When the orbital periods of a planet they form a simple fraction (that is, both numbers are integers), they are said to be in harmony. Using the Cheops space telescope (Characterizing Exoplanets Satellite), operated by the European space agency, astronomers from the universities of Bern and Geneva, Switzerland, found such a harmonic relationship between five out of six exoplanets orbiting the Estrela TOI-178, more than 200 light years from us.
The first observations of this solar system were made using NASA's Transiting Exoplanet Survey Satellite (Tess) in transit and pointed to three exoplanets, with two of them orbiting very close. But the results were considered "inconclusive", which led to further investigations.
“We observed the system with additional instruments, such as the Espresso spectrograph (Echelle Spectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations, Echelle Spectrograph for Stable Spectroscopic and Rocky Exoplanet Observation) based on the ground at the Paranal Observatory of the European Southern Observatory (ESO, European South Observatory) in Chile, but the results were inconclusive, ”says Adrian Leleu, an astronomer at the Center for Space and Enablability of the Universe at the University of Bern.
“After analyzing the data from eleven days of observation of the system with Cheops, there seemed to be more exoplanets than we had initially thought”, says Leleu. The team identified a possible solution with five exoplanets and decided to invest another day of observation in the system to confirm. They found that there were in fact five exoplanets present with orbital periods of about 2, 3, 6, 10 and 20 days, respectively.
The discovery of a system with five exoplanets would, in itself, be a remarkable fact. But what intrigued the astronomer was that they seemed to be in harmony. “Our theory implied that there could be an additional exoplanet in this harmony; however, its orbital period needed to be very close to 15 days, ”he said.
A new observation with Cheops confirmed that the system around TOI-178 has, in fact, six exoplanets. Thanks to the precision of the telescope's instruments, combined with data from Tess, scientists were able to measure the periods and size of exoplanets 1,1 to 3 times the size of Earth. And then they had another surprise.
“In the few systems that we know of with such harmony, the density of exoplanets constantly decreases as we move away from the star. In the TOI-178 system, a dense and terrestrial exoplanet like Earth appears to be very close to an exoplanet with half the density of Neptune followed by a very similar one to Neptune, ”says Kate Isaak, scientist at the Cheops Project at NASA.
“The system has therefore proved to be something that challenges our understanding of the formation and evolution of planetary systems,” says Leleu.
Source: Eureka Alert