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Last February, the European Southern Observatory (ESO) announced the discovery of a small exoplanet around the closest star to the Solar System, Proxima Centauri. This discovery was made possible by ESPRESSO, an instrument installed a few years ago at the Very Large Telescope in Chile. The team making this announcement includes Romain Allart, a Trottier postdoctoral researcher at iREx. Here he answers our questions about ESPRESSO, this recently discovered exoplanet around Proxima Centauri, and his contribution, which consisted of developing a new method to correct the data obtained with the instrument. Artist’s rendering of Proxima Centauri d, a small exoplanet roughly twice the mass of Mars discovered in February 2022 by a team including iREx postdoctoral researcher Romain Allart. credit: ESO/L. Calçada. Romain Allart, iREx Trottier postdoctoral researcher. credit: M-Eve Naud. iREx: What’s so special about the newly discovered exoplanet around Proxima Centauri? Romain: Proxima Centauri is the nearest star to our Solar System. Other planets were already known around this star, but the one announced by our team, called Proxima Centauri d, is one of the smallest exoplanets ever discovered. Its mass could be as low as a quarter of Earth’s mass or about twice that of Mars. We suspected these tiny planets existed, but it’s a real engineering feat to detect them, so we know very little! This planet is about 35 times closer to its star than Earth is to the Sun, and orbits it every 5 days or so. It is too close to its host to be in the so-called habitable zone: receives too much energy to be able to hold liquid water on its surface. So it is a very different planet from ours! iREx: Tell us about ESPRESSO. Why is this instrument so effective at detecting exoplanets? Romain: ESPRESSO, for Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, is an instrument installed in 2018 at the Very Large Telescope in Paranal, Chile. The VLT has four telescopes with 8.2 meter diameter mirrors. ESPRESSO can harness light from any of these four large telescopes, or all four together, allowing it to achieve unique performance. Like its predecessor HARPS, also installed in Chile but at a different location, ESPRESSO is a spectrograph using the radial velocity (or tachometry) method. This method involves detecting the small periodic motions of a star created by the presence of a planet in orbit around it. The less massive a planet is, the less important this wobble is and the smaller the radial velocity (towards us) that can be measured with a spectrograph. To be able to identify a planet as low-mass as Proxima Centauri d, we need to have an instrument that can detect a variation in velocity of only 40 centimeters per second (1.44 kilometers per hour), which is remarkable. We hope that in the future ESPRESSO will be able to detect even smaller variations, on the order of 10 centimeters per second! The 4 telescopes of the VLT in Paranal, Chile, where the ESPRESSO instrument is installed. Credit: ESO/B. Tafreshi (twanight.org). iREx: What is your contribution to this discovery? Romain: I developed a method to improve the results obtained with ESPRESSO and other similar spectrographs. To reach an accuracy of 10 centimeters per second, it is necessary to be able to use all available information while minimizing the sources of contamination. This method allows reducing the pollution created by the Earth’s atmosphere. It can be applied to the data obtained for Proxima Centauri by ESPRESSO. iREx: Could you summarize the correction method you have developed? Romain: While completing my PhD in Geneva, Switzerland I developed this method, which I have just presented in detail in a scientific paper to be published in the journal Astronomy & Astrophysics. Our goal was to develop a simple, automatic method with few parameters to adjust to remove the signature of the Earth’s atmosphere in data from spectrographs such as ESPRESSO. To do this, we constructed a simplified model of the Earth’s atmosphere by combining its physical properties with the meteorological conditions during the observations. This model can then be fitted and subtracted from the spectra observed by the ESPRESSO instrument. By removing a large part of the signal from our planet’s atmosphere, our method allows us to obtain more information about the radial velocity variation of the star, and therefore, about the possible presence of planets around it. It can be used in future analyzes with ESPRESSO and other similar instruments. iREx: What work did you have to do to get these results? How would you feel if we saw you work on it? Romain: This work, as is often the case in astronomy, consists of writing an algorithm in the Python programming language, a language widely used in industry as well. If you saw me working, you’d just see someone typing lines of code into a computer! It may seem a bit boring or off-putting at first, but Python is very easy to use and there is a lot of help online. I like to think that this tool allows me to harness the power of modern computers to better understand the universe! iREx: Why are you interested in this type of project? Why should the public care? Romain: These kinds of works, more technical, are necessary for the good functioning of an astronomical instrument and are the necessary tools for great scientific discoveries. The correction method we have developed will be included in ESPRESSO data reduction and offered to the scientific community. It will improve the quality of all studies done with ESPRESSO, which is very motivating! iREx: What are the next steps? Romain: The results of my work show a clear improvement in the quality of the results for Proxima Centauri. Combined with other methods of analysis, we are confident that it will confirm the existence of Proxima Centauri d. This kind of independent confirmation is a critical step in any scientific discovery. We also adapt our method for another instrument to which iREx contributes, the NIRPS spectrograph. This instrument will have the same role as ESPRESSO, but will observe in the near infrared, along with its big brother HARPS, which is installed at the 3.6-meter telescope in La Silla, Chile. So there is still a lot of work to do! Note: This interview has been edited for clarity. About studies Connections Link to the ESO Press Release on the discovery of Proxima Centauri d.

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