HD 209458 b (Osiris)
Hot Jupiter · First planet seen crossing its star
HD 209458 b was the first exoplanet caught passing in front of its star, and the first whose atmosphere was detected. The star’s heat is so intense that the planet’s gas is boiling away into space, forming a comet-like tail.
- 159 light-years awayEven light – the fastest thing there is – needs 159 years to get here.
- 3.5 daysYear
- Atmosphere escapingSpecial
Take a closer look
A puffed-up giant
HD 209458 b is about 40% wider than Jupiter but weighs less. Fierce heat from its nearby star has puffed up its atmosphere like a soufflé in an oven. That makes it a very light, fluffy world for its size – less dense than any planet in our Solar System.
A tiny dip in light
When the planet crosses its star, it blocks only about 1.5% of the starlight – like a moth flying in front of a distant streetlamp. Telescopes measure this tiny dip every three and a half days, which tells scientists how big the planet is and how it moves.
Forever day, forever night
Because it orbits so close, scientists think the planet is “locked” with the same side always facing its star, just as the Moon always shows Earth the same face. One side would have endless scorching daytime, while the other would sit in never-ending night.
How we found out
- 1999
Two teams watch the star dim slightly as the planet passes in front of it – the first transit ever seen.
- 2001
The Hubble Space Telescope detects sodium in its atmosphere – the first look at the air of a planet around another star.
- 2003
Astronomers discover a huge cloud of hydrogen streaming away from the planet.
How can we tell what gases a far-away planet has?
When the planet passes in front of its star, a little starlight shines through its atmosphere. Each gas absorbs its own special colours, leaving a fingerprint in the light.
See it yourself
The star needs binoculars or a small telescope, in Pegasus.
From the encyclopedia
Read the full article on Wikipedia ↗Got it?
Mark HD 209458 b (Osiris) as explored to earn 10 XP.