About a hundred light-years from Earth, astronomers have found six special exoplanets (planets outside ‘our’ solar system), all with a size between that of Earth and Neptune. Astronomers suspect that planets of this size orbit more than half of all Sun-like stars in the universe, but what is special about the new find is that all six planets are in resonance with each other. This means that their cycle times are precisely coordinated, like a clock. For example, when one of the planets orbits the sun once, the other orbits the sun twice. A kind of long-distance relationship that probably only occurs in 1 percent of all planet groups.
The astronomers found the planets with NASA satellite TESS and the European satellite Cheops. She shared their discovery this week in the scientific journal Nature.
“It is great that the worlds now discovered teach us something about the formation of planets,” says Ewine van Dishoeck, astronomer at Leiden University and not involved in the study. Although planets of this size are not unique, it is still a mystery how and under what circumstances exactly they form. “When a planet group forms, the planets generally come into resonance with each other at the very beginning, but usually become unbalanced over time. All it takes is one large planet, say a Jupiter, or a star to pass by and the careful balance is upset; the planets change orbits. Then you can study a planet, but perhaps the planet only moved to its current position much later and you still do not know under what circumstances it was formed.”
The fact that the newly discovered planets are still in resonance probably means that their arrangement has remained intact since their formation about a billion years ago. “That makes it a wonderful lab for astronomers who investigate exactly how planets form.”
Scorched Jupiters
What astronomers do know is that planets probably form as clumps in a doughnut-shaped disk of gas and dust around a young star. The dust grains slowly grow into larger objects and eventually new worlds are formed.
Astronomers search for planets outside ‘our’ solar system by studying the light of stars. When a planet passes in front of its star as seen from Earth, the planet blocks some starlight. That method is called the transit method. Over the past thirty years, more than five thousand distant worlds have been found: from gigantic, scorched Jupiters to planets orbiting two stars.
Using this transit method, a team of astronomers, led by Rafael Luque from the University of Chicago, found signals from exoplanets around star HD 110067 with NASA’s space detector TESS in 2020. This is a bright star relatively close to Earth in the constellation Coma Berenices. It was still unclear how many planets were involved and what their properties were. “The disadvantage of TESS is that the telescope only scans the same part of the sky once every two years,” Luque said in a press conference. Two years later, the astronomers took another look at HD 110067 with TESS, but their questions remained largely unanswered.
9,114 days
“That’s when we decided to use ESA’s Cheops space telescope,” says Luque. TESS is good at scanning large areas of the sky, while Cheops can observe in detail. The team first found the inner three planets. It was striking that they resonate with each other. The outermost of the three takes 20.519 days to orbit the star, which is about 1.5 times as long as the middle planet’s orbital period of 13.673 days. That is 1.5 times as long as the orbital period of the innermost planet: 9.114 days. Small disturbances in the orbits of the three planets revealed the presence of the outer three planets. The outer planet takes about 54 days to complete a circuit.
Anyone who knows the mass and size of a planet can calculate its density and thus say something about the composition of the planet. The now discovered stars have relatively low densities, suggesting that they have hydrogen-rich atmospheres. Van Dishoeck: “It would be interesting to study this in more detail in the future, for example with the James Webb space telescope.”