Stars orbiting planets
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Stars with Orbiting Planets: Prevalence and Discovery
Research from large-scale surveys like the Kepler Mission and the Keck Planet Search has shown that most stars in our galaxy have planets orbiting them. These planets come in a wide range of sizes and orbital distances, and systems with multiple planets are common. Many of these planets are similar in size to Earth, and a significant number are found in the habitable zone, where conditions could allow for liquid water and potentially life. Estimates suggest that about 22% of Sun-like stars have Earth-size planets in their habitable zones, and up to 17–20% of stars may have gas giant planets within 20 AU of the star Borucki2016Cumming2008Petigura2013.
Types of Planetary Systems: Single and Binary Stars
Most known exoplanets orbit single stars, but planets can also orbit binary star systems. These circumbinary planets orbit around both stars in a binary pair. Surveys like SPOTS and discoveries such as the Kepler-47 system have confirmed that planets can form and persist even in the complex environments around binary stars. The frequency of giant planets orbiting binary stars appears to be similar to that around single stars, though such planets are less common at wide separations Thalmann2013Orosz2012.
Methods of Detection: Transits, Radial Velocity, and Direct Imaging
Planets are detected using several methods. The transit method, used by missions like Kepler and TESS, identifies planets when they pass in front of their host star, causing a slight dimming. The radial velocity method detects the gravitational pull of a planet on its star, causing the star to wobble. Direct imaging, though challenging, has revealed planets in wide orbits, such as the three Jupiter-like planets orbiting HR 8799, which move in counterclockwise orbits and have masses between 5 and 13 times that of Jupiter Kane2020Marois2008.
Orbital Characteristics: Periods, Eccentricities, and Architectures
The orbits of planets around stars vary widely. Some planets have short orbital periods, while others take hundreds of days or more to complete an orbit. Multi-planet systems are often nearly coplanar, with several planets orbiting in the same plane. The occurrence of Earth-size planets is relatively constant with increasing orbital period up to about 200 days. In some cases, especially for giant planets orbiting evolved stars, the orbits can be more eccentric, possibly due to tidal interactions as the host star evolves Mulders2018Grunblatt2018.
Frequency and Distribution of Planets
Statistical analyses indicate that at least 42% of Sun-like stars have nearly coplanar planetary systems with seven or more exoplanets, and the fraction of stars with at least one planet within 1 AU could be as high as 100%. The occurrence rate of potentially habitable, rocky planets around Sun-like stars is estimated to be up to 18%. The distribution of planet sizes shows a gap between super-Earths and sub-Neptunes, likely due to atmospheric loss processes Petigura2013Mulders2018Kunimoto2020.
Conclusion
Stars commonly have planets orbiting them, with a wide diversity in system architectures, planet sizes, and orbital characteristics. Both single and binary stars can host planetary systems, and planets are found at a range of distances from their host stars, including in the habitable zone. Ongoing and future surveys continue to refine our understanding of how common planets are and the variety of systems that exist in our galaxy.
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