47 Ursae Majoris

47 Ursae Majoris, often abbreviated as 47 UMa, is a solar twin, yellow dwarf star approximately 46 light-years away in the constellation of Ursa Major. As of 2008, it has been confirmed that two extrasolar planets orbit the star. Because of this, 47 Ursae Majoris was listed as one of top 100 target stars for NASA's former Terrestrial Planet Finder mission.

Distance and visibility

47 Ursae Majoris is located fairly close to our solar system: according to astrometric measurements made by the Hipparcos astrometry satellite, the star exhibits a parallax of 71.04 milliarcseconds, corresponding to a distance of 14.1 parsecs. With an apparent magnitude of +5.03, it is visible to the naked eye under good conditions.

Stellar components

47 Ursae Majoris has a similar mass to that of our Sun. It is slightly more metal-rich than the Sun, having around 110% of the solar abundance of iron. With a spectral type of G1V, it is also slightly hotter than the Sun, at around 5,882 K. 47 Ursae Majoris has an absolute magnitude of +4.29, implying it has a visual luminosity around 60% greater than the Sun.

Like the Sun, 47 Ursae Majoris is on the main sequence, converting hydrogen to helium in its core by nuclear fusion. Based on its chromospheric activity, the star may be around 6,000 million years old, though evolutionary models suggest an older age of around 8,700 million years.

Planetary system

In 1996 an extrasolar planet was announced in orbit around 47 Ursae Majoris by Geoffrey Marcy and R. Paul Butler. The discovery was made by observing the change in the star's radial velocity as the planet's gravity pulled it around. The measurements were made by observing the Doppler shift of the star's spectrum. The planet, designated 47 Ursae Majoris b was the first long-period extrasolar planet discovered. Unlike the majority of known long-period extrasolar planets, 47 Ursae Majoris b has a low-eccentricity orbit. The planet is at least 2.62 times the mass of Jupiter and takes 1,095 days to orbit its star. If located in our solar system, it would lie between the orbits of Mars and Jupiter.

In 2001, preliminary astrometric measurements made by the Hipparcos probe suggest the orbit of 47 Ursae Majoris b is inclined at an angle of 63.1° to the plane of the sky. If these measurements are confirmed, this implies the planet's true mass is around 2.9 times that of Jupiter. However, subsequent analysis suggests that the Hipparcos measurements are not precise enough to accurately determine the orbits of substellar companions, and the inclination and true mass remain unknown.

A second planet, designated 47 Ursae Majoris c was announced in 2002 by Debra Fischer, Geoffrey Marcy and R. Paul Butler. The discovery was made using the same radial velocity method used to detect the first planet. According to Fischer et al., the planet takes around 2,594 days to complete an orbit. This configuration is similar to the configuration of Jupiter and Saturn in our solar system, with the orbital ratio (close to 5:2), and mass ratio roughly similar.

Subsequent measurements failed to confirm the existence of the second planet, and it was noted that the dataset used to determine its existence left the planet's parameters "almost unconstrained". Analysis of a longer dataset spanning over 6900 days suggests that while a second planet in the system is likely, periods near 2,500 days have a high false alarm probability, and the best fit model gives an orbital period of 7586 days at a distance of 7.73 AU from the star. Nevertheless, the parameters of the second planet are still highly uncertain. On the other hand, the Catalog of Nearby Exoplanets gives a period of 2190 days, which would put the planets close to a 2:1 ratio of orbital periods, though the reference for these parameters is uncertain: the original Fischer et al. paper is cited as a reference in spite of the fact that it gives different parameters, though this solution has been adopted by the Extrasolar Planets Encyclopaedia.

Simulations suggest that the inner part of the habitable zone of 47 Ursae Majoris could host a terrestrial planet on a stable orbit, though the outer regions of the habitable zone would be disrupted by the gravitational influence of the planet 47 Ursae Majoris b. However, the presence of a giant planet within 2.5 AU of the star may have disrupted planet formation in the inner system, and reduced the amount of water delivered to inner planets during accretion. This may mean any terrestrial planets orbiting in the habitable zone of 47 Ursae Majoris are likely to be small and dry. As of 2008, there were two METI messages sent to 47 Ursae Majoris. Both were transmitted from Eurasia's largest radar — 70-meter Eupatoria Planetary Radar. The first message, the Teen Age Message, was sent on September 3 2001, and it will arrive at 47 Ursae Majoris in July 2047. The second message, Cosmic Call 2, was sent on July 6, 2003, and it will arrive at 47 Ursae Majoris in May 2049.

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