Cal Poly physics professor David Mitchell has discovered a number of planets throughout his career, but never one that shouldn’t exist — until now. Mitchell’s discovery raised questions about the process by which planets are formed, something he hopes will lead to more answers about how other planets, stars and solar systems like our own come into existence.
Mitchell is an observational astronomer, studying planets that orbit red giants huge, bright stars with low surface temperatures that are in their last phases of life. While it’s common to find planets that orbit other stars, called extrasolar planets or exoplanets in some cases, Mitchell’s newly discovered planet formed under circumstances that do not typically allow for planet formation.
The planet orbits a red giant in a binary system, a system where two stars orbit each other. Those two stars, Mitchell said, are very close to each other. While planets form as byproducts of nebulas collapsing and creating stars, the gravitational pull of a neighboring star lingering too close is enough to disrupt the planet-making process altogether. That’s what makes the existence of this newly discovered planet so puzzling.
“How you could possibly end up with a planet there, when the process shouldn’t even be possible with a star nearby? We don’t really have an explanation for it,” Mitchell said.
Cal Poly is the only American university successfully researching planets that orbit red giants, but scientists in Germany, Australia and Japan are working with Mitchell on his research. Sabine Reffert, an astronomer at the Center for Astronomy at Heidelberg University in Germany and lead of the project, is one of Mitchell’s primary collaborators.
According to Reffert, one explanation for the planet’s existence is that the two stars could have formed after the planet was already in place. Another theory, she said, is that the planet formed after the star.
“There is the possibility of second generation planet formation, which implies that the planet is much younger than the star, in contrast to regular planet formation theory where planets form in the disks around young stars and thus have approximately the same age as their host stars,” Reffert said in an email to Mustang News.
Mitchell has also studied extrasolar planets with his students. Though no students are involved with this project, some, like aerospace engineering sophomore Elle Glad, spent last summer at Cal Poly’s observatory collecting data on roughly 400 extrasolar planets.
“[Mitchell] works so hard in the specific areas that he loves and he’s good at it,” Glad said. “I think we’re naturally curious people and I think that more knowledge about our universe is just interesting. It helps us learn more about our planet and why we’re here.”
There aren’t many groups researching these kinds of extrasolar planets, Mitchell said, so his discovery is one of the first of its kind.
“The planets we’re finding are some of the first data points we have. It’s not like we have this big collection of information about planets going around these stars,” Mitchell said. “So in that sense, in our small way, every planet we discover is interesting, just because we have so few of them we know about.”
Not having an explanation for this planet’s existence can actually help advance scientific discoveries, Mitchell said.
“If we can’t explain why it’s there … that’s where the good discoveries, the good theories come from,” Mitchell said. “You have to change what you think is happening.”
Mitchell said there’s only one other known instance of a planet that was formed in close proximity with a neighboring star. The fact that there are now two, means it’s more likely there’s a piece of planet and star formation we don’t understand.
“If there’s one example, you can imagine that it’s just some crazy thing happened,” Mitchell said. “But if there’s two of them, then maybe it’s not some freak thing, there’s some fundamental piece we don’t understand about the process.”
Now that the planet has been discovered, it’s up to other astronomers to find out how and why it can exist. Mitchell said that’s out of his field of expertise, so it will likely be taken on by a scientist studying star formation. While this discovery is still in its early stages, Mitchell hopes it will be beneficial for learning more about the formation of stars and planets.
“No one planet is ever that important because we know of thousands of planets now,” Mitchell said. “But I think the fact that there’s more than one of these means there’s something there.”