Six sub-Neptunes discovered 100 light years away: sky and telescope

Astronomers have discovered six sub-Neptune exoplanets dancing in unison around the same distant star, solving a major mystery in the process. The ancient, pristine system opens a window into how planetary systems form. The team's findings are published in Nature.

In 2020, NASA's Transiting Exoplanet Survey Satellite (TESS) helped astronomers observe dips in the brightness of the star HD 110067. These dips are the calling card of transiting planets. But how many planets were there exactly? Astronomers' initial analysis of the data suggested two, one with an orbital period of 5.6 days and another with an undetermined orbit.

However, when they re-observed HD 110067 with TESS two years later, this interpretation no longer fit the data. “That's when we decided to use Cheops,” says Rafael Luque (University of Chicago), referring to the European Space Agency's exoplanet characterization satellite. “We went looking for signals among all the potential periods those planets could have.”

It quickly became clear that there was a third planet in the system. The trio of planets (called HD 110067 b, c and d) are in orbital resonance, meaning that their orbital periods are closely related. The inner planet takes 9.1 days to orbit, the next one takes 1.5 times as long, 13.7 days, and the third takes 1.5 times as long, 20.5 days. These are known as resonances 3:2.

The planets' linked orbits mean they are locked into a rhythm and align every few orbits, creating duo transits, the astronomical equivalent of a musical chord. This phenomenon is what caused the initial confusion.

However, the discoveries did not end there, as a three-planet system still did not fit perfectly. Instead, Luque's team dug deeper and discovered three more planets (HD 110067 e, f and g) hidden in the data. There is a 4:3 resonance between e and f, as well as between f and g. They all have radii greater than those of Earth but smaller than those of Neptune, which places them in the sub-Neptune category.

“Cheops gave us this resonant configuration that allowed us to predict all the other periods,” says Luque. “Without that detection by Khufu, it would have been impossible.” Is he Cheops discovers a system of six planets for the second time in the three years since its launch; the first is known as TOI 178.

“I think it's a wonderful discovery,” says Carole Haswell (Open University, UK), who was not involved in the research. The resonance also sheds light on the system's past. “It probably means that the planetary system has been locked in this configuration for most of the star's life,” she says.

Most planetary systems are thought to form in a resonant configuration, but this is a delicate balance that is easily upset by the influence of giant planets, passing stars, or even large impacts. “We believe that only about 1% of all systems remain in resonance,” says Luque. “(HD110067) shows us the pristine configuration of a planetary system that has survived intact.”

We could soon learn more about these planets. “They are a great prospect for a technique called transmission spectroscopy, which can tell us about the composition of planets' atmospheres,” says Haswell. HD 110067 is the brightest known system with four or more planets, and the James Webb Space Telescope is well suited for such a task.

Understanding planets like this sextet is important work, since sub-Neptunes are the most common type of planet in the universe. Figuring out how they form could help us explain why we don't have one in our own solar system and, in turn, whether that's related to how we ended up here to ask these questions in the first place.