Planetary system · 1 known planet · Pegasus

HD 209458

Its star: above the horizon after darkHighest after dark 22:01 · south · 58°

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Where is HD 209458?

Times for LondonChange city

Look south, about 58° up at 22:01 in Pegasus.

Rises: 14:20
Highest: 22:06
Sets: 05:52
Best after dark: 22:01 · 58°

Can I see HD 209458?

No. The planets of HD 209458 can’t be seen with the eye, binoculars or any amateur telescope: they are lost in their star’s light. What you can do is point toward the star.

From London tonight, the star HD 209458 is above the horizon after dark. It’s highest after dark at 22:01, in the south at 58°.

Why is HD 209458 interesting?

HD 209458 has 1 known planet, discovered in 1999 by the radial velocity method.

HD 209458 b is a giant planet that orbits in under five days: a “hot Jupiter”.

What can you see?

Eyes

Direction only

Not visible. The star is around magnitude 7.6.

Binoculars

Visible as a faint star with a chart.

Telescope

Easy to see as a star. The planets themselves can’t be seen with any backyard telescope.

What you’re likely to see, not what a space telescope photographs.

About

HD 209458 is a Sun-like star about 150 light-years away in Pegasus. Its planet, HD 209458 b, nicknamed Osiris, made history twice.

In 1999 it became the first exoplanet seen passing in front of its star, dimming the starlight slightly every 3.5 days. In 2001 it became the first exoplanet with a detected atmosphere, when astronomers found sodium in the starlight filtering through it.

The planet is a hot Jupiter so close to its star that its atmosphere is slowly boiling away, leaving a trail of hydrogen behind it.

HD 209458 b has about two-thirds of Jupiter’s mass but is noticeably larger than Jupiter, puffed up by the heat of its star. Every 3.5 days it blocks about 1.5% of the star’s light, enough for careful amateurs with cameras to measure.

Its escaping atmosphere forms a tail somewhat like a comet’s, streaming away from the star.

The HD 209458 system

Known planets of HD 209458 (catalog values from exoplanet.eu)
Planet	Mass	Radius	Orbit	From star	Found	Method
HD 209458 b	0.69 M♃	1.38 R♃	3.52 days	0.0475 AU	1999	Radial Velocity

M⊕ / R⊕ = Earth masses / radii; M♃ / R♃ = Jupiter. Masses from radial velocity are often minimum masses. Data: The Extrasolar Planets Encyclopaedia, exoplanet.eu (CC BY 4.0).

Questions

Can I see HD 209458 b?

No, but you can see its host star in binoculars.

Can amateurs detect its transit?

Yes. With a telescope, a camera and careful measurement, amateurs have recorded the star dimming by about 1.5% during a transit.

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Sources: The Extrasolar Planets Encyclopaedia (CC BY 4.0); star chart: Wikidata (CC0). Positions computed for your place and time with astronomy-engine. Data & Licences