Usually stars and their planets are aligned in the same direction. The planets orbit the central star in its direction of rotation and mostly parallel to the stellar equator. But now astronomers have discovered a planetary system that steps out of line: the star K2-290 rotates in the opposite direction to its two planets and their orbits are tilted 124 degrees from its equator. Models suggest that such a drastic deviation could only be caused by the disruptive effect of a nearby second star on the primordial cloud of this system. In fact, the researchers discovered such a “disruptive star” at K2-290.
There is order in the solar system: the earth and its neighbors all orbit the sun in the same direction and on one level and this is roughly at the level of the solar equator – even if there is a shift of a few degrees. But there are also planetary systems that are more chaotic. Some exoplanets orbit their stars on a steeply inclined orbit or even retrograde – against its direction of rotation. “If observations reveal such misalignments between the stellar rotation and the orbital movement of a planet, it is mostly attributed to the fact that the original alignment was disturbed by gravitational turbulence after planet formation,” explain Maria Hjorth of Aarhus University and her colleagues. In this scenario the planet is thrown out of its old orbit by the influence of another, heavier planet in the system or the disruptive effect of a passing star.
When a companion star disturbs the primeval cloud
But there is also the possibility that a disturbing influence is already deflecting the protoplanetary disk around a young star. According to the models, this can happen, for example, in a binary star system through the gravity effect of a distant companion star. “An ideal example of this scenario would consist of a coplanar system of several planets and a backward rotating star,” the astronomers say. Because such a configuration cannot easily be explained by the mere influence of a planet or the subsequent disruptive effect of a second star. “So far, however, no definitive examples of this type have been found,” say the researchers. It is true that there are a few cases of multi-planetary systems in which all planets are equally deflected. But a companion star could never be found in these and the central star did not rotate retrograde either.
This is different with the K2-290 system now described by Hjorth and her colleagues. Since observations with the Kepler Space Telescope in 2014, it has been known that there are two planets around the sun-like main star in this triple star system. The inner planet K2-290b orbits the star in a good nine days and has three times the radius of the earth – it is therefore a so-called “hot sub-Neptune”. The outer planet K2-290c needs around 48 days for one orbit and, with around eleven times the earth’s radius and 246 earth masses, is more similar to Jupiter. In which direction these two planets orbit their star and how around the star rotates, the astronomers have now investigated with the help of high-resolution optical spectroscopy.
Tilted orbit and retrograde star
To do this, the team observed which parts of the light the transits of the planets in front of their star swallowed in which phase. “If the stellar rotation and the orbital movement of the planet are in the same direction, then during the first half of the transit the planet blocks the blue-shifted light emanating from the side of the star that is turning towards us,” they explain. “In the second half of the transit, it is primarily the red-shifted light component that is blocked.” Using the spectrometers on several earth-based telescopes, they checked whether this is the case with K2-290, including the Very Large Telescope of the European Southern Observatory in Chile.
The observations showed that the two planets of K2-290 orbit on a uniform plane. However, this orbital plane is strongly deflected compared to the central star: It is tilted by 124 degrees and the star rotates in the opposite direction to the direction of its two planets, as Hjorth and her colleagues report. This configuration is difficult to explain by turbulence after planet formation. Instead, the team sees K2-290 as possibly the first clear example of a planetary system that was disrupted by a companion star in its early stages. “The unique aspect of K2-290 is that a companion star has already been found and that it has properties that make it a good candidate for disrupting the protoplanetary disk,” state the astronomers. In supplementary model simulations, they prove that gravity resonances between the central star of K2-290 and the companion star could actually have generated the observed configuration of the system. “The architecture of the K2-290 system shows that we cannot take it for granted that stars and their protoplanetary disks are always well aligned,” emphasize Hjorth and her team.
Source: Maria Hjorth (University of Aarhus) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2017418118