Is lightning a phenomenon only occurring on Earth? Or is it universal? How can the knowledge we learnt from Solar System lightning help with discovering lightning on exoplanets and to understand these very different worlds? The next two entries will be devoted to the work on extraterrestrial lightning carried out by Gabriella Hodosán, LEAP PhD student, under the supervision of LEAP PI Dr Christiane Helling, in collaboration with various LEAP group members. In Part I we apply results of lightning surveys of several Solar System planets, including Earth, to different groups of extrasolar planets. Part II will be about a specific planet, HAT-P-11b and the possibility of lightning detection in its atmosphere.
Lightning is one of the most spectacular phenomena on Earth. It has interested not just scientists but the general public for thousands of years. However, it is not a unique phenomenon to Earth. It has been observed before on several Solar System planets, such as Jupiter, Saturn, or Uranus and Neptune. Spacecraft like Cassini, Galileo, New Horizons or the Voyagers provided us with breath-taking images of the outer Solar System, including images and measurements of lightning occurring on the gas giant planets (Fig. 1).
Since the late 90s, thousands of exoplanets have been discovered. These exoplanets show a large diversity (Fig. 2) in sizes, masses, even distances to the host star, much different to our Solar System planets: Jupiter-size planets orbiting other stars at the distance of Mercury; planetary systems with several planets inside the orbit of Mercury; terrestrial planets several times bigger than Earth, but still rocky and not made of gas. Could these planets host lightning in their atmospheres? Let’s look at Earth and Saturn. They have different composition, different sizes, masses, different atmospheres. And still, they both show lightning activity. So why couldn’t it occur on exoplanets?
In our work, we were focusing on the statistical side of lightning occurrence on Solar System planets, then extrapolated to extrasolar objects. Lightning climatology explores the spacial and temporal distribution of lightning. It uses the quantity of flash rate [flashes/unit time, e.g. flashes/hour] or flash density [flashes/unit time/unit surface area, e.g. flashes/hour/km2] to quantify this distribution. It is a tool to estimate the lightning activity on the surface of an object. This is important in order to estimate the total energy released of lightning flashes, and to determine whether the signatures produced by lightning would be observable from Earth.
Lightning observing networks net the whole surface of the Earth and satellites continuously look for lightning flashes from near Earth orbits. Lightning monitoring is important because of the hazards (e.g. forest fires, large scale power outage, fatalities) it causes. Measurements from Earth provide the largest data sets we can work with. In our study we analyzed data from the Lightning Imaging Sensor (LIS)/Optical Transient Detector (OTD), which are optical instruments on board of satellites, and from two ground based radio networks, the Sferics Timing and Ranging Network (STARNET) and World Wide Lightning Location Network (WWLLN). Figure 3 shows an example of lightning climatology maps produced from LIS/OTD data. It shows an average of lightning occurrence over the period of 1995-2013. It shows clear trends of more lightning over continents than over oceans, and more lightning over lower latitude regions than higher latitude regions.
Lightning occurs not only in thunderclouds but in volcano plumes as well (e.g. see work by our group, and the eruption of Eyjafjallajökull). Based on literature research we collected flash densities from two volcano eruptions: Mt Redoubt in 2009 and Eyjafjallajökull in 2010. Some interesting fact: both eruptions show orders of magnitude larger flash densities than what thunderclouds produce. On average the LIS/OTD data showed a 2×10-4 flashes/km2/h flash density while Eyjafjallajökull showed 0.1 – 0.32 flashes/km2/h and Mt Redoubt produced up to 2000 flashes in an hour over a square kilometer.
Venus, Jupiter and Saturn
Is there lightning on Venus? The long debated question has not yet been fully answered, however, more and more clue indicate the existence of such phenomenon on Earth’s sister planet. If lightning does exist on Venus, it is probably not very energetic and it appears deep within the atmosphere, since no optical observation of it has been made to date. Radio data from the Venus Express mission, however, shows a possible ~10-11 flashes/km2/hour flash density.
Jupiter and Saturn are more interesting, since, apart from Earth, these are the only planets in the Solar System where lightning has been observed directly. We used published data from the Galileo (1997, Jupiter), New Horizons (2007, Jupiter) and Cassini (2009, 2011, Saturn, e.g.) spacecraft. Figure 4 shows and example lightning distribution map for Jupiter. It shows and increased lightning activity around the +- 50-degree latitude regions, most probably due to the increased affect of internal heating on convection and cloud formation. Until 2009, only radio signals of lightning were observed on Saturn. The so-called SEDs (Saturnian Electrostatic Discharges) are short and strong radio bursts from lightning, detected by e.g. the Voyagers and Cassini spacecraft. The two giant gas planet, on average, shows a flash density of 10-6 – 10-7 flashes/km2/hour.
Now the most exciting part: How does this all relate to exoplanetary research? The idea behind our paper is to use the statistics from Solar System planets to estimate a possible lightning occurrence on exoplanets with similar environments to those described in the previous sections. E.g. let’s take possible ocean planet: Kepler-62f. Then apply the flash density derived for Earth for above oceans: ~5 x 10-5/km2/hour. Combining this, we estimate the flash density on Kepler-62f, throughout its whole surface, to be the same: ~5 x 10-5/km2/hour. Another example is to apply the flash densities in order to estimate the total lightning occurrence during the transit of a planet. 55 Cancri e is a close-in rocky planet, most probably cover by lava. How much lightning could occur during its transit on the projected disk of the planet, if we assume continuous flashing from volcano activity? Depending on which eruptions we consider as the template, 55 Cnc e could produce as many as 108-1012 flashes on its whole disk during its 1.5-hour transit. This enormous amount of lightning would produce radio emission that might be observable during the transit.
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