Understanding the Diversity of Planetary Atmospheres

Volume 81 in the Space Sciences Series of ISSI

Thanks to the observation of a growing number of planetary atmospheres, we are at the dawn of a major scientific revolution in atmospheric and climate sciences. But are we ready to understand what will be discovered around other stars? 

This book brings together 15 review chapters that study and provide up-to-date information on the physical and chemical processes that control the nature of atmospheres. It identifies commonalities between various solar system atmospheres, analyzes the dynamic processes behind different atmospheric circulation regimes, and outlines key questions remaining in solar system science.

This volume results from a Workshop organized at ISSI, on November 12-16, 2018, with the support of the Europlanet Research Infrastructure of the EU.

This volume is dedicated to the memory of Adam P. Showman, a creative thinker, brilliant scientist, pioneer and leader in the study of the diversity and dynamics of planetary atmospheres.

This book is edited by F. Forget, O. Korablev, J. Venturini, T. Imamura, H. Lammer, M. Blanc

This volume is co-published in Space Science Reviews in the Topical Collection “Understanding the Diversity of Planetary Atmospheres” (Partial Open Access) >>

Hard Cover Book >>

Reading Terrestrial Planet Evolution in Isotopes and Element Measurements

Volume 80 in the Space Sciences Series of ISSI

This volume takes an interdisciplinary approach to the evolution of terrestrial planets, addressing the topic from the perspectives of planetary sciences, geochemistry, geophysics and biology, and solar and astrophysics.
The review papers analyze the chemical, isotopic and elemental evolution of the early Solar System, with specific emphasis on Venus, Earth, and Mars. They discuss how these factors contribute to our understanding of accretion timescales, volatile delivery, the origin of the Moon and the evolution of atmospheres and water inventories of terrestrial planets. Also explored are plate tectonic formation, the origin of nitrogen atmospheres and the prospects for exoplanet habitability.The papers are forward-looking as well, considering the importance of future space missions for understanding terrestrial planet evolution in the Solar System and beyond. Overall, this volume shall be useful for academic and professional audiences across a range of scientific disciplines.

This volume is based on an interdisciplinary international Workshop organised by Europlanet and ISSI, which took place at ISSI, in Bern (Switzerland) during October 22 and 26, 2018 where about 48 leading scientists discussed the issues presented here.

The papers are edited by H. Lammer, B. Marty, A. Zerkle, M. Blanc, H. O’Neill, T. Kleine

This volume is co-published in Space Science Reviews in the Topical Collection “Reading Terrestrial Planet Evolution in Isotopes and Element Measurements” (Partial Open Access) >>

Hard Cover Book >>


The Tidal Disruption of Stars by Massive Black Holes

New Topical Collection published in Space Science Reviews (partial Open Access)

A new collection of 14 review articles has been completed and is available online in Space Science Reviews, a printed book version will be published as volume 79 of the Space Sciences Series of ISSI.

For several decades, astronomers have speculated that a hapless star could wander too close to a super massive black hole (SMBH) and be torn apart by tidal forces. It is only with the recent advent of numerous wide-field transient surveys that such events have been detected in the form of giant-amplitude, luminous flares of electromagnetic radiation from the centers of otherwise quiescent galaxies. These discoveries span the entire electromagnetic spectrum, from γ-rays through X-rays, ultra-violet, optical, infrared, and radio. A small number of events launch relativistic jets. These tidal disruption events (TDEs) have caused widespread excitement as they can be used to study the properties of quiescent, otherwise undetectable, SMBHs; the populations and dynamics of stars in galactic nuclei; the physics of black hole accretion including the potential to detect relativistic effects near the SMBH; and the physics of (radio) jet formation and evolution in a pristine environment. For scientific questions concerning quiescent SMBHs, TDEs are unique probes beyond the local universe. TDEs can also occur around active galactic nuclei (AGNs), although uniquely identifying such an event on top of a bright AGN is difficult.

Currently, the diverse emission properties of flares associated with TDEs is not fully understood. This challenge is being addressed by a sharp increase in observational work and theoretical modelling. Over the next few years, the largest growth areas will likely be in the greatly expanded surveys of the transient sky, and in new numerical modeling techniques. Together these will reveal how SMBHs shine by ripping apart orbiting stars and swallowing the stellar debris.

In light of this foreseen growth, many new researchers are expected to enter the field. Therefore, the time was deemed ripe to compose a comprehensive overview of the state of the art in this rapidly-evolving field. This topical collection was planned and launched at a workshop held at the International Space Science Institute (ISSI) in Bern on 8–12 October, 2018.

