When the primordial interstellar clouds of gas and dust begin to coalesce to form solar systems and stars, they start to spin. Gravity and other forces cause the matter to rotate much like a whirlpool or tornado. As the larger proto-stars and proto-planets take shape, they will usually continue to follow the same path or direction. Or do they?
Can A Planets Orbit Be Reversed?
Since 1995, over 500 extrasolar planets have been discovered. Within the last few years, astronomers have seen that — in a few cases — the star is spinning in one direction, and the planet is orbiting in the opposite direction.
“This is extremely unusual, and it’s even more unusual because the planet is very close to the star,” said Frederic Rasio, a theoretical astrophysicist at Northwestern University. “How can a star be spinning one way and the planet is orbiting in the opposite direction? It’s crazy. It violates our most basic theory of planet and star formation.”
The planets he is referring to are typically giant planets called “hot Jupiters” that orbit incredibly close to their star. Attempting to solve how these ‘hot Jupiters’ got so close to their stars led Rasio and his team to also explain their paradoxical orbits. Details of their discovery are published in today’s issue of the journal Nature.
Using a new gravitational physics program on a supercomputer, his team was able to perform large-scale computer simulations which was the first to accurately model how a hot Jupiter’s orbit can flip and go in the direction opposite to the star’s spin. They found that by adding more planets into the system, the gravitational anomalies from those planets have a mutual effect.
“Once you have more than one planet, they perturb each other gravitationally,” Rasio said. “It’s interesting because that means whatever orbit they were formed on isn’t necessarily the orbit they will stay on. These mutual perturbations can change the orbits, as we see in these extrasolar systems.”
In explaining the peculiar configurations of certain extrasolar systems, the researchers have also added to our understanding of planetary system formation and reflected on what their findings mean for our solar system.
Our Solar System Is Special?
“We thought our system was common in the universe, but from day one everything has looked strange in the extrasolar planetary systems,” Rasio said. “That makes us the oddball in reality. Learning about these other systems provides context on how special our solar system is”.
Rasio said his team used orbital mechanics to solve the problem; the same kind of physics NASA uses to send satellites around our solar system.
“It was a beautiful problem,” said Naoz, “because the answer was there for us for so long. It’s the same exact physics, but no one realized it could explain hot Jupiters and flipped orbits.”
“Doing the calculations was not easy,” Rasio said, “Some of the approximations used previously by others were not quite right. We were doing it right for the first time in 50 years, thanks in large part to the persistence of Smadar.”
“It takes a smart, young person who first can do the calculations on paper and develop a mathematical model and then turn it into a computer program that solves the equations,” Rasio added. “This is the only way we can produce real numbers to match the actual measurements taken by astronomers.”
How Does It Happen?
In their model, the researchers assume a star like our sun, and a system with two planets. The inner planet is a gas giant much like Jupiter, and initially it is far from the star, where gas giants are thought to form. The outer planet is also just as large but is much further out. Its gravity interacts with the inner planet, perturbing it and shaking up the system.
The effects on the inner planet are weak at first but build up over a long period of time, resulting in two significant changes in the system: the inner gas giant orbits very close to the star and its orbit is in the opposite direction of the star’s spin. This happens because the two orbits are exchanging angular momentum, and the inner one loses energy via tidal forces.
The gravitational coupling between the two planets causes the inner planet to go into an eccentric, needle-like orbit. It has to lose a lot of angular momentum, which it does by dumping it onto the outer planet. The inner planet’s orbit gradually shrinks because energy is lost through tides, pulling in close to the star and producing a hot Jupiter. During this process, the orbit of the planet reverses.
About a quarter of these hot Jupiter systems found show ‘reversed’ orbits. The Northwestern model needs to be able to produce both flipped and non-flipped orbits, and it does, Rasio said.
Read more at the National Science Foundation