First Transiting Exoplanet (2000): HD 209458 b Crosses Its Star

The first transiting exoplanet, HD 209458 b, was caught in September 1999 by David Charbonneau and colleagues, who saw its Sun-like star dim for a few hours as the planet passed in front of it. The result, published in January 2000, gave the first measured size of an exoplanet and launched the transit method.

What the paper found

• Two transits. On 9 and 16 September 1999 the star HD 209458 faded for several hours, at the times predicted by the planet’s orbit.
• A puffy gas giant. Assuming a star of 1.1 solar radii and masses, the planet’s radius is 1.27 ± 0.02 times Jupiter’s, on an orbit tilted 87.1 ± 0.2 degrees.
• A 3.5-day year. The two transits alone gave an orbital period of 3.5250 ± 0.003 days, consistent with radial-velocity data.
• First density of an exoplanet. With the mass from radial velocities, the planet’s average density came out at about 0.38 grams per cubic centimeter, lower than Saturn’s, plus the first surface gravity of a planet beyond the Solar System.

How they did it

The transit method looks for a small, regular dip in a star’s brightness when a planet crosses its face. HD 209458 was already known from radial-velocity measurements (Keck’s HIRES and the ELODIE spectrograph) to have a close-in giant planet, and David Latham and Michel Mayor suggested the photometric search in August 1999. The team used the STARE Project camera, a small Schmidt camera with a 286 mm focal length and a 6-degree field, built to hunt for transits. They observed on ten nights between 29 August and 16 September 1999 and compared the star with neighbors in the same images to remove atmospheric effects.

Why it mattered

Radial velocities give only a planet’s minimum mass. A transit adds the planet’s size and its orbital tilt, so the true mass and density follow. That showed for the first time that a “hot Jupiter” like 51 Pegasi b really is a gas giant. Gregory Henry, Geoffrey Marcy, Paul Butler and Steven Vogt independently reported transits of the same star in the same journal issue.

Transits also let starlight filter through a planet’s atmosphere. In 2002 HD 209458 b became the first exoplanet with a detected atmosphere, through sodium absorption measured with the Hubble Space Telescope.

What we know now

The transit method became the main way to find exoplanets. NASA’s Kepler mission confirmed more than 2,600 planets this way, and TESS continues the survey across the whole sky. Transit spectroscopy with the James Webb Space Telescope now reads atmospheres in detail, starting with the detection of carbon dioxide on WASP-39b.

Who wrote it

David Charbonneau and David Latham were at the Harvard-Smithsonian Center for Astrophysics, and Timothy Brown, who led the STARE transit-search camera, at the High Altitude Observatory in Boulder, Colorado. Michel Mayor, co-discoverer of 51 Pegasi b, contributed the orbit from the Geneva radial-velocity team.

Read the original

Charbonneau, D., Brown, T. M., Latham, D. W. and Mayor, M. (2000). “Detection of Planetary Transits Across a Sun-like Star.” The Astrophysical Journal Letters 529: L45. doi:10.1086/312457. Free to read on arXiv. Semantic Scholar lists about 1,300 citations (October 2026).