The ULg is the first francophone Belgian university to equip itself with a robotic telescope for astronomical observations. Named TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope), it has been installed in Chile, on the mountain of Silla, in the arid inlands of the Atacama desert. A territory which is hardly hospitable for life forms other than astronomers, who can observe the starry skies in perfect conditions almost throughout the whole year. The ESO (European Southern Observatory) has moreover installed its living quarters there since 1969.
© ULg E. Jehin
TRAPPIST will help the ULg’s astrophysicists to better understand the formation and evolution of planetary systems, and in two ways: by searching for and studying planets located beyond our solar system (exoplanets) on the one hand, and on the other by observing the comets which gravitate around the Sun.
With a diameter of 60cm, TRAPPIST has the advantage of being completely autonomous. It can thus be operated from Liège, which enables travel costs and on site living expenses to be avoided. Moreover, as the telescope belongs to the ULg, the astrophysicists will be able to carry out their research throughout the year in a domain in which are habitually required fierce negotiations concerning equipment operating time, argued over by numerous international teams. The project’s financing has principally been assured by the F.R.S.-FNRS, but also by the FNS (the Swiss National Scientific Research Fund).
Within the framework of its new research, the Liège team, lead by Professor Pierre Magain and the researchers Michaël Gillon et Emmanuël Jehin, has linked up a team of Swiss astronomers, amongst them Didier Queloz, a professor at the University of Geneva, who in 1996 discovered, together with his colleague Michel Mayor, the first planet gravitating around a star other than the Sun
The majority of professional observatories are the fruit of colossal investment, made possible through international collaboration. Astronomers must thus share the use of telescopes and can only benefit from them for time periods fixed in advance, sometimes hardly a few hours. Emmanuël Jehin is delighted to have available a telescope dedicated to a precise programme: ‘In the present case, we will be able to use the telescope for as much time as we desire, which is very important to successfully carry out our projects, which necessitate long observation times.’ ‘For this type of research,’ adds Pierre Magain, ‘it is important to have available a telescope which is ours to use exclusively. For example, for the comets, certain of them are known but a lot arrive without a warning cry. And there we have to be capable of observing them at the opportune moment. As for the exoplanets, we have to be able to observe them at the precise moment they are in transit, when they pass in front of their star. Such transits last several hours, even a wholenight.
Entirely robotised and autonomous, TRAPPIST will be capable of being operated from Liège and will not require much human presence at the site. ‘It even has its own weather station which will allow the cupola to be closed when the weather worsens,’ specifies Pierre Magain. The telescope is equipped with a CCD camera combined with a double filtered runner wheel, one to observe the planets, the other to observe the comets. It is enough to choose the filter in advance according to what will be observed. The camera then records the captured images, ‘which represents a considerable mass of observation,’ explains Michaël Gillon. ‘We will collect hundreds of images, or several gigabytes, each night.’ A pre-processing of the data will be carried out on site, before the main results are sent to Liège for the final analysis. The investment climbs to around 300,000 Euros and results from a partnership between the F.R.S.-FNRS, the ULg, and the University of Geneva.
© ULg E. Jehin
The project was born from an idea Michaël Gillon, which germinated during his post-doctoral research at the Geneva Observatory. The Swiss researchers very quickly showed an interest in the project. ‘When TRAPPIST took concrete shape they made available the building of their old telescope, with a dome of 5 metres in diameter. For this research we will work in total collaboration with them.’
This is fine recognition as the Geneva Observatory’s scientists are considered as the world leaders in this domain. And for good reason, as they discovered the first exoplanet in 1995. ‘They didn’t rest on their laurels,’ states Emmanuël Jehin. ‘They continued to polish their technologies and found other planets.’ There exist several techniques to detect them. The Swiss team mainly uses the measurement of the variation of radial velocity (see below). The Liège researchers for their part use the transit method. The convergence of the data harvested by the two methods allows a lot of information about the planets detected to be obtained. There will thus be a constant exchange of data, information, techniques and knowledge.
Situated in the Atacama desert at some 2400 metres above see level and 600 kilometres north of Santiaga, Silla at first site is not a very welcoming region. Nevertheless it brings happiness to astronomers. The climatic conditions offer around 300 clear nights a year, ‘compared with two nights in Belgium,’ rematks Michaël Gillon, with some irony. A godsend for scientists who have their heads in the stars. It is thus not by chance if the Atacama desert already houses several of the ESO’s telescopes and instruments, including the Very Large Telescope (VLT), the New Technology Telescope (NTT) and the HARPS spectrometer HARPS (High Accuracy Radial velocity Planet Searcher).
Photo: the ESO site at Silla. TRAPPIST is installed in the smallest domes in the centre of the image. © ESO
Since the end of the 1960s Silla is one of the world’s most important astronomical sites. Many new exoplanets are detected and studied there.
The two objectives of the TRAPPIST mission are integrated in the domain of astrobiology (or exobiology) and aim at a better understanding of one of the universe’s greatest mysteries, in other words the origin of life and its evolution (terrestrial as much as extraterrestrial). The study of comets and exoplanets are two very different paths to take in order to find the key.
We are at the beginning of the 1990s. For a very long time we have known the planets of our solar system and their positions. We are also aware that we are situated in one of the suburbs of our galaxy, itself a tiny speck of dust in a universe of unimaginable proportions, a universe of billions of other galaxies in which shine hundreds of billions of stars. And if it is completely conceivable to imagine the existence of other stars, nothing has scientifically proved it. We had to wait until 1995 for observational confirmation of these suppositions.
The astronomers Didier Queloz and Michel Mayor from the Geneva observatory detected for the very first time a planet gravitating around a star other than our Sun. Since then over 400 exoplanets have been discovered, and are all of a mass greater than that of the Earth. For the most part they orbit around close-by stars, situated at a maximum of a few dozen light years away.
To succeed in detecting the presence of exoplanets, astrophysicists principally fall back on so called indirect methods. Planets in effect emit practically no light by themselves and only reflect that of their stars, the reason why it is much more difficult to observe planets than stars. The method by which the majority of exoplanets have been detected is the measurement of variations in a star’s velocity, thanks to the Doppler-Fizeau effect. In the same manner that a planet is attracted to a star, a star is attracted to a planet. They thus both gravitate around the same centre of gravity and the same orbital period. But, given the much greater mass of a star, the resulting speed for the latter is a lot lower. For example the speed imparted to the Sun by the Earth’s influence is hardly 0.4km/h.
The movements of a star are thus influenced by the presence of planet orbiting around it, which provokes a periodic discrepancy of its position, an oscillation. It is this oscillation which is measured by the method of radial velocities. Depending on this discrepancy it is possible to estimate the orbit of the planet as well as to deduct a minimal value for its mass. This method is all the more highly capable when the planet has a heightened mass and is close to its star, the reason why the majority of planets discovered up until now are gas giants with relatively short orbital periods.
The transit method used by TRAPPIST is also an indirect method, based on measuring a star’s apparent light intensity. A periodic slight reduction of it could reveal the passage of a planet, which eclipses a small part of the star. From that point onwards astrophysicists can gain a lot of information about the planet, notably its size. They can also study its atmosphere. Combined with the radial velocity method they allow the planet’s mass to be measured and its structure and composition to be given up. That is why the majority of our knowledge about these exoplanets comes from the tiny fraction of them (around 10%) which transit in front of their star.
To this date the transit method has allowed the detection and study of several dozen gas giants, three ice giants similar to Neptune and two intermediary planets between the Earth and Neptune, called ‘super-Earths.’ One of the objectives of this research is certainly to discover planets with similarities to ours, with a view to one day perhaps being able to ascertain that we are not alone in the Universe.
TRAPPIST will essentially use the transit method to detect and study exoplanets, and will do so in complete collaboration with the Geneva team and with other projects, such as the CoRoT space mission. TRAPPIST will grant particular importance to the smallest stars, the red dwarves, which throw out much less heat than our Sun. The habitable zone around these stars is much closer to them, and planets studied in this zone could thus be detected relatively easily. On these planets, if there is water, it could be present in liquid form and be a first factor favourable to life.
The planned outcome is thus to study a vast range of planets, and certain of them could have characteristics close to that of our Earth.
The term ‘comet’ comes from the Greek (Komêtês) and signifies ‘long-haired’ because of their appearance. If they have been observed for centuries and they have always impressed human beings because of their sudden appearance and their strange aspect, it has only been since recently that we have been able to deepen our knowledge of them.
At the heart of the comet’s crown is found the comet’s core, mainly a block of frozen water (80%) and dust of diverse chemical composition, which can range from 1 to 50km in diameter. The tail(or coma) develops when the comet approaches the Sun. it is made up of gas (atoms, molecules) and dust originating from the core and released by the action of solar radiation.
Comets are thus celestial bodies composed of water (in the form of ice), carbon dioxide, methane, ammonia, particles of agglomerated dust and more complex organic molecules such as amino acids. Their creation dates back to the very beginnings of the solar system and they were at the origin of planet formation. ‘Besides their role in the creation of the planets,’ explains Emmanuël Jehin, ‘one theory puts forward the argument that could have contributed to the appearance of oceans on planet Earth and to the provision of organic compounds. At the beginning of life, 4.5 billion years ago, the Sun was a lot more luminous than it is today, which allows us to suppose that any liquids were vaporised and that the earth was just a rocky planet, with no life. It was only later, 3.8 billion years ago, that some volatility in the solar system generated a rain of several billions of comets and meteorites, of which many smashed into our plant. Given their ice and organic molecule composition, we could argue that they are at the origin of our oceans and thus contributed to the appearance of life on Earth.’
Today’s comets are thus residues from the formation of the solar system, frozen since 4.5 billion years ago in the cold regions well beyond Pluto. A study of them thus enables a better understanding of the history of our solar system and our planet.
TRAPPIST will contribute to deepening our knowledge of comets. The telescope will study their chemical composition thanks to a series of special filters developed by NASA. When comets approach the Sun they become illuminated because of the melting of surface ice and the throwing out of gas and dust liberated into space. The researchers of the ULg’s comet physics group will on the basis of these observations study the detailed evolution of the chemical composition of the ejected gases. ‘With a large telescope you can only study comets for one or two nights a year only,’ points out Emmanuël Jehin. ‘Here we can do what astronomers who work in this field dream about: track and study each one from week to week as they near and grow more distant from the Sun. We can in this way determine their evolution and composition and observe if the abundance changes according to their nearing the Sun. In accumulating measurements on a dozen or so comets by year we can highlight the different classes of comet from the point of view of their composition.’
Département d'astrophysique, géophysique et océanographie (AGO)
Pr Pierre Magain, +32 4 3669753, Pierre.Magain@ulg.ac.be
Michaël Gillon, 32 4 3669743, Michael.Gillon@ulg.ac.be
Emmanuël Jehin, +32 4 3669726, email@example.com
ULg Relations extérieures et Communication, Tel +32 4 366 52 18, firstname.lastname@example.org