Astronomers have discovered a star with a scorching Jupiter and a chilly super-Jupiter in orbit

In the constellation Ursa Major, about 300 light years from Earth, is the sun-like star HD118203 (Liesma). In 2006, astronomers discovered an exoplanet (HD 118203 b) similar in size and twice as massive as Jupiter, orbiting very close to Liesma (7% of the distance between the Earth and the Sun), making it a “hot Jupiter.” In a recent study, an international team of astronomers announced the discovery 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 at Nicolaus Copernicus University (NCU) in Torun, Poland – led the study, which recently appeared in the journal Astronomy & Astrophysics. He was joined by researchers from the School of Astronomy and Astrophysics and the Center for Exoplanets and Habitable Worlds at Pennsylvania State University (PSU), the Instituto de Astrofísica de Canarias, the Agencia Espacial Española (AEE), the Instituto de Astrofísica de Andalucía (IAA-CSIC) and the Center for Astrophysical Investigations at the National Center for Supercomputing Applications (NCSA).

According to their study, the planet (HD 118203 c) has up to 11 times the mass of Jupiter and orbits its parent star at a distance of 6 AU (six times the distance between the 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 G-type yellow 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 (an estimated 5 billion years).

Henry Draper's Astrograph (1891), donated by the Harvard College Observatory in 1947. Photo credit: Andrzej Romanski

Although astronomers have known for nearly twenty years that a fairly massive planet orbits HD 118203, it was only confirmed in 2006 by radial velocity measurements (Doppler spectroscopy). However, these measurements showed a linear trend, suggesting that there may 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 Prof. Andrzej Niedzielski, a co-author of the study, explained in an NCU news report:

“However, Doppler observations suggested that this was not the end of the story, that there might be another planet out there. So we immediately included this system in our observation programs. First, we tracked the object as part of the Torun-Pennsylvania Exoplanet Research Program, which we conducted in collaboration with Professor Aleksander Wolszczan, using 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 observing the star with the Telescopio Nazionale Galileo (TNG) at the Roque de los Muchachos Observatory. But first they had to rule out the possibility that there were other planets hidden in the system. “I analyzed photometric observations made with the Transiting Exoplanet Survey Satellite space telescope. They showed that there are no other planets around HD 118203 larger than twice the size of Earth and therefore not massive enough to be relevant for studying the dynamics of the system,” said Julia Sierzputowska – astronomy student and co-author of the study.

By 2023, the team obtained solid data from a super-Jupiter with a wide orbit, showing that HD 118203 was a hierarchical planetary system. Prof. Maciejewski said:

“Patience pays off. The new observations from March 2023 proved crucial in determining the planet's orbital parameters. Since a planet takes several years to orbit its star, we were able to combine our Doppler observations with available astrometric measurements to unambiguously determine its mass. This allowed us to build a complete model of this planetary system and study its dynamic behavior.”

Astronomers at NCU have discovered a new planet in the constellation Ursa Major. Photo credit: Andrzej Romanski

The configuration is peculiar: one planet orbits its star closely (forming a pair), while a second is far enough away to form another pair with the first. Although both planets are massive and have rather elongated orbits, their mutual gravitational influence does not destabilize the system over the eons. According to their study, this is due to the effects of general relativity, which prevents the planets from constantly changing the shape of their orbits and their orientation in space.

This makes HD 118203 one of only a handful of hierarchical systems known to astronomers, which will help develop theories about how massive planets form. This, in turn, will allow astronomers to learn more about the formation and evolution of the gas giants in our solar system – Jupiter, Saturn, Uranus and Neptune. The international team also plans to continue collecting data on this system in the hope of finding more exoplanets.

Further reading: NCU News, Astronomy & Astrophysics

Like this:

How Load…