CoRoT-9b

Lore & Background

CoRoT-9b was discovered by combining observations from the CoRoT satellite, which detects the small dip in starlight as a planet transits its parent star, with radial velocity measurements from the HARPS instrument on the 3.6 m Telescope at the European Southern Observatory. Its presence was confirmed by observations from several ESO telescopes. The discovery was announced on St. Patrick's Day in 2010, following 145 days of continuous observations in summer 2008.

The planet has a mass of 0.84 times that of Jupiter and a radius of 1.05 times that of Jupiter, giving it a density of 96% that of water and a surface gravity 1.93 times that of Earth. A search for rings and satellites using the Spitzer space telescope returned negative results.

Its atmosphere is presumably dominated by hydrogen and helium, with up to 20 Earth masses of other elements including water and rock at high temperatures and pressures. The planet lies at a distance from its star where a strong increase in albedo occurs as temperature decreases, due to condensation of reflective water clouds. This suggests its atmosphere may be locked into one of two states: a cloudless state with temperatures between 380 K and 430 K, or covered in water clouds with temperatures in the range 250 K to 290 K. The discovery paper classifies it as a class II ("water cloud") or class III ("clear") atmosphere planet under the Sudarsky extrasolar planet classification.

Reader's Guide

CoRoT-9b holds significance as the first temperate giant exoplanet discovered via the transit method, enabling astronomers to study the atmosphere of a temperate giant planet for the first time. This allows examination of cloud composition, atmospheric composition, temperature distributions, and some details of the planet's interior. Its large orbital distance (0.36 AU) and 95-day period made it an outlier among known transiting planets at discovery, bridging the gap between hot Jupiters and cooler giants. The potential for two distinct atmospheric states—clear or water-cloud-covered—offers a natural laboratory for understanding cloud formation and albedo effects in exoplanet atmospheres. The planet's density (96% that of water) and composition (hydrogen and helium with up to 20 Earth masses of heavier elements) provide constraints on interior models of temperate giant planets. Its discovery demonstrated the capability of combining space-based transit photometry (CoRoT) with ground-based radial velocity follow-up (HARPS) to find and characterize long-period transiting planets. The negative search for rings and satellites with Spitzer further refines our understanding of this system.

Did You Know?

• CoRoT-9b has the largest distance of nearest approach (0.36 AU) of any known transiting planet at the time of its discovery.
• Its transit lasts 8 hours, and its orbital period is 95 days.
• The planet's atmosphere may be either cloudless (380–430 K) or covered in water clouds (250–290 K).
• A search for rings and satellites around CoRoT-9b using the Spitzer space telescope found none.