Six planets that orbit their star in a coordinated dance have been discovered by scientists, who say the finding could help shed light on why planets in our own solar system move to their own beat.
The newly discovered planets orbit a star that sits about 100 light years away in the constellation Coma Berenices, with a mass about 20% smaller than our Sun.
Not only is their makeup different from planets within our solar system, but their movements appear to be tied together: the team said the time it takes one planet to travel around the star was related to that of the next planet by a neat ratio.
“This system has this very delicate resonant configuration, which has been preserved for billions of years,” said Dr Rafael Luque, co-author of the research from the University of Chicago.
The team said such “resonance” should be common within planetary systems, arising from gravitational interactions between planets that begin as they form.
However, in reality only about 1% of observed planetary systems show resonance – and even fewer involve as many as six planets moving in a coordinated fashion.
The rarity, the researchers said, could be down to other systems, including our own, which experience events that distort planetary orbits – for example, the formation of massive planets such as Jupiter or Saturn, or impacts by meteorites.
The team added that the newly discovered planets sit close to their star, with temperatures of 170-650C, and have diameters two to three times that of Earth but smaller than Neptune, making them “sub-Neptunes”. The masses of the planets and their densities were elucidated using ground-based measurements.
While their composition remains unknown, the team said the planets probably had a solid core of rock, ice or even iron, and were enveloped in hydrogen and helium.
“Even though we have found so many planets like these ones outside of the solar system we do not know much about them,” said Luque.
Luque added that with six sub-Neptunes of varying sizes, temperatures and masses around the same, bright star, astronomers now had a way to explore how and why such planets differed, without having to take into account a host of possible influences such as different origins, evolutionary paths, or starting materials.
The team said they had their first inklings of the new system in 2020, when dips in the brightness of a star known as HD 110067 were detected by Nasa’s Transiting Exoplanet Survey Satellite – or Tess – revealing planets were passing in front of it.
Further data from Tess has revealed that one planet had a nine-day orbit while another took 13 days to orbit the star. Subsequent data from the European Space Agency’s Characterising Exoplanet Satellite (Cheops), suggested yet another planet took 20.5 days to orbit the star.
The team realised these orbits formed neat ratios: the first planet from the star makes three orbits in the time it takes the second planet to make two orbits, and the second planet makes three orbits in the time it takes the third planet to make two orbits.
The discovery led the team to propose that the orbits of the other three planets in the system also would be related by simple ratios.
Further observations confirmed they were right.
“I was kind of shocked and delighted,” said Dr Hugh Osborn, a co-author from the University of Bern.
The study is published in the journal Nature.
• This article was amended on 30 November 2023 to correct the name of the star HD 110067 after an earlier version transposed the last two digits.
