47 Ursae Majoris b – a Gas and an Extrasolar Planet

47 Ursae Majoris b (abbreviated 47 UMa b) is a gas planet and extrasolar planet located in the constellation Ursa Major, approximately 46 light-years from Earth. It is a gas giant exoplanet that revolves around a G-type star. In January 1996, the planet was discovered in a long-period orbit around the star 47 Ursae Majoris, and as of 2011, it is the innermost of three known planets in its planetary system. It has a mass of 2.53 Jupiters, takes 3 years to complete one orbit of its star, and is located 2.1 AU away from it. It has at least 2.53 times the mass of Jupiter.


In 1996, it was announced that it had been discovered. NameExoWorlds, a process for giving proper names to certain exoplanets and their host stars, was launched by the International Astronomical Union in July 2014. The new names were chosen through public nomination and voting. The IAU announced in December 2015 that the winning name for this planet was Taphao Thong. The Thai Astronomical Society of Thailand submitted the winning name. Taphaothong was one of two sisters mentioned in a Thai folktale.


47 Ursae Majoris b, like the majority of known extrasolar planets, was discovered by detecting changes in its star’s radial velocity as the planet’s gravity pulls the star around. Taphao Thong was discovered by detecting changes in its star’s radial velocity caused by the planet’s gravity. This was accomplished by observing the Doppler shift of Chalawan’s spectrum.

Following the discovery of the first extrasolar planet, Dimidium, around a Sun-like star, astronomers Geoffrey Marcy and R. Paul Butler searched their observational data for signs of extrasolar planets and soon discovered two: Taphao Thong and 70 Virginis b. Taphao Thong was discovered in 1996.

Orbit and mass

47 Ursae Majoris b orbits its star at a distance of 2.13 AU and completes one revolution in 1,089 days. It was the first long-period planet discovered orbiting a main-sequence star. The eccentricity of 47 Ursae Majoris b’s orbit is low, in contrast to the majority of known long-period extrasolar planets. The planet has a 2:5 orbital resonance with the outer planet 47 Ursae Majoris c, which is similar to Jupiter and Saturn’s configuration in our solar system. Furthermore, the mass ratio of the two planets is similar to the mass ratio of Jupiter and Saturn.

The radial velocity method used to detect 47 Ursae Majoris b has the limitation of only providing a lower limit on the planet’s mass. Preliminary astrometric measurements indicate that the planet’s orbit is inclined at a 63.1° angle to the plane of the sky. If confirmed, this would imply that the planet’s true mass is approximately 2.9 times that of Jupiter. In any case, the mass cannot be significantly greater than the lower limit, or the system will become unstable.

Physical characteristics

Given its mass, 47 Ursae Majoris b is most likely a gas giant with no solid surface. Because the planet was only discovered indirectly, its radius, composition, and temperature are unknown. Its surface gravity is likely to be 6–8 times that of Earth due to its mass. Assuming a composition similar to Jupiter and a chemical environment close to equilibrium, the planet’s upper atmosphere is expected to contain water clouds rather than the ammonia clouds typical of Jupiter.

Although 47 Ursae Majoris b is outside the habitable zone of its star, its gravitational influence would disrupt the orbits of planets in the habitable zone. Furthermore, it may have hampered the formation of terrestrial planets and reduced water delivery to any inner planets in the system. Planets in the habitable zone of 47 Ursae Majoris are therefore likely to be small and dry.

It has been proposed that light reflections and infrared emissions from 47 UMa b, combined with tidal influence, could warm any moons in orbit around it to the point where they could be habitable, despite the planet being outside the normally accepted habitable zone.