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Astronomers discover new six-planet system

Astronomers have discovered a new six-planet system (Thibaut Roger/NCCR PlanetS/PA)
Astronomers have discovered a new six-planet system (Thibaut Roger/NCCR PlanetS/PA) Astronomers have discovered a new six-planet system (Thibaut Roger/NCCR PlanetS/PA)

Astronomers have discovered a rare six-planet star system.

The planets orbit their central star in a rhythmic beat, a rare case of an “in sync” gravitational lockstep that could offer deep insight into planet formation and evolution, researchers say.

The star is smaller, and slightly dimmer than the Sun, and the six “sub-Neptunes” – possibly smaller versions of Neptune in our solar system – move in a cyclic rhythm.

According to the experts, this orbital waltz repeats itself so precisely it can be readily set to music.

The star, HD110067, is 100 light-years away in the northern constellation of Coma Berenices, and had perplexed researchers for years.

Now scientists, including those at the University of Warwick, have revealed the true architecture of this unusual system using Nasa and European Space Agency (Esa) spacecraft.

The analysis was led by University of Chicago scientist Dr Rafael Luque, who said: “This discovery is going to become a benchmark system to study how sub-Neptunes, the most common type of planets outside of the solar system, form, evolve, what are they made of, and if they possess the right conditions to support the existence of liquid water in their surfaces.”

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 which suggested planets were passing in between the star and the spacecraft.

A preliminary analysis revealed two possible planets – one with a year (the length of 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, and analysis ruled out the original interpretation but presented two additional possible planets.

Much was still unknown about the planetary system, until scientists across the world – including those at the University of Warwick – joined the investigation.

They used data from Esa’s Characterising Exoplanet Satellite (Cheops), hoping to determine the orbital periods of these faraway planets.

While multi-planet systems are common in the Milky Way, those in a tight gravitational formation known as “resonance” are observed by astronomers far less often.

In this case, the planet closest to the star makes three orbits for every two of the next planet out – called a 3/2 resonance – a pattern that is repeated among the four closest planets.

Among the outermost planets, a pattern of four orbits for every three of the next planet out (a 4/3 resonance) is repeated twice.

Thomas Wilson, from the Department of Physics 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.”

Researchers say the planets – two to three times the size of Earth – are likely to have been performing this same rhythmic dance since the system formed billions of years ago.

Dr Luque said: “We think only about 1% of all systems stay in resonance, and even fewer show a chain of planets in such configuration.”

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

Planets around stars tend to form in resonance but can easily have their orbits thrown around.

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

Therefore, multi-planet systems preserving their resonance are rare.

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 using the James Webb Space Telescope (JWST) and the Esa’s future Ariel telescope.

Mr Wilson added: “All of these planets have large atmospheres – similar to Uranus or Neptune – which makes them perfect for observation with JWST.

“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 – 170C to 530C – which would make it very difficult for life to exist.”

Hannah Osborne, a PhD student at UCL’s Mullard Space Science Laboratory and a co-author of the study, said: “The system itself is a key discovery for exoplanet science: because all six planets are in a resonant chain we know that the architecture of the system can’t have changed much since its formation, so by studying HD110067 we get a rare window into the past to understand how these types of systems may have formed and evolved.”

– The findings are published in the Nature journal.