--- title: "51 Pegasi b | Intro to Astronomy" description: "51 Pegasi b is the first exoplanet found orbiting a Sun-like star, a hot Jupiter that changed how Intro to Astronomy explains planet formation." canonical: "https://fiveable.me/intro-astronomy/key-terms/pegasi" type: "key-term" subject: "Intro to Astronomy" unit: "Unit 14" --- # 51 Pegasi b | Intro to Astronomy ## Definition 51 Pegasi b is a hot Jupiter exoplanet orbiting the Sun-like star 51 Pegasi every 4.2 days. In Intro to Astronomy, it is famous for being the first exoplanet discovered around a Sun-like star. ## What It Is 51 Pegasi b is an exoplanet in Intro to Astronomy, specifically a hot Jupiter orbiting the star 51 Pegasi. It is best known because it was the first planet found around a Sun-like star, announced in 1995 by Michel Mayor and Didier Queloz. The planet itself is not small or rocky like Earth. It is a gas giant with a mass similar to Jupiter, but it sits much closer to its star than Jupiter does to the Sun. Its orbit is only about 0.05 astronomical units from the star, and it completes one trip in about 4.2 days. That tiny orbit is what makes it a classic hot Jupiter. That setup surprised astronomers because earlier planet formation ideas mostly assumed gas giants had to form far from their stars, where it is cold enough for ices to help build large cores. 51 Pegasi b forced astronomers to rethink that picture. Either giant planets can form in unexpected places, or they can form farther out and then migrate inward after forming. The way it was found matters just as much as the planet itself. Astronomers detected 51 Pegasi b using the radial velocity method, which looks for the star’s tiny back-and-forth motion caused by the planet’s gravity. The star does not sit perfectly still, it wobbles around the system’s center of mass, and that wobble shows up as slight shifts in the star’s spectral lines. In a class setting, 51 Pegasi b is usually used as the first big example of how exoplanets are discovered and why our solar system is not the only possible blueprint. It is a simple name attached to a huge shift in astronomy: planets outside our solar system are real, common, and much more diverse than once expected. ## Why It Matters 51 Pegasi b matters because it is one of the clearest examples of how one discovery can reshape an entire astronomy unit. Before it was found, planet formation models were mostly built from the solar system, so gas giants were expected to live far from their stars. This planet breaks that pattern right away. It also gives you a concrete case for connecting theory to observation. You are not just memorizing that exoplanets exist, you are seeing how astronomers infer a planet from the motion of its star. That makes 51 Pegasi b a useful bridge between stellar spectra, gravity, and orbital mechanics. The planet is also a reminder that a system can be physically real without looking familiar. A Jupiter-sized planet in a four-day orbit sounds impossible if you only picture the solar system, but exoplanet discoveries show that migration and other formation processes can produce very different architectures. In Intro to Astronomy, this term often shows up when the course shifts from our solar system to comparison with other planetary systems. It helps you explain why exoplanet science changed from a search for rare oddities into a field that reveals how planets form and rearrange themselves. ## Connections ### Exoplanet 51 Pegasi b is an exoplanet, which just means a planet outside our solar system. This term is broader than the specific planet, so it is the category you use when you are talking about planets around other stars in general. 51 Pegasi b is famous because it was the first exoplanet found around a Sun-like star, not because it is the only one. ### Hot Jupiter 51 Pegasi b is a textbook hot Jupiter because it is a gas giant that orbits extremely close to its star. That close orbit makes it hot and gives it a very short year. When you compare it to Jupiter, the surprise is not just size, but location, since Jupiter sits much farther from the Sun in our own system. ### [Radial Velocity Method](/intro-astronomy/key-terms/radial-velocity-method) Astronomers detected 51 Pegasi b by measuring the star’s wobble with the radial velocity method. The planet’s gravity pulls the star slightly toward and away from Earth, which changes the star’s spectral lines through the Doppler effect. This is one of the clearest examples of how exoplanets can be found without directly imaging them. ### [Core Accretion](/intro-astronomy/key-terms/core-accretion) 51 Pegasi b pushed astronomers to rethink core accretion models, which had mostly been used to explain how gas giants form farther from a star. The usual idea is that a solid core forms first, then attracts gas. A giant planet so close to its star made people ask whether it formed elsewhere and migrated inward after formation. ## On the AP Exam A quiz question on 51 Pegasi b usually asks you to identify what kind of planet it is, how it was detected, or why it changed planet-formation models. You might also get a spectral graph or a description of a star’s wobble and need to connect that motion to the presence of an orbiting planet. In short-answer work, this term is useful when you explain