55 Cancri e
Reference data for 55 Cancri e: 0.737-day orbit, 8.0 Earth masses, 1.88 Earth radii, magma-ocean dayside, and JWST thermal emission spectra.
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 |
|---|---|---|---|---|---|
| Distance | 41.05 ly | ± 0.04 ly | Gaia DR3 trigonometric parallax of host star | definitive | ESA Gaia Archive — Data Release 3 |
| Orbital Period | 0.736547 d | ± 0.000001 d | Multi-epoch transit or astrometric orbital fitting | definitive | NASA Exoplanet Archive |
| Semi-Major Axis | 0.01544 AU | ± 0.00005 AU | Keplerian third-law solution from stellar mass and orbital period | definitive | NASA Exoplanet Archive |
| Planetary Mass | 7.99 M⊕ | ± 0.33 M⊕ | Radial velocity or photodynamical transit-timing variations | definitive | NASA Exoplanet Archive |
| Planetary Radius | 1.875 R⊕ | ± 0.029 R⊕ | Transit depth photometry combined with Gaia stellar radius | definitive | NASA Exoplanet Archive |
| Equilibrium Temperature | 1958 K | ± 15 K | Radiative equilibrium calculation from bolometric stellar insolation | definitive | NASA Exoplanet Archive |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots 55 Cancri e (Janssen) — Ultra-Hot Super-Earth at RA 08h 52m 35.81s, Dec +28° 19′ 50.9″ within Cancer, 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
Originally discovered via radial velocity variations in 2004, 55 Cancri e was initially thought to have a 2.8-day orbital period until Rebekah Dawson and Daniel Fabrycky demonstrated in 2010 that the true period was a daily sampling alias of just 0.7365 days (17 hours and 41 minutes). Subsequent space observations with MOST and Spitzer immediately detected its transits across the bright K0IV-V host star 55 Cancri A (Copernicus), making it the brightest star known to host a transiting super-Earth.
Skimming just 0.0154 astronomical units from its star—less than 1/25th of Mercury's distance from the Sun—this 8.0-Earth-mass, 1.88-Earth-radius world endures equilibrium temperatures near 1,958 K and dayside temperatures exceeding 2,200 K. In 2024, infrared secondary eclipse spectroscopy from JWST NIRCam and MIRI revealed a lower-than-expected dayside brightness temperature (~1,800 K) along with absorption features near 4.5 microns, pointing toward a volatile secondary atmosphere of carbon monoxide or carbon dioxide continuously outgassed from a global magma ocean.
Formally named Janssen by the IAU, 55 Cancri e is the innermost member of a five-planet system (planets e, b, c, f, and d) spanning orbital distances from 0.015 to 5.5 astronomical units, accompanied by a wide red dwarf companion star (55 Cancri B) at 1,065 astronomical units. Secular interactions with the inner warm Jupiter 55 Cancri b help maintain the ultra-short-period world's alignment with the stellar equator.
Practical Visual Observing Guide
- Host star 55 Cancri A shines at magnitude V = 5.95 in Cancer, easily visible to the naked eye under dark skies and in binoculars.
- Planetary transit depth is extremely shallow (~350 ppm) over 95 minutes, requiring precision space photometry.
- Radial velocity semi-amplitude is ~6.0 m/s with an ultra-fast 17.68-hour period.
Catalog Cross-Identifications & Aliases
Common Aliases: Janssen · 55 Cnc e · HD 75732 e · Rho1 Cancri e
| Catalog Authority | Identifier |
|---|---|
| NASAEXOPLANET | 55 Cancri e |
| SIMBAD | 55 Cancri e |
Related Objects, Comparisons & Guides
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.
WASP-76 b — Molten-Iron-Rain Ultra-Hot Jupiter
Reference data for WASP-76 b: 1.85 Jupiter radii, 1.81-day orbit in Pisces, ESPRESSO iron-condensation asymmetry, and glory effect studies.
HD 189733 b — Cobalt-Blue Hot Jupiter Data
Reference profile for HD 189733 b: 2.22-day orbit, 2.4% transit depth in Vulpecula, atmospheric silicate clouds, and JWST H2S detection.
Exoplanet Transit & Radial Velocity Methods
How astronomers detect and characterize exoplanets using photometric transit depth ((Rp/R*)²), Doppler radial velocity (M sin i), and JWST transmission spectra.
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.
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)
- 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