Pi Mensae b (π Men b, π Mensae b), also known as HD 39091 b, is an extrasolar planet^[3] approximately 60 light-years away in the constellation of Mensa. The planet was announced orbiting the yellow main-sequence star^[4] Pi Mensae in October 2001.

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Pi Mensae b

Discovery
Discovered by	Jones et al.[1]
Discovery site	Anglo-Australian Telescope
Discovery date	15 October 2001
Detection method	Doppler spectroscopy
Orbital characteristics[2]
Semi-major axis	3.31+0.13 −0.15 AU
Eccentricity	0.642+0.0007 −0.0006
Orbital period (sidereal)	5.719 ± 0.001 years (2,088.86 ± 0.37 d)
Inclination	54.4°+5.9° −3.7°
Longitude of ascending node	278.5°+6.7° −9.8°
Time of periastron	2456540.34±0.75
Argument of periastron	331.02°+0.23° −0.22°
Semi-amplitude	193.98+0.38 −0.38 m/s
Star	Pi Mensae
Physical characteristics[2]
Mass	12.3+1.2 −1.4 MJ

Close

On October 15, 2001, a team of astronomers including Jones, Butler, Tinney, Marcy, Penny, McCarthy, Carter, and Pourbaix announced the discovery of one of the most massive extrasolar planets that have yet been observed. It was discovered by the Anglo-Australian Planet Search team, using a Doppler spectrometer mounted on the Anglo-Australian Telescope.

Pi Mensae b has a very eccentric orbit and takes 5.72 years to revolve around the star. The semi-major axis of the planet's orbit around the star is 3.31 AU. This planet passes through the star's habitable zone at periastron (1.19 AU) while at apastron, it passes to around Jupiter-Sun distance (5.44 AU). The gravitational influence of this planet would disrupt the orbit of any potentially Earth-like planet.^[2][5]

Pi Mensae b is over ten times more massive than Jupiter,^[6] the most massive planet in the Solar System. It will have 10 times the surface gravity of Jupiter alone and could be incandescent (glowing).^{[citation needed]}

In 2020, the true mass of Pi Mensae b was measured to be 14.1 M_J via astrometry. Since this is greater than 13 Jupiter masses, the object could be considered a brown dwarf.^[7] The most recent astrometric results as of 2022 have revised this mass estimate slightly downward, to 12.3 M_J.^[8][2]

The plane of orbit of Pi Mensae b is strongly inclined to equatorial plane of the star, with the misalignment equal to 24±4.1°.^[9]

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