Some particular aspects of our solar system – an enveloping cloud of comets, dwarf planets in strange orbits and, if there really is, a possible new planet far from the sun – have been linked to the close approach of another star from our early childhood system that threw things helter-skelter.
But are stellar flybys really capable of overturning planets, comets, and asteroids by reshaping entire planetary systems?
Astronomers at Berkeley University and Stanford University believe they have found a smoking gun.
A planet orbiting a young binary star may have been disturbed by another pair of stars that skated too close to the system 2 or 3 million years ago, soon after the planet was formed from a swirling disc of dust and gas.
If confirmed, this reinforces the arguments that nearby stars help to carve planetary systems and can determine whether or not they host stable-orbiting planets.
"One of the mysteries of the study of the exoplanets is that we see systems where the planets are misaligned, even if they are born in a flat and circular disk," said Paul Kalas, assistant professor of astronomy at the University of California. Berkeley University. "A cosmic tsunami may have hit those systems and rearranged everything, but we have no proof. Our article provides rare evidence of observation for any of these overflights gently influencing one of the planetary systems of the galaxy. "
Astronomers are already looking for a stellar flyby in the past of our solar system, but since this probably happened 4.6 billion years ago, most of the evidence has disappeared. The star system studied by astronomers, identified only by HD number 106906 and located about 300 light-years from Earth towards the constellation of Crux, is very young, about 15 million years old. only years old.
Kalas and Robert De Rosa, a former postdoctoral fellow at the University of Berkeley and currently a researcher at Stanford's Kavli Institute for Particle Astrophysics and Cosmology, describe their findings in an article accepted for publication in the journal. Astronomical Journal and now available online.
Kalas, who studies young, newly formed planetary systems to try to understand what happened in the early years of our own solar system, first focused on HD 106906 in 2015 after having discovered that his planet was gigantic in an extremely unusual orbit. The planet, named HD 106906b, has a mass of about 11 Jupiters and revolves around HD 106906 – recently revealed to be a binary star – in an orbit inclined at about 21 degrees from the plane of the disc containing all the rest of the matter l & # 39; star. Its current location is at least 738 times farther from its star than the Earth is from the sun, or about 18 times farther from its star than Pluto is from the sun.
About 2 to 3 million years ago, in a young, newly formed planetary system, a planet was at risk of being expelled from the system due to gravitational interactions with the binary central star ( left panel). A pass close to another binary star (not shown) in the same group gave the planet an extra kick that stabilized the orbit and saved it from some ejection (right panel). (Video of Paul Kalas)
Kalas used both the Gemini Planet Imager on the Gemini telescope in the Chilean Andes and the Hubble Space Telescope to take a closer look at HD 106906 and discovered that the star also possessed an asymmetrical comet belt. The strange orbit of the planet and the fact that the dust disk itself is asymmetrical indicates that something has disturbed the young system.
Kalas and his colleagues, including De Rosa, suggested that the planet had been driven out of its solar system by interactions with another unprecedented planet in the system or by a passing star. Kalas and De Rosa now believe that both have occurred: the planet was projected into an eccentric orbit when it came dangerously close to the central binary star, a scenario proposed in 2017 by the theoretician Laetitia Rodet and her collaborators at the Grenoble Observatory in France. Repeated gravitational kicks from the binary would have quickly ejected the planet into interstellar space, but passing stars saved the planet by moving its orbit a safer distance from the binary.
The Gaia Space Observatory provided them with the data needed to test their hypothesis. Launched in 2012 by the European Space Agency, Gaia collects accurate measurements of the distance, position and movement of 1.3 billion stars in the Milky Way, a catalog 10,000 times larger than his predecessor, Hipparcos.
Kalas and De Rosa collected information about Gaia regarding 461 stars in the same group as the HD 106906 and calculated their positions back in time – reversing the cosmic clock, so to speak – and discovered that another system of binary stars could have approached enough 3 million years ago. to change the planetary system.
"What we did here is actually to find the stars that could have given the HD 106906 an extra kick, a second kick that would make it durable, as would a hypothetical planet. Nine in our solar system, "said Kalas. .
They also discovered that the binary star had followed a trajectory about 5 degrees from the system disk, making it even more likely that the encounter had a strong and lasting impact on the HD 106906.
This double kick can be important for stabilizing planets, asteroids and comets around the stars, Kalas said.
"Studying the HD 106906 planetary system, it's like going back in time to observe the cloud of Oort comets form around our young sun," he said. "Our own giant planets have gravitationally pushed countless comets over great distances. Many were ejected completely and became interstellar objects like'Uumuamua, but others were influenced by passing stars. This second shot made by a stellar flyover can detach the orbit of a comet from any new encounter with the planets, thus avoiding the prospect of its ejection. This chain of events has preserved for billions of years the most primitive material of the solar system in a very cold gel.
Kalas hopes that future observations, such as an updated catalog of Gaia measurements, will clarify the importance of HD 106906 overflight.
"We started with 461 suspects and found two who were at the crime scene," he said. "Their exact role will be revealed as we gather more evidence."
The work was supported by the National Science Foundation (AST-1518332), the National Aeronautics and Space Administration (NNX15AC89G) and Nexus for the Science of Exoplanet Systems (NExSS), a research coordination network sponsored by the Directorate of Science and Technology. NASA Science Mission (NNX15AD95G).