EXO-LIGHTNING Part I: what can we learn from the Solar System?

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).

Lightning on Jupier (right top and bottom) and Saturn (left bottom). (Credit: NASA/JPL-Caltech/SSI, NASA/Galileo)

Figure 1. Lightning on Jupiter (right top and bottom) and Saturn (left bottom). (Credit: NASA/JPL-Caltech/SSI, NASA/Galileo)

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?

Diversity of exoplanets and brown dwarfs on a mass-radius and distance-density plot (Hodosán et al. 2016).

Figure 2. Diversity of exoplanets and brown dwarfs on a mass-radius and distance-density plot. The lines on the top plot indicate the potential chemical composition of the bodies based on their obtained mass and radius  (Hodosán et al. 2016).

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.

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.

Figure 3. LIS/OTD lightning climatology map averaged from 1995-2013 (Hodosán et al. 2016).

Figure 3. LIS/OTD lightning climatology map averaged from 1995-2013 (Hodosán et al. 2016).

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.

Figure 4. Lightning distribution on Jupiter. Triangles: Galileo data from 1997. Circles: New Horizons data from 2007.

Figure 4. Lightning distribution on Jupiter. Triangles: Galileo data from 1997. Circles: New Horizons data from 2007.

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.

Exoplanets

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.

In Part II we introduce our original idea of estimating whether a weak and tentative radio signal observed on the mini-Neptune, HAT-P-11b, could have been caused by lightning or not.

If you have an opinion, please leave a comment below.

For more details check out the original paper on ADS:

 Hodosán, G.; Helling, Ch; Asensio-Torres, R.; Vorgul, I.; Rimmer, P. B., MNRAS, 461, 3927, 2016

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Why exoplanets should have ionospheres and brown dwarfs chromospheres

Do exoplanets have an ionosphere? What does a brown dwarf need to form aurorae even without a companion? Isabel Rodriguez-Barrera and colleagues, including Christiane Helling and former member Craig Stark from the LEAP group, investigated whether it is possible to create a magnetized plasma, a medium composed of positive and negative charges but with an overall neutral electric property. The production of magnetized plasma would allow the creation of ionospheres and electromagnetic phenomena such as aurorae.

The ionosphere is the upper part of a planetary atmosphere created by the ionizing effects of stellar UV and X-ray radiation. Its importance inheres in atmospheric electricity and radio wave propagation, but also the shielding of the inner atmosphere from stellar UV radiation. The following study showed what conditions a planetary atmosphere (that is an atmosphere from a planet or a planet like object like a brown dwarf) should fulfil to produce an ionosphere or, in case of brown dwarfs, a chromosphere, such as the spectacular example of the solar chromosphere.

Radio, X-ray and Hα emission from brown dwarfs have been observed in the recent years (e.g. 2MASS J10475385+2124234 by Route & Wolszczan 2012; 2MASS J13153094-2649513AB by Burgasser et al. 2013). Similar observations are not yet available for extrasolar planets. In case of the Sun, observations of these emissions (radio, X-ray and Hα) are tracers of the solar chromosphere. These observations suggest that brown dwarfs contain ionized gas and host very strong magnetic fields, which are both needed to explain, for example, the radio emission. The aim of our study is to identify ultra-cool objects (with effective temperatures less than ~3000 K; Fig. 1.) that are most susceptible to processes leading to instabilities that trigger the emergence of strong plasma, a neutral state of matter composed of equal number of positive and negative ions.

Our theoretical work proposes a method of analysing the ionization and magnetic coupling state of objects with ultra-cool atmospheres. Our particular interest focuses on late M-dwarfs, brown dwarfs and giant gas planets.

Figure1. M-dwarfs, brown dwarfs and giant gas planets in comparison. Teide 1 is an example for a late M-dwarf, GD 165B for a cloud-forming brown dwarf of spectral type L, Gliese 229B is a cooler cloud-forming brown dwarf of spectral class T, and Jupiter is the example for a giant gas plane.

Figure 1. M-dwarfs, brown dwarfs and giant gas planets in comparison. Teide 1 is an example for a late M-dwarf, GD 165B for a cloud-forming brown dwarf of spectral type L, Gliese 229B is a cooler cloud-forming brown dwarf of spectral class T, and Jupiter is the example for a giant gas plane.

To determine the fraction of atmosphere that can be ionized, first Rodriguez-Barrera et al. consider thermal ionization only. Thermal ionization results from collisions between the gas particles according to the local gas temperature, therefore here we do not consider external ionizing affects from companions. Sources of possible irradiation are the host star in the case of planets, and a white dwarf in the case of a white dwarf – brown dwarf binary (for example WD0137-349B, Casewell et al. 2015). Such external affects can be later compared to the results of our reference study. We use the Drift-Phoenix model atmosphere grid where the local atmospheric structure is determined by the following global parameters: Teff (effective temperature), log(g) (surface gravity) and [M/H] (metallicity).

