Hunting exoplanets from Chile

by Matías Díaz, Ph.D. student at University of Chile.

Edited by Zaira M. Berdiñas and John Strachan.

As an astronomer your life consists (mostly) on developing computer software, writing papers to show the results of your research and applying for telescope time. The latter means you apply for telescope time by writing proposals that describe the project you want to carry out and the scientific motivation behind it. If your proposal is accepted you go directly to the telescope to observe your stars. Then, you come back to the office and analyze the new data acquired applying and fitting models to detect new planet candidates. I’m interested in detecting ‘rocky’ small planets around bright stars, so for this purpose I use the radial velocity method for exoplanet detection using both the Carnegie Planet Finder Spectrograph (PFS) at Las Campanas Observatory and the High Accuracy Radial velocity Planet Searcher (HARPS) at La Silla Observatory.

As I live in Chile, my journey starts when I leave Santiago in the morning to go to the airport and take a short flight (~45 min) to La Serena (~500 km north of Santiago). Once I arrive at La Serena I take the transport provided by the observatory. It takes about 2 hours to get to either La Silla or Las Campanas Observatory from La Serena. From the road, the landscape changes as you leave the city, with a nice view of the coast and the Pacific Ocean, the Andes mountains and the Atacama desert.

Telescopes’ domes at La Silla Observatory. Credits: Matías Díaz.

Each observatory is different, but they all have common things such as a hotel to accommodate the staff and visiting astronomers and a big dining room/kitchen. If you like watching movies or reading a book there is a good catalog you might want to check (especially if it snows!).

If it is your first time observing, you arrive one or two nights in advance to learn how to use the instrument, familiarize with the telescope/instrument control software and to prepare the observations (list of targets, strategy, etc). During my nights off, if it’s clear, I take my camera to shoot pictures of the starry southern sky!

Starry night with the 6.5 m Magellan Telescope of Las Campanas Observatory in the foreground. Note the Large Magellanic CloudCredits: Matías Díaz.

One of the best things about observing is the food. It includes all sorts of meals and desserts (a lot of chilean “manjar“!). Usually, before sunset we all get together in the dining room to have dinner. This is a good opportunity to meet and talk with other astronomers from around the world. Depending on the observatory, on Sundays they will serve special meals like “empanadas” or chilean barbecue.

Normally, my ‘day’ starts at 4 p.m. when I have to initialize the calibrations. The night officially starts after sunset and ends when the Sun is up again, so I go to bed around 8 a.m. La Silla, for example, has a big control room with workstations dedicated to the different telescopes. In fact, the ESO-3.6m telescope, in which HARPS is installed, is about 3 km away from the control room. The high-resolution spectrograph HARPS is located in a separate room where the temperature and pressure are stabilized. These conditions allow the instrument to achieve precision below 1 m/s. But, what this number means? This precision is the key to detect Earth-mass exoplanets. A planet orbiting a star causes the star’s spectral lines (i.e. the specific pattern of lines that each star produces in a spectrograph) to move in time. The shift of the star’s spectral lines is proportional to the mass of the planet and 1 m/s precision ensures we can detect small exoplanets with masses similar to that of the Earth.

As I was saying, I operate the instrument from the control room. The Telescope Operator controls the telescope while I set up the target list and all the exposure parameters of the instrument. For HARPS, there is a great reduction system that allows you to visualize how your data is processed as soon as the exposure finishes. The number of stars I observe in a typical night varies depending on factors such as weather conditions and brightness of the stars. Typically I am able to observe between 30 and 40 stars in one night.

Control room of the Magellan 6.5 m Clay telescope at Las Cumbres Observatory. The instrument Planet Finder Spectrograph (PFS) is controlled and monitored from the computers on the left. Credits: Matías Díaz.

Observing with the Planet Finder Spectrograph (PFS) at 6.5m Magellan telescope is a bit different. The control room is in the same building as the telescope. Also, PFS is mounted on a platform next to the telescope. I control the instrument from the control room. Here everything is less automated as I have to choose the target and tell the Telescope Operator to move the telescope. Then I adjust the correct parameters in the instrument control panel for the current star. As soon as the exposure finishes I have to tell the Telescope Operator the new target, change the instrument’s values, and so on. When the night is over, I transfer the data to a computer in the USA where it is stored and ready to be processed. Then, small shifts (1/1,000 of a pixel of the detector) in the spectral lines are measured with respect to a reference spectrum (template) to compute the radial velocity of the star. This measurement will reveal (or not) the presence of a planet and its minimum mass, along with other orbital parameters such as its period or eccentricity.

I usually add to all this a decent amount of Heavy Metal music while I observe. It also helps you to stay awake.

The lodge next to the hotel where astronomers meet for dinner or relax after a working night at Las Campanas Observatory. Credits: Matías Díaz.

That’s it. The process repeats until your final night when after a good sleep you go down in the transport that will take you to the airport. Once there, I always make sure to celebrate another successful observing run with a Pisco sour!

Then, when back in the office I add the new measurements to the previously gathered data, analyze the new data set and if a candidate meets the criteria for a detection and publish this new result.

About the author

Matías Díaz

Matías is a last year Ph.D. student at Universidad de Chile currently working on the detection of small exoplanets around nearby stars with FGK spectral types. His expertise are the high-precision radial velocity observations. He has used the Carnegie Planet Finder Spectrograph (PFS) at the 6.5 m Magellan/Clay telescope in Las Campanas Observatory and HARPS at the ESO 3.6m telescope in La Silla Observatory to carry out his program.

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