The editors would like to thank all authors and coordinators for their hard work in creating this collection, the staff at ISSI for their friendly and generous support, and the future readers of this topical collection who will no doubt answer many of the TDE puzzles that so far remain unresolved.

Peter G. Jonker, Iair Arcavi, E. Sterl Phinney, Elena M. Rossi, Nicholas C. Stone & Sjoert van Velzen (Guest Editors)


Introductory Article: 
Peter G. Jonker, Iair Arcavi, E. Sterl Phinney, Elena M. Rossi, Nicholas C. Stone & Sjoert van Velzen. Editorial to the Topical Collection: The Tidal Disruption of Stars by Massive Black Holes. Space Sci Rev 217, 62 (2021). https://doi.org/10.1007/s11214-021-00837-4

Complete Topical Collection in Space Science Reviews >>

Auroral Physics

New Topical Collection in Space Science Reviews (partial Open Access)

The new article collection on “Auroral Physics” is designed to provide a comprehensive review of our current scientific understanding of the terrestrial aurora, both observational and theoretical, with an emphasis on developments since a previous collection devoted to the terrestrial aurora, “Auroral Plasma Physics” (Space Science Reviews volume 103, 2002, by G. Paschmann, S. Haaland, and R. Treumann). That collection emphasizes introductory and background material which is not repeated in the current set. 

The term “aurora borealis”, or “northern dawn” dates back centuries and refers to emissions of light from the otherwise-dark nighttime atmosphere, usually occurring in the polar regions except during highly disturbed periods. Aurora is distinct from airglow, which is a weak and relatively unstructured emission in the thermosphere caused by chemical reactions and ionization driven primarily by solar UV illumination during the day. In contrast, auroral emissions are the result of excitation of neutral atoms and molecules in the upper atmosphere by collisions with charged particles, typically which originate in the magnetosphere and precipitate along geomagnetic field lines with energies of hundreds of eV to tens of keV.

Popular descriptions of the aurora including in the media, dictionaries, encyclopedias and even textbooks often claim that the aurora is caused by “particles from the sun striking the upper atmosphere”. It is well established in auroral science that such a description is not accurate except in very limited cases. While it is certainly true that particles from the sun – the solar wind – provide the energy that drives the aurora, the widely varying morphologies and behaviors of the aurora are the result of a complex chain of events that take place within Earth’s magnetosphere or at its boundary, the magnetopause. The end result of this chain – excitation of neutrals by charged particles – is also well-established fact, as are many other aspects described in this collection. However, due to the vast region of the magnetosphere that is magnetically conjugate to the auroral ionosphere, and the difficulty in sampling it with a single or even multiple spacecraft, the nature of the magnetospheric generator responsible for driving individual auroral forms still remains one of the most elusive aspects of the aurora.

Planning for this Topical Collection took place during a workshop hosted in August 2018 by the International Space Science Institute (ISSI), in Bern (Switzerland), and attended by more than 40 members of the international scientific community.

This Topical Collection will be reprinted as the Volume 78 in the Space Sciences Series of ISSI and is edited by David Knudsen, Joe Borovsky, Tomas Karlsson, Ryuho Kataoka and Noora Partmies.

Introductory article: 
David Knudsen, Joe Borovsky, Tomas Karlsson, Ryuho Kataoka and Noora Partmies. Editorial: Topical Collection on Auroral Physics, Space Sci Rev 217, 19 (2021). https://doi.org/10.1007/s11214-021-00798-8

Complete Topical Collection in Space Science Reviews >>


Understanding the Diversity of Planetary Atmospheres

New Topical Collection published in Space Science Reviews

Ten planetary atmospheres are currently studied in the solar system and many more will be characterized in the coming years as we remotely observe exoplanets. Are we ready to understand what we will discover around other stars? The examples of the solar system are probably not sufficient to let us anticipate the diversity of exo-atmospheres. To prepare this revolution, it is nevertheless very interesting to make the most of what we have learned so far, to identify commonalities between the different solar system atmospheres, and to make out the remaining key questions in our understanding of the known planetary atmospheres.

Toward these goals, a special article collection on “Understanding the Diversity of Planetary Atmospheres” has been published in the journal Space Science Reviews. It was prepared during a Workshop organized at ISSI, on November 12-16, 2018, with the support of the Europlanet Research Infrastructure of the EU. Nearly 40 scientists from Europe, USA, Russia, China, Japan, Israel and Australia attended this meeting, including planetary scientists, experts in the origins of atmospheres, planetary interior, aeronomy, escape, climatologists, and astronomers. The diversity of expertise proved to be very fruitful to discuss the diversity of atmospheres.

The 15 articles of this topical collection present rich, up-to-date views on many hot scientific questions relating to planetary atmospheres and the climate of exoplanets. By compiling this collection we hope to provide a solid reference for anyone willing to work towards understanding the diversity of planetary atmospheres. 

