HD 40307 d is an exoplanet orbiting the K2.5V-type main‑sequence star HD 40307, located approximately 42 light‑years (13 parsecs) from Earth in the constellation Pictor. The planet was detected in 2008 by the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph using the radial‑velocity method and is part of a multi‑planet system that includes at least six confirmed planets.
Discovery
- Date announced: 2008 (as part of a three‑planet detection around HD 40307)
- Method: High‑precision radial velocity measurements obtained with the HARPS instrument at the La Silla Observatory, Chile.
- Discovering team: Mayor et al., part of the HARPS consortium.
Host star (HD 40307)
- Spectral type: K2.5V (orange dwarf)
- Mass: ~0.77 M☉ (solar masses)
- Radius: ~0.71 R☉ (solar radii)
- Effective temperature: ~4 800 K
- Metallicity: Slightly sub‑solar ([Fe/H] ≈ –0.31)
Orbital characteristics
- Orbital period: 20.43 days (± 0.01 days)
- Semi‑major axis: ≈ 0.13 AU (≈ 19.5 million km)
- Eccentricity: Low; best fit value ≈ 0.07 (consistent with a near‑circular orbit)
- Inclination: Not directly measured; radial‑velocity data provide only a minimum mass (M sin i).
Physical characteristics
- Minimum mass (M sin i): ≈ 9.5 M⊕ (Earth masses), classifying the planet as a super‑Earth or mini‑Neptune.
- Radius: Unmeasured; estimates depend on composition assumptions and orbital inclination.
- Equilibrium temperature: ≈ 600–700 K, reflecting its close proximity to the host star; exact value depends on albedo and atmospheric properties.
Context within the system
HD 40307 d is the third planet from the star in order of increasing orbital distance, situated between HD 40307 c (period ~9.6 days) and HD 40307 e (period ~34.6 days). The system’s architecture, featuring several low‑mass planets in compact, near‑circular orbits, has been of interest for studies of planetary formation and dynamical stability.
Observational status
- The planet has been confirmed solely by radial‑velocity data; no transits have been detected to date, and direct imaging is currently unfeasible.
- Follow‑up observations continue to refine its orbital parameters and to search for additional, lower‑mass companions in the system.
Scientific significance
HD 40307 d contributes to the population of super‑Earths detected around nearby, bright, low‑mass stars, providing a benchmark for comparative exoplanetology and helping to constrain models of planetary formation around metal‑poor, K‑type stars.