However, the authors of today’s paper argue that since the signal is not polarized, it is unlikely that it resulted from this type of cyclotron emission. In 2010, no signal was observed when they re-observed the planet. This means the signal was produced by a transient phenomenon if it was real. The authors then ask the question of how many lightning flashes would be needed to produce the observed signal. The lightning flashes are measured in terms of the flash density, which is the number of lightning events per area per time. Taking the energetic properties from the lightning on Saturn measured by Cassini, they estimate that a flash density of ~ 105 km-2 hour -1 is needed for the radio signal to reach the Earth (left panel in Figure 1), which is about 107 higher than that on Saturn and 106 greater than the highest flash density observed within the USA. Based on lightning energetic properties known from our Solar system, this is an unrealistically high value.  The authors also compute the optical counterpart and find the enormous lightning storms would be as bright as the host star (right panel in Figure 1).

The estimated lightning flash densities (left) and the apparent magnitude of the optical flashes (right) that could produce the observed radio pulses from HAT-P-11b. Left: the horizontal line is the average values of the radio flux ~3.87 mJy and the vertical dashed lines are the beginning and end of the eclipse when the planet is blocked by the star. Right: estimated with optical properties of lightning on Earth (blue) and on Saturn (red).

 

As previously mentioned, lightning changes the chemistry by producing molecules that would not be abundant in equilibrium. Applying Saturn-like lightning to HAT-p-11b, the main lightning product, hydrogen cyanide (HCN), can linger in the atmosphere for 2–3 years after the lightning storm. With strong enough wind to mix the gas, the abundance of HCN can be detectable (~10-6 cm-3) in the upper atmosphere. The variation of the molecule can potentially be identified in the infrared spectra.

As the radio signal on HAT-p-11 remains unsolved, the authors recommend observing radio emission at a higher frequency of a few tens of MHz, where it is more probable to observe lightning, and infrared observations can reveal spectral features from lightning tracers such as HCN. If both are observed at the same time, it could be strong evidence of the first detection of “exo-lightning”.

 

About Shang-Min Tsai

I am a 3rd year PhD student at the University of Bern and part of the Exoplanets & Exoclimes Group led by Prof. Kevin Heng. We are developing various open-source tools to study exoplanets. I work on modelling of atmospheric chemistry and dynamics. When I am not coding or debugging, I enjoy basketball and playing board games.

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