The Waltz Telescope – Searching for Exoplanets in the Backyard of Heidelberg
by Dane Späth (Universität Heidelberg), July 2024
Anyone who regularly deals with the latest research on exoplanets often becomes aware of the largest and most powerful telescopes in the world. These include the James Webb, Kepler and TESS space telescopes. But the most modern earthbound telescopes in Chile, Hawaii or on the Canary Islands are also regularly featured in press releases.
These ultra-modern instruments and their discoveries are the great driving force behind exoplanet research. However, due to the limited observation time available, they can often only examine a fraction of the stars in our night sky. Larger-scale sky surveys in search of distant planets, for example using NASA’s TESS telescope, examine almost all the brighter stars, but can generally only find planets with very short orbital periods of a few days to a few weeks. Although these are (surprisingly) common, a look at our own solar system reveals that a large proportion of the planets remain hidden. While the inner rocky planets with orbital periods of 88 days (Mercury), 224 days (Venus), 365 days (Earth) and 686 days (Mars) could still be detected in exceptional cases, the large gas and ice giants with periods of 11.8 years (Jupiter), 29.4 years (Saturn), 84 years and 165 years (Neptune) cannot be found. For these objects, astronomers need patience and long-term access to suitable telescopes.
Such an ambitious project will soon go into operation at the Landessternwarte (state observatory) located on the Königstuhl mountain, in Heidelberg’s backyard, so to speak. A group of mechanics, astronomers and students from Heidelberg University have been working there for some time on equipping the Waltz telescope, which has been observing the sky over Heidelberg since 1906, with the latest technology for detecting exoplanets. With a primary mirror measuring 72 cm in diameter, the Waltz telescope, named after its founder Katharina Böhm (née Waltz), was once one of the largest telescopes in the world, but of course cannot hold a candle to modern observatories. Nevertheless, it is excellent for observing relatively bright stars up to magnitude 6. This brightness class corresponds roughly to the darkest stars that are still visible to the naked eye under good observation conditions.
The Waltz telescope, which went into operation in 1906, still observes the night sky in Heidelberg today. Image: Joshua Jost
However, the Waltz project is not starting from scratch, but builds on a long-term planetary search of over 370 stars, which began in 1999 at the Lick Observatory in California. Due to a technical defect, the project had to be temporarily discontinued in 2011, although data was temporarily recorded at other observatories for particularly interesting stars. The project is now being revived in Heidelberg. The data from the Lick Observatory has already yielded 15 new exoplanets and a large number of planet candidates for which the data situation is not yet sufficient to confirm the planetary companions beyond doubt. These candidates also include planets with relatively long orbital periods. However, as the observation time for most stars has so far been no longer than 12 years, planets with longer orbital periods have so far been very difficult or even impossible to detect.
This is exactly what the Heidelberg astronomers hope to achieve with the Waltz project. By combining the data from the Lick Observatory and the Waltz telescope, a time span of 25 years is already covered. As the Waltz telescope will only observe these 370 stars and will be operated on a long-term basis, it is hoped that planets with orbital periods similar to Saturn’s orbital period will be found.
Another special feature of the Waltz project is that the 370 stars to be observed are red giant stars. These stars are many times larger than our own Sun. At least at the moment, because our Sun will also develop into such a giant star in the distant future. The giants have up to a hundred times the radius of our Sun. Because of this size, they are very bright (at the same distance). In fact, many red giants are among the brightest stars to be found in the night sky. Among the 370 stars to be observed at the Waltz telescope are Aldebaran and Pollux, both of which are among the 20 brightest stars.
In around 5 billion years, the Sun itself will develop into a red giant and expand its envelope many times over. Models show that Mercury and Venus will probably be swallowed up by it during this expansion process. Whether the Earth will share this fate cannot yet be determined beyond doubt, as further processes (such as tidal forces) also play their part. How strongly and in what way a planetary system is influenced by the development of its star is precisely the scientific question to which the Heidelberg astronomers want to add a piece of the puzzle.
Another special feature is that the giant stars to be observed with the Waltz telescope are on average significantly more massive than our Sun (they have around 1-3 times the mass). These stars can only be studied well in their giant stage using the known methods for discovering distant planets. Before that, planets around these stars remain hidden from us. The Waltz project is therefore investigating a completely different mass range of stars than most other projects. The hope is to understand whether the planets around more massive stars are perhaps quite different from the planets we have known so far around Sun-like stars.
The Waltz telescope will use the radial velocity method to detect other planets around these giant stars. High-resolution spectra of the stars will be recorded regularly. The idea is that when a planet orbits a star, the star is also forced into a (significantly smaller) orbit. This is easy to see, as it is not only the star that attracts the planet by gravitational force, but conversely the planet also attracts the star. However, as the star has significantly more mass, its resulting orbit is considerably smaller. Nevertheless, the star also has an orbital velocity that causes the spectral lines of the star to periodically shift to longer or shorter wavelengths. This is known as the Doppler effect. It is precisely this shift in the spectral lines that can be measured using a spectrograph and used to reconstruct the mass and orbit of the planet.
For this purpose, the historic Waltz telescope was equipped with a modern spectrograph, which was mainly developed and built by students. The spectrograph is fed with light from the telescope through an optical fiber. The light beam is then bundled into parallel beams (collimator) by a parabolic mirror and beamed onto an Echelle grating. This is a reflection grating arranged in steps (hence the term Echelle, from French: ladder). This grating produces a diffraction pattern, i.e. the light is divided into different diffraction orders. This diffraction pattern is then guided via several reflections on different mirror surfaces to a prism, which scatters the diffraction orders, which previously ran in one plane, vertically once again in order to generate a suitable format for a square CCD detector (in technical jargon this is referred to as cross-dispersion). The 2D diffraction pattern is then recorded by a camera on a modern CCD detector and digitized. This principle is used by almost all modern spectrographs for planet searches.
Model of the Echelle spectrograph. Image: Tala et al. (2016, https://ui.adsabs.harvard.edu/link_gateway/2016SPIE.9908E..6OT/arxiv:1608.06090)
In order to minimize thermal fluctuations, most modern spectrographs operate in vacuum tanks under very stable pressure and temperature conditions – an extremely complex and costly process. However, as this is too expensive for smaller spectrographs such as the Waltz, the Heidelberg group uses the fine absorption lines of a glass flask filled with iodine gas instead. This is inserted into the beam path of the telescope and leads to an unmistakable fine pattern of dark iodine lines in the measured star spectrum. As the position of the lines can be measured very precisely in the laboratory, they can be used to calibrate the spectrum. This does not achieve the accuracy in the measurement of the radial velocity of 10 cm/s that would be necessary to find a second Earth. However, accuracies of 3 – 5 m/s can be achieved, which is sufficient for red giant stars. This is because these bloated stars themselves lead a very restless life: they get bigger and smaller. This pulsation of the star itself causes a measurable radial velocity variation and unfortunately in most cases completely swallows up the signal from light planets like our Earth. Therefore, it is mostly only possible to detect heavy planets around such giant stars.
One of the first star spectra (Aldebaran) from the new spectrograph.
The telescope and the spectrograph are completely controlled by a computer. In addition to the scientific results, it is also intended to give Heidelberg students the opportunity to gain observational experience. True to the motto “Young students should work at the observatory as early as possible!”, which is attributed to the observatory’s founding director, Max Wolf. In the long term, there are even plans to operate the telescope completely remotely or even robotically. However, a few final teething troubles (or sometimes rather signs of ageing) still need to be eliminated.
The team has been recording its first data from the night sky in Heidelberg since the end of 2022. Initial tests show that the telescope achieves the required precision. The telescope is currently undergoing a final “makeover”. All mirrors have been recoated to guarantee the best possible use of starlight. After that, the Waltz telescope will be one of only a handful of telescopes in Germany to set out in search of our extrasolar neighboring planets. We can look forward to exciting results “Made in Germany”.