Planet Occurrence Rates

By Joey Schmitt (Planet Hunter team)

Planet Hunters will soon start work on a new, important question in the field of exoplanets: how common are planets around other stars? This question has become a hot topic in exoplanets, but Planet Hunters has one major, unique advantage. Planet Hunters are sensitive to planets with just one or two transits. The automated computer algorithms require three or more transits; otherwise, they would be overloaded with spurious signals. This allows Planet Hunters to explore much longer periods than the rest of the field.

Until now, Planet Hunters have been looking for planets one quarter at a time. This has been successful in discovering more than 60 new planet candidates and two new confirmed planets (and counting). However, this one-quarter-at-a-time method doesn’t let us figure out how common planets truly are.

Planet Hunters will be moving from this quarter-focused method to a star-focused method with Planet Hunters 2.0. Instead of showing a few quarters of data for all Kepler stars, we will be showing all quarters of data for some stars. This will allow us to determine how common planets really are around these stars. (But don’t worry. Whenever we get a download of fresh data from the new K2 mission, these new light curves will take priority.)

The Planet Hunters team has decided to first show all the light curves for all the red dwarf stars. These stars are much smaller than the Sun, live for tens of billions of years or more, and have habitable zones very close to the star. They’re the best chance to find habitable, Earth-like worlds. Red dwarfs are also the most common type of star in the universe, making up about 70% of all stars. Kepler has only observed about 4,000 red dwarfs consistently, so we hope to finish this project over the course of just a few months (but keep in mind that the peer-review process can take longer). If we’re successful, we will do the same thing for the tens of thousands of Sun-like stars.

The biggest challenge in exoplanet statistics is to know how many planets we’re missing. However, we can actually figure this out by creating “synthetic data”. To non-scientists, this might sound like nonsense, but this is an extremely important tool that scientists use all the time. We must “inject” synthetic transits of planets of various sizes and periods into real light curves and let the Planet Hunters users classify them. This allows us to know how effective we are at finding these planets and correct for how many we’re missing.

For example, if Planet Hunter volunteers detect 50 of 100 synthetic Earth-size planets at a period of 300 days, then we know that if we detect 5 true Earth-like, 300-day planets, there are actually about 10 of them. Unfortunately, in order to correct (with any sort of scientific certainty) for the number of planets that we all may miss, we must inject a large number of synthetic planets into the real data.

This project will roll out with the release of our new site. The Planet Hunters team is excited about this new project and wants you to know that you will be helping answer one of the most important questions in astronomy: how common are planets in the Milky Way?