Find here the Editorial of the Topical Collection on Understanding the Diversity of Planetary Atmospheres (Open Access), Space Sci Rev 217, 51 (2021). https://doi.org/10.1007/s11214-021-00820-z

This collection is dedicated to the memory of Adam P. Showman, a creative thinker, brilliant scientist, pioneer and leader in the study of the diversity and dynamics of planetary atmospheres.


François Forget, Oleg Korablev, Julia Venturini, Takeshi Imamura, Helmut Lammer & Michel Blanc (Guest Editors)

Find here the complete Topical Collection Understanding the Diversity of Planetary Atmospheres in Space Science Reviews >>

This Topical Collection will be reprinted as the Volume 81 in the Space Sciences Series of ISSI.

How Water Explains Missing Planets

by Arian Bastani, NCCR PlanetS, University of Bern

Space exploration telescopes have revealed that planets between the size of 1.3 and 2.4 Earth radii seem to be comparatively rare. Scientists under the lead of the International Space Science Institute and the National Centre of Competence PlanetS have found a remarkably simple explanation.

Since 1995, scientists have found over 4000 planets outside the boundaries of our solar system. Some very small, with as little as a third of Earth’s radius. Other very large, up to 20 times wider than our home planet. These extremes are rare, however. Most known planets measure between 1 and 4 times the radius of Earth.

Within this range of common planet sizes, two were found particularly often: Planets with 1.3 and 2.4 Earth radii. “Sizes between these two peaks are much less common and thus form the so-called radius-valley”, Julia Venturini, lead author and researcher at the International Space Science Institute explains. In a study published in the journal Astronomy and Astrophysics, she and her collaborators have now demonstrated why this could be the case.


Water or no water

“We found that it has to do with the formation of planets”, Venturini explains “namely the regions in which the planets form”. Previous studies had been able to account for the radius-valley, but only by limiting the formation of the planets to a specific region around their star. Within this region, no condensed water exists. Thus, such planets would be dry. But, as Venturini points out: “This is at odds with planet formation theory. Planets form very easily beyond the ice line (the cold region of around the star beyond which water condenses), accrete plenty of water, and then typically migrate inwards, ending up closer to the star”.

An illustration of the planet formation within and beyond the ice-line. The planets that form further out grow more massive due to the accumulation of larger icy pebbles. After formation, the planets move closer to the star. Credits: Julia Venturini


The solution that Venturini and her colleagues came up with imposes no such limitations on the planets’ formation location. “We found that planets which form only out of dry rocky material stay much smaller than ones that also accumulate ice as they grow”, she explains. “This has to do with the different collisional properties of rocks and ice”. Using computer models, they could reproduce the radius-valley based on these distinct formation regions, separated by the so-called ice-line. Thus, the first common planet size of around 1.3 Earth radii comes from dry terrestrial planets and the second group around 2.4 Earth radii mostly consists of water-rich worlds.


Sketch of the two peaks of common planet sizes with dry and water-rich planets, as well as the radius-valley between them, as computed by Venturini et al. (2020). Credits: Julia Venturini


Time and new telescopes will tell

“These results could help us with preliminary characterizations of planets beyond our solar system”, Venturini hopes. But they first have to be confirmed. With the development of ever more sophisticated telescopes, such as the planned Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) of ESA, the compositions of faraway planets could be revealed in more detail and would thus allow to test the results of Venturi and her colleagues.

Julia Venturini is a postdoctoral research fellow at the International Space Science Institute (ISSI)

Reference: Julia Venturini, Octavio M. Guilera, Jonas Haldemann, Mari­a P. Ronco, Christoph Mordasini: The nature of the radius valley: Hints from formation and evolution models, A&A 643, L1 (2020) https://www.aanda.org/10.1051/0004-6361/202039141

Press Release NCCR Planet S (28 October 2020)

Contact: Dr. Julia Venturini, International Space Science Institute, Telephone + 41 31 631 48 86
Email: Julia.Venturini@issibern.ch

3He-rich Solar Energetic Particles Observations at Parker Solar Probe

Report from ISSI Team #425 Origins of 3He-Rich Solar Energetic Particles led by R. Bucik and J.F. Drake 

Left: Mass histogram for 3He-rich SEP event on 2019 April 20 shows small but clear 3He peak. Right: Jet observations at the west limb from the SDO/AIA. Adapted from Wiedenbeck et al. (2020).

3He-rich solar energetic particles (SEPs) are one of the most peculiar and least explored particle populations in the heliosphere with a tremendously enhanced abundance of the 3He nuclide and ultra-heavy elements (e.g., Pb) by a factor up to 104 above the solar corona or solar wind. One reason for the current lack of understanding of 3He-rich SEPs is the small size of these events. Recently launched Parker Solar Probe (PSP) is able to approach the solar sources of 3He rich SEPs at distances (~0.05 au; 1 au ~ 1.5⨉108 km) that have never been reached before. On 2019 April 20-21, the IS⊙IS energetic particle suite on PSP made its first observations of 3He-rich SEPs. 3He-rich SEPs were observed at energies near 1 MeV/nuc in association with energetic protons, heavy ions, and electrons. At the time of 3He-rich SEP observations, the spacecraft was near 0.46 au. The event was also detected by ULEIS and EPAM on Advanced Composition Explorer (ACE) spacecraft, located near Earth, at 0.99 au from the Sun. The average intensity at ~ 1 MeV/nuc was a factor ~4 greater at PSP than at ACE, which might be attributable to a 1/r2 dependence of the fluence, where r is a distance from the Sun. At that time, PSP and ACE were both magnetically connected to a location near the west limb of the Sun. Remote sensing measurements showed the presence of a type III radio burst and also a helical unwinding jet from this region of the Sun. This activity, which is commonly associated with 3He-rich SEP acceleration on the Sun, originated from the active region number AR 12738. We also searched for smaller 3He-rich SEP events that are not observable near the Earth but might have been detectable closer to the Sun because of the expected strong radial dependence of the intensities of SEP events impulsively released from localized sources. Although no such events were detected during the first two orbits of PSP, this search will be continuing as PSP moves progressively closer to the Sun, and as solar activity increases. These observations should enable IS⊙IS to make significant progress in understanding small 3He-rich SEP events.


Animation of the Jet observations at the west limb from the SDO/AIA. Adapted from Wiedenbeck et al. (2020).



Wiedenbeck M.E., Bucik R., Mason G.M., Ho G.C., Leske R.A. et al., 3He-rich Solar Energetic Particle Observations at Parker Solar Probe and Near Earth, Astrophys. J. Suppl. Ser. 246, 42, 2020.

Saturn’s Huge Moon Titan Drifting Away Faster Than Previously Thought

Report from ISSI Team #411 The ENCELADE Team: Constraining the Dynamical Timescale and Internal Processes of the Saturn and Jupiter Systems from Astrometry led by V. Lainey

Did you know that the Earth’s moon distance is increasing at about 3.8 cm/year because of the tides the Moon raises on our own planet? Thanks to Newton and his law of gravity, entailed the proper explanation of tides: the side of the Earth that is closer to the Moon is more attracted than its opposite side. As a consequence, the Earth takes an elongated shape like a football. Distance increase comes then as a consequence of friction inside the oceans essentially. Both shape distortion and orbital variation rate are expected to get significantly lower with distance, since tides are a consequence of gravitation.

A giant of a moon appears before a giant of a planet undergoing seasonal changes in this natural color view of Titan and Saturn from NASA’s Cassini spacecraft. (Image Credit: NASA/JPL-Caltech/Space Science Institute)

Since tidal theory is universal, researchers have applied it over the last 50 years to predict the orbital evolution of many moons. From the evolution of the four big Galilean satellites of Jupiter to the small moons of Mars, Phobos and Deimos, the same theory was used underneath. Recently and in the context of the Cassini mission, the ISSI ENCELADE Team led by Valery Lainey in collaboration with a team from the University of Bologna tried to quantify from observations the orbital expansion of Titan, the largest Saturnian moon, under Saturn’s tides. Surprisingly, they found that Titan is escaping Saturn’s gravity at a large pace of 11 cm/year, more than a hundred times faster than expected from theoretical models. Even more surprising, such expansion rate is larger than for moons closer to Saturn, in complete contradiction with classical tidal theory! But the study demonstrates a perfect agreement with the prediction of a new tidal mechanism, suggested only four years ago by Jim Fuller (Caltech) and co-authors. Such so-called “tidal lock mechanism” suggests that Titan may have formed way closer to Saturn, than commonly believed. This result brings an important new piece of the puzzle for the highly debated question of the age of the Saturnian system.

Like the classical tidal theory for terrestrial objects, tidal lock mechanism is a universal physical mechanism for giant planets. In principle, it could be at play in way other systems were giant planets are involved, starting with the Jupiter system itself.

More Information can be found here: News Release June 8, 2020, NASA JPL Caltech >>

Lainey, V., Casajus, L.G., Fuller, J. et al. Resonance locking in giant planets indicated by the rapid orbital expansion of Titan, Nature Astronomy, 2020. https://doi.org/10.1038/s41550-020-1120-5