how astronomers infer invisible objects from data. If a prompt asks why exoplanets expanded our view of planetary systems, 51 Pegasi b is the go-to example because it shows both the detection method and the surprise result. You may also need to compare it with Jupiter or with the solar system’s layout to show why it was unexpected. If your instructor uses discussion or writing prompts, you can describe 51 Pegasi b as evidence that planetary systems come in very different arrangements from our own. That makes it a strong case study for migration, hot Jupiters, and the limits of early planet-formation theories. ## 51 Pegasi b vs Jupiter 51 Pegasi b is often confused with Jupiter because both are gas giants, but they are not the same kind of example. Jupiter is a planet in our solar system, while 51 Pegasi b is an exoplanet orbiting another star. The big difference in class discussions is orbit: Jupiter is far from the Sun, but 51 Pegasi b is packed very close to its star. ## Key Takeaways - 51 Pegasi b is a gas giant exoplanet orbiting the Sun-like star 51 Pegasi. - It was the first exoplanet discovered around a Sun-like star, announced in 1995. - It is a hot Jupiter, which means it orbits extremely close to its star and has a very short period. - Astronomers found it with the radial velocity method by measuring the star’s wobble. - Its discovery changed planet-formation ideas by showing that planetary systems can look very different from the solar system. ## FAQs ### What is 51 Pegasi b in Intro to Astronomy? 51 Pegasi b is a hot Jupiter exoplanet orbiting the star 51 Pegasi. It is famous because it was the first planet found around a Sun-like star, and it helped launch modern exoplanet astronomy. ### How was 51 Pegasi b discovered? It was discovered with the radial velocity method. Astronomers measured tiny shifts in the star’s motion as the planet’s gravity pulled it back and forth, which revealed the planet’s presence. ### Why is 51 Pegasi b called a hot Jupiter? It is called a hot Jupiter because it is a gas giant like Jupiter, but it orbits very close to its star. That close orbit makes it much hotter than Jupiter and gives it an extremely short year. ### Is 51 Pegasi b in our solar system? No, it is not in our solar system. It orbits the star 51 Pegasi, which is outside the Sun’s planetary system, so it is an exoplanet. ## Related Study Guides - [14.4 Comparison with Other Planetary Systems](/intro-astronomy/unit-14/4-comparison-planetary-systems/study-guide/FlAWVl3dO6OUxhat) - [21.6 New Perspectives on Planet Formation](/intro-astronomy/unit-21/6-perspectives-planet-formation/study-guide/M9FtmwqSExzUg3Wt) - [21.5 Exoplanets Everywhere: What We Are Learning](/intro-astronomy/unit-21/5-exoplanets-everywhere-learning/study-guide/Sf4M8iZR55UxuuLR) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/intro-astronomy/key-terms/pegasi#resource","name":"51 Pegasi b | Intro to Astronomy","url":"https://fiveable.me/intro-astronomy/key-terms/pegasi","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/intro-astronomy/key-terms/pegasi#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:22:21.332Z","isPartOf":{"@type":"Collection","name":"Intro to Astronomy Key Terms","url":"https://fiveable.me/intro-astronomy/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/intro-astronomy/key-terms/pegasi#term","name":"51 Pegasi b","description":"51 Pegasi b is a hot Jupiter exoplanet orbiting the Sun-like star 51 Pegasi every 4.2 days. In Intro to Astronomy, it is famous for being the first exoplanet discovered around a Sun-like star.","url":"https://fiveable.me/intro-astronomy/key-terms/pegasi","inDefinedTermSet":{"@type":"DefinedTermSet","name":"Intro to Astronomy Key Terms","url":"https://fiveable.me/intro-astronomy/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is 51 Pegasi b in Intro to Astronomy?","acceptedAnswer":{"@type":"Answer","text":"51 Pegasi b is a hot Jupiter exoplanet orbiting the star 51 Pegasi. It is famous because it was the first planet found around a Sun-like star, and it helped launch modern exoplanet astronomy."}},{"@type":"Question","name":"How was 51 Pegasi b discovered?","acceptedAnswer":{"@type":"Answer","text":"It was discovered with the radial velocity method. Astronomers measured tiny shifts in the star’s motion as the planet’s gravity pulled it back and forth, which revealed the planet’s presence."}},{"@type":"Question","name":"Why is 51 Pegasi b called a hot Jupiter?","acceptedAnswer":{"@type":"Answer","text":"It is called a hot Jupiter because it is a gas giant like Jupiter, but it orbits very close to its star. That close orbit makes it much hotter than Jupiter and gives it an extremely short year."}},{"@type":"Question","name":"Is 51 Pegasi b in our solar system?","acceptedAnswer":{"@type":"Answer","text":"No, it is not in our solar system. It orbits the star 51 Pegasi, which is outside the Sun’s planetary system, so it is an exoplanet."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Intro to Astronomy","item":"https://fiveable.me/intro-astronomy"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/intro-astronomy/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 14","item":"https://fiveable.me/intro-astronomy/unit-14"},{"@type":"ListItem","position":4,"name":"51 Pegasi b"}]}]} ```