It’s like a normal solar system … only completely backwards.
When solar systems form, both the parent star and all planets come from the same molecular cloud. When the cloud collapses, it spins out of conservation of angular momentum, just like a figure skater pulling on its arms. The cloud maintains this spin when it flattens out into a protoplanetary disk, and so the spin of the central star is aligned with the direction of the orbits of its planets.
Easy right?
But apparently the K2-290 system discovered by Maria Hjorth and Simon Albrecht from the Stellar Astrophysics Center at Aarhus University didn’t get any attention in its Astro101 class. This system has two planets that are both orbiting in the same direction, but those orbits are almost entirely in the opposite direction of their star.
“We have found a very fascinating planetary system,” said Hjorth. “There are two planets that orbit the star in almost opposite directions when the star rotates on its own axis.” This is different from our own solar system, where all planets rotate in the same direction as the rotation of the sun. “
Astronomers have discovered such “backward” systems before, but this time they are two planets and another distant orbiting star.
This star is key to explaining what went wrong with K2-290. When the system first formed, the distant star may pull on the early protoplanetary disk, causing it to tip so badly that it overturned completely.
Study co-author Rebekah Dawson of Pennsylvania State University said: “In every planetary system it is assumed that the planets form in a spinning, circular disk of material that swirls around a young star a few million years after the birth of the star, called a protoplanetary disk. Usually the disk and star rotate in the same way. However, if there is a neighboring star (where “neighbor” means within a light year or so in astronomy), the gravitational force of the neighboring star can tilt the disk. “
What does this mean for future exoplanet hunting? Simon Albrecht from the Stellar Astrophysics Center in Aarhus summed it up best: “I find our results encouraging, as we have found another aspect of the system architecture in which planetary systems have a fascinating variety of configurations.”
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