Key Takeaways:
- Only about 0.5% of Sun-like stars have planets that transit from our view.
- Larger stars and planets with closer orbits have higher transit probabilities.
- Earth-like planets have a low (0.47%) transit probability.
- “You have to observe for many years to know for sure,” said William Welsh.
So, there is no single percentage to answer the question because it depends on how big the orbit is. For a planet whose orbital period is just a few days, the probability could be 10 to 20 percent or more. But for an Earth-like world at 1 astronomical unit (our planet’s distance from our star) with a period of 365 days, the probability is only 0.47 percent. Thus, the percentage ranges, roughly, from 0.5 percent to 15 percent for a star like the Sun. Because NASA’s Kepler mission team is most interested in planets within their stars’ habitable zones — where liquid water might exist on these worlds’ surfaces — the probability is close to that half-percent mark.
Another important aspect to keep in mind: Transits of planets in large orbits are rare events. You get just one transit per year of a planet in an Earth-like orbit. So, not only is the probability low of ever seeing a transit, but for the vast majority of the time the planet will not be transiting. You have to observe for many years to know for sure.
San Diego State University