Located in the constellation Ursa Major, approximately 300 light years from Earth, is the Sun-like star. HD118203 (Liesma). In 2006, astronomers detected an exoplanet (HD 118203b)) of similar size and twice as massive as Jupiter that orbits very close to Liesma (7% of the distance between Earth and the Sun), making it a “hot Jupiter”. In a recent study, an international team of astronomers announced the detection of a second exoplanet in this system: a Super Jupiter with a wide orbit around its star. In short, they discovered a “cold Super Jupiter” on the outskirts of this system.
Gracjan Maciejewski, associate professor at the Institute of Astronomy of Nicolaus Copernicus University (NCU) in Torun, Poland – led the study, which recently appeared in the journal Astronomy and AstrophysicsHe was joined by researchers from the Department of Astronomy and Astrophysics and the Center for Exoplanets and Habitable Worlds At Pennsylvania State University (PSU), the Institute of Astrophysics of the Canary Islands, Spanish Space Agency (EEA), the Institute of Astrophysics of Andalusia (IAA-CSIC), and the Center for Astrophysical Studies to the National Center for Supercomputing Applications (National Statistics Centre).
According to their study, the planet (HD 118203 c) has up to eleven times the mass of Jupiter and orbits its parent star at a distance of 6 AU (six times the distance between Earth and the Sun) with a period of 14 years. Astronomers discovered the parent star in 1891 using the Draper telescope, now located at the NCU Institute of Astronomy in Piwnice, near Torun. Liesma is a yellow G-type dwarf (like our Sun), but 20% more massive and twice as large. Astronomers estimate that the star and its entire planetary system are slightly older than the Sun (about 5 billion years).
Although astronomers have known for almost twenty years that a fairly massive planet orbits HD 118203, it was only confirmed by Radial velocity (Doppler spectroscopy). However, these measurements indicated a linear trend that indicated there could be a companion planet with a wider orbit. The presence of another planet would indicate that the system has a hierarchical orbital architecture, which could help astronomers learn more about the origins of hot Jupiters. As Professor Andrzej Niedzielski, co-author of the study, explained in a paper published in the journal Astronomy & Astrophysics. News from NCU:
“Doppler observations, however, indicated that it was not the end of the story, that there could be another planet out there. Therefore, we immediately included this system in our observing programs. At first, as part of the Torun-Pennsylvania exoplanet research program, conducted in collaboration with Professor Aleksander Wolszczan, we tracked the object with one of the largest optical instruments on Earth, the nine-meter Hobby-Eberly telescope in Texas.”
The results were so promising that the international team continued to observe the star using the Galileo National Telescope (TNG) at the Roque de los Muchachos Observatory. But first it was necessary to rule out the possibility that more planets were hiding in the system. “I analysed the photometric observations obtained with the Transiting Exoplanet Survey Satellite space telescope, which showed that there were no other planets around HD 118203 that were larger than twice the size of Earth and therefore not massive enough to be relevant for studying the dynamics of the system,” says Julia Sierzputowska, an astronomy student and co-author of the study.
In 2023, the team obtained robust data from a super-Jupiter with a wide orbit, proving that HD 118203 was a hierarchical planetary system. Professor Maciejewski said:
“Patience pays off. New observations collected in March 2023 proved crucial in determining the planet’s orbital parameters. Moreover, since a planet takes several years to orbit its star, we were able to combine our Doppler observations with the available astrometric measurements to unambiguously determine its mass. This allowed us to build a complete model of this planetary system and study its dynamic behaviour.”
The configuration is peculiar, as one planet orbits close to its star (forming a pair) while a second orbits far enough away to form another pair with the first. While both planets are massive and have fairly elongated orbits, their mutual gravitational influence does not destabilize the system over eons. According to their study, this is due to the effects of General relativitywhich prevents the planets from constantly changing the shape of their orbits and their orientation in space.
This makes HD 118203 one of the few hierarchical systems known to astronomers, which will help address theories about how massive planets form. This, in turn, will allow astronomers to learn more about the formation and evolution of our Solar System’s gas giants: Jupiter, Saturn, Uranus, and Neptune. The international team also plans to continue collecting data on this system in the hopes of finding additional exoplanets.
Further reading: NCU News, Astronomy and Astrophysics
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