WASP-39 b (Bocaprins)
Coordinates, Gaia DR3 distance (213 pc), 4.055-day period, 0.28 M♃ mass, 1.27 R♃ radius, and JWST CO2/SO2 photochemical spectrum of WASP-39 b.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Orbital period | 4.0552941 d | ± 0.000003 d | SuperWASP, HST, TESS & JWST transit timing | definitive | NASA Exoplanet Archive |
| Planetary radius | 14.24 R⊕ | ± 0.45 R⊕ | 2.1% transit depth across 0.90 R☉ G8 V star (1.27 ± 0.04 R♃) | definitive | NASA Exoplanet Archive |
| Planetary mass | 89 M⊕ | ± 9.5 M⊕ | SOPHRIE & CORALIE radial velocity (0.28 ± 0.03 M♃ — almost identical to Saturn) | definitive | NASA Exoplanet Archive |
| Equilibrium temperature | 1166 K | ± 14 K | JWST panchromatic transmission & thermal emission modeling | definitive | JWST / MAST Science Archive & Mission Documentation |
| Host star distance (Gaia DR3) | 213.3 pc | ± 1.5 pc | Gaia DR3 astrometric parallax (4.688 ± 0.033 mas, 695.7 ly) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots WASP-39 b (JWST Benchmark Saturn Exoplanet) at RA 14h 29m 18.42s, Dec -03° 26′ 40.2″ within Virgo, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
WASP-39 b (officially named Bocaprins by the IAU) is a puffy, Saturn-mass gas giant orbiting a quiet G8 V star 213.3 ± 1.5 parsecs (696 light-years) away in eastern Virgo. Discovered in 2011 by the Wide Angle Search for Planets (SuperWASP) consortium, WASP-39 b has almost the exact same mass as Saturn (0.28 Jupiter masses, or 89 Earth masses) but orbits just 0.0486 AU from its host star with a 4.055-day period, heating its atmosphere to 1,166 K and inflating its radius to 1.27 Jupiter radii (mean bulk density only 0.18 g/cm³).
Because low surface gravity and high temperature give WASP-39 b an enormous atmospheric scale height (~900 km) around an inactive host star, the international Transiting Exoplanet Community Early Release Science team selected WASP-39 b as the flagship test case for all four near-infrared spectrographs on the James Webb Space Telescope.
In August and November 2022, JWST observations of WASP-39 b delivered the first unambiguous detection of carbon dioxide (CO2 at 4.3 microns, at 26-sigma significance!) in any exoplanet atmosphere, followed by the first detection of photochemical sulfur dioxide (SO2 at 4.05 microns) produced when stellar ultraviolet light splits water vapor and reacts with hydrogen sulfide in the upper atmosphere—alongside water (H2O), carbon monoxide (CO), sodium (Na), and potassium (K).
Practical Visual Observing Guide
- The G8 V host star WASP-39 shines at visual magnitude V = 12.11 in eastern Virgo (about 6° northeast of Zeta Virginis / Heze).
- Visual observers can spot the 12.1-magnitude host star in a 130–150 mm (5–6 inch) telescope at 100×–150×, while its deep 2.1% (0.023-magnitude) transit dip is readily captured by amateur 200 mm CCD/CMOS rigs.
Catalog Cross-Identifications & Aliases
Common Aliases: Bocaprins · GSC 04980-00761 b
| Catalog Authority | Identifier |
|---|---|
| NASAEXOPLANET | WASP-39 b |
| SIMBAD | WASP-39 b |
| WIKIDATA | Q4052920 |
Related Objects, Comparisons & Guides
HD 209458 b (Osiris First Transiting Planet)
Coordinates, Gaia DR3 distance (48.3 pc), 3.5247-day transit period, 1.359 R♃ inflated radius, and atmospheric escape data for HD 209458 b.
K2-18 b (Temperate Sub-Neptune in Leo)
Coordinates, Gaia DR3 distance (38.07 pc), 32.94-day habitable-zone orbit, 8.63 M⊕ mass, 2.61 R⊕ radius, and JWST CH4/CO2 spectrum of K2-18 b.
51 Pegasi b (Dimidium First Hot Jupiter)
Coordinates, Gaia DR3 distance (15.46 pc), 4.2308-day orbital period, Nobel-winning radial-velocity discovery, and observing guide for 51 Pegasi b.
Saturn (Ringed Giant & Cassini Division)
JPL orbital parameters (9.5826 AU, 29.457 yr), ring tilt cycle, Cassini Division resolution, and telescope observing guide for Saturn.
Apparent vs Absolute Magnitude Explained
Understand the logarithmic Pogson astronomical magnitude scale, apparent visual magnitude (m), absolute magnitude (M at 10 parsecs), and bolometric corrections.
Trigonometric Parallax, Parsecs & Gaia DR3
How trigonometric parallax (d_pc = 1 / p_arcsec), the distance modulus equation, Hipparcos, and Gaia DR3 microarcsecond astrometry measure cosmic distances.
Confirmed Exoplanets & Host Systems Catalog
Curated NASA Exoplanet Archive reference catalog of benchmark exoplanets: Proxima Centauri b, TRAPPIST-1 e, 51 Pegasi b, HD 209458 b, K2-18 b, and WASP-39 b.
Scientific Sources & Dataset Provenance
- NASA Exoplanet Archive (2026-10-01 snapshot): Akeson, R. L., et al. 2013, PASP, 125, 989; NASA Exoplanet Archive (IPAC/Caltech) — NASA Exoplanet Science Institute (NExScI / Caltech / IPAC)
- JWST / MAST Science Archive & Mission Documentation (Cycle 1–4 Public Releases (2022–2026)): Rigby, J., et al. 2023, PASP, 135, 048001 (The Science Performance of JWST as Characterized in Commissioning) — NASA / ESA / CSA / STScI
- ESA Gaia Archive — Data Release 3 (DR3 (2022-06-13)): Gaia Collaboration, Vallenari, A., et al. 2023, A&A, 674, A1 (Gaia Data Release 3) — European Space Agency / Gaia DPAC