Aarhus Universitets segl

5 billion years old hot Saturn discovered with the NASA space telescope TESS

Researchers from Aarhus University have analyzed data from NASA's space telescope TESS and the Danish-led SONG telescope in a large international collaboration led by Daniel Huber, assistant professor at the Institute for Astronomy (IfA) of the University of Hawaii at Manoa. They have discovered and examined an exoplanet - TOI-197.01. The newly discovered exoplanet is a Saturn-like gas planet that orbits around its star with a period of only 14 days. The precise parameters for the planet could be determined, as a detailed study of the star could be performed from so-called star quakes. The studies show that the planet is a gas planet with a density of only 1/13 of the Earth's density and a mass that is approx. 60 times the Earth's mass. The age of the planet is slightly older than the Earth with and age of a bit more than 5 billion years and with the results of this study, TOI-197.01 is one of the best described exoplanets of this type to date.

Link to the article: https://arxiv.org/pdf/1901.01643.pdf (First author: Daniel Huber, University of Hawaii)

Last year, NASA launched a satellite (Transiting Exoplanet Survey Satellite, TESS) destined to find planets around stars other than the Sun, so-called exoplanets. The first exoplanets have now been discovered with TESS, but to be sure that there is no misinterpretation of the measurements, all the newly discovered exoplanets are examined further with other telescopes. One of these possible candidates was selected for follow-up observations with among others the Danish-led SONG (Stellar Observations Network Group) telescope in Tenerife. SONG was the first telescope that made follow-up observations among the ground bases telescopes. By measuring the movement of the star around which the exoplanet revolves, one can confirm the existence of the planet and gain important knowledge of the properties of the planet, among other things its mass and surface temperature. The special thing about the star that the new planet revolves around is that it also shows signs of so-called "star quakes", and these quakes can be used to examine the star itself to measure e.g. the size and age of the star.

PhD student at Stellar Astrophysics Centre at Aarhus University Emil Knudstrup is highly involved in the work of finding and investigating exoplanets and mentions:
It is really exciting to be involved in characterizing a newly discovered exoplanet that revolves around a distant star. I think I have been fortunate to be able to work with TESS data, while also being able to observe the star with one of our own telescopes.

An illustration of the TOI-197.01 exoplanet system. The hot Saturn-like gas planet orbits a sun-like star that vibrates, as illustrated by the curvy lines on the star. (Illustration: Gabriel Perez Diaz, Instituto de Astrofísica de Canarias).

"In my work with TOI-197, I have used some theoretical models that I compare to the light curve we have from TESS and in this way, I can determine some of the physical parameters of the planet. As the planet passes in front of the star, it will shadow some of the star's light and depending on how large the planet is, it will shade more or less of the star's light. So if we know the size of the star, we can determine the size of the planet.” Explains Emil, who is a co-author of the article and he continues:
"From the light curve we can determine the planet's radius as a fraction of the radius of the star and for this system the ratio turned out to be approx. 3%. With the exact star radius determined from oscillations in the star, we could translate this into a planet radius of about 9 times the Earth's radius. This corresponds to a planet that has the size of Saturn and since we have measured the mass we can determine the density, which for this planet is about 1/13 of Earth's density.

Light curve from TESS of the star TOI-197 during the time the planet passes in front of the star and thus shadows part of the star's light. The black points are data from TESS and the blue curve is the model that best describes the observed data. The scattering of the black spots around the blue curve is mainly due to noise in the measurements from TESS. (Figure: Emil Knudstrup)

The exoplanet, which has been named TOI-197.01 (TOI stands for TESS Object of Interest), is about the size of Saturn, but is much closer to its star than Saturn is to the Sun. This means that TOI-197.01 probably doesn't have the characteristic rings that we know from Saturn in the Solar System. By contrast, the newly discovered exoplanet, due to the distance to the star, will be a lot warmer and is therefore called a "hot Saturn". The planet has a short period of only 14 days, so it takes the planet 14 days to orbit the star ones.

