By Nils Feldmann, Project Coordinator - Science Outreach & Diplomacy
Having been captivated by the idea of distant worlds – and potentially even life – existing somewhere beyond our solar system for millennia, humankind currently finds itself in a very exciting time for exoplanet research.
Since the discovery of the first planets beyond our Solar System, also known as “exoplanets”, in 1992 by Aleksander Wolszczan and Dale Frail, as well as the seminal discovery of “51 Pegasi b” – the first confirmed exoplanet to be in orbit around a Sun-like star – by Swiss astronomers Michel Mayor and Didier Queloz in 1995, researchers have detected thousands of these extrasolar worlds, thereby not only confirming their existence beyond any doubt, but also revealing a plethora of fascinating insights regarding the properties of individual planets, their atmospheres, and the planetary systems in which they are located.
Driven by increasingly powerful ground- and space-based telescopes, as well as sophisticated observational techniques, the field has subsequently grown considerably over the past two decades, and is currently transitioning from an era of discovery to one of physical and chemical characterization.
In this context, Swissnex in China had the pleasure of welcoming four distinguished experts from the National Center of Competence in Research (NCCR) PlanetS, the Chinese Society of Space Research (CSSR), and Sun Yat-sen University during the fourth edition of its “nexFrontier” webinar series, which was held on 25 November 2021, to provide precious first-hand insight into a few of the latest developments of this fascinating, and rapidly evolving, area of research.
Space Instrumentation Development Within the PlanetS Domains
Professor Nicolas Thomas, Professor of Experimental Physics at the University of Bern, Principal Investigator of the Color and Stereo Surface Imaging System (CaSSIS), and member of the NCCR PlanetS, kicked off the webinar with a short overview of the development of space instrumentation within the exoplanet domain, and within planetary sciences in general.
Beginning with the observation that the “combination of ground-based observation, numerical modelling, and space-borne investigations has completely changed our perception of planetary formation and evolution within the past quarter of a century,” Prof. Thomas contends that there are two factors, which were crucial to the discovery of the first exoplanet:
1. Having the foresight to appreciate that stars could be influenced by the planets moving around them; and,
2. Developing the instrumentation to observe the motion induced by these planets.
With the latter evidently continuing to constitute a vital element of exoplanetary research today, Prof. Thomas proceeded to provide a few examples that illustrate why Switzerland, via the NCCR PlanetS, continues to be at the forefront of this expanding field.
Instrumentation for Extra-Solar Objects:
CHaracterising ExOPlanets Satellite: “CHEOPS”
Launched in December 2019, CHEOPS is not only special because it constitutes the first Swiss-led mission of the European Space Agency (ESA), but it is also the first space mission dedicated to studying known exoplanets. This is significant because by knowing exactly when and where to point the satellite to catch an exoplanet as it transits – passes in front of – its host star, scientists can obtain extreme-precision brightness measurements to observe highly precise transit light curves, from which an exoplanet's radius can be deduced. These measurements can in turn be combined with mass determinations made from previous ground-based spectroscopic surveys to enable researchers to accurately calculate a given planet’s density, and, by extension, to constrain its composition.
To learn more about CHEOPS, click here.
PLAnetary Transits and Oscillations of stars: “PLATO”
Scheduled to launch in 2026, this DLR (German Aerospace Center)-led ESA mission in particular aims to detect and characterize Earth-like rocky extrasolar planets that orbit in what is known as the “habitable zone” (a zone where liquid water can exist on the surface of a celestial object) around sun-like stars. The University of Bern is in turn supporting this mission by producing the flight model hardware.
To learn more about PLATO, click here.
Instrumentation for Solar System Objects
In addition to the aforementioned extra-solar missions, Prof. Thomas explained that the NCCR PlanetS is also deeply involved in the development of instruments to study solar system objects, as illustrated by their involvement in the “BepiColombo” and “Comet Interceptor” missions.
BepiColombo
Launched in October 2018, this joint ESA-JAXA (Japan Aerospace Exploration Agency) mission aims to study Mercury, Mercury is a particularly interesting object of study, because as an end-member in our own solar system, it can, in some ways, help researchers test some of the models of exoplanet formation and evolution. This is also in part because Mercury possesses the best surface properties of any terrestrial planet in our Solar System, as well as has an unusual magnetic field and geophysics, and, in the process, attempts to test theories that were developed by the current leader of the NCCR PlanetS, Prof. Dr. Willy Benz. For this, the University of Bern is contributing two experiments to the mission, including the most complex one: “BELA”, the first European laser altimeter built for interplanetary flight.
Mercury in turn constitutes a particularly interesting object of study, because as an end-member in our own solar system, it can, in some ways, help researchers to test some of the models of exoplanet formation and evolution. This is also in part because Mercury possesses the best surface properties of any terrestrial planet in our solar system, as well as has an unusual magnetic field and geophysics.
