Key parameters at a glance
- A radius of 15.58 Earth radii
- A mass of 232.02 Earth masses
- Surface gravity around 0.96 g
- An orbital period of 3.525 days
- Semi-major axis 0.0471 AU
- Equilibrium temperature 1,459 K (1186 °C)
- Distance from Earth 157.54 light-years
- Earth Similarity Index 0.077
- Travel time at Voyager 1 speed 2,778,192 years
HD 209458 b Planet Profile
Physical Specs
Classification
Planet Type
Gas Giant
Discovery
Size rank in cohort
Rank by radius
#192of 1771
top 10.8%
This planet
15.58R⊕
Gas Giant median
13.52R⊕
Nearest-size peers
| Metric | Earth | HD 209458 b | Jupiter |
|---|---|---|---|
| Radius (R⊕) | 1.00 | 15.58 | 11.21 |
| Mass (M⊕) | 1.00 | 232.02 | 317.83 |
| Density (g/cm³) | 5.51 | 0.36 | 1.33 |
| Surface Gravity (g) | 1.00 | 0.96 | 2.53 |
| Insolation (S⊕) | 1.00 | 587.10 | 0.037 |
Alternative Mass Estimates
| Method | Value | Interpretation |
|---|---|---|
| Best measured mass | 232.016 M⊕ | Direct mass measurement (not derived). |
| Minimum mass (M sin i) | 222.154 M⊕ | Lower bound from radial-velocity fitting. |
Star Catalogue Identifiers
HD
HD 209458
HIP
HIP 108859
TIC
TIC 420814525
Gaia DR2
Gaia DR2 1779546757669063552
Gaia DR3
Gaia DR3 1779546757669063552
System
HD 209458
Percentile among Gas Giant cohort
Orbit & Habitability
Orbital Dynamics
Year Length
A year here lasts approximately 3.52 Earth days (1.0% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.0471 AU.
Transit Fingerprint
Depth
1.500 %
Duration
3.072 h
Impact parameter b
0.507
Rp / R★
0.120860
Transit midpoint (BJD)
2,451,659.9374
Photometric dip
Transit produces a flux drop of 15,000 ppm lasting ≈ 3.07 h.
Extended Orbital Architecture
Planet / star radius ratio
0.120860
Semi-major axis / stellar radius
8.810
Impact parameter (b)
0.507
RV semi-amplitude (K)
84.700 m/s
Transit mid-time (BJD)
2,451,659.9374
Long. of periastron (ω)
0.00°
Sky-projected obliquity (λ)
-1.60°
Angular separation (arcsec)
0.97500
Eq. Temperature
1,459K
(1186 °C)
Insolation (S⊕)
587.10
Earth = 1.00
Earth Similarity (ESI)
0.077
0 = alien, 1 = Earth-identical
Inside the inner HZ boundary (too hot for liquid water) — the dayside is scorched.
Detection Signatures
Transit photometry
A transit dip has been observed from at least one telescope.
Radial velocity
Stellar wobble has been measured spectroscopically.
Discovery Paper
Primary reference
Henry et al. 2000Instrument
HIRES Spectrometer
Publication
2000-01
Observation locale
Ground
Discovery cohort
Planets confirmed in 1999 at W. M. Keck Observatory (4 shown).
Host System: HD 209458
Spectral Class
G-type yellow (Sun-like)
Effective Temperature
6,091 K — hotter than the Sun (5,778 K)
Estimated Age
3.10 Gyr — younger than the Sun (4.6 Gyr)
Stellar Radius
1.190 R☉
Stellar Mass
1.230 M☉
Metallicity [Fe/H]
0.01
Extended Stellar Properties
Surface gravity (log g)
4.450 dex
Stellar density
1.040 g/cm³
Systemic radial velocity
-14.74 km/s
Rotational v·sin i
4.40 km/s
Rotation period
10.65 days
Activity index (log R'HK)
-5.000
Metallicity ratio
[Fe/H]
Multi-band Host Photometry — 11 bands
Astrometric Data
Parallax
20.675 mas
Total Proper Motion
34.568 mas/yr
PM Right Ascension
29.58 mas/yr
PM Declination
-17.89 mas/yr
Galactic Cartesian (pc)
x = 0.826 · y = -0.462 · z = 0.324
Equatorial (J2000)
RA 330.79502° · Dec 18.88424°
Galactic ℓ, b
76.753° · -28.527°
Ecliptic λ, β
340.350° · 28.724°
HTM-20 index
1553919534
Observation Record
Photometric series
27
RV measurements
5
Stellar spectra
2
Transmission spectra
9
Emission spectra
35
Archive notes
6
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