SONG-manager and co-author of the article Mads Fredslund Andersen explains:
With the SONG telescope, we can very accurately measure the star's movement relative to us here on Earth. By looking at the intensity distribution of the light the star emits, we can determine the star's radial velocity, that is, the velocity in the viewing direction seen from Earth towards the star. As the planet and star revolve around their common center of gravity, the star will move a little back and forth as the planet orbits the star. On TOI-197, we have measured this with the SONG telescope and the star is moving back and forth with a speed of up to 50 km/h. The variation occurs with a period of 14 days and it tells us that the planet weighs about 60 times the Earth's mass or approx. 70% of Saturn's mass. Put into context, we have measured that a star there which is approx. 300 light-years away moves slightly back and forth with speeds similar to driving a car in the city.

The very precise parameters determined in this study of the exoplanet are only possible because the star vibrates with the so-called star quakes. These vibrations or oscillations bring information about the interior of the star to the surface where astronomers can observe the oscillations and from these determine fundamental parameters such as size, age and mass of the star. When, as has been done here with the SONG telescope, measuring the gravitational influence that the planet exerts on the star, one can also determine the planet's mass. From the star quakes, scientists could determine the age of the star to be a bit more than 5 billion years old and slightly heavier and slightly larger than the sun.

Mads continues:
"It is not easy to determine the fundamental parameters of stars with great precision, but with these star quakes we have a method that is very robust and very accurate. It is a bit special that for this system, we have a planet around a star that shows clear star quakes, so we can determine the interesting parameters of the star and planet very accurately." And he continues: "With SONG, we actually measure these oscillations in other stars, but the TOI-197 is too faint for us to use the SONG telescope to observe the oscillations, but then it is perfect that we can instead contribute with radial velocities of the star, which can tell us something about the planet."

Radial velocity variations measured with the SONG telescope of the star TOI-197. The black points are the measurements from SONG and the blue curve is the theoretical model that Emil has determined and which matches best with the measurements. (Figure: Emil Knudstrup)

The SONG project was initiated as a collaboration between Aarhus University, the University of Copenhagen and the Instituto de Astrofísica de Canarias to build the first telescope of the SONG network in Tenerife. The telescope and the running cost were funded by the Villum Foundation, the Carlsberg Foundation, the Instituto de Astrofísica de Canarias, the Independent Research Fund Denmark, the European Research Council, Science and Technology (AU), Copenhagen University and the Danish National Research Foundation. The latter also finances the Stellar Astrophysics Centre at Aarhus University, which manages the operation the SONG project. Later, the National Astronomical Observatories of China joined the project and started building the second SONG telescope on the Tibetan plateau in China. However, local conditions have considerably delayed the completion of this telescope. Now five Australian universities; the University of Southern Queensland, the University of Sydney, the University of New South Wales, Monash University and the Australian National University have also become partners in the SONG project and the third observatory in the network is soon a reality. It is important to have a network of telescopes distributed all over the Earth, as one can follow individual stars over long periods of time, despite the change locally between night and day. The five universities in Australia, with a recently announced grant from the Australian Research Council, together with the Department of Physics and Astronomy (Aarhus University) and the Carlsberg Foundation have joined forces to finance the new SONG telescopes for the network.

TESS was launched on April 18, 2018 and began to make scientific measurements from the summer of 2018. The telescope is oriented so that during the first year it will measure almost all of the southern hemisphere and then it turns 180 degrees and looks at the northern hemisphere in the following year. It is expected that TESS will find several thousands of new exoplanets and scientists expects that a few hundred of those will be of the size of the Earth.

Photo Tenerife of the SONG telescope. In the background is the iconic volcano Teide. (Photo: Mads Fredslund Andersen)

TESS and exoplanets:
Ph.d.-student: Emil Knudstrup, 87155134, emil@phys.au.dk

SONG-manager: Mads Fredslund Andersen, 21527146, madsfa@phys.au.dk

Lead author: Daniel Huber, (808) 956-8573, huberd@hawaii.edu

More information on the SONG project:

Information on the TESS mission: