HAT-P-11b (or Kepler-3b) is an extrasolar planet orbiting the star HAT-P-11. It was discovered by the HATNet Project team in 2009 using the transit method, and submitted for publication on 2 January 2009.

This planet is located approximately 123 light-years (38 pc) distant from Earth.^[4]

The HATNet Project team initially detected the transits of HAT-P-11b from analysis of 11470 images, taken in 2004 and 2005, by the HAT-6 and HAT-9 telescopes. The planet was confirmed using 50 radial velocity measurements taken with the HIRES radial velocity spectrometer at W. M. Keck Observatory.^[1]

At the time of its discovery HAT-P-11b was the smallest radius transiting extrasolar planet discovered by a ground based transit search and was also one of three previously known transiting planets within the initial field of view of the Kepler spacecraft.^[1]

There was a linear trend in the radial velocities indicating the possibility of another planet in the system.^[1] This planet, HAT-P-11c, was confirmed in 2018.^[2]

This planet orbits about the same distance from the star as 51 Pegasi b is from 51 Pegasi, typical of transiting planets. However, the orbit of this planet is eccentric, at around 0.198, unusually high for hot Neptunes. HAT-P-11b's orbit is also highly inclined, with a tilt of 103⁺²⁶
₋₁₀°.^[5] degrees relative to its star's rotation.^[6][7] The planet is probably composed primarily of heavy elements with only 10% hydrogen and helium by mass, like Gliese 436 b.^[1]

On 24 September 2014, NASA reported that HAT-P-11b is the first Neptune-sized exoplanet known to have a relatively cloud-free atmosphere and, as well, the first time molecules, namely water vapor, of any kind have been found on such a relatively small exoplanet.^[8]

In 2009 French astronomers observed what was thought to be a weak unpolarized radio signal coming from the exoplanet, but it was not observed in a repeat observation in 2010.^[9] If the signal was real, then it was probably due to intense lightning storms with similar properties as ones on Saturn.^[10]

In December 2021 evidence of a magnetosphere was discovered in HAT-P-11b that could be the first ever in any exoplanet.^[11][12][13]

• Gliese 436 b
• HATNet Project
• HAT-P-7b
• Kepler Mission
• Tau Boötis b another extra solar planet with evidence of magnetic fields
• YZ Ceti another extra solar planet with evidence of magnetic fields
• HD 209458 b another extra solar planet with evidence of magnetic fields