Six pack of sub-Neptune exoplanets hang tight around nearby star

Their resonant orbits have remained unchanged for some 4 billion years

Scientists have discovered a rare six-exoplanet system orbiting a nearby bright star.

Reported in Nature this week, the discovery is particularly valuable because the planets' orbital configuration shows that the system has been largely unchanged since its formation around four billion years ago.

The team of international researchers first suspected a group of exoplanets orbiting star HD110067, situated around 100 light-years away from Earth in the northern constellation of Coma Berenices, in 2020. Measurements using the Transiting Exoplanet Survey Satellite (TESS) – an MIT research facility run on behalf of NASA – detected dips in the star's brightness, suggesting planets blocking the star's light.

The first look suggested two possible planets with different orbital periods. Two years later, TESS observed the same star again, but with more data ruling out the two-planet hypothesis.

With much of the TESS data proving difficult to interpret or making little sense, the team led by Rafael Luque, a postdoctoral researcher at the University of Chicago, decided to reach for another toolkit – the European Space Agency's Characterising Exoplanets Satellite, or Cheops for short.

"We decided to use Cheops. We went fishing for signals among all the potential periods that those planets could have," Luque said in a statement.

Yet instead of two planets, a series of data models showed there were six planets with radii somewhere between Earth's and Neptune's, technically referred to as "sub-Neptunes" in astrophysics terminology.

By studying the three innermost planets, researchers found the orbits of all six, ranging from around nine days for the innermost planet to around 54 days for the outermost.

The relatively low masses of the planets suggest large, hydrogen-rich atmospheres, they said.

The planets also seem to exert regular gravitational forces on each other sufficient to produce resonant orbits, which suggests that the exoplanet system is likely to have remained unchanged since its birth at least four billion years ago, the researchers said.

Luque said the results were unusual and invited further research. "We think only about 1 percent of all systems stay in resonance. It shows us the pristine configuration of a planetary system that has survived untouched."

Maximilian Günther, ESA project scientist for Cheops, highlighted that among the only three known six-planet resonant systems, the latest result is the second to be found by Cheops, which has only been operating for three years. ®

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