Astronomers Discover New Planet in Ursa Major Constellation

Astronomers from the Nicolaus Copernicus University in Torun have discovered a new planet in the constellation Ursa Major. Its mass is 11 times that of Jupiter, it orbits its star every 14 years, and its temperature does not exceed -100 degrees Celsius.

The scientists described the cosmic discovery in the journal Astronomy and astrophysics.

A discrete star with a massive planet

Astronomers are dealing with an extremely massive exoplanet, with a mass 11 times that of Jupiter, the largest planet in our solar system. It orbits its parent star every 14 years and is located six astronomical units away.

“We cannot see a planet, but we can spot the star around which it orbits, with a small telescope of only 10 cm. The physical parameters of the star are similar to those of the Sun. The data indicate that it is 20% more massive and twice as large as the Sun. Interestingly, it has already completed the stage of evolution in which the Sun is currently; it has a backyard of 5 billion years behind it. We can therefore estimate that this age also corresponds to that of the entire planetary system,” explains Dr. Habil.

Gracjan Maciejewski, a professor at North Carolina National University and head of the research group at the Institute of Astronomy at North Carolina National University, explains: “It is located on the northern side of the sky in the constellation Ursa Major and has the designation HD 118203, because it was first listed in Henry Draper’s star catalogue under this number. The telescope used to make observations for this catalogue more than a century ago is now in our Piwnice Observatory near Torun.”

The Draper Telescope

The Draper Telescope is one of the world’s first astrographs, or photographic recorders of celestial phenomena. It was built in 1891 in memory of the American spectroscopic physicist Henry Draper, who died prematurely, with whom his wife Anna Maria supported the ambitious program of the Harvard Observatory, led by Edward C. Pickering, to develop a catalog of photographic and photovisual luminosities of stars and their spectral classification.

Over 60,000 photometric and spectral images of the sky were taken with this telescope at Cambridge, and it contributed so much to Pickering’s intention that the compiled inventory containing nearly a quarter of a million stars was called the Henry Draper Catalogue. The “HD” star designations are still in use today and are familiar to astronomers everywhere.

How did Draper’s astrograph come to Piwnice? In the autumn of 1947, construction began on the first observation pavilion of the NCU astronomical observatory, with a rotating dome five metres in diameter. Two years later, an astrograph sent from Cambridge moved in and, after the necessary adaptations, began its regular observation work.

Today, this interesting monument, unique in the world, has become an attraction for visitors to the Institute of Astronomy of the National University of Poland in Piwnice.

Patience pays off

For nearly 20 years, astronomers have known that the star HD 118203 orbits a fairly massive planet. In 2006, the first gas giant was discovered, with a mass twice that of Jupiter, orbiting the star in a tight orbit in just six days.

“Doppler observations, however, showed that the story did not end there, that there could be another planet. That is why we immediately included this system in our observation programs,” explains Professor Andrzej Niedzielski, co-author of the discovery.

“First, as part of the Torun-Pennsylvania Exoplanet Research Program, conducted in collaboration with Professor Aleksander Wolszczan, we followed 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 Torunians, with Spanish collaborators, continued observations of the star in the Canary Islands, using the Italian Galileo telescope. This observatory was equipped with the best instrumentation designed to discover planets.

“However, eight years of research have not allowed us to determine what type of object it is,” adds Professor Niedzielski.

It took another seven years for the Torun astronomers to obtain irrefutable proof that they were dealing with a planet.

“Patience is rewarded,” says Professor Maciejewski. “The new observations collected in March 2023 proved crucial in determining the orbital parameters of the planet. Moreover, since it takes several years for a planet 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 dynamical behavior.”

But before that, it was necessary to make sure that there were no other planets hiding in the system. This task was taken on by Julia Sierzputowska, an astronomy student.

“I analyzed photometric observations obtained with the Transiting Exoplanet Survey Satellite, showing that there were 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,” explains Julia Sierzputowska.

Planetary Tandem

It turned out that astronomers had discovered a hierarchical planetary system.

“This is a special configuration in which one planet forms a close pair with its star, and a second planet orbits the pair in an orbit wide enough to, as it were, form another pair with the first,” explains Professor Krzysztof Goździewski, who conducted detailed numerical studies of the system’s dynamics.

Both planets are massive and orbit rather elongated orbits. Despite this, their mutual gravitational influence does not destabilize the system on a time scale of millions of years.

“We have shown that this is due to effects arising from the general theory of relativity. Without these effects, the planets would behave like nervous springs, constantly changing the shape of their orbits and their orientation in space,” adds Professor Goździewski.

Cosmic Answers

Astronomers admit that knowledge of the formation and evolution of planetary systems still hides many fundamental unknowns. Hierarchical systems like HD 118203, of which only a dozen examples are known, allow us to probe hypotheses on the formation of massive planets.

“It is interesting to ask how these planetary configurations evolve,” says Professor Maciejewski. “Although from our perspective – that of the inhabitants of the solar system – they are rather ‘exotic’, it seems important to know more about systems with massive gas planets in order to better understand our closest ‘astronomical backyard’.”

“Our work does not stop here. We continue to observe and analyze the data. There is still a chance to make other planetary discoveries,” says Professor Niedzielski. “It is a great satisfaction to be able to involve students and doctoral students in this interesting and important research.”