Extrasolar planets observed by students
Two Swiss astronomers were awarded the Nobel Prize in Physics in early October 2019 for their discovery of planets outside our solar system. But how does one learn to discover new planets? Two physics students and a Bachelor’s student from the Department of Interdisciplinary Natural Sciences at ETH Zurich recently completed an independent project in which they searched for extrasolar planets with the study telescope.
In astrophysics, data and mathematical methods are fundamental. The measurement of an exoplanet, as extrasolar planets are also called, involves not only observation with a telescope but also algorithms, data analysis, models and statistics. This sounds very theoretical at first, but in practice every measurement – if made from Earth – also depends on weather and environmental influences. This practical experience, as one of the students remarks, was essential for the later interpretation of the data.
From idea to independent project
Several Bachelor’s students at ETH Zurich independently made contact with ETH professor Sascha Quanz, head of the research group Exoplanets and Habitability with ideas for a semester project in astrophysics. The students – Sven Kiefer, Adrian Gheorghe and Thomas Birbacher – now agree that it was well worth it. Students often hesitate to turn to professors with project ideas on their own initiative, and this, the three think, is unfortunate. In fact, their idea of observing extrasolar planets with the external pagestudy telescope on the highest building of the ETH Hoenggerberg Campus in Zurich soon found sympathetic ears.
What can be observed?
Today, astrophysicists explore new exoplanets with large space telescopes such as “TESS”. These telescopes are not disturbed by factors such as clouds, light pollution and air turbulence. To observe exoplanets from the surface of the Earth presents completely different challenges. The choice for the students’ semester project fell to the observation of an already known large “Hot Jupiter”, since a smaller planet could hardly be observed with the study telescope. In searching for unknown exoplanets, however, the same methods can later be employed with more powerful telescopes.
What is a Hot Jupiter transit?
“Hot Jupiters” are gas giants, similar to Jupiter in our solar system, but have orbits of only a few hours to a few days. They block the light of the star they orbit during so-called transit – similar to a solar eclipse – and this periodic darkening can be measured with a telescope. From a scientific perspective, a transit has to be unambiguously measured three times in order to confirm the existence of an exoplanet. The frequent transits of a Hot Jupiter increase the chance that students will be able to complete their observations and derive a convincing result within a single semester. From a perspective in outer space, for example, planet Earth would only block out our Sun once a year. Even with the very best weather conditions, therefore, one would have to observe from that perspective for at least three years to confirm the existence of the Earth. It is easy to understand, then, that most already discovered exoplanets have short orbits.
Will the observation be successful?
Adrian Gheorghe and Sven Kiefer chose the winter months to observe the northern hemisphere. Since last winter was almost continuously cloudy, it took a lot of perseverance and an effective methodology to see their project through. In March they were finally able to make ten nights’ worth of measurements. Two of these ten were successful – despite the disturbing influence of the nearby airport! The measurements clearly showed the Hot Jupiter known as “HAT-P-44 b” reducing the brightness of the star it orbits by approximately two percent.
Their good methodology and intermediate successes motivated them to stick with it as well as the freedom and the support within the research group. Luca Tortorelli, who has also been the responsible teaching assistant for the study telescope in the past few years and the doctoral candidate Silvan Hunziker supported the students during their project.
Whether it would actually be possible to observe an exoplanet from the ETH Hönggerberg campus using the transit method was still open at the beginning of the project.
Observation supported by methodology and algorithms
After only two years of study at ETH, the students enjoyed doing this independent research. They wrote algorithms that helped them in making their observations, combining data from reference databases of already detected exoplanets with weather forecasts. Whenever the algorithms suggested an opportunity for observation, they spent the night by the telescope. They took between 100 and 200 pictures per night. These observations fed into Tomas Birbacher’s semester project, which aimed to improve the analysis of the collected data.
30 to 50 GB of data were collected every night of observation. Thomas Birbacher analysed these, searching for the approximately two-hour transit of the Hot Jupiter. The slight deviation of the light curve that occurs during the transit is too small to be detected by the human eye.
16 million stars – which one are we hunting for?
Data covering 16 million stars (suns) could be found in the reference database. The students compared these data with the stars in the images they had produced using the telescope in order to find out which stars they had photographed. Specially created digital filters were used to reduce the background noise caused by non-optimal weather conditions. Thomas Birbacher programmed these filters specifically for the study telescope. The importance of the interplay between observation, technical equipment, clever algorithms, statistics and specific data analysis in constructing scientific knowledge rapidly became clear in this process.
Caption: The black data points in the upper part of the graph show the measured brightness of the star as a function of time. The blue line shows the light curve, taking into account unavoidable measurement errors. At the beginning of the curve, the planet is still next to the star. The normal brightness of the star (= 1) is measured as a reference. In the middle of the curve, the planet is directly in front of the star and produces the maximum darkening of about two percent of its light intensity. At the end of the curve the planet has completely passed by the star, and one can see the normal maximum brightness again. The planet passes in front of the star every few hours, as the time axis (Time) shows.
The lower part (residuals) shows the total measurement data minus the upper part – that is, the parts of the measurement that were not used for the so-called “fit” in the upper part.
Good preparation: “Astrowoche” and Physics Lab
Some of the students had already acquired practical experience during the Astrowoche, a practical course offered by the Department of Physics, and the Physics Lab 3+4. There they quickly realised how much fun these challenges were for them. All three are now specialising in exoplanet research for their Master’s degrees.