Rodriguez-Barrera et al. show that ultra-cool atmospheres with high Teff , or with high metallicity and low log(g) have large fraction of atmospheric volume where plasma processes occur, and so they are the best candidates for radio, X-ray and Hα emissions, observed from various objects as was mentioned above.

Figure 2. The volume fraction of the atmosphere that is thermally ionized, V^th_gas/V_atm, for f_e>10-7 and for M-dwarf, brown dwarf and gas giant planet atmospheres. f_e measures the extent to which a gas is ionized. (Rodriguez-Barrera et al. 2015)

Figure 2. The volume fraction of the atmosphere that is thermally ionized, V^th_gas/V_atm, for f_e>10-7 and for M-dwarf, brown dwarf and gas giant planet atmospheres. f_e measures the extent to which a gas is ionized. (Rodriguez-Barrera et al. 2015)

M-dwarfs have a considerable degree of ionization throughout the whole atmosphere, the degree of thermal ionization for a L-dwarf is low but high enough to seed other local ionization processes like Alfven ionization (see Stark et al. 2013) or electrostatic discharges, such as lightning, as seen on Fig. 2.

We show that the first criterion to form chromospheres, ionospheres or an aurora on an extrasolar planets or brown dwarf, namely a small but sufficient degree of ionization, can be fulfilled by thermal ionization alone without the need for additional processes. But is it possible to magnetise this ionized plasma? The second part of our study says yes! The results also give an idea of how well the different types of atmospheres can be magnetized (Fig. 3): The minimum threshold for the magnetic flux density required for electrons and ions to be magnetised is smaller than typical values of the global magnetic field strengths of a brown dwarf and a giant gas planet. This means the ionized plasma inside the atmosphere can be magnetised quite easily. A considerably lower magnetic flux density is required for magnetic coupling of the atmosphere in the rarefied upper atmosphere than in the dense inner atmosphere, meaning magnetising the plasma in the upper atmosphere is easier than in the inner parts of the atmosphere. The magnetic coupling works equally for electrons and atomic ions like Mg+ and Fe+ (Fig. 3).

Figure 3. The magnetic flux density required for electrons, Be (lower set of lines), and ions, Bi (upper set of lines), to be magnetically coupled to a background magnetic field in the object (B=10 G - giant gas planets (GP), B=103 G M-dwarfs (MD), brown dwarfs (BD); black horizontal/vertical lines). If B>Bi (or B>Be) the gas is magnetized by the external magnetic field B. (Rodriguez-Barrera et al. 2015).

Figure 3. The magnetic flux density required for electrons, Be (lower set of lines), and ions, Bi (upper set of lines), to be magnetically coupled to a background magnetic field in the object (B=10 G – giant gas planets (GP), B=103 G M-dwarfs (MD), brown dwarfs (BD); black horizontal/vertical lines). If B>Bi (or B>Be) the gas is magnetized by the external magnetic field B. (Rodriguez-Barrera et al. 2015).

To sum it up, our results suggest that it is not unreasonable to expect ultra-cool atmospheres (M-dwarfs and L & T brown dwarfs) to emit Hα or even in radio wavelength. We showed that, in particular, the rarefied upper parts of the atmospheres fulfil quite easily the plasma criteria despite having low degrees of ionization also in the case of giant gas planets. Therefore the results suggest that an ionosphere may emerge also in brown dwarf and giant gas planet atmospheres, and that the built-up of a chromosphere on brown dwarfs is likely too. Both effects will contribute to atmospheric weather features and to space weather occurrence in extrasolar, planet-like objects. An interesting result is that ultra-cool atmospheres could also drive auroral emission without the need for a companion’s wind (e.g. aurora on Earth triggered by solar wind) or an outgassing moon (e.g. aurora on Jupiter is triggered by its outgassing moon Io).

fig4

Figure 4. The dominating thermal electron donors for a subset of effective temperatures for log(g)=3,0 and solar element abundances, against the local gas pressure (Rodriguez-Barrera et al. 2015).

We further studied which of the gas species might be the best electron donors. Na+, K+ and Ca+ are the dominating electron donors in low-density atmospheres (low log(g), solar metallicity) independent of Teff. Mg+ and Fe+ dominate the thermal ionization in the inner parts of M-dwarf atmospheres. Molecules remain unimportant for thermal ionization. Chemical processes (e.g. cloud formation, cosmic ray ionization) that affect the abundances of Na, K, Mg, Ca and Fe will have a direct impact on the state of ionization in ultra-cool atmospheres.

For more details check out the original paper on ADS:

 Rodríguez-Barrera, I.; Helling, Ch; Stark, C. R. and Rice, A. M. 2015, MNRAS 454, 3977

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