Great enthusiasm for small stars
TRAPPIST-1 is the name of the star around which seven planets and many current research projects revolve. Cool dwarf stars are the new favourites when searching for earth-like, life-friendly planets out in space. PlanetS teams are among the world leaders in this field of research.
Brice-Olivier Demory is enthusiastic, “with our expertise here at the University of Bern we can investigate aspects of the TRAPPIST-1 system which few other research groups in Europe are able to.” The professor at the Center for Space and Habitability (CSH) is a co-discoverer of the planetary system 40 light years away. Actually, what astronomers really want to discover is a planet identical to earth, orbiting a star like the sun at the same distance as our planet on which indications of life can be identified. But this is not possible with today’s instruments, so an alternative is small stars like TRAPPIST-1, where it is easier to observe small planets similar to the earth.
“The TRAPPIST-1 planets are currently our best candidates for Earth-like, life-friendly conditions outside our solar system,” says Brice-Olivier Demory. The star itself is about ten times smaller than the sun and its surface temperature is much lower, so that is why it is considered an ultra-cool red dwarf. The seven earth-sized planets orbiting it were discovered because, as we see them, they pass right in front of the star and slightly obscure it. These so-called transits indicate the existence and size of the planets. Since the star radiates only weakly, the planets have a moderate surface temperature, although they are very close to the star and can thus be observed despite their small size.
The journey of discovery began in April 2007. Brice-Olivier Demory, who did his doctorate at the University of Geneva, travelled to the Saint-Luc Observatory above Nendaz together with Michaël Gillon, a Belgian postdoc. “We ate a fondue, had a discussion and had the telescope set to automatically observe the red dwarf Gliese 436,” Demory recalls. “When we went to the computer at the end of the night, we saw a transit in the data”. Further investigations showed that the object is only about the size of Neptune – the smallest planet that had previously been observed using the transit method. The star Gliese 436 is about half the size of the sun.
The success in the Valais mountains motivated Michaël Gillon to look at even smaller stars in the search for even smaller planets. With a telescope specially built for this purpose called TRAPPIST located in Chile, a team under his leadership discovered three planets the size of the earth orbiting a red dwarf. The star was named TRAPPIST-1. NASA’s Spitzer space telescope located four more planets. “Before TRAPPIST-1, nobody believed that this kind of planet could be found among such stars,” says Brice-Olivier Demory: “We have opened a new door for exoplanet research.” The researchers are convinced that they will soon find many more Earth-like planets located near ultra-cool stars.
New telescope in Mexico
Under the direction of Brice-Olivier Demory, a new telescope in Mexico will search for similar objects in the northern hemisphere sky this autumn. The 1.3 million Swiss franc project was financed by the CSH of the University of Bern, the National Centre of Competence in Research (NCCR) PlanetS and the Universities of Cambridge and Geneva, and is operated in cooperation with the Mexican “Universidad Nacional Autónoma”. The name of the project is “Saint-Ex” which officially stands for “Search and chAracterIsatioN of Transiting Exoplanets”. As an avid aviator, Demory is above all reminiscent of the writer and pilot Antoine de Saint-Exupéry. The Saint-Ex team at CSH developed the software for the scientific operation of the automatic telescope, which can be controlled via remote control from the University of Bern.
Another exoplanet project will benefit from the observations of the Saint-Ex telescope. The CHEOPS space telescope has been developed and assembled at the University of Berne over the last five years. CHEOPS (Characterising Exoplanet Satellite) will also observe transits. The launch of the mission under the joint leadership of Switzerland and the European Space Agency (ESA) is scheduled for 2019. “Saint-Ex can pre-select CHEOPS targets to ensure that we use the satellite sensibly and don’t waste time on things we can do from the ground,” explains Brice-Oliver Demory.
CHEOPS will be used to observe stars which are known to be orbited by exoplanets. The details that can be ascertained by means of transits are astonishing. This is shown by the investigations of the TRAPPIST-1 system. For example, Simon Grimm of CSH succeeded in calculating the masses of the seven planets. Although the size of a planet can be derived relatively directly from the decrease in star brightness during its transit, the determination of mass is much more complicated. Theoretically, it can be calculated from orbit deviations, because the planets interfere with each other. In practice, however, this seemed impossible for such a diverse and compact planetary system as TRAPPIST-1 until Simon Grimm set to work with a computer code, which he developed himself. While the codes of other research teams proved to be too slow, the CSH scientist was able to solve the 35-dimensional problem and determine the mass of the TRAPPIST-1 planets with an accuracy of up to 10 percent.
“If you know how large and how heavy the planets are, you can calculate their density,” explains Simon Grimm. The team discovered that all TRAPPIST-1 planets consist mainly of rock. The fourth planet is most similar to Earth in terms of size, density and amount of radiation it receives from its star. There could also be water in liquid form on its surface, which is necessary for life as we know it.
Lively discussion among theorists
According to calculations, some of the TRAPPIST-1 planets contain up to five percent water – a result that is currently causing more than a little distress among the theorists at CSH and NCCR PlanetS. “In our computer simulations, the planets orbiting around such cool dwarf stars have either no water at all or a much higher water content of 20 to 30 percent,” says Yann Alibert, Astrophysics professor at the University of Bern. “However, we are able to reproduce other properties of the system, such as the size of the planets, in the model with little trouble.” At one of their regular meetings, Alibert and four other scientists from CSH and NCCR PlanetS met in a conference room – one researcher has come from Zurich, while another colleague from Heidelberg participates in the meeting via Skype. Using coloured graphics, the astrophysicists show how planets are formed in their models from a disk of gas and dust in the first millions of years. “Everyone tries to explain how the TRAPPIST-1 system works with a slightly different approach,” says Yann Alibert. Now the group wants to work out what consequences the different scenarios have and how they can be tested.
The researchers are particularly interested in the atmosphere of the exoplanets as this is where researchers are likely to find traces of life. Kevin Heng, Director of CSH, is a specialist in exoplanetary atmospheres and has received a grant of two million euros from the European Research Council for a new project. “The study revolves around small planets orbiting small stars and is therefore called EXOCLEIN,” explains Kevin Heng. “If you find many more such exoplanets, the question naturally arises as to whether they are habitable, which is by no means obvious.” Even if the planets resemble the size and mass of the Earth, their parent stars are clearly different from the Sun. Red dwarfs are not only smaller and cooler, they are also more active in the ultraviolet range. “There are some important differences between Trappist 1 systems and our system,” explains Kevin Heng: “The goal of the EXOKLEIN project is to understand the effects of these differences on the planetary atmospheres and thus understand the entire system.” Only then can biosignatures be correctly detected in the atmosphere.
With the James Webb space telescope, which is to be launched in 2021 as the successor of the Hubble space telescope, it might be possible to observe the atmosphere of small planets orbiting red dwarf stars. “This will be difficult, but it cannot be ruled out,” says Brice-Olivier Demory. But for the time being, he is looking forward to the forthcoming commissioning of the telescope in Mexico and the CHEOPS launch. (bva)
This article was published in May 2018 in the UniPress magazine of the University of Bern.