To learn more about BepiColombo, click here.
Comet Interceptor
In a similar vein, comets are also considered to be valuable objects of study to better understand planetary formation processes, as they are thought to have an interrelated origin with planets, and are therefore often considered to be leftover planetesimals, which are known as the “building blocks of planets”. To help drive this area of research forward, Switzerland is leading the development of the spacecraft’s main imaging system: “CoCa”.
To learn more about the Comet Interceptor, click here.
What to Expect in the Next 25 Years
Finally, Prof. Thomas concluded by providing a brief glimpse of what to expect in the far future with the envisioned launch of the “Large Interferometer for Exoplanets” (Life) telescope, which, as part of ESA’s Voyage 2050 framework, will aim to “characterize terrestrial exoplanet atmospheres and search for life outside the solar system.”
To learn more about the LIFE mission, click here.
Habitable Exoplanet Search by Astrometry
Following the opening speech by Prof. Thomas, Prof. Dr. Ji Wu, President of the Chinese Society of Space Research (CSSR) and Former Director-General of the National Space Science Center (NSSC) of the Chinese Academy of Sciences (CAS) provided a brief introduction into the main methods used to look for exoplanets, with a particular emphasis on the search for habitable exoplanets.
Beginning with a short definition of a few key concepts, Prof. Wu underlined that in order for a planet to be considered as “habitable”, three main criteria need to be fulfilled:
1. It must be a terrestrial (i.e., not gaseous), “Earth-like” (in size) planet
▪ If it is too small, it will not be able to hold an atmosphere, due to the lack of gravity
▪ If it is too big, only small forms of life will be able to live on it, due to the strong gravitational forces
2. It must be located in the “habitable zone”
▪ I.e., liquid water can exist on its surface
3. It must be orbiting a Sun-type star
To find these Earth-like exoplanets, researchers have a variety of methods at their disposal. Nonetheless, as explained by Prof. Wu, most discoveries have been made with one of the following two approaches:
1. Radial velocity method
▪ Aims to detect subtle changes in the radial (line-of-sight) velocity of a star, which are caused by the fact that a planet’s orbit offsets the planetary system’s center of gravity, thereby causing its host star to "wobble" back and forth.
▪ This method can also be used to determine a planet’s mass.
2. Transit method
▪ Aims to detect the periodic dip in stellar light caused by a planet passing in front of – “transiting” – the face of its host star.
▪ This method can also be used to determine a planet’s radius.
However, despite the fact that there are an estimated (according to the so-called “Drake Equation” ) 2 billion habitable Earth-like planets in our universe, and between 50 and 1,000 in our galaxy alone, researchers have, until now, only been able to find massive, Jupiter-like planets, or planets with extremely short periods (less than 100 days) – both of which would not be able to sustain humans (the gravitational forces on massive, Jupiter-like planet would be far too strong. Short periods on the other hand imply that a planet is orbiting extremely close to its host star, and is therefore too hot).
According to Prof. Wu, this can be explained by the fact that there is currently no suitable method for discovering Earth-like exoplanets, because although the transit or radial velocity methods excel at detecting so-called “edge-on” planets – i.e., planets whose elliptical plane is facing in the direction of the observer – they cannot be used to discover exoplanets orbiting in any other constellation, such as “face-on”.
To circumvent this limitation, Prof. Wu and his team are therefore turning to a different approach, known as “astrometry”. This method consists of trying to “measure tiny changes in the star's position as it wobbles around the center of mass of the planetary system,” thus making it possible to not only detect “edge-on” exoplanets but also “face-on” ones, as well as those with other inclination angles. However, until now, researchers have struggled to achieve the high level of precision needed for this method, and as a result, only one exoplanet has been discovered using this method so far.
According to Prof. Wu, this is bound to change in the near future however, thanks to the planned launch of the China National Space Administration’s (CNSA) “Closeby Habitable Exoplanet Survey” (CHES) mission, which aims to discover Earth-like planets orbiting nearby stars via high-precision astrometry.
Strange New Worlds: Planets Beyond the Solar System
Following this in-depth explanation of astrometry by Prof. Wu, Prof. Dr. Monika Lendl, Astrophysicist and Assistant Professor at the University of Geneva, as well as a member of the NCCR PlanetS, continued the webinar by providing additional information regarding the discovery and characterization of extra-solar planets, as well by presenting a few noteworthy results that they recently obtained from the CHEOPS mission.
Taking a step back, Prof. Lendl began by providing a helpful analogy to illustrate just how difficult it is to detect an exoplanet. Specifically, according to Prof. Lendl, when trying to detect a large, Jupiter-sized planet, one can imagine being located in Geneva and attempting to detect a candle, or a firefly, that is sitting next to a bright lighthouse on the other side of Switzerland, near the Bodensee (approximately 300 km away).
