Planetary System
The star is orbited by one known planet, designated Gliese 436 b. The planet has an orbital period of 2.6 Earth days and transits the star as viewed from Earth. It has a mass of 22.2 Earth masses and is roughly 55,000 km in diameter, giving it a mass and radius similar to the ice giant planets Uranus and Neptune in the Solar System. In general, Doppler spectroscopy measurements do not measure the true mass of the planet, but instead measure the product m sin i, where m is the true mass and i is the inclination of the orbit (the angle between the line-of-sight and the normal to the planet's orbital plane), a quantity that is generally unknown. However, for Gliese 436 b, the transits enable the determination of the inclination, as they show that the planet's orbital plane is very nearly in the line of sight (i.e. that the inclination is close to 90 degrees). Hence the mass quoted is the actual mass. The planet is thought to be largely composed of hot ices with an outer envelope of hydrogen and helium, and is termed a "hot Neptune".
GJ 436 b's orbit is likely misaligned with its star's rotation. In addition the planet's orbit is eccentric. Because tidal forces would tend to circularise the orbit of the planet on short timescales, this suggested that Gliese 436 b is being perturbed by an additional planet orbiting the star.
| Companion |
Mass | Semimajor axis |
Orbital period |
Eccentricity | Radius |
|---|---|---|---|---|---|
| b | 0.041 ± 0.05 MJ | 0.0291 ± 0.0004 | 2.64385 ± 0.00009 | 0.150 ± 0.012 | 4.327 ± 0.183 R⊕ |
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