Hat-P-11B

It was discovered by the HATNet Project team in 2009 using the transit method, and submitted for publication on 2 January 2009.

HAT-P-11b / Kepler-3b

Size comparison of Neptune with HAT-P-11b (gray).
Discovery
Discovered by	Bakos et al.
Discovery site	Cambridge, Massachusetts
Discovery date	2 January 2009
Detection method	Transit (HATNet)
Orbital characteristics
Apastron	0.0637+0.0020 −0.0019 AU
Periastron	0.0413+0.0018 −0.0019 AU
Semi-major axis	0.05254+0.00064 −0.00066 AU
Eccentricity	0.218+0.034 −0.031
Orbital period (sidereal)	4.887802443+0.000000034 −0.000000030 d
Inclination	89.05+0.15 −0.09
Time of periastron	2454957.15+0.17 −0.20
Argument of periastron	19+14 −16
Semi-amplitude	10.42+0.64 −0.66
Star	HAT-P-11
Physical characteristics
Mean radius	4.36±0.06 R🜨
Mass	23.4±1.5 ME
Mean density	1,440 kg/m3 (2,430 lb/cu yd)
Surface gravity	1.20 g

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

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.

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.

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

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°. degrees relative to its star's rotation. The planet is probably composed primarily of heavy elements with only 10% hydrogen and helium by mass, like Gliese 436 b.

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.

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. If the signal was real, then it was probably due to intense lightning storms with similar properties as ones on Saturn.

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

• 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