Smaller, Ground-Based Telescopes can Study Exoplanet Atmospheres too

The next step to understanding exoplanets is to understand their atmospheres better. Astronomers can determine a planet’s mass, density, and other physical characteristics fairly routinely. But characterizing their atmospheres is more complicated.

Astronomers have had some success studying exoplanet atmospheres, and spacecraft like the James Webb Space Telescope and the ESA’s ARIEL mission will help a lot. But there are thousands of confirmed exoplanets with many more to come, and the Webb has many demands on its time.

Can smaller, ground-based telescopes play a role in understanding exoplanet atmospheres?

Everything we know about exoplanets we learn from light. An exoplanet signals its presence by slightly dimming the light from its star. Astronomers can also gauge the exoplanet’s mass, size, and density from starlight, as the planet tugs ever so slightly on its star, causing a minute change in the starlight. It takes powerful, light-gathering telescopes and instruments to measure these fluctuations in the light.

Studying exoplanet atmospheres requires a similar level of technological power and sophistication. Astronomers can determine some of an exoplanet’s atmospheric composition by examining the starlight as it passes through the planet’s atmosphere. They do this with spectrographs.

The world’s largest observatories have high-resolution spectrographs on them. For example, the ESO’s Very Large Telescope (VLT) has the ESPRESSO spectrograph. But there are many smaller telescopes than large telescopes like the VLT. The authors of a new paper think that some of those smaller telescopes, when equipped with a powerful spectrograph, could advance the study of exoplanet atmospheres.

The four Unit Telescopes that make up the ESO's Very Large Telescope, at the Paranal Observatory> Image: By ESO/H.H.Heyer [CC BY 4.0 (], via Wikimedia Commons
This image shows the four Unit Telescopes that make up the ESO’s Very Large Telescope, at the Paranal Observatory> Image: By ESO/H.H.Heyer [CC BY 4.0 (], via Wikimedia Commons

The paper’s title is “Exoplanet atmospheres at high resolution through a modest-sized telescope. Fe II in MASCARA-2b and KELT-9b with FIES on the Nordic Optical Telescope.” Aaron Bello-Arufe, a Ph.D. student in the Exoplanet Group at the Technical University of Denmark, is the lead author. The journal Astronomy and Astrophysics has accepted the paper and will publish it.

FIES stands for FIbre-fed Echelle Spectrograph. It’s an Echelle spectrograph like the ESPRESSO instrument on the much larger VLT. It’s the latest instrument added to the Nordic Optical Telescope (NOT,) a 2.56-metre telescope at La Palma in the Canary Islands. “Our work demonstrates the feasibility of investigating exoplanet atmospheres with FIES, potentially unlocking a wealth of additional atmosphere detections with this and other high-resolution spectrographs mounted on similar-size telescopes,” the authors write.

The Nordic Optical Telescope (NOT) telescope at Roque de los Muchachos Observatory in June 2001. The telescope's primary mirror is 2.56 meters but the FIES spectrograph adds to the telescope's capabilities. Image Credit: By Bob Tubbs - Own work, Public Domain,
This image shows the Nordic Optical Telescope (NOT) telescope at Roque de Los Muchachos Observatory in June 2001. The telescope’s primary mirror is 2.56 meters, but the FIES spectrograph adds to the telescope’s capabilities. Image Credit: By Bob Tubbs – Own work, Public Domain,

MASCARA-2b, also known as Kelt-20b, is a hot Jupiter orbiting an A-type star. Astronomers discovered it in 2017. In 2022, astronomers announced the discovery of water vapour and carbon monoxide in the planet’s atmosphere.

Astronomers discovered the other star in this study, Kelt-9b, in 2016. It’s also a hot Jupiter and also orbits an A-type star.

“In this work, we seek to demonstrate the capability of spectrographs at 2-m class telescopes to characterize exoplanetary atmospheres at high resolution,” the authors write. They focus on MASCARA-2b and Kelt-9b because astronomers have studied both of these hot Jupiters with more powerful spectrographs, giving them something to compare their results to. Hot Jupiters are also prime targets for spectroscopy: they’re large and close to their stars, making the light easier to examine as it passes through the atmospheres.

An artist's illustration of an ultra-hot Jupiter orbiting close to its star. Image Credit: Gabriel Pérez Díaz, SMM (IAC).
This artist’s illustration shows an ultra-hot Jupiter orbiting close to its star. Image Credit: Gabriel Pérez Díaz, SMM (IAC).

Previous research found iron in these planets’ atmospheres using spectrographs like HARPS-N on the larger 3.58-meter Telescopio Nazionale Galileo. “Our goal is to use FIES to replicate the detection of atmospheric neutral and ionized iron (Fe I and Fe II) in the transmission spectra of these extremely hot planets claimed by recent works,” the authors write.

The team observed two transits of MASCARA-2b and one of Kelt-9b. They detected atmospheric Fe II in the atmospheres of both planets. They also detected Fe I in both atmospheres, but they aren’t as confident that that signal stands out from the noise. “Although it is plausible that these signals are of planetary origin, we argue that we need more data before claiming a detection of Fe I due to the presence of additional high-S/N signals…” they write.

The authors are confident that smaller ground-based telescopes with modern spectrographs can successfully study exoplanet atmospheres.

“Our results demonstrate the feasibility of studying atmospheres at high spectral resolution with FIES/NOT,” they write in their conclusion. “FIES has the stability and precision required for characterization of exoplanet atmospheres,” they write. FIES is in a separate building from the NOT, and its isolated thermally and mechanically.

FIES has been upgraded since these observations were taken and is even more capable now. “Additionally, we note that the data analyzed in this work is prior to the recent recoating of the FIES collimator and folding mirrors. With the new mirror coatings, FIES has become 60–140% more efficient,” the authors explain.

These improvements bode well for the ongoing study of exoplanet atmospheres. The world’s smaller telescopes don’t always grab the headlines or capture gorgeous images worthy of a front page. But they’re used by working astronomers every day, often in concert with one another.

“More generally, this proof-of-concept study opens the door for characterization of exoplanet atmospheres with other modest-size telescopes equipped with similar spectrographs.” The authors point out that there “… is a large suite of high-resolution spectrographs on 2-m class telescopes whose potential for characterization of exoplanet atmospheres remains largely unexplored…”

As NASA’s TESS (Transiting Exoplanet Survey Satellite) finds more and more exoplanets and more and more hot Jupiters, these smaller 2-m class telescopes will have more and more targets. The authors think these smaller ‘scopes can become an essential part of exoplanet science.

“As NASA’s TESS mission keeps delivering numerous targets transiting bright stars,” the authors write, “the availability of multiple spectrographs capable of this work will be crucial to exploring the composition and dynamics of exoplanet atmospheres and understanding the systematics of different instruments.”