Jennifer Tucker

Steven Gentile

14 March 2002

The Planets of 47 UMa

Throughout the Milky Way Galaxy there lies billions of stars. Within each of these stars there is the possibility for planets. It was not until 1995 that earthlings were able to detect these stars that contained planets. The planets that have been found have been Jovian type planets mainly because it takes a relatively brief amount of time to detect Jovian type planets due to the tidal tug on the star. Two of these Jovian size planets reside in the solar system of 47 Ursae Majoris (47 UMa). The star 47 Ursae Majoris, approximately 46 light years from Earth, resides in Ursa Major section of the galaxy and is located right under the bottom portion of the Big Dipper. The coordinates to find the star are +40º 25’46” and 10h 59m 29s. The star, roughly seven billion years old, is classified as a yellow dwarf and has a surface temperature of 5800K, which is the same surface temperature of the Sun. It can be seen with the naked eye on a dark night, but easier to see with the use of binoculars. In 1996, G. Marcy and R.P. Butler discovered two planets that orbit 47UMa; these two planets are HD95128 (47 UMa b) and HR4277 (47 UMa c). With these planets, calculations of the luminosity of the star, temperature of the planets, the planets’ distances from Earth, the planets’ distances from their star, and the period were found. These, and other characteristics of the planets, were explored and debated on whether it was possible to contain forms of life.

It would be difficult to determine the similarities and differences between Earth and extra-solar planets if the formulas that allow us to calculate specific qualities of the planets and their star were not available. Important qualities include the luminosity of the star, the semimajor axis, the albedo, and the temperature of the planets orbiting that star. The formula required to determine the luminosity of the star is; in which represents the luminosity of the star, represents the luminosity of the Sun, and M represents the absolute magnitude of the star. The luminosity of the Sun is known to be watts. The absolute magnitude of the star can be found using its predetermined spectral type. The spectral type of a star classifies it according to subtle features in the stars spectrum. Star 47 UMa’s spectral type is G1V; applying this information to the chart on page 477, figure 19-16 of the Astrophysics textbook Universe: Fifth Edition determined that the absolute magnitude was 5.1. When the values for the luminosity of the sun and star 47 UMa’s absolute magnitude are substituted into the equation the luminosity of the star is found to be watts. The semimajor axis, or the distance of the planet from its star, is converted to meters by multiplying the given distance, in AU’s, by Earth’s semimajor axis, meters. The resulting values for the semimajor axis for planet UMa b is meters, and for UMa c is meters. The albedo of a planet is the fraction of sunlight that it reflects. The albedo for both planets in orbit around 47 UMa is unknown, so instead we chose two albedos for both planets, representing the two extremes of the planet. The two albedos modeled after Jovian planets of our own solar system are, for UMa b, 0.6, and for UMa c, 0.56. The two albedos modeled after terrestrial planets of our own solar system are, for UMa b, 0.17, and for UMa c, 0.2.The luminosity of the star, the planet’s semimajor axis and the planet’s albedo are all important because they are used to determine the temperature of the planets in orbit around it. The formula required to determine the temperature of the planets, UMa b and UMa c, is . The calculated temperatures of the planets using the albedos similar to those of Jovian planets from our solar system are, for UMa b, 140K, and for UMa c, 110K. The calculated temperatures of the planets using the albedos similar to those of terrestrial planets from our solar system are, for UMa b, 170K and for UMa c, 125K. Sources of error in or calculations include rounding error and the use of a hypothetical albedo. Rounding error in any stage of the progression calculating the temperature would affect the resulting value. The hypothetical albedos are inaccurate representations of the planets actual albedos, as they were specifically to be a basis of comparison to the terrestrial and Jovian planets in our solar system.

There are many different factors that contribute to the possibility of life on planets. First of all, the heat of the planet has to be of a certain analogous to Earth’s temperature. Second of all, the planet must be void of migration. Also, the planet must have the specific elements necessary to produce life. The Jovian like planets 47 UMa b (2.41_Jup in mass) and 47 UMa c (.76_Jup in mass) are very similar to the Jovian planets in our solar system. They have conditions that suggest that the solar system is similar to our solar system. Like Saturn and Jupiter, they are very cold. The atmospheres of the planets might contain gases that are homologous to Jupiter and Saturn. The eccentricities, or how elliptical the planets’ orbit around the star are, of the planets are both less then 0.1. Because the eccentricities are so small, 47 UMa b and 47 UMa c make a nearly perfect circular orbit around their star, 47 UMa. A circular orbit, and the fact that the two planets are a distance to their star that is similar to Jupiter and Saturn’s distance to the sun indicates that the two planets have not migrated in like other planets that have been detected have. The fact that the two planets are very similar to Saturn and Jupiter suggests that there might be a habitable zone around 47 UMa like our solar system. Within the orbits of 47 UMa b and 47 UMa b might be a planet like earth that contains water, warm temperatures, and livable conditions that are similar to Earth’s conditions. Because the temperatures of the two planets orbiting 47 UMa are similar to the Jovian planets in our solar system, it is possible that there is an Earthlike planet in 47 UMa.

In conclusion, the temperature of 47 Ursae Majoris and its mass (approximately 1.03 times that of the sun) all indicate that the solar system of 47 UMa is analogous to our solar system. The planets orbiting the star share a great number of characteristics with planets from our own solar system, Jupiter and Saturn, and possess several unique qualities as well. Most of the extra-solar planets found are comparable to Jupiter and Saturn in size, but are too close to their star to suggest evidence of life. It is because of migration that the planets are close to the star. Because it does not seem as if 47 UMa b and 47 UMa c migrate towards the sun significantly, it is possible that the star is habitable for life. The solar system of 47 UMa is one of the more likely solar systems to actually support the existence or presence of living organisms.