HD 209458 b

HD 209458 b is a hot Jupiter orbiting the Sun-like star HD 209458. Its transits enabled the first direct measurements of an exoplanet’s size and atmosphere.

Transit photometry: A method that detects a planet by measuring the periodic dimming of its star as the planet crosses the stellar disk. The planet’s 1999 transit made this detection method’s promise unmistakable.
Hot Jupiter: A gas giant on a close orbit around its star, with an atmosphere heated to high temperatures. HD 209458 b is a well-studied example of this planetary class.
Michel Mayor: A Swiss astronomer who shared the 2019 Nobel Prize in Physics for discovering an exoplanet orbiting a Sun-like star. His radial-velocity work helped establish the methods that found this system.
Exoplanet atmosphere: The gaseous envelope surrounding a planet outside the Solar System. This planet’s atmosphere became the first detected beyond the Solar System.
Sodium in exoplanet atmospheres: Sodium absorption is a measurable spectral feature produced when starlight passes through an exoplanet’s atmosphere. Sodium absorption provided the first detection of an exoplanet atmosphere, in this planet.
Planetary transit: An event in which a planet passes in front of its star from the observer’s viewpoint. Repeated transits reveal this planet’s radius and orbital period.
Didier Queloz: A Swiss astronomer who co-discovered 51 Pegasi b, the first exoplanet found around a Sun-like star. His discovery helped spur searches that identified HD 209458 b.
Atmospheric scale height: The vertical distance over which atmospheric pressure decreases by a factor of the mathematical constant e. Its inflated atmosphere makes spectral absorption easier to measure.
Evaporating exoplanet: An exoplanet losing atmospheric material into space under intense stellar heating or radiation. Ultraviolet observations revealed an extended hydrogen envelope escaping from this planet.
Transmission spectroscopy: A technique that analyzes starlight filtered through a planet’s atmosphere during transit. Atmospheric absorption changes the transit spectrum of this planet.

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