Despite the challenges associated with this task, astronomers have not only been able to detect over 4,000 exoplanets so far but, by combining various techniques, particularly the radial velocity- and transit methods, they are also able to significantly constrain their composition.
As previously mentioned, the characterization of these newly discovered worlds currently constitutes a key focus for exoplanet research. This in turn requires, the ability to measure precise radii, which is exactly what the CHEOPS space mission was designed for. To illustrate what this means in reality, Prof. Lendl subsequently presented the following research highlights:
ν² Lupi
While studying two small exoplanets – “ν² Lupi b” and “ν² Lupi c” – orbiting the naked-eyed star “ν² Lupi”, the CHEOPS mission was not only able very precisely measure their radii, but thus enable researchers to subsequently characterize their composition, but it also unexpectedly spotted a third planet – dubbed “ν² Lupi b” – transiting ν² Lupi. Even more excitingly, this newly discovered exoplanet has a period of over 100 days, thereby making it the first long-period low-mass planet known to transit a naked-eye star.
To learn more about ν² Lupi, click here.
TOI-178
While studying a curious planetary system known as “TOI-178”, which, according to initial observations made by NASA’s “Transiting Exoplanets Survey Satellite” (TESS) mission, hosted three transiting planets, CHEOPS revealed that TOI-178 in fact harbors at least 6 exoplanets with extremely short periods. Not only that, but the period ratios of the outer 5 planets follow an 18:9:6:4:3 pattern, thereby forming a unique resonant chain of planetary orbits. Their sizes and masses do not follow such an orderly pattern, however, thereby challenging current planet formation theories.
To learn more about TOI-178, click here.
WASP-189b
Prof. Lendl subsequently introduced a slightly different type of science that they have been doing with CHEOPS, which relates to the characterization of the planetary atmosphere. Specifically, when studying WASP-189b, the team conducted their observations when the planet passed behind its host star, thereby enabling them to deduce the planet’s temperature, based on the amount of light emitted by the planet that was blocked during that time.
To learn more about WASP-189b, click here.
To conclude, Prof. Lendl provided a brief overview of what is to come in exoplanetary research, as scientists move from studying hot planets and gas giants to attempting to discover extrasolar terrestrial planets in the habitable zone around sun-like stars with ESA’s “PLAnetary Transits and Oscillations of stars” (PLATO) mission, to study the atmospheres of small, cool planets with ESA’s “Atmospheric Remote-sensing Infrared Exoplanet Large” (Ariel) survey, and, finally, to study the atmospheres of terrestrial planets with the next generation of space telescopes.
A Brief History of Planetary Systems
To close the webinar, Prof. Dr. Shang-Fei Liu, Associate Professor at the School of Physics and Astronomy and Chinese Space Station Telescope Center for the Guangdong-Hong Kong-Macau Greater Bay Area of Sun Yat-sen University, provided an insightful overview of the formation and evolution of planetary systems.
Beginning with a short historical perspective, Prof. Liu explained that the most widely accepted scenario that explains the formation and evolution of our solar system was initially proposed in the 18th century by the German philosopher Immanuel Kant and is known as the so-called “nebular hypothesis”. According to this theory, the solar system was formed from a spinning disk of dust and gas – also known as a “protoplanetary disk” – in which the tiny grains of dust gradually grew in size to eventually form the cores of planets, or, in the case of terrestrial planets, the planets themselves.
To test this model, researchers subsequently used powerful ground-based and space-based instruments to look for and observe planets around young stars, as well as sophisticated computer models to better understand the physics behind the planetary formation. Interestingly, these modern methods have largely confirmed Kant’s original proposition.
In this context, determining the interior composition of planets is a particularly important area of research, as this provides scientists with vital clues to the history of how these celestial bodies were formed. As such, there is a considerable emphasis on studying the planets within our solar system, because unlike with extrasolar planets, we can send spacecraft to orbit these planets, thereby enabling scientists to, for example, measure their gravitational field. This data is valuable, as it can subsequently be used to interpret their interior structure.
These measurements can in turn also lead to big changes in our understanding of planetary formation. One example of this was the discovery that, contrary to the standard theory, which assumed that Jupiter possessed a tiny, metallic core, gravity measurements gathered by NASA’s Juno mission suggested that it instead has a diluted, “fuzzy” core. To explain this unexpected discovery, Prof. Liu, together with colleagues from the NCCR PlanetS, proposed that Jupiter’s formation may have been disrupted by a head-on collision with an impactor 10 times the mass of Earth. This question however remains open to this day.
Finally, Prof. Liu concluded with a brief overview of the current efforts to better understand the formation of white dwarfs. This is an important topic because over 95% of the stars in our solar system will eventually turn into white dwarf at the end of their nuclear burning stage. In this context, researchers are particularly interested in the mechanism by which white dwarfs accrete planetary material from the surrounding planetary system, as well as in determining the composition of this material.
Helpful links
Webinar Recording: click here.
Information on exoplanet science provided by ESA: