HD 209458

HD 209458

system

HD 209458 is a Sun-like star 158 light-years away in Pegasus, and the most historic exoplanet host of all: its hot Jupiter, HD 209458 b ('Osiris'), was the first exoplanet ever seen transiting its star (1999) and the first found to have an atmosphere (2001).

Open in 3D viewSee HD 209458 in the live, interactive 3D map

Key facts

Type
Sun-like star + 1 hot Jupiter
Distance
158 light-years
Confirmed planets
1
Claim to fame
First transit + first atmosphere

Science

Classification

Type
Single Sun-like (G0V) system
Central star
HD 209458 — G0V, ~6091 K, 1.23 M☉

Planets

Confirmed planets
1 · HD 209458 b, a transiting hot Jupiter
Historic firsts
First transit (1999); first exoatmosphere (2001)

Distance

Distance
158 light-years · in Pegasus, the Winged Horse

How we know

Note
One confirmed planet (NASA Exoplanet Archive, checked 2026-07-07). Because it transits with a measured mass, its size and density are genuinely measured. The nickname 'Osiris' is informal, not an IAU name. Placed at its real galactic position, 158 light-years away.

Discovery & history

Name
A Henry Draper catalogue star (HD 209458); its planet b carries the unofficial nickname 'Osiris' (not an IAU name), for the Egyptian god, after its evaporating tail.

The planet was found by radial velocity in 1999 and, weeks later, caught crossing the star's face — the first exoplanet transit ever observed, which gave the first true size and density of a world beyond the Sun. In 2001 Hubble detected sodium in its atmosphere (the first exoatmosphere), and later an escaping hydrogen 'comet tail'.

Images

Artist's impression of the gas giant HD 209458 b ('Osiris') orbiting its Sun-like star — an artist's concept, not a photograph.
NASA/ESA/STScI, G. Bacon (public domain) · Wikimedia Commons
HD 209458 b emerging from behind its star — the first exoplanet seen transiting and the first found to have an atmosphere. Artist's concept.
NASA (public domain) · Wikimedia Commons

Images from open archives · credit shown per image