The statement that most stars have planets is based on statistical estimates and observations of a limited sample of stars. Astronomers have detected thousands of exoplanets (planets outside our solar system) to date, suggesting that planets are indeed common in the universe. However, it is important to note that the methods used to detect exoplanets have limitations, which contribute to the fact that many planets remain unseen.
One of the primary techniques for detecting exoplanets is the transit method. This method involves observing a star for regular, slight dips in its brightness caused by a planet passing in front of it, blocking a fraction of the star's light. However, this method is biased towards detecting planets that orbit their stars in such a way that they pass in front of them from our vantage point. This means that only a small fraction of planets will have their transits aligned with our line of sight, making them detectable through this method.
Other detection methods, such as the radial velocity method or gravitational microlensing, also have limitations. They may be more sensitive to larger planets or planets in specific orbital configurations, making it more challenging to detect a representative sample of planets around stars.
Moreover, the sheer vastness of the universe means that astronomers can only observe a small fraction of stars. There are estimated to be billions of stars in our galaxy alone, and there are billions of galaxies in the observable universe. Astronomers have limited resources and observational capabilities, which restrict the number of stars they can study in detail.
As technology advances and new detection methods are developed, astronomers continue to improve their ability to find and characterize exoplanets. Future missions and telescopes, such as the James Webb Space Telescope, will provide even more data and potentially increase our understanding of exoplanet populations. However, it is likely that many planets will still remain unseen due to the limitations of current and future observational techniques.