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Geometric patterns in space: representation of the orbits of the planetary system around HD110067

Photo: Thibaut Roger / NCCR Planets

Several planets orbit the star, which is about a hundred light-years away, HD110067 – but how many and on which orbits has been a mystery to experts until now. Detailed observations by an international research team with the European space telescope "Cheops" have now provided the solution.

According to this, there are a total of six planets that orbit the star in resonance – i.e. in unison. This is highly unusual. It means that the orbits have remained stable for about eight billion years, the scientists report in the journal Nature.

Three years ago, NASA's Tess space telescope detected faint changes in the brightness of the star HD110067, which is slightly smaller than our Sun at 80 percent the diameter. At over eight billion years old, however, it is almost twice as old. Changes in brightness occur regularly when planets pass in front of the star as seen from Earth and therefore darken it somewhat. Most of the more than 5500 known planets around other stars have betrayed themselves through such transits.

At least two planets must be orbiting HD110067, the astronomers concluded from the "Tess" data, but the brightness fluctuations did not really fit possible orbits. Further measurements with »Tess« in 2022 also did not bring any clarity. Rafael Luque of the University of Chicago and his colleagues have therefore taken a closer look at the star with the help of "Cheops".

Undisturbed resonances

The space telescope, which was launched in December 2019, does not monitor the brightness of many stars at once, unlike Tess, but observes individual stars with high accuracy for which planets are already known in order to obtain detailed information about these systems. This allowed us to specifically search for signals that match potential orbits," explains Luque.

Using the "Cheops" data, the team was initially able to detect three planets with orbital periods of 20.5 days, 13.7 days and 9.1 days. These periods are each in an integer ratio of 3:2 to each other. "Resonances" is what astronomers call such relationships. Computer models of the formation of planetary systems show that planets often form in orbits that resonate with each other.

However, such conditions can be thrown out of balance by a variety of influences – a nearby star or impacts from asteroids. "However, the architecture of this planetary system has remained almost unchanged since its birth," Luque and his team emphasize. In this way, the unusual system provides an insight into its formation history that remains hidden from other stars with their orbits altered by perturbations.

Mini-Neptune, not super-Earths

After the transits of the three planets had been clarified, further initially inexplicable changes in the star's brightness remained in the data from "Cheops". Luque and his colleagues explained it to themselves by using a trick: Since the architecture of the system is still in its original state, other planets should also move in resonance with each other, they thought. In this way, they were able to explain all observations with three other planets with orbital periods of 30.8 days, 41.1 days and 54.8 days.

The six planets are all between two and three times the size of Earth. Additional observations with the Very Large Telescope of the European Southern Observatory in Chile also provided the team with information about the masses of the planets. This revealed another peculiarity: Apparently, the six companions of HD110067 are not "super-Earths", i.e. larger rocky planets, but a kind of miniature version of the planet Neptune with extensive atmospheres.

This makes HD110067's system an ideal observation object for the James Webb Space Telescope, according to the experts. With its large mirror, it can determine the composition of the planet's atmospheres and provide valuable information about the nature of such mini-Neptunes.