HD 209458 b
HD 209458 b is an exoplanet that orbits the solar analog HD 209458 in the constellation Pegasus, some 157 light-years (48 parsecs) from the Solar System. The radius of the planet's orbit is 0.047 AU (7.0 million km; 4.4 million mi), or one-eighth the radius of Mercury's orbit (0.39 AU (36 million mi; 58 million km)). This small radius results in a year that is 3.5 Earth-days long and an estimated surface temperature of about 1,000 °C (1,800 °F; 1,300 K). Its mass is 220 times that of Earth (0.69 Jupiter masses) and its volume is some 2.5 times greater than that of Jupiter. The high mass and volume of HD 209458 b indicate that it is a gas giant.
Size comparison of HD 209458 b with Jupiter | |
| Discovery | |
|---|---|
| Discovered by | D. Charbonneau T. Brown David Latham M. Mayor G.W. Henry G. Marcy R.P. Butler S.S. Vogt |
| Discovery site | High Altitude Observatory Geneva Observatory |
| Discovery date | September 9, 1999 |
| Radial velocity | |
| Orbital characteristics | |
| 0.04707 AU (7,042,000 km) | |
| Eccentricity | <0.0081 |
| 3.52474859(38) d 84.59396616 h | |
| Inclination | 86.710°±0.050° |
| 2452826.629283(87) JD | |
| 83 | |
| Semi-amplitude | 84.27+0.69 −0.70 m/s |
| Star | HD 209458 |
| Physical characteristics | |
Mean radius | 1.359+0.016 −0.019 RJ |
| Mass | 0.682+0.014 −0.015 MJ |
Mean density | 0.333+0.014 −0.013 g/cm3 |
| 9.2 m/s2 (0.94 g) | |
| Albedo | 0.096±0.016 (geometric) |
| Temperature | 1499±15 K (1,226 °C; 2,239 °F, dayside) 972±44 K (699 °C; 1,290 °F, nightside) |
HD 209458 b represents a number of milestones in exoplanetary research. It was the first of many categories:
- a transiting extrasolar planet
- The first planet detected through more than one method
- an extrasolar planet known to have an atmosphere
- an extrasolar planet observed to have an evaporating hydrogen atmosphere
- an extrasolar planet found to have an atmosphere containing the elements oxygen and carbon
- one of the first two extrasolar planets to be directly observed spectroscopically
- The first extrasolar gas giant to have its superstorm measured
- the first planet to have its orbital speed measured, determining its mass directly.
Based on the application of newer theoretical models, as of April 2007, it is thought to be the first extrasolar planet found to have water vapor in its atmosphere.
In July 2014, NASA announced finding very dry atmospheres on HD 209458 b and two other exoplanets (HD 189733 b and WASP-12b) orbiting Sun-like stars.
HD 209458 b has been nicknamed "Osiris" after the Egyptian god. This nickname has been acknowledged by the IAU, but as of 2023 it has not yet been approved as an official proper name.