Billions and Billions of Stars — Part One

Astronomy PhD candidate Christian Clanton is first author on a pair of papers recently published in the Astrophysical Journal. Clanton carefully compared and combined results from two different planet discovery methods, microlensing and radial velocity, to make the most comprehensive estimate to date of the frequency of planets around low-mass stars (M dwarfs).

Given that M dwarfs comprise the majority (roughly 70%) of all stars, his conclusions have far-reaching implications for the prevalence of planets in the Galaxy.

Furthermore, the methodology he developed in these studies provides researchers with a powerful tool in their quest to determine the census of extrasolar planets. His advisor, B. Scott Gaudi, professor, astronomy, and Thomas Jefferson Chair for Space Exploration and Discovery, is co-author.

Clanton found that these collective results suggest that there are at least two planets per M dwarf in the Galaxy. “Since the majority of stars are M dwarfs, this implies that planets are very common in our Galaxy,” Clanton said.

He also finds that giant planets like our own Jupiter and Saturn are not extremely rare around low-mass stars, as had previously been thought. Indeed, some 15% of all M dwarfs host giant planets. While they are still less common than around more massive stars like our Sun, Clanton’s work demonstrates that giant planets can indeed form around low-mass stars.

What enabled Clanton’s findings was not an influx of new data, but rather development of sophisticated new methods to analyze existing data. “I took previous results from several disparate methods, and figured out how to compare and combine them in a robust way,” he said.

His results demonstrate how this process—combining results from several experiments, each utilizing different techniques—leads to more robust and far-reaching conclusions about the frequency of exoplanets. Clanton’s approach is something that has never been done before.

“Combining the results of these two methods results in a picture of exoplanets that is far more complete,” Gaudi said. “What makes it so interesting is that it enables us to say something very general about how common planets are around low-mass stars.”

“The power of combining different methods is that it gives the broadest picture that is achievable by providing the ability to use ALL the information, not just part of it,” Clanton added.

One could say that it is akin to the elephant analogy. One person feels one part, but has no idea of what they are feeling until they talk to each other.

Or, as Gaudi said, “Envision a dark room lit by only very small lights, then turning on the overheads!

“People don’t want to do this because it’s hard. Christian first had to learn the details of all of these methods—then find a way to combine them.”

Part Two: Payoff – what was learned?

Using methods developed in his first paper, Clanton examined the results of different planet detection techniques. He found that the microlensing and radial velocity results are clearly consistent — a fact that was far from clear in the past.

“One of the motivations for my project,” Clanton said, “was to show that, although the results from these two methods seemed to be in conflict, their difference was only due to the practical limitations of these experiments. When these limitations are properly addressed, we found consistency.

“The other main result—that one out of every six M dwarfs hosts a giant planet — demonstrates that giant planets are more common around M dwarfs than anyone previously believed. My next project is to use my methodology to include the results from another exoplanet detection technique, direct imaging, to determine if there are even more planets further out — one out of six may be too low!”

— Sandi Rutkowski