Rare six-planet star system discovered

By Stephen Beech via SWNS

A rare six-planet star system has been discovered - with a layout unchanged for billions of years.

The star, HD110067, is 100 light-years away in the northern constellation of Coma Berenices and has been perplexing astronomers for years.

Now scientists, including University of Warwick academics, have revealed the true architecture of the unusual system using NASA and ESA spacecraft.

They explained that, usually, the orbits of planets are disrupted over time. But those orbiting HD110067 have remained unscathed.

The planets have large atmospheres – similar to Uranus or Neptune - and are much hotter than Earth, according to the research team.

The first indication of planets orbiting the strange star system came in 2020, when NASA’s Transiting Exoplanet Survey Satellite (TESS) detected dips in the star’s brightness that suggested planets were passing in between the star and the TESS spacecraft.

A preliminary analysis revealed two possible planets: One with a year - the time it takes to complete one orbit around the star - of 5.64 days, and another with an unknown period at the time.

Two years later, TESS observed the same star again.

Analysing all data ruled out the original interpretation but instead presented two additional possible planets, changing the picture of the planetary system completely.

Much was still unknown about the planetary system when Doctor Rafael Luque, of the University of Chicago in the US, and scientists around the world - including the Warwick team - joined the investigation.

They used data from the European Space Agency’s (ESA) CHaracterising ExOPlanet Satellite (CHEOPS), hoping to determine the orbital periods of the distant planets.

The CHEOPS data proved key tp confirming a third planet in the system and the team were then able to unlock the whole system.

It was now clear that the three planets were in a pattern of orbits known as an ‘orbital resonance’.

For example, an outer planet takes 20.52 days to orbit, which is extremely close to 1.5 times the orbital period of the next planet with 13.67 days. That in turn is almost exactly 1.5 times the orbital period of the inner planet.

Dr. Thomas Wilson, a senior research fellow at the University of Warwick, said: “By establishing this pattern of planet orbits, we were able to predict other orbits of planets we hadn’t yet detected.

"From this we lined up previously unexplained dips in starlight observed by CHEOPS and discovered three additional planets with longer orbits.

"This was only possible with the crucial CHEOPS data.”

He explained that orbitally resonant systems are extremely important to find because they tell astronomers about the formation and subsequent evolution of the planetary system.

The researchers said that planets around stars tend to form in resonance, but can easily have their orbits thrown around.

For example, a massive planet, a close encounter with a passing star, or a giant impact event can all disrupt the careful balance.

As a result, many of the multi-planet systems known to astronomers are not in resonance, meaning that multi-planet systems preserving their resonance are rare.

Dr. Luque said: “We think only about one percent of all systems stay in resonance.

"That why HD110067 is special and invites further study. It shows us the pristine configuration of a planetary system that has survived untouched.”

Maximilian Günther, ESA project scientist for CHEOPS, said: “As our science team puts it: CHEOPS is making outstanding discoveries sound ordinary.

"Out of only three known six-planet resonant systems, this is now the second one found by CHEOPS, and in only three years of operations."

He said HD110067 is the brightest known system with four or more planets. Since those planets are all sub-Neptune-sized with likely larger atmospheres, it makes them ideal candidates for studying the composition of their atmospheres.

Dr. Wilson added: “All of these planets have large atmospheres - similar to Uranus or Neptune - which makes them perfect for observation.

"It would be fascinating to test if these planets are rocky like Earth or Venus but with larger atmospheres - solid surfaces potentially with water.

"However, they are all much hotter than Earth, 170 to 530 degrees Celsius, which would make it very difficult for life to exist.”