by Ruth Titz-Weider (DLR), October 2024
Orbiting the star HD 110067, slightly over 100 light years from here, six planets were discovered using the space telescopes TESS and CHEOPS (Luque et al, nature 2023). The exceptional thing are the orbital periods of these planets, because they are in small whole-number ratios to each other, for example 3:2 or 4:3. In the time planet c takes for three orbits, planet b has gone around the star twice. In music, one would call this frequency ratio a fifth, and the ratio 4:3 a fourth.
The different resonances in the HD 11067 system. Credit: ESA
This harmony of orbit times has probably been established since the system’s formation and was not disrupted by catastrophic events. So we can assume that this configuration has been stable for 8 billion years. Scientists had previously found other planet system in harmonic arrangement. but for HD 110067, this is true for the whole chain of 6 planets, and therefore it could shed some light on the formation and evolution of multiple planet systems.
Because HD 110067 is a very bright star – brightness of 8 magnitudes – this star in the constellation “Hair of Berenike” will continue to be a hgihly interesting observation target in future, that will likely be observed with the James Webb Space Telescope.
The process of discovery and characterization happened over quite a long time and with several telescopes. It began with the NASA space telescope TESS (Transiting Exoplanet Survey Satellite), that found brightness variations in the star HD 110067, which were attributed to two planet candidates, so-called TESS Objects of Interest (TOI). Two years later, in February and March 2022, TESS observed the star a second time. The combined data revealed two planets. The first one, HD 110067 b (the first discovered planet around another star always gets the letter “b”) showed an orbital period of 9 days, the second planet HD 110067 c takes 13.6 days for its orbit. But there remained transit events that could not be interpreted for the time being.
ESA‘s space telescope CHEOPS (Characterising Exoplanet Satellite) was able to untangle this riddle a bit, and confirmed a third planet HD 110067 c, which takes 20 days for an orbit. But the data for each TESS sector still contained some further dips in the light curve, and therefore hints for possible further transit events that could not yet be accounted for. Under the assumption that the harmonic arrangement could be continued, researchers could match two transits – which were from the two TESS sectors and thus well-separated in time – to a fourth planet: HD 110067 e with a period of 30.8 days. Then, there were two single transits, which the researchers suspected to be the result of two other planets “f” and “g”. From mathematical considerations regarding stable orbits and possible combinations of orbit periods that could be integrated into the existing data without inconsistency, they derived two more planets: HD 110067 f with 41 days and HD 110067 g with 54.7 